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SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA' The purposes of these Powered By Docstoc

                                      P. C. SCRUTON2
                                       Tulsa, Oklahoma

   The Mississippi Delta is building because the rate at which sediments are supplied is faster
than the combined effect of the rate at which they are removed by waves and currents and the
rate the delta area is subsiding. River load is discharged into a variety of depositional en-
vironments in the Gulf of Mexico. These environments are inter-related, gradational, and
persist both laterally and through some depth of water. Sedimentary deposits being formed
in these environments have different lithologic and petrographic characteristics but also are
related and gradational. Differences between environmental factors and processes produce
the lithologic and petrographic differences and also produce correlative differences in deposi-
tional rates. Continuing delta growth causes seaward migration of these environments and
leads to the formation of deposits, or sediment units, of large areal extent which bear some
predictable relationships to each other.
   Sedimentary units which are almost entirely the products of river-borne material in order
of seaward occurrence are : (1) "marsh deposits," (2) "delta front silts and sands," (3) "pro-
delta silty clays," and (4) "offshore clays." These units are deposited approximately concentric
to distributary mouths and are analogous to the topset, forset, and bottomset beds of classic
delta description. Their characteristics depend to a large extent on the distance from source
and depth of water. Another unit, "marginal deposits," generally occurs between the deltaic
clays and outlying sands from marine sources. Marginal deposits have characteristics transi-
tional between the clays and the outlying sands.

                INTRODUCTION                     learn more about the basic principles of
  The Scripps Institution of Oceanog-            sedimentation and to develop criteria for
raphy, with a grant from the American            recognition of environments of deposi-
Petroleum Institute, has been studying           tion in older rocks.
the recent sediments in the Gulf of Mex-           The eastern side of the Mississippi
ico and their environments of deposition.        River Delta was chosen as one of the
The purposes of these studies are to             regions to be examined, because sedi-
                                                 ments are being deposited there very
   'Contribution from the Scripps Institution    rapidly by the distributaries or branches
of Oceanography, New Series No. 792. This        of the river. All of the beds formed by
investigation was supported by a grant from
the American Petroleum Institute, Project         delta building over the pre-existing con-
51.                                               tinental shelf have been investigated, but
      Stanolind Oil and Gas Company, Tulsa,       marine beds have received the emphasis
Oklahoma. Acknowledgments : Appreciation          because the completed delta is deposited
is extended to F. P. Shepard, Director of
Project 51, and to R. S. Arthur, M. N. Bram-      predominantly below the surface of the
lette, D. G. Moore, F. B. Phleger, and D. M.      Gulf of Mexico.
Poole for their numberless helpful sugges-           Five separate trips were made to col-
tions and comments made both during the           lect data at different seasons of the year
course of investigation and the preparation of
the manuscript. Special thanks also are due       and at different high and low stages of
John G. Bartram and Parke A. Dickey for           river discharge. A total of more than
their encouragment and help with the manu-        five months was spent in the delta. Over
script and to the Stanolind Oil and Gas           six hundred and fifty sample locations
Company which generously made time avail-
able to complete the work.                        were occupied, and cores were collected
22                                P. C. SCRUTON
at most of them. The gross lithology of mouths. These sediments which were
the cores has been studied in the field either deposited immediately or were
and at the Scripps laboratory. Studies transported laterally by waves and
were made of the coarse fraction (sedi- currents and later deposited now consti-
ment retained on 0.062 mm sieve) as tute the great mass of Tertiary and
described by Shepard and Moore (1954)         Quaternary shales and sandstones in the
and by grain size analyses. In addition a     Gulf Coast region. Slow and probably
large number of measurements have             almost              crustal continuous
been made, at different seasons of the        during this time has carried much of
year, on the currents and the tempera-        this material to great depths where it has
ture, salinity, turbidity, and chemistry of   been buried by younger deposits. At
the water. These measurements aid in          different times during this period, the
understanding the environments of depo-       sea has regressed southward and trans-
sition.                                       gressed northward. The net result has
  The Mississippi Delta has been studied      been a southward regression of
by many other geologists and engineers.       shore, however, because sediments were
Exhaustive reviews of the literature on       deposited more rapidly than subsidence
the delta have been published by R. J.        occurred. The modern Mississippi Delta
Russell (1936) and Russell and Dohm           represents one of the types of deposits
(1936). Trowbridge (1930), Russell and        formed during this long history of depo-
Russell    (1939), and Morgan (1951)          sition and subsidence.
have included extensive bibliographies in       Waxing and subsequent waning of
their publications. The most recent pub-      Pleistocene glaciation affected the Mis-
lications have been by Fisk (1944, 1952,      sissippi River and the modern delta in
1954), Morgan (1951, 1953), and Bates         two different ways. First, the advance of
(1953).                                       the ice sheet into northern United States
  The purpose of this report is to de-        blocked the channels of several north-
scribe briefly the physical processes and     ward-flowing rivers, such as the Miss-
factors operating in the depositional en-     ouri River in Montana and the Monon-
vironments and the different kinds of         gahela           in            River
                                                                    Pennsylvania, and
sediments of the Mississippi Delta, and       forced them to cut new channels into
to show how these processes and factors       Mississippi River drainage. This created
produce the different sediments. The          one of the world's great river systems
sediments vary in a regular manner sea-       and gave the Mississippi River a vastly
ward from shore but are relatively uni-       expanded drainage basin from which to
form parallel to the shore. The seaward       draw elastic sediment for transport to
variations of lithology, grain size, and      the Gulf of Mexico.
coarse fraction are the basis for sub-          Second, during maximum expansion of
division   of                 sedimentary
                                    the       the continental ice sheets in the Pleisto-
wedge into lithologic units. Another re-      cene, the shoreline must have regressed
port is planned to present more detailed      seaward to one of its most southern
descriptions of the sediments and the         points. When the ice melted, sea level
environments of deposition.                   was raised and the shoreline trans-
                                              gressed northward. The sea advanced
                                              rapidly over a surface previously ex-
  During Teritary and Quaternary              posed to subaerial conditions into a val-
times many rivers in the southern part        ley which had been eroded by the en-
of the United States flowed south into        larged river to the lowered baselevel of
the Gulf of Mexico. They transported          the Pleistocene sea. Thus, in the present
vast amounts of sediments and dis-            Lower Mississippi Valley, the sea prob-
charged them into the ocean at the river      ably entered a relatively vvide trough and
         SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                              23

formed an embayment or estuary (pl. I).           pushed rapidly southward because the
Earlier deposits of the Mississippi and           river supplied sediment faster than it
other rivers of the area were submerged           could be removed by waves and currents
and subjected to further erosion, win-            and faster than the area was subsiding.
nowing, and transportation by waves and             As deposits of the Mississippi River
currents. Off the coast of Mississippi and        have grown seaward         in     post-glacial
east of the modern delta there now exist          time, the positions of the river mouths
extensive sheets of sand sorted in this           have not stayed in one place. Whenever
manner from sediments originally de-              the accumulation of sediments has over-
rived from rivers to the east. These sands        extended the river, the resulting- gradient
contain a different mineral assemblage            disadvantage has caused a shifting of
than the Mississippi River sands and are          the direction of discharge. Many such
called Eastern Gulf sands.                        shifts in position of the river mouths
  During stabilization of sea level the           occurred during building of the shore-
Mississippi River with its great flow of          line to its present position. The Teche
fresh water and heavy load of sediment            and Lafourche are older subdeltas west
began building new land southward. This           of the present one. The St. Bernard
land was formed and the shore line was            Subdelta lies farther north and built

  ARKtANSAS                                  IMBRICATING DELTAS
                                             COASTAL LOUISIANA
 ALEXANDRIA            AND OLDER             -: DELTA SEQUENCE :-
PLEISTOCEN               LAND
                                        CD BAYOU LAROSE 0 MARINGOUIN
                                        0 COCODRIE   TECHE
                                        0 LAFOURCHE 0 PLAQUEMINES -
                                        ST. BERNARD 0 MODERN
                                         BATON                              OLD SHORE LINE
                      VALLEYn'           ROUGE                             ( 5000 t YRS. AGO )


   SHORE LINE                                      10-110111                            MODERN
                                                         loo;                           SHORE

                PLATE I.Step-by-step development of the Mississippi River's delta
                                complex in coastal Louisiana.
24                               P. C. SCRUTON
eastward into the present Chandeleur         Delta are representative of some of those
Island area. The Plaquimines (Forts)         constituting the great mass of Gulf  .

subdelta was built mostly near the site      Coast Tertiary sediments. Building of
of the present active delta (pl. I).         the modern delta is one of a series of
  Rapid seaward advance of the shore-        similar events which has filled the Post-
line of each of the subdeltas during their   Glacial Mississippi estuary and built out
constructional phases is followed by a       the coast of Louisiana. Therefore, the
slow retreat when the river is diverted to   modern delta also should be representa-
a different area of discharge. This shore-   tive of the individual wedges which
line retreat may in part be the result of    make up this large deposit (pls. I and
compaction of sediments and regional         II).
subsidence, but it is also the result of
marine erosion, winnowing, and removal                  SEDIMENTARY PROCESSES
of previously deposited sediments. This        At the point where a large river meets
is the destructional phase of a delta's      the sea many powerful and conflicting
history. Extensive sand areas on the sea     physical forces and processes control the
floor south of the Teche and Lafourche       distribution and deposition of sediments.
subdeltas and east of the Chandeleur         The work described in this report was
Islands are the result of sand concentra-    undertaken to get more and better data
tion during this destructional phase.        on these conditions and their effects on
  These sand sheets mark the limit of        the sediments. It is hoped that by recog-
seaward advance of one delta and may         nition of the processes producing the
he overlapped and buried by the sedi-        lithologic, structural, and organic char-
ments of younger deltas. Because these       acteristics of the sediments of the mod-
deltas were built upward to sea level in     ern delta, it may be possible to better
water which gradually deepened to the        interpret ancient sediments deposited in
south, they each consist of a wedge or       similar deltas.
embankment of sediment thickening sea-          Forces producing water motion and
ward (pl. 2). The coastal area of Louisi-    physical processes that are recognized
ana consists of an imbrication of several    in the Mississippi Delta include the fol-
such wedges. Plate I shows the step-by-      lowing :
step development of the deltas in coastal
Louisiana ; plate II shows the morphology        Seaward flow of fresh water due to
of the modern delta and its relationship         conserved momentum
to older deltas and the pre-existing con-        Currents in and around distributary
tinental shelf and slope.                        mouths, due to density differences, mix-
   A part of the sand area east of Breton        ing between masses of fresh and salt
and the Chandeleur islands and north of          water, and also due to hydraulic head
the present delta is shown in figure 1.          Tidal currents
It is most important to remember that            Wind waves and longshore currents due
this mass of sand is not being deposited         to waves
by the present delta, but was produced           Large scale vvind-driven currents
by wave and current erosion and win-             Regional, semi-permanent currents
nowing of sediments deposited as the             Differential settling of particles
earlier St. Bernard subdelta. Still              Turbidity currents and slumping
 farther northeast near Cat Island, this          Diffusion.
residual St. Bernard sand body merges
into the larger mass of Eastern Gulf         These forces and processes are acting
sands previously described.                  simultaneously to supply and distribute
     Deposits of the modern Mississippi      varying amounts of different kinds of

                            11,1# 'II 1.              !        I

                                i'll° .° .' 1: I ' .
                                   d'ollo .11. °'.
                               ' .1 p * . ' /I

 á                      !I's
 to                     ii.'   .1               0-
       -        .   i?°.11°P.1:11;\:/°:
                                                '         1....1.1

                               ,0411°,11. 1
                    :                  1

                               ilf         :I
                                                      I    I

            i       1

           x. ..I   I              1


   PLATE II.Morphology of the Modern (Balize) Delta of the Mississippi River and its relationships to
older deltas and the continental shelf and slope. Distances seaward toward deposits of the older deltas ex-
tended are unknown. Detailed cross section X-X' shows spatial relations between (a) marsh deposits (includ-
ing natural levees), (b) shallow water delta front silts, (e) delta front sands and silts formed off large
distributaries, and (d) pro-delta silty clays. (e) indicates the horizon of offshore clays.
26                                                      P. C. SCRUTON
               89.2      8,10.


                                                                                                   MISSISSIPPI RIVER DELTA

                                                                                  DISTRIBUTION OF SEDIMENT UNITS
                                                                                               .     1951-1953


                                                                                              DELTA FRONT

                                                                                     teASS4   SILTS 0 SANO5
                                 OuTTO NTSII                                                                                                          g90:
                                                                                              pRo DE,L,TAA,,SILTT

                                                                                                                      MOLDER DEPOsiTs
        SCUT   ST PASS                                                               LAOFFSHORE CLAYS

                                                                                              CIAT4MISWITYEENR EfAISNTÉCLGtfTt'S'D

                                                                                                           STATuTE MILES
                                                                                                       CONTOURS IN FATHOMS

                                               69'00'      8E150I         &PAO'       88'3,                  88.20'                    88,         ee 00.

     FIG. 1.Note : Varied cross-hatching in offshore clay and marginal deposits indicates
                                   different mineral suites.

sediments. They also react on each other                                enough so that only fresh water flows in
so that their effects may cancel or be                                  the distributary channels. Under these
additive. In addition they exhibit fluctua-                             conditions      the             entire                 cross-sectional
tions of intensity with periods that range                              areas of the channels are occupied by
from very short to annual or longer,                                    seaward-moving fresh water and sedi-
some of which occur regularly and some                                  ment. Bed load, suspended load, and all
at random. In all of the processes in-                                  sediments deposited in                                     the        channels
volving water movement, turbulence also                                 under previous, lower discharge condi-
is an important factor. Despite this great                              tions are transported out to the ends of
complexity it is possible to recognize                                  the distributaries and delivered to the
dominant processes in some areas and to                                 Gulf. The topographic form of the larger
interpret their effects.                                                distributaries, including depth, width, and
   A great volume of fresh water, loaded                                both the distributary mouth bars and
with sediment ranging in size from sand                                 flaring distributary mouths, are devel-
to clay, flows into the Mississippi Delta                               oped under these high discharge condi-
where the main river channel breaks up                                  tions. Distributary mouth bars and the
into the numerous large and small dis-                                  flaring natu-ral levees at the seaward
tributaries. Each of these distributaries                               ends of the distributaries are formed
has a different rate of flow which, of                                  where the light fresh water meets and
course, varies greatly between different                                flows out over more dense Gulf water
seasons of the year.                                                    beneath. River water is buoyed up by
   During about one-third of each year,                                 the sea water. The layer of river water
the discharge of the Mississippi is great                               is thinned in this manner and therefore
       SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                     27

is forced to spread laterally to accom-       Mexico, conditions also are complex.
plish the same discharge. The distribu-       The general effects of the physical proc-
tary mouth bars and flaring natural           esses controlling sedimentation in the
levees are the streamlining topographic       Gulf during flood stages are as follows.
forms which develop in response to this       The moving mass of fresh water with its
dynamic situation.                            load of sediments flows into the Gulf
  During the remaining two-thirds of          out over sea water which has a greater
the year, discharge of the river is low       density. Movement of the sea water is
enough so that the entire capacity of the     the result primarily of waves, tides,
distributary channels is not required to      winds, and regional currents, but also is
conduct river water seaward. Under            produced by the fresh water being dis-
these conditions dense salt water creeps      charged. In several different but inter-
over the bars and flows upstream in the       related marine environments, the sedi-
distributary channels beneath seaward         ments settle to the bottom to form the
flowing river water. The salt water en-       delta deposits. Under low river stages,
croaches landward for distances up to         this chain of sedimentary events is initi-
about 125 miles. These distances con-         ated in the river at the upstream end of
stantly vary with the rate of river flow,     the salt wedge rather than in the Gulf.
the tides, and the winds. Intrusion of sea       Because the light river water flows out
water into the distributaries forms what      on top of the salt water, its velocity   is
is called the "salt wedge." The top sur-      not     completely   checked   immediately.
face of the salt wedge is continually         Sediment deposition begins as soon as
eroded along its length by mixing with        the river water passes the crest of the
river water, so that river water may          bar and leaves the confines of the dis-
already contain appreciable quantities of     tributary channel. In addition, bed load
salt when it finally is discharged into the   and material eroded from the channel
Gulf. Bed load of the river cannot be         bottom is moved up to the bar crest,
transported seaward of the upstream end       shoved over into the Gulf, deposited, and
of the salt wedge, because there is no        helps      to build the bar seaward. Sus-
downstream current present on the bot-        pended particles of different size, weight,
tom. In the same way much material            and shape settle at different velocities.
being transported seaward in suspension       Rapidly settling sand is abundant in the
is deposited when it is trapped by settling   samples collected close to the channel
into the underlying salt wedge. These         mouths, although sand is the least abund-
soft oozy deposits, formed in the river       ant material in suspension. Silt and clay
channel during low water, are flushed         are also settling to the bottom with the
out into the Gulf over the distributary       sand, however, so that even the first
mouth bars during succeeding flood            deposits seaward of the bar crests con-
stages, however.                              tain all three kinds of material and are
   Because of these processes which are       not just clean sand. This can be seen
 the result of "system dynamics" there        schematically in figure 2, which was
 is an annual fluctuation in both size        drawn to show the relationships between
 characteristics and amount of sediment       the change in suspended load, size com-
 delivered to the Gulf which is independ-     position of sediments, and rate of depo-
 ent of any other factors. The greatest        sition.
 volume of coarsest material is trans-            Finer sediment is retained in suspen-
 ported to the Gulf during high water.         sion, in part because of turbulence, and
 Less material of finer size is transported    is more widely distributed by the decel-
 during the remainder of the time.             erating river flovv and by wind, tidal,
  At the mouths of the distributaries          regional, and other currents. Diffusion
 where the river meets the Gulf of             associated with large, horizontal eddies
28                                             P. C. SCRUTON

                                              MISSISSIPPI RIVER DELTA

                                       RELATIONSHIP BETWEEN CHANGE
                                    IN SUSPENDED LOAD, SIZE COMPOSITION
                                         OF SEDIMENTS, AND RATE OF
                                           DEPOSITION (SCHEMATIC)
            SEA LEVEL


                        UNIT VOLU
                        OF WATER

                                             AL,                              AL,

                                                                        Lo      INITIAL SUSPENDED LOAD
                                                                        at_     CHANGE IN SUSPENDED LOAD

                                                                                CONTACT BETWEEN THE RIVER
                                                                                WATER AND THE UNDERLYING
                                                                                SALT WATER.



                                              RATE OF
                                      H,. H, DEPOSITION

     FIG.   2.Open blocks represent a unit volume of water with the amount and kind of sus-
pended sediment shown schematically by the enclosed smaller blocks. Solid vertical arrows
from the small blocks are proportional to the settling velocities of sand, silt, and clay. Initial
sediment load (L.) is continuously decreased in amount and kind by the change in load
(AL). The amounts of sediment deposited and the proportions of sand, silt, and clay in
bottom deposits are related to settling velocities and distance f rom river mouth. Outward
changes in the size composition of bottom sediments are shown by blocks Hi, H2, and H. The                  41

heights of these blocks are proportional to the rates of deposition which decrease seaward.

also may be important. Particle size                        growth of marine organisms and by
characteristics of the transported load                     lateral migration of sands and silts from
change with increasing distance from                        outside sources along the sea bottom on
distributary mouths, and smaller parti-                     the shelf. These are moved by waves
cles become relatively more abundant                        and currents. In this area, they are com-
both in suspension and in the deposits.                     ing from the north and east toward the
The suspended load decreases constantly                     south and west.
during transportation, and the direction                      Relative importance of the different
of distribution changes with fluctuating                    processes that produce these general re-
currents. Therefore, a lesser amount of                     lationships varies              the area             within
sediments is spread over a larger area,                     studied. Differences in processes between
and the rate of deposition decreases with                   subareas is discussed below following a
increasing distance from the source. Ad-                    description of the sediments. There it
ditional material of marine, not river,                     is shown how differing processes may
origin is added to the sediments by the                     produce the same type of lithology.
                SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                                                            29

                                SEDIMENTS                                          In general, near the distributary
                       General Features                                           mouths the sediments are largely derived
   The vertical sequence of types of                                              from the river and contain very little
deposits forming                   water at marine-derived material. In deeper water
depths on the front of the actively farther from the distributary mouths, as
building delta corresponds to the strati- the rate of deposition of river sediments
graphic sequence making up the com-                                               decreases, the slower rates of supply of
pleted delta wedge. As the delta builds                                           marine-derived material become increas-
seaward, sediment characteristics at any                                          ingly important, until, farther out to sea,
particular horizon are incorporated into                                          shells and current-transported sand and
the wedge and their relative position is                                          silt from outside areas constitute much
maintained as modified by later com-                                              of the deposited sediment. Several char-
paction and structural movements. Char-                                           acteristics of the sediments will be de-
acteristics of the sediments are analog-                                          scribed which vary systematically with
ous to those of the classic delta sequence                                        increasing distance from shore and depth
consisting of topset, foreset, and bottom-                                        of water along any traverse outward.
set beds. However, no sharp planes of                                             These characteristics are gross lithology,
separation exist and initial dips are at                                          texture (grain size), coarse fraction
very low angles. All available data indi-                                         composition, and color.
cate that this relationship holds around                                            Gross lithology is most complex and
the entire Mississippi Delta.                                                     variable in the shallow water near the

                                                                     MISSISSIPPI RIVER DELTA

                                           CHANGE IN MAJOR COARSE FRACTION COMPONENTS
                                              WITH INCREASING DISTANCE FROM SHORE

                        A. NORTH PASS TO OPEN GULF                                      B. BAPTISTE COLLETTE SUBDELTA
                                                                                                TO BRETON SOUND

                                                                SKELETONS                           AGGREGATE
                                                            OF MARINE                             WOOD                    MARINE
                                                        ORGANISM                   75                  AND PLANT          ORGANISMS
                                           AND PLANT                                                     FIBERS
                    (MISSISSIPPI                                   (EASTERN        50
                        RIVER                                        GUL F
                                                                    MINERAL                        TERRIGENOUS SAND
                      MINERAL               TERRIGEN,O, SAND
          8 25                                                       SUITE,        25             (MISSISSIPPI RIVER
                        SUITE   I                                                                   MINERAL SUITE,

           100                      SAND                             lE ASTERN    100
                 (MISSISSIPPI RIVER                                     GULF
                    MINERAL SUITE              SILT                   MINERAL
           75                                                          SUITE,
                                                                                   75              SILT
    .1"    25
                                                                                 r 2,              CLAY

            O                                                                       O
                                      INCREASING DISTANCE FROM                                 INCREASING DISTANCE FROM
                                        SHORE TO EAST                                             SHORE TO NORTH

  FIG. 3.Note changes in amounts of sand with increasing distance from the delta. Com-
position of the sand also changes; terrigenous (land-derived) components decrease, marine-
derived components increase, and eastward f rom North Pass the mineral suite changes.
30                               P. C. SCRUTON
coast and becomes more uniform and           which were produced by deposition pre-
homogeneous     with   increasing water      vious to the present delta cycle. These
depth and distance seaward. Near the         beds are called "older deposits." The
outer edge of deltaic deposition, litholo-   first three of the listed units are the
gic structures change character and          most important in delta growth. They
gradation into the outlying sands begins.    are deposited approximately concentric
Grain size varies in somewhat the same       to the distributary mouths. In general
manner (fig. 3). It generally is coarsest    their characteristics depend on distance
and most variable near the delta shore,      from source and depth of water. The
becomes finer and more uniform out-          fourth sediment unit, marginal deposits,
ward at greater depths, reaches a mini-      as the name suggests, occurs at the outer
mum, and with greater distance seaward       edge of the delta deposit where river-
on the shelf, again becomes variable and     derived silts and clays are mixed in a
coarse until the outlying sands are          transition zone with the bottom sands
reached. Within the coarse fraction (fig.    from outside the delta.
3) the general progression is a decrease       The distribution of these sedimentary
in the amounts of terrigenous (detrital)     units on the eastern side of the Missis-
sand to very small percentages of the        sippi Delta is shown in figure 1. Bound-
total deposit. Land-derived woody mate-      aries of the units were drawn after study
rial also decreases to the eastward paral-   of the core samples. Since changes in
leling its decrease in suspension shown      sediment character are gradational and
by Riley (1937). Beyond the sand mini-       only small changes occur in short dis-
mum (which may have considerable             tances, the exact locations of boundaries
areal extent) quantity of detrital sand      must be somewhat arbitrary. The order
again increases. Mineralogy and grain        of discussion of sediment units is from
size relationships show that this sand is    the shore outward (youngest to oldest as
derived from the outlying shelf deposits,    they would appear from the top to the
however. Remains of marine organisms         base of a section through the completed
also become more abundant, increase          delta wedge, see fig. 4). Figure 5 is a
toward the outer sands, and in some          fence diagram that shows both vertical
areas, replace woody material. Colors        and lateral relationships between sedi-
of surface sediments generally vary from     ment units northeast of the delta.
yellowish brown or brown near shore              Description of Sediment Units
through olive gray to light olive gray
with increasing distance seaward. Colors       Marsh deposits.-Deposits of
below the surface zone also appear to        marshes (including the natural levees)
vary systematically from brownish gray       lie at the top of the completed cycle of
near shore through olive gray and gray-      delta deposition and are a portion of the
ish black to dark greenish gray and olive    topset bed (fig. 1). They are the only
gray offshore.                               deposits in the active delta formed above
     Areas within which these different      mean sea level and are deposited under
characteristics are relatively uniform       both fresh and brackish water condi-
have been mapped as sediment units. The      tions. These beds are built up mainly
sediment units, in order of seaward oc-      from deposition of suspended material
currence, are called: "delta front silts     carried into the marshes during high
and sands," "pro-delta silty clay," "off-    river stages by overtopping of natural
shore clays," "marginal deposits," and       levees and also by growth and subse-
"sands." Within the delta itself, lying      quent deposition of marsh vegetation.
above the marine sequence, is another        Overtopping of the natural levees dur-
sedimentary unit, "marsh deposits." In       ing high river stages occurs particularly
addition there are deposits       exposed    in conjunction with high tides, because
                                  MISSISSIPPI         RIVER     DELTA

                         COLUMNAR SECTION DEVE LOPED
                                DURING     DELTA              BUILDING
                                          (SCHEMATIC )
                                                              MARSH DEPOSITS

        SEA LEVEL-          -                                               OFF LARGE
            OFF SMALL
           DISTRIBUTARIES                                      ./////i/
            DELTA FRONT
           SILT AND SAND
                                  --                                        DELTA FRONT
                                                                            SILT AND SAND


            SILTY CLAY

                                                                            SILTY CLAY


                                           //////             PRO-DELTA SILTY CLAY

           MAY OCCUR                       -    EITHER
                                                                            OFFSHORE CLAY


                                                                            MARGINAL DEPOSIT
                                           DEPOSITS           =MINI

                                    fig        SANDY-SILTY-CLAY              SHELL S

                                               SILTY CLAY a         1   1
                                                                             PLANTS a
               SILT                            CLAYEY SILT                   PLANT ROOTS




  FIG. 4.Shows the vertical distribution of sediment units in the completed delta. Be-
cause of areal variations, some differences exist in the vertical sequence from place to place.
Either of the sequences may occur at the indicated positions.
32                                P. C. SCRUTON
                                   MISSISSIPPI RIVER DELTA

                           SPATIAL RELATIONS OF SEDIMENT UNITS

                                PASS A LOUTRE TO BRETON SOUND

                                                                           MARSH DEPOSITS

                                                                           DELTA FRONT SILTS 4. SANDS

                                                                           PRO-DELTA SILTY CLAYS

                                                                             MARGINAL DEPOSITS


                                                                           OLDER DEPOSITS

FIG. 5.Fence diagram showing three-dimensional relations between sediment units in the
inshore portion of the delta area. Marsh deposits lie at the top of the completed sequence.

raising of the sea level partially im-           clayey silt3 or silty clay but also contain
pounds river-flow causing river level            appreciable quantities of sand (fig. 6).
to rise even more. Hurricanes, which             They frequently are heterogeneous in
may raise sea level several feet around          texture, and usually contain many small
the delta, also cause general flooding of
the marshes. Probably a relatively small             Sand-silt-clay terminology is the system
amount of material is brought in in this         recently suggested by Shepard (see Shepard,
                                                 F. P. (1954) Nomenclature based on sand-
manner, however.                                 silt-clay ratios : Tour. Sedimentary Petrology,
  Marsh deposits consist principally of          v. 24, fig. 7, p. 157).

34                                  P. C. SCRUTON
lenses of silt or sandy silt near the base   fragments are rare. In addition to detri-
of the deposit (fig. 7). They also tend to   tal sand and small fragments of plant
be somewhat coarser on the low natural       material, there is an abundance of plants
levees than in the marsh farther from        and plant roots buried in situ (fig. 7).
distributaries. The coarse fraction (sedi-   These larger plant remains bond the de-
ment coarser than 0.062 mm) contains         posit into a coherent mat.
an abundance of woody material in addi-        In general, marsh deposits are quite
tion to the detrital sand; shells or shell   thin over the young land surface of the


40                      " 20                                  40
     FIG. 7.Marsh deposits (scale in centimeters). Note abundant, large, and randomly
placed plant remains. Also change of lithology with depth (B) from marsh deposits toward
underlying delta from silt and sand type.
            SEDIMENTS OF THE EAS7'ERN MISSISSIPPI DELTA                                 35

eastern Mississippi Delta. Natural levees      tively small amount of river water and
are the thickest parts of the marsh; their     sediment and initially discharge their
height (and thickness) above mean sea          load into very shallow water. The water
level is determined initially by the height    depths in which silts and sands off small
of flood waters and by tidal rise. This        distributaries are being deposited range
initial thickness of natural levee deposits,   from a few inches to approximately six
and other parts of the marsh as well, is       feet with an average of three to four
increased by continued addition of sedi-       feet. Wave and current action continually
ment during compaction of underlying           sort and retransport the material. Cores
beds.   -                                      show that under these conditions, the
  Delta front silts and sands.Delta            thin delta front deposits consist mostly
front silts and sands occur as a generally     of alternating layers of coarser material,
continuous deposit of variable thickness       well sorted sandy silt, and finer material,
seaward of the mouths of the numerous          silty clay or clayey silt. The coarser
delta distributaries (fig. 1). These de-       layers of sandy silt generally show sort-
posits are composed almost entirely of         ing with the coarsest material at the
river-derived material. There is more          bottom and an upward gradation to finer
silt than sand in the delta front deposits,    material. The individual coarse layers
although the general impression seems          range in thickness mostly from three to
to have been that they are mostly sand.        20 cm, but some are thicker. Because of
The triangle diagram in figure 6 shows         small variations in components or texture,
that in most of the analyzed samples the       they exhibit in the cores distinct and per-
sand content is less than 50 percent.          sistent fine lamination throughout. Some
Most of the sand present is very fine. In      of this fine bedding is horizontal and
the area of delta front silts and sands        regular, some is cross bedded, and some
shown in figure 1 there are no beaches         is wavy and distorted (fig. 8). The cross
around the delta.                              bedding occurs in relatively thin sub-
  In all samples, woody material is pres-      zones (up to two cm thick) with inclina-
ent and in some it is abundant. Shell          tions up to 25 degrees from the horizontal.
fragments, Foraminifera, and ostracods            The layers of finer material, clayey
are rare and entirely absent in some           silt and silty clay, usually are thinner
samples. Averages of coarse fraction           than the sandy silt beds, ranging in
analyses (sediment coarser than 0.062          thickness from two to 10 cm. The tex-
mm) of thirteen samples showed 87.7            ture of these layers tends to show a sys-
percent terrigenous sand, 6.7 percent          tematic change with the finest sediment
wood and plant fibers, and the remainder       near the center of the layer. Individual
mostly mica and a ferruginous (limo-           clayey layers often include a few thin
nitic) aggregate of silt and clay which        beds or lenses of silt. There are also
apparently is authigenic and persists even     some lenses, composed of fine bits of
after treatment with sodium hexameta-          woody material, with thicknesses from
phosphate.                                     a few millimeters to 50 centimeters.
   Delta front silts and sands deposited       Morgan    (1953)    called        material    this
in shallow water, usually off small dis-       "coffee grounds." Locally in the bays
tributaries, overlie all other marine beds     scattered oyster or other shells occur at
of the delta. They grade downward into         this general horizon.
both pro-delta silty clays and the silts         Cores taken in closely spaced core
and sands formed off the major distribu-       holes show that the alternating layers
taries. For this reason, they are consid-      of sandy silt and silty clay change in
ered part of the delta's topset bed.           thickness and appearance laterally within
   The smaller distributaries carry a rela-    short distances. Individual laminations
36                               P. C. SCRUTON




FIG. 8.Delta front silts and sands (scale in centimeters). Note delicate lamination, small
     scale cross bedding, and alternating zones of coarse and fine materials (0-60).
          SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                   37

do not seem to persist as much as a yard,      tend to be more massive than those
although the layers can be traced several      formed in shallow water and to have less
times this distance. In most cores if the      sharply defined vertical changes in tex-
coarser silt layers are thick, the alternat-   ture, to be somewhat coarser although
ing finer clay layers are thin, and where      less well sorted, and are considerably
the clayey layers are thick, the silt layers   thicker. They also occupy different posi-
tend to be thin. Despite the lack of areal     tions in the delta wedge, so that they
uniformity in the alternations of silt and     differ in their spatial relations with pro-
clay layers, they appear to have been de-      delta silty clays. The reasons for these
posited, at least in some places, in small     differences are that the larger distribu-
depositional cycles. These cycles may be       taries carry more and coarser sediment,
related to the annual fluctuations in kind     deposit this material at faster rates, and
and amount of sediment because of              build their parts of the delta into deeper
variation in the river discharge, but this     water. The high rate of deposition in
record is obscured by the annual wave          deeper water reduces the winnowing and
cycle. The shallow delta front deposits        retransporting effects of currents and
usually contain several superimposed al-       waves. The sediments are distributed
ternations of coarse and fine layers; the      more according to the settling velocities,
range in total thickness is from approxi-      with coarsest material nearest the chan-
mately one to six feet, averaging about        nel mouths. Such deposits tend to have
four feet thick.                               fewer laminations and less sharply de-
  Off the mouths of smaller distribu-          fined textural changes because they are
taries, the thin beds of delta front silts     not washed, sorted, and retransported in
and sands grade downward into pro-             shallow water. As these deposits build
delta silty clay by progressive increase       upward to sea level, however, and the
in thickness of the clayey zones, de-          water depth decreases, their character-
crease in thickness of silt zones, and ab-     istics become like those formed off small
sence of fine lamination in the silts. As      distributaries. Plate II (detail) shows the
the small distributaries continue to build     relationships between the delta front sedi-
their coalescing deltas seaward, the re-       ments and pro-delta silty clays.
cently deposited delta front sediments            Sand off the channel mouth is coarsest
along the channels are eroded so that          and best sorted on the crests of the bars
underlying clays constitute the channel        where the water is shallow, and it gen-
floors.                                        erally becomes progressively finer with
  Delta front silts and sands deposited        increasing depth of water and distance
in deeper water off the mouths of major        from thc channel mouth, both seaward
distributaries underlie a zone of silts        and laterally. The coarsest material is
and sands of the shallow water type            deposited seaward of the bars when the
(fig. 4). They are deposited on the steep-     river is in flood stage and fresh water is
est seaward slopes of the delta, corre-        flowing seaward at all depths in the
spond in some ways to a coarse phase of        channels. Delta front samples when the
pro-delta silty clays, and represent a         river flow is high range from very fine
part of the delta's foreset bed system.        sand to medium silt size (median diam-
Because of their relative coarseness and       eters 0.12 mm to 0.02 mm). At other
relationship to distributory mouths, how-      seasons samples are finer than this, both
ever, they are mapped as a unit with the       upstream and seaward of the bar crests,
silts and sands of shallow water.              although bar crest material may be quite
  Delta front sediments off the larger         coarse at any season.
distributaries differ in several respects        Both coarse and fine zones occur in
from those formed in shallow water, usu-       thesilts and sands off large distribu-
ally off the small distributaries. They        taries. Cores of the coarser zones (sands,
38                                P. C. SCRUTON
silty sands, and sandy silts) show that        posit, therefore, may be convex down-
they are relatively thick and quite homo-      ward in cross section, relatively flat on
geneous vertically and textural changes        top, and be asymmetrical about the chan-
are gradual. The zones of finer texture        nel extension.
(clayey silt, silty clay) also may be rela-      Delta front silts and sands off the
tively thick and contain numerous thin         larger distributaries grade both laterally
layers of silt. Some bedding consisting        and downward into pro-delta silty clays.
of one to two cm thick layers of sand or       The grain size progressively decreases,
silt may be present. There is very little of   and the lithology becomes less complex
the small scale cross bedding and sharply      and variable.
pronounced vertical cyclic variation that         Much of the material referred to by
was described in similar sediments de-         Russell and Russell (1939) as "natural
posited in shallow water.                      levee deposits" and by Fisk (1952) and
  Total thickness of the delta front de-       Fisk, McFarlan, et al (1954) as "sand
posits off any large distributory is re-       fingers" or "bar sands" is deposited in
lated to the maximum depths of water           this delta front environment. As demon-
in which they were deposited initially.        strated by South and Southwest Passes
Under existing discharge conditions, the       (fig. 1), a large distributary tends to build
maximum depth at which delta front silts       seaward in a straight line. As it moves
and sands are found is. about 60 feet off      ahead, it         its channel into the
Southwest Pass, 35 feet off North Pass,        coarser material previously deposited,
and 10 to 12 feet off Main Pass. The           and wherever the distributary channel
depths increase in order of percentage of      is deep enough, it will erode through the
total river discharge carried by these         recent delta front deposits. However, the
distributaries. Seaward slopes of the          sediment in these deposits is not all
delta front off the distributaries are rela-   clean sand but contains much silt and
tively uniform (concave upward, maxi-          clay both in the sandy zones and in the
mum of about two degrees in 12 to 30           alternating finer-grained zones (fig. 6).
feet of water, decreasing seaward) so             Pro-delta silty clays.On the east side
that depth limits (and thickness) of the       of the Mississippi Delta, pro-delta silty
deposits now forming apparently are re-        clays (pro : in front of, before) are
lated principally to distributary dis-         being deposited in a continuous belt
charge (cf. Humphreys and Abbot,               varying in width from about one to ten
1876; Holle, 1952). This relationship          miles (fig. 1). The belt extends from
would not hold, of course, if these dis-       lower Breton Sound seaward to depths
tributaries were discharging into shal-        as great as 30 fathoms. Similar silty
lower water.                                   clays occur off Southwest Pass and must
   These deposits remain in the deltaic        surround the entire active delta. They
wedge as elongate sandy masses marking         correspond to the foreset beds of classic
the old courses of large distributaries.       delta descriptions, and probably are the
The form of delta front silts and sands        "soft muds" of earlier workers (Shaw,
in cross section perpendicular to their        1913).
long axes is highly variable. They tend           Pro-delta silty clays consist typically
to be deposited symmetrically about the        of homogeneous silty clay or clayey silt
channel mouth, to he widest at the top,        with a trace of sand-sized Particles. A
and to narrow with increasing depth.           glance at the histogram in figure 6 hows
Comparisons of old and recent charts           that on the average they are about half
show, however, that distributary mouths,       silt and half clay. Actual percentages in
uncontrolled by artificial levees, shift       an average of the samples analyzed were
from side to side of the stabilized axes       sand 1.6 percent, silt 42.1 percent, and
of channel elongation. The sandy de-           clay 56.3 percent. The coarse fraction
        SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                    39

 (coarser than 0.062 mm) contained the        coarser material, either clean silt or
following kinds of material : terrigenous     clayey silt, which appear in core sections
sand 45.3 percent, wood and plant fibres      as continuous laminae or lenses (fig. 9).
26.5 percent, aggregates (mostly fer-         These laminae normally are two to three
ruginous) 21.9 percent, shells and shell      mm thick, but vary from a fraction of a
fragments 3.3 percent, Foraminifera 0.9       millimeter to over one centimeter. The
percent, and mica 1.6 percent. In addition    bounding surfaces of the laminae are
there were a few echinoid spines, os-         smooth and sharp, and individual lami-
tracod valves, and diatom frustules. At       nae have relatively constant thickness.
and near the surface the silty clays are      These coarser silt beds are rarely them-
soft and fluid, but they stiffen with in-     selves laminated, and no regular textural
creasing depth of burial. Colors (based       gradations, such as "graded bedding,"
on comparison with the Rock Color             were detected in the microscopic ex-
Chart, Goddard, 1951) of the surface          aminations of core sections. The thin silt
material near shore are brown or yel-         beds occur singly or in sets, where they
lowish brown and become brownish g-ray        are separated by variable thicknesses of
with depth in the core. Farther offshore,     silty clay. Cores taken in holes only three
surface colors are olive gray to light        feet apart show that sets of laminae
olive gray and at depth in the core are       usually can be correlated, but individual
dark gray, olive black, or black. Still       laminae can not.
farther seaward color at and beneath the        These laminations are best developed
surface is olive gray.                        in shallower water and apparently are
   The silty clays are deposited mainly       formed by the winnowing effect of wave
by almost continuous settling of sus-         action. They become progressively rarer,
pended material from the water above.         thinner, and less distinct in deeper water,
Sediment-charged river water flowing          until   at depths of 20 to 25 fathoms
out over gulf water beneath gradually         laminations of any kind are rare in pro-
loses its remaining load consisting of        delta silty clays.
silt, clay, a little sand, and bits of wood     As in all sediment units around the
and plants. This occurs during lateral        delta, there is a progressive gradation
distribution of river water and while         in the material from near shore out to
mixing with salt water proceeds. Distri-      deeper water. On any single traverse
bution and character of silty clays, there-   outward from shore, sand and silt con-
fore, are controlled largely by the cur-      tent of samples decreases and clay con-
rents distributing river water and by dis-    tent increases. Where sand has not been
tance from the sources. Currents prin-        transported into the area from an outside
cipally responsible for this distribution     source, sand content in the silty clays
are tidal currents, wind-driven currents,     is less than one percent within a mile
regional semi-permanent currents, and         of the one fathom curve (fig. 1). An ex-
currents in and around the distributary       ception to this is off Southwest Pass
mouths due to conserved momentum of           where sand persists somewhat farther
river water, mixing of fresh and salt         seaward because of the large volume of
water, and hydraulic head. Off the            sediment and water carried by that dis-
large distributaries turbidity currents or    tributary.
slumping probably also occur.                    Thickness of pro-delta silty clays de-
  The pro-delta silty clays are a quite       pends on the depth of water into which
homogeneous body of mixed silt and            the delta is built. Their total thickness
clay. They are mostly clayey silt on the      increases from 15 feet or less near Main
shoreward side and silty clay to seaward.     Pass in shallow water to about 175 feet
In shallow water the homogeneity is           near North Pass in deeper water. Silty
interrupted by thin beds of slightly          clays make up the large bulk of the delta
40                                 P. C. SCRUTON


                60                                                                 14
FIG. 9.Pro-delta silty clays (scale in centimeters). Note zones of thin silt laminae separated
                             by thicker zones of smooth silty clay.

wedge and thicken seaward as the delta          outward and downward into offshore
is extended into deeper water,                  clays or marginal deposits (figs. 1, 4).
   Pro-delta silty clays normally grade         However, as the delta grew northeast
       SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                    41

in shallow water it decreased the size      isms than is found in sedimentary units
of the channel between Main Pass and        closer to shore. Average percentages
Breton Island (fig. 1) . This accelerated   are : terrigenous sand 55.9 percent, fer-
tidal currents in the channel and an area   ruginous aggregates of clay and silt
of scour and erosion rather than deposi-    particles (authigenic) 17.0 percent, wood
tion was created temporarily. Some pre-     and plant fibers 8.9 percent, Foramin-
vious deposits have been removed and        ifera 10.3 percent, echinoids 3.6 per-
pro-delta silty clays are advancing sea-    cent, shells and shell fragments 3.0 per-
ward directly over older deposits. Posi-    cent, and all other material 1.3 percent.
tion of the small unconformity is shown       These clays are the finest-grained sedi-
in figure 4. The lower part of the silty    ments found in the active delta. They are
clays above this unconformity contains      smooth, homogeneous, and relatively Stiff
laminae of coarser material, sand and       (fig. 10). Lamination of any sort is rare
shell fragments, derived from adjacent      and if present, consists of thin concen-
older beds. These abnormal laminae are
coarsest, thickest, and most numerous at
the base. The sediments grade upward                                  26
into typical pro-delta silty clays.
  Offshore clays.On the east side of
the Mississippi Delta, offshore clays are
being deposited in an area up to 20 miles
wide (fig. 1). They are found mostly in
water deeper than .20 to 25 fathoms. It
is not known yet with any accuracy how
far out to sea or to what maximum depth
south of the delta the offshore clays ex-
tend, but it is probable they go into
depths of at least 500 to 600 fathoms on
the continental slope. They are formed
by the normal settling of fine river-de-
rived clay and silt onto the ocean floor.
To this material are added shells of
marine organisms and sand or other
sediment transported by currents from
outside sources. Offshore clays are equi-
valent to the classical bottomset beds of
delta literature.
  The triangle diagram of offshore clay
,samples (fig. 6) shows that all the
samples analyzed contained some silt and
most of them a little sand. The average
of samples analyzed for coarse fractions
was clay 66.9 percent, silt 27.2 percent,
and sand 5.9 percent. The increase in
percent of sand over that found in pro-
delta silty clays (1.6 percent) is from
samples collected near the outer margins
of offshore clays and is due to sand
transported in from outside sources by
                                              4I                      57
currents. Coarse fraction analyses show     FIG. 10.Offshore clay (scale in centimeters).
much more material from marine organ-          Texture is uniform and fine-grained.
42                                 P. C. SCRUTON
trations of shells, echinoid plates, other     pension, whereas the coarse materials
shell fragments, and some silt and sand.       are introduced periodically along and
Color of the clay is light olive gray at       near the bottom during more extreme
the surface, and changes to olive gray or      conditions.
dark greenish gray within three to 10           The most characteristic feature of
cm into the deposit. Landward, the clays       marginal deposits is the presence of curi-
grade into the silty clays of the advanc-      ous pockets, lumps, or irregular lenses
ing delta. Offshore clays overlie mar-         which give a mottled appearance to core
ginal deposits and grade laterally into        sections (fig. 11). These lumps or pock-
them northward and eastward (figs. 1,          ets are of coarse material in a fine ma-
4) where clay is mixed with sediment           trix or fine material in a coarse matrix.
from outside sources.                          Normally they are about one to cm in
  Where measurements have been possi-          diameter, but they range up to about four
ble, offshore clays on the east side of        cm in diameter. Texture of the lumps
the Mississippi Delta are thin. Eleven         and pockets may differ sharply from the
miles east of Pass a Loutre Light on           matrix or there may be intergradation.
the landward edge of the area in which         Various activities of burrowing organ-
they occur, cores showed only seven feet       isms apparently produce these features.
of the clay on top of marginal deposits.       The deposits also often consist of a more
Cores taken still farther east found the       or less homogeneous mixture of sand and
clay even thinner. Other cores taken           clay with only a little silt. Irregular vari-
northeast of Pass a Loutre showed the          ations in the proportions of sand, silt,
clay somewhat thicker. Variations in           and clay and in the degree of mixing
thickness are controlled by distance from      cause a vaguely mottled or zoned appear-
shore, prevailing directions of currents       ance. In some marginal areas there are
carrying suspended sediment (Scruton           also thin irregular beds of sand contain-
and Moore, 1953), and irregularities of        ing many shell fragments. Color of these
the original topography of the sea floor.      deposits usually is light olive gray at the
  Marginal deposits.The name "mar-             surface and olive gray at depth.
ginal deposits" is applied to the mass of        The triangle diagram of figure 6 shows
more heterogeneous sediment which oc-          that the percentages of sand, silt, and
curs on the shelf surrounding the ad-          clay vary greatly between individual
vancing delta embankment. These de-            samples. An average of 35 samples was
posits floor most of Breton Sound and          sand 30.6 percent, silt 44.5 percent, and
form an almost continuous belt between         clay 24.9 percent. The coarse fraction
the silty clays and clays and the sands ly-    (coarser than 0.062 mm) is mostly detri-
ing farther seaward (fig. 1).                  tal sand. However, remains of marine
  The material in marginal deposits            organisms are common, and bits of wood
ranges in size from sand to silty clay         and plant fibers are common near the
and varies both vertically and laterally.      delta but rare to the east (fig. 3). Shells
the sand is brought in from nearby sandy       and woody material usually are scattered
bottoms, silt and clay are derived mainly      through the sediment, but may be con-
from the Mississippi River, so that there      centrated in lenses or layers. Marginal
are .tivo sotir-ces of elastic sediments for   deposits east of the dashed line in figure
thésebeds. In general the fine and coarse      1 contain Eastern Gulf sand. West of
cornp-orients of marginal deposits also        this line they contain Mississippi River
seem fo be introduced at different times       type sand.
and by different processes. The fine-            Marginal deposits show a general gra-
grained components are mainly the result       dation in character outward from shore.
of relatively constant settling from sus-      Textures generally become coarser. The
       SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                          43


FIG. 11.Marginal deposits (scale in centimeters). Note sandy lumps and lenses, irregular
  and indefinite stratification, indistinct contacts, random variations of texture, and shells.
44                               P. C. SCRUTON
general outward textural gradation is          Sands.-The widespread sands shown
shown in the sand-silt-clay triangle of      on figure 1 are not being brought in by
figure 6 by a continuous series from silty   the present Mississippi River. Sands of
clay or clayey silt to sand. Within this Mississippi River type (Goldstein, 1942)
general outward gradation many excep- have been concentrated, principally from
tions occur, however, and there usually older delta deposits, by marine erosion
is a "patchy" distribution of different and winnowing on the shallow Gulf
kinds of sediment. Thus, silty sand may floor. During and following winnowing,
occur in an area that is mainly clayey this sand has been transported by waves
silt. Average grain size of the sand frac- and currents, concentrated in some
tion also increases outward in some places, and distributed thinly over other
areas. An increase in shell material usu- large areas. Sand of the Eastern Gulf
ally accompanies the increase in detrital province also is widespread. This sand
sand.                                      has remained unburied since the post-
   Lithologic structures also change out- glacial sea level rise as described previ-
ward. Development of pockets, lumps, ously. Mineralogy of Eastern Gulf type
and irregular lenses is poor near shore, sand seems to show that its ultimate
but they became prominent seaward as source was the Southern Appalachian
sand content increases. Thickness of metamorphic belt (Goldstein, 1942).
sandy zones and the number of irregular
sandy bodies increase at the expense of                 RATE OF DEPOSITION
silty clay. In core samples this is the        Depositional environments of the Mis-
way the outward textural gradation into      sissippi Delta are controlled by : (1)
sand occurs.                                 sources of sediments, (2) effective proc-
   Marginal deposits have been pene-         esses, and (3) rates of deposition. Rate
trated completely in Breton Sound and        of deposition is a property of the en-
the eastern offshore area. The minimum       vironment that depends, to some degree,
thickness in Breton Sound is approxi-        on the other two. However, rate of depo-
mately one foot, and lithology of under-     sition exerts primary control on sedi-
lying deposits indicates they probably       ments by selecting or limiting the effec-
are related to the St. Bernard Subdelta.     tive processes.
In the eastern offshore area a minimum         The rate of deposition of any one
thickness of about two feet was found        kind of material can determine or limit
and the underlying- material was shelf       the amounts, kinds, or characteristics of
sand. Maximum thickness of marginal          other materials that are deposited. For
deposits of the present delta cycle is un-   example, a high rate of clay deposition
known but probably is not great.             may prohibit the local existence of all
  Long cores in marginal deposits show       but a few certain kinds of organisms,
the general tendency is for sand content     make it impossible for wave action to
to increase with depth in the deposit.       develop clean beach sands, or for slow
Marginal depOsits grade downward into        chemical processes to occur at the sedi-
sand by the same changes that occur in       ment-water interface. By influencing bio-
the horizontal, that is, by an increase in   logical activity and chemical reactions,
numbers, size,    or thickness of sandy      these limits also may affect both the kind
lumps, pockets, layers, and lenses. Be-      and extent of later physical and chemical
cause the sand is derived from adjacent      diagenetic modifications.
sand bodies, thickness and character of        Depositional environments change out-
marginal deposits may give an index to       ward from the Mississippi Delta, and
the proximity, character, and thickness      the changes produce the different belts
of these sands.                              of sediments that encircle the delta. Rate
           SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                                                               45

                                                             MISSISSIPPI RIVER DELTA

                                                            COMPARISON OF RATES OF
                                        DEPOSITION, LITHIC TYPES, AND CLAY CONTENT

           A. NORTHWEST FROM MAIN PASS MOUTH                                               NORTH FROM NORTH PASS MOUTH

                       DISTANCE FROM ORIGIN                                                       DISTANCE FROM ORIGIN
                         (NAUTICAL MILES)         BRETON SOUND                                       (NAUTICAL NILES)
                                                MARGiNAL DEPOSITS
                                        PRO   E_TALLy.Tsv


                      LIMiTS OF MPPEO UNITS
                       PERCENT CLAY                                                              X PERCENT CLAY
                     U RATE OF DEPOSITION                                                        o RATE OF DEPOSITION


   FIG. 12.Profiles are along corresponding lines A and B of figure 1. Curves and mapped
limits of sediment units show the similarity of the sequence of lithologic types, rates of
deposition, and clay contents outward from the delta between profiles. "Outcrop" widths of the
units, and the distances they extend seaward vary around the delta f rom profile to profile,

of deposition is one of the factors that                                 silts and sands. Within the area of pro-
changes outward and exerts much con-                                     delta silty clays, rates of deposition are
trol over the sediments.                                                 very high and decrease rapidly with
   Close approximations to rates of depo-                                increasing distance from distributary
sition have been obtained in the Missis-                                 mouths. Depositional rates of pro-delta
sippi Delta by comparing depth charts                                    silty clays on these traverses range from
constructed at different times (depth                                    greater than one foot per year to about
changes between surveys being con-                                       0.15 feet per year. Rates of deposition
verted to rates of deposition expressed                                  of marginal deposits are slower (less
as feet per year). The shorter the pe-                                   than about 0.15 feet per year), but also
riod of time represented between depth                                   are uniform between these two regions.
charts, the more real are the rates of                                   The curves of clay content also show
deposition. Two curves of rate of depo-                                  similarities, increasing with increasing
sition plotted against distance from the                                 distance from distributary mouths to
nearest principal distributary mouth are                                 maxima which coincide approximately
shown in figure 12, and are along- cor-                                  with breaks in the rate of deposition
responding lines A and B of figure                                  1.   curves. These similarities between curves
Limitations of survery coverage made it                                  show there is a distribution of rate of
necessary to use different periods of                                    deposition around the delta that corre-
time. Boundaries of sediment units and                                   sponds to the distribution of sediment
clay content of surface samples along                                    types.
the lines of section also are shown.                                       Figure 13 generalizes on these find-
  Curves A and B of figure 12 are simi-                                  ings. As the delta advances seaward,
lar in shape and somewhat different in                                   the rate of deposition at any particular
slope except in the areas of delta front                                 point increases with approach of the
46                                     P. C. SCRUTON

                                      MISSISSIPPI      RIVER    DELTA

                              OFFSHORE MIGRATION OF DEPOSITIONAL
                               ENVIRONMENTS SHOWING RELATIONSHIP
                               BETWEEN TIME AND LITHIC SURFACES
                                       ( SCHEMATIC)
                                                    RATE OF
                                                DECREASES                                        OFFSHORE
                                                                                                                       SEA LEVEL--
                                                                     ELTA FRONT SILT AND SUR O

                                                                                           PRO DELTA SILTY GI..
                                                                                                       MODERN DEPOSITIOÑAL

                                                                    ORIGINAL DEPOSITIONAL SURFACE

                                                                                        ENVIRONMENT OF
                    TIME SuRFACES
                                                                                       ANSDA L DENPAO.S.I.TASL iOPPSOORE CLAYS

                    LiTHIC SuRFACES                                         MBA

   FIG. 13.Deposition on the delta's seaward slope builds the bottom toward sea level and-
causes seaward migration of the different depositional environments so that the sediment units
they produce are extended. Rate of deposition in any area increases as the shore builds out
until the depth of water is relatively shallow (shown schematically by the dashed horizontal
line). Deposition continues af ter this, but the rate decreases and is very low in the marshes.
distributary mouths. The rate rises to a                      relationships between time and lithic sur-
maximum determined by location with                           faces in the delta.
respect to the large distributaries. The                            LOCAL RELATIONS BETWEEN SOURCES,
water shoals also, so that winnowing                                PROCESSES, AND RATES OF DEPOSITION
by wave and current action becomes
more effective. At some depth of water                           Although there is a similar distribu-
(also determined partly by distance from                      tion of sediments and rate of deposition
large distributaries) winnowing begins                        outward from the delta and although
to decrease the rate of deposition, and                        processes are generally similar around
continued building of the delta toward                         the delta, there are local variations in
sea level increases the effectiveness of                       processes or in relative effectiveness of
wave and current action. This phase is                         processes around the delta. These differ-
recorded by the lithology of the upper                         ences in processes or their effectiveness
part of pro-delta silty clays and by delta                     around the delta produce the differences
front silts and sands formed in shallow                        in     depth of water or distance from
water. The rate of deposition continues                        sources at which similar sediments occur
to decrease after formation of the marsh                       (fig. 1).
and may be very slow in the higher                               Conditions along three traverses out-
marshes or behind well developed natural                       ward from shore are described below
levees. Decrease in rate of deposition of                      to show how the basic processes are
clastic material in the marsh results in the                   modified and how the similar sediments
greater relative importance of vegetation                      are being produced. These traverses are
in the sediments. Figure 13 also shows the                     (fig. 1) : (1) eastward from North Pass
        SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                      47

mouth, (2) seaward along line B, and           water. Northward in deeper water semi-
(3) northwest along line A.                    permanent currents have little direct ef-
  The outward sequence of deposits off         fect, but reversing tidal currents, wind
the Mississippi Delta is best illustrated      currents, and residual river flow are
eastward from North Pass mouth (fig.           important. Tidal currents flowing to and
1) . The gradation of sediments is being       fro between the open Gulf" and Breton
produced through a broad range of depth        Sound are accelerated because of con-
and distance from sources in a series of       striction between the deltaic embank-
environments in which relatively gradual       ment and the submarine extension of the
variations of water properties and effec-      Chandeleur Arc. Higher speed of tidal
tive physical processes occur. In this    currents has a three-fold effect, each
area residual river velocity, wind, and   tending to reduce the rate of deposi-
semi-permanent currents are the agents tion of river-derived sediment and to
principally responsible for sediment dis- increase seaward change of sediment
tribution. These agents cause fluctuations     properties. First, these currents restrict
in direction and distance of transporta-       seaward migration of the surface turbid
tion. Numerous small valleys and depres-       zone by influencing its direction of move-
sions on the seaward slope of the delta        ment. In this way they reduce the supply
wedge suggest also that turbidity cur-         of fine sediments to the bottom. Sec-
rents or slumping occur intermittently.        ond, tidal currents also tend to prevent
Increasing distance from river source          deposition of fine-grained material which
and fluctuating direction of transporta-       is supplied, and finally, they spread later-
tion produce decreasing rates of elastic       ally the sand thrown into suspension by
sediment supply seaward and greater            waves on the adjacent shallow sand bot-
relative abundance of organic remains.         tom. The effects of wind currents and
Depth also increases and results in less-      residual river flow (similar in this area
ened wave action and more uniform              to the two-layer estuarine circulation
bottom conditions. Marginal deposits on        such as described by Ketchum, 1953) are
the shelf east of the delta are forming        additive to tidal currents and generally
through slow supply of river-derived           increase or decrease tidal current speeds,
sediments because of distance and direc-       depending on instantaneous conditions.
tion from source, slow supply of Eastern       The combined effect of these currents,
Gulf or Mississippi type sand because          together with a relatively high rate of
of water depth, and a slow continuing          supply of sand derived from the Chande-
supply of shells. Tidal currents seem to       leur area to the north, is producing a
have little net effect because of low re-      narrow zone of pro-delta silty clay and
lief of shelf topography, although in-         a relatively wide and thick marginal de-
shore they aid in supplying sediment           posit. Increased speed of tidal currents
from the Cubits Gap and Baptiste Col-          toward the northwest because of greater
lette distributary systems.                    constriction of the channel prevents
   Northward from the Cubits Gap dis-          sedimentation at present directly off
tributary system towards Gosier Island         Main Pass mouth, and the currents have
 ( just west of line B, fig. 1), the outward   scoured into pre-existing deposits (fig.
sequence of deposits, although similar to      1).
the eastward series, is compressed in             Along line A of figure 1 deposits and
length and does not include typical off-       conditions near the delta generally are
shore clays. Delta front silts and sands       similar to those just described, although
are typical of the deposits off small dis-     tidal effects are altered somewhat by
tributaries forming under conditions of        topography. In the Breton Sound mar-
decreasing rate of deposition in shoaling      ginal area (west and        northwest          o
48                                P. C. SCRUTON
Breton Island), however, the processes         destroyed after formation. Prominent
are different. Tidal currents and surface      lamination generally is absent in the
flow from the Cubits Gap and Baptiste          deeper parts of pro-delta silty clays and
Collette systems carry suspended sedi-         in offshore clays because water depth
ment into Breton Sound, particularly prohibits extensive winnowing by waves
during late spring and early summer. and the sediment is uniformly fine-
Current speeds decrease in the open grained. Typical structures of marginal
sound and generally are weak, and the deposits apparently are produced by bur-
suspended sediment settles. Intermittent rowing organisms. The various condi-
strong northerly winds commence in tions of origin of marginal deposits all
September, continue through April, and         combine to produce a slow rate of depo-
produce high waves in the shallow              sition. This permits the relatively abun-
sound. These waves resuspend much of           dant faunas that live on them to destroy
the previously introduced fine sediment.       most of the laminations formed by peri-
Currents, also produced by these winds,        odic introduction of coarse sediment or
offset normal tidal oscillations, flow sea-    by winnowing.
ward continously from the sound during                         SUMMARY
the storms, and remove most of the fine
resuspended material gained during pre-         Recent studies of the Mississippi Delta
vious periods of calm. Sand and silt are       make it possible to better describe many
derived locally from the islands and           features of the sediments and the gen-
from reworking of the shallowest areas         eral character of some of the funda-
by waves and also are distributed under        mental sedimentary processes. In some
these extreme wind conditions.                 cases, the relationships between these
                                               two can be shown. The processes are
     FORMATION OF MINOR LITHOLOGIC             complex and usually are difficult to iso-
                STRUCTURES                     late, so that their detailed definitions and
     Similarities and differences between      relationships with the sediments require
these three areas and other data sug-          much more work in the area.
gest origins for the characteristic bed-          Environmental factors and processes
ding and other minor lithologic struc-         in the Mississippi Delta determine the
tures of the sediment units (figs. 7-11).      kinds and amounts of materials that are
Structures of shallow water delta front        supplied and deposited in a given time.
silts and sands seem to be produced by         The depositional rate of any one kind
winnowing and bottom transportation by         of sediment also influences the amounts
waves, tidal currents, and high river dis-     and kinds of all other materials that are
charge, and by variations of supply due        deposited. Amounts and kinds of ma-
to seasonal river fluctuations. The deli-      terials in the sediments also influence
cate laminae are preserved, despite de-        which post-depositional chemical modi-
creasing rate of deposition, because the       fications can occur. Therefore, total rate
environment is unfavorable to organ-           of deposition and rate of deposition of
isms. Thicker vertical variations in tex-      each sediment component are derived
ture in delta front silts and sands off        environmental factors of great impor-
large distributaries seem to be caused by      tance in controlling petrography.
seasonal discharge variations in river           Environments in the active Mississippi
flovv, but the fine laminae, like those in     Delta generally produce a rate of depo-
pro-delta silty clays, seem to be the result   sition that is greater than the rate of
of periodic winnowing by waves and             subsidence. They      also simultaneously
lateral transportation by tidal or other       produce a wide range of types of sedi-
 currents. Rate of deposition is high, or-     ments that bear definite spatial and
 ganisms are scarce, and laminae are not       lithologic relationships to each other.
        SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                         49

Environments migrate seaward because            (3) the absence in some 'places of one of
of delta growth (a form of marine re-           the structural elements.
gression), and develop distinct but inter-         Knowledge of the processes and the
related sedimentary units of large lateral      types of sediments they produce permits
extent.                                         interpretation of the flux of environ-
  All of the structural elements of clas-       ments represented by a vertical section
sic delta description are present on the        in more ancient deposits. Thickness of
east side of the modern Mississippi             the total section originally measured the
Delta. Bottomset beds are represented           water depth into which the delta ad-
by offshore clays, foreset beds by pro-         vanced, but this later has been modified
delta silty clays and delta front silts and     by compaction and subsidence. A thick
sands off large distributaries, and top-        section of clay low in sand content sug-
set beds by marsh deposits and shallow          gests the position of these beds is a con-
water delta front silts and sands. There        siderable distance from a sand body,
are differences between model lake deltas       but a thick sandy sequence of either
and the large marine delta, however, that       marginal or pro-delta silty clay type de-
are due largely to marine influences, tex-      posits suggests nearness to equally thick
tures of sediments, and scale of the phe-       sand bodies formed during the delta's
nomenon. Main differences on the con-            constructional cycle. Because        of the
tinental shelf that result from marine          source of sand for the outlying sand
processes and density differences are :         bodies, active delta growth in the im-
(1) the presence of multi-source de-            mediate area is not necessary for their
posits underlying bottomset beds, (2)           formation. These beds can form during
much smaller seaward slope of foreset           long periods of delta inactivity and later
beds on the delta face than the 10 de-          may be buried by a local renewal of
grees to 25 degrees found in lakes, and          delta building.

                                    REFEREN CES
BATES, C. C., 1953, A rational theory of delta formation Am. Assoc. Petroleum Geologists

    Bull., v. 37, pp. 2119-2162.
DoHm, C. F., 1936, Petrography of two Mississippi River subdeltas : Louisiana. Geol. Survey
   Bull. 8, pp. 339-396.
FisK, H. N., 1944, Geological investigation of the alluvial valley of the lower Mississippi
    River : Mississippi River Commission, Vicksburg, December.
      , 1952, Geological investigation of the Atchafalaya Basin and the problem of Mississippi
     River diversion : Mississippi River Commission, Vicksburg, April.
Fisic, H. N., MCFARLAN, E., JR., KOLB, C. R., AND WILBERT, L. J., JR., 1954, Sedimentary
    framework of the modern Mississippi Delta : Jour. Sedimentary Petrology, v. 24, pp.
GILBERT, G. K., 1885, The topographic features of lake shores U. S. Geol. Survey Fifth Ann.
   Rept., pp. 1883-1885.
      , 1890, Lake Bonneville: U. S. Geol. Survey Mon. I, 438 pp.
GODDARD, E. N., 1951, Rock-Color Chart : prepared by the Rock-Color Chart Committee, dis-
   tributed by Geol. Soc. America, 2nd printing.
GOLDSTEIN, A., JR., 1942, Sedimentary petrologic provinces of the northern Gulf of Mexico :
   Jour. Sedimentary Petrology, v. 12, pp. 77-84.
HOLLE, C. G., 1952, Sedimentation at the mouth of the Mississippi River : Proc. 2nd Conf.
   Coastal Engineering, Houston, 1951, pp. 111-129.
HUMPHREYS, A. A. and ABBOT, H. L., 1876, Report upon the physics and hydraulics of the
   Mississippi River : Corps of Engineers, U. S. Army Prof. Paper 13, 691 pp., Appel,
   A-M, Pls. 1-25.
KETCHUM, B. H., 1953, Circulation in estuaries : Proc. 3rd Conf. Coastal Engineers, Cam-
   bridge, Mass., 1952, pp. 65-76.
MORGAN, J. P., 1951, The occurrence and origin of the mud lumps at the mouths of the
    Mississippi River : Rept. to U. S. Corps of Engineers dated 5-7-51.
50                                    P. C. SCRUTON
MORGAN, J. P., VAN LOPIK, J. R., and NicHoLs, L. G., 1953, Trafficability and navigability of
     delta-type coasts, occurrence and development of mudflats along the western Louisiana
     coast Louisiana State -University Geol. Repts., Tech. Rept. No. 2.
RILEY, G. A., 1937, The significance of the Mississippi River drainage for biological con-
   ditions in the northern Gulf of Mexico : Jour. Marine Res., v. I, pp. 60-74.
RUSSELL, R. D., 1937, Mineral composition of Mississippi River sands : Geol. Soc. America
   Bull., v. 48, pp. 1307-1342.
RUSSELL, R. J. 1936, Physiography of lower Mississippi River delta.:           Louisiana Geol.
     Survey Bull. 8, pp. 3-199.
RUSSELL, R. J., and Dori m, C. F., 1936, Bibliography : Louisiana State Geol. Survey Bull.
     8, p. 279-338.
RUSSELL, R. J., and RUSSELL, R. D., 1939, Mississippi River delta sedimentation : Recent

   Marine Sediments (P. Trask, ed.). Tulsa, pp. 153-177.
SCRUTON, P. C. and MOORE, D. G., 1953, Distribution of surface turbidity off Mississippi delta :
   Am. Assoc. Petroleum Geologists Bull., v. 37, pp. 1067-1074.
SHAW, E. W., 1913, The mud lumps at the mouths of the Mississippi : U. S. Geol. Survey,
  Prof. Paper 85 B, pp. 11-27.
SHEPARD, F. P. and MOORE, D. G., 1954, Sedimentary environments differentiated by coarse
   fraction studies : Am. Assoc. Petroleum Geologists Bull., v. 38, pp. 1792-1802.
TROWBRIDGE, A. C.,     1930, The building of the Mississippi delta : Am. Assoc. Petroleum
     Geologists Bull., v. 14, pp. 867-901.

  Parke A. Dickey: It seems to me that on which we based our interpretation of
the work that Dr. Scruton and his associ- ancient rocks were vastly oversimplified.
ates are doing is of great significance not For example, we were all taught that
only to petroleum geology in particular waves operate in shallow water and it is
but also to the whole field of geology in therefore in shallow water close to the
general. It seems also to me that it is coast that sands are winnowed of their
fulfilling in a very precise manner the fines. Further, the sand was always sup-
objectives of the project of Recent Sedi- posed to be dropped first as the river
ments in the Northern Gulf of Mexico             entered the stiller water of the ocean,
that was conceived by the research com-          and mud was dropped farther out. The
mittee of the A.A.P.G. and which has             result of both these considerations was
now, become project no. 51 of the A.P.I.         to indicate that sand is deposited close to
These men are dissecting the Mississippi         shore and mud farther out. Both of those
delta and determining its anatomy and            ideas are still basically true, but they
physiology, and to carry this analogy a          cannot be applied in a simple manner to
little further, they are making it possible      the interpretation of an environment. In
to recognize the different organs that           this particular area that was being dis-
compose the delta by means of their litho-       cussed, the mud is being deposited near-
logic character.                                 est shore in just a few feet or inches of
  For example, I have frequently seen in         water, and the best body of sand they
the course of sub-surface work, dark             found was the farthest out and in the
gray shales with thin interlaminated fine        deepest water. Finally, they are establish-
.ilty sands in irregular lumps and lenses.       ing a depositional sequence of first one
This particular lithology is common              and then the next and then another lith-
where ever oil is found, and yet it is not       ology, which may be characteristic of
described as far as I know in any text-          deltas and which will permit us to rec-
book. In this paper it is described and          ognize this environment whenever we
identified as this marginal mud-sand             see it in the sub-surface. As soon as this
facies of the delta.                             is done I'm sure that most of us are
  Another thing that this study is bring-        going- to go back to our well records and
ing out is that many of the older concepts       cores with new understanding and new
        SEDIMENTS OF THE EASTERN MISSISSIPPI DELTA                                   51

ability to interpret what they mean.          it was virtually impossible to distinguish
  G. E. Murray: I'm a little confused by      between sediment types. This area be-
some of the terminology used by Dr.           came, in effect, a circulus, a wastebasket
Scruton in his paper. I would like for        term. We couldn't distinguish more pre-
him to explain for me and perhaps for         cisely than simply "mud."
some of the other audience his meaning           W. H. Twenhofel: I have nothing but
of the term "mud."4 I was rather under        praise for the type of work that Doctor
the impression that mud was a textural        Scruton is doing, but some of his nomen-
term that was not in very good repute of      clature disturbs me. Mud, according to
late, and this introduction has me a little   my understanding- of the word is a mix-
bit stumped. Just what would a muddy          ture of clay, silt, or both -clay and silt
sand, for instance, be ?                      with water. If one mixes clay with water,
  P. C. Scruton: We've been under a           the product is mud ; silt mixed with water
certain amount of fire for the use of these   is mud, both in any proportions mixed
terms. We feel that their use is justified    with water also make mud. The use of
so long as the meaning of the term is         the word in any diagram with mud rank-
defined, however. The term "mud" refers_      ing in equality with silt, clay, and sand
to an area on the sand, silt, clay triangle   is very unfortunate. One knows what silt,
where definite percentages of sand, silt,     clay and sand mean, but no one can
and clay occur. It simply is a textural       evaluate the physical composition of mud.
term, as you imply, and connotes a cer-         If it is intended that the use of the
tain percentage of sand, silt, and clay.      word is merely a preliminary one in the
  We made a number of tests of material       study of the sediments, the use may be
to see how clearly we could distinguish       excused, but in a published paper the
between various textural types before         word mud should not be used in parallel
we chose these names, and we found            position with the words silt, clay and
that there was a large central area where     sand. These three words have very defi-
    The terminology objected to .here has     nite meanings. This is not true of the
been abandoned. See footnote 3.               word mud.

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