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					Alice Kwok

Sedimentology and Stratigraphy

10-18-11

            Depositional History and Coastal Environments of the Merced Formation

Abstract

       The Merced Formation can be broken into a series of five facies: offshore, backshore,

foreshore, nearshore and embayment. It was deposited late in the Cenozoic and subsequent uplift

and movement along the San Andreas Fault has exposed it at Thornton Beach in Daly City. It’s

2000 meters thick and characterized by a series of transgressive and regressive cycles.

Introduction

       The Merced Formation is 2000 meters thick and composed of loosely consolidated

marine and non-marine deposits located south of San Francisco and deposited in the Pleistocene

(Clifton et al. 1988, Figure 1). It nonconformably overlies the Franciscan Complex and is

overlain by an angular unconformity of the Colma Formation. The formation consists of

sequences formed by was subject to transgressive and regressive eustatic sea level fluctuations

[INSERT REF HERE]. There were multiple shifts due to sea level change from the open ocean

to embayment (Clifton et al. 1988) and it eventually filled in. The Merced Formation is exposed

from Thornton Beach to Fort Funston spanning Daly City to San Francisco respectively.

Through observation and utilization of field techniques we interpreted visible shifts from

transgressive to regressive sea levels and the resulting sedimentological evidence that was left

behind in order to understand the prograding vertical sequence throughout the studied areas to

see changes in the direction of flow. In order to understand the processes undertaken at Thornton

Beach an actualistic model was needed. Located outside of Half Moon Bay, the study area at
Pescadero is composed of modern fluvial deposits of sand and clay (Ian Lesser, et. al). We

sampled sediments and viewed the foreshore, backshore, embayment and modern sediments at

this location. The focus of this report is to describe the coastal facies of the Merced Formation

and the modern environment and interpret the processes that formed them.

Facies Description and Interpretation

Intro

        There are 5 study sites in total. The first four are a series of beds lumped into sequences J,

O, R, and S exposed from Thornton Beach to Fort Funston. Sequence J exhibited an overall

regression and coarsening upward of sediments (figure 2). Sequence O illustrates a coarsening

upward sequence of sediments and R and S show an embayment and eventual filling in of an

embayment.

Offshore

        The offshore facies was observed at Thornton Beach (Figure 2) is characterized mainly

by fine sand, silt and clay. It's a low energy environment exposed at the base of sequences J and

O at the base at Thornton Beach to Fort Funston. Sequence O is about 3 meters thick and has a

strike of N70E and dip of 25NW. The offshore facies of J is olive gray in color and composed of

more mud than silt with clay lenses coarsens upward from the base. It contains very fine-grained

and well-sorted sand with storm layers of clam shells. In the offshore transition zone of J we saw

a coarsening upward of sediment and the appearance of concretions in a massive sand layer. As

it coarsens upward the color became lighter and contained more sand than mud.

        The offshore facies of Sequence O, subsequence O1, is composed of silt, mud, and flecks

of charcoal. It is greenish black in color and, like Sequence J, the sediment in this sequence is

massive with no clear lamination and is composed of fine sand and and silt. Shell storm layers
are present in addition to clam shells that were deposited in “life” position. In sequence O, sub-

sequence O3 (Figure 2) the foreshore facies is characterized by parallel sand and gravel layers

and coarsening upward. The sediment is poorly sorted and has a variety of colors. It contains

sub-angular to sub-rounded grains of gravel with siliciclastic grains of quartz and feldspar.

Nearshore

       Nearshore facies were observed in Sequence O and J (figure 2) of the Merced Formation.

Visible structures were medium-scale cross beds. The reddish-orange crossbedded sands were

interbedded with gravel. Sands were composed of well-sorted quartz and feldspar. Sediments of

this facies ranged from pebble-sized to medium-grained sized sand and were sub-angular to sub-

rounded. Overall the sediment size is coarse grained and orange-red in color. The nearshore is

characterized as a high-energy depositional setting wherein coarse sediments are commonly

transported and deposited. The gravel and pebble layers present in Sequence O were seen in

crossbeds as preserved evidence of megaripple migration.

Foreshore.

       In sequence O, sub-sequence O3 (figure 2) the nearshore facies is characterized by

parallel sand and gravel layers. The sediment is composed of quartz and feldspar grains of sand,

ranging from sub-angular to sub-rounded grains. Processes at the foreshore are dominated by

wave action and the foreshore of Sequence O contains parallel lamination.

       Processes at the foreshore are dominated by wave action and after the high-energy waves

crash and deposit the gravel layers, the velocity of the water decreases and deposits the finer

sand above it (Hiroki, Terasaka, 2005). The foreshore of Sequence O contains parallel lamination

and fines upward as it adjust from a wave-dominated environment to a more tidally affected area.

Backshore
       Unconsolidated sand and eolian dunes characterize the backshore. Sediments of the

backshore facies were observed in the bottom of Sequence O, R, S and the modern environment

at Pescadero Beach. Sediments of the backshore facies in Pescadero felt soft in texture and are

composed of quartz, chert, feldspar, and shells fragments that were very well sorted. The

backshore facies exposed in Sequence O, subsequence O4 (figure 2) was laminated and

composed of very fine quartz and feldspar-rich grains of sediment. Sequence R contains

massively bedded sands of quartz and feldspar. A soft, laminated sand layer we called

“trampolite” and volcanic ash were exposed in Sequence S. The volcanic ash was white in color

with a chalky, powdery texture while the trampolite was light grey and very well sorted with

visible offset of its heavy mineral laminations.

       Wind processes--with the exception of storms, which bring nearshore and offshore

sediments inland-- dominate the backshore (Mitchum, 1978). The backshore of sequence O was

primarily eolian dunes that were weathered by wind and deposited before being truncated by the

next sequence of beds. Sequence R’s massive bedded sands were also eolian dunes that were

massively crossbedded. The trampolite beds in Sequence S were interpreted to be due to

bioturbation of the beds from “trampling” of the then-soft sediment by animals native at the time

of deposition. The rockland ash—a non-marine unit could only have been preserved in its

entirety if outside the water and is evidence that the sea level regressed long enough for the ash

to deposit undisturbed.

Embayment

       Sequence R, S, contained preserved remnants of embayments and the modern

environment contained real-time deposition of material (figure 2). An embayment is a bay that is

characterized by inflow of sediments from its marine environment as well as fluvial deposits
(Hein, 2011). It can be broken up into four constituents: tidal channels, tidal flats, mudflats and

marshes with varying levels of mud, silt, and organic matter in each locale.

       Embedded in Sequence R, we observed beds of ripples overlain by mud in massive

greenish black beds layer striking 009 and dipping 39NE and another bed striking southwest.

Above these series of beds we saw a distinct olive-black to olive-gray layer with a mottled brown

color. It has a silty texture and is well sorted, composed of silt, mud and organic matter.

       At the time of deposition, Sequence R was deposited over a short period of time as

evidenced by the bioturbation of sediments below the ripples but perfect preservation of the

ripples themselves, which indicates that the organisms living at the time did not have time to

bioturbate the beds. The change in paleocurrent direction indicates that there was bimodal

transport in the shallow environment to a tidally dominated environment. Vertical bioturbation of

roots were visible in the olive black mottled layer. This was most likely a marsh above a tidal

channel in the embayment and the plants bioturbated the mud and silt beneath.

       At Pescadero, the modern embayment wasn’t visibly connected to the open ocean and

exposed along the inland side of Highway 1 was observed to have a tidal channel (Butano

Creek), tidal flat, mudflat and marsh. The tidal flats contained more sand than mud and had a

gritty texture and were olive gray in color. The mudflats and marsh contained primarily mud and

little sand and felt much smoother in comparison. The colors of the sediments varied from olive

black to olive grey in color and the entire landscape was covered with vegetation.

       It is the nature of an embayment to fill with sediments as observed in Sequence R and S

and is currently underway in the modern environment as it is being overlain by sand carried by

wind from the backshore of Pescadero Beach. The vertical sequence of R and S coarsened

upward as the mud and silt is overlain by sand.
Conclusion

       Within Sequence J, we saw evidence of the offshore facies and the nearshore. Sequence

O featured the offshore, nearshore, foreshore and backshore facies. Sequence R featured

embayments with backshore sediments overlain by dunes and Sequence S contained an

embayment and backshore facies. Through study of the facies we can see the change from a

wave-dominated environment to tidally dominated environment and the subsequent associated

sediments found in each environment.

       Apparent shifts were seen in the


References

Hiroki, Yoshihisa, 2005. Wavy lamination in a mixed sand and gravel foreshore facies
of the Pleistocene Hosoya Sandstone, Aichi, central Japan. Sedimentology vol 52, pg 65-
75.

Hein, Christopher J. et al., 2011. Evolution of a Pharaonic harbor on the Red Sea:
Implications for coastal response to changes in sea level and climate. GEOLOGY, July
2011.

Clifton, H. Edward, R. E., 1988. Analysis of Eustatic, Tectonic, And Sedimentologic
Influences on Transgressive and Regressive Cycles in the Upper Cenozoic Merced
Formation, San Francisco, California.

Mitchum, Jr., 1978. Seismic Stratigraphy and Global Changes of Sea Level, Part 3:
Relative Changes of Sea Level from Coastal Onlap.

Gallagher, Edith L., 2002. A note on Megaripples in the Surf Sone: Evidence for their
Relation to Steady Flow Dunes. Marine Geology, v. 193, pg. 171-176.




Figures
Figure 1. Location map of the study area along Thornton Beach to Fort Funston
                             (Clifton, et al. 1987)
Figure 2. Stratigraphic columns of the Merced Formation (Clifton, et al. 1987)
Figure 3. The modern environment at Pescadero Beach.

				
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posted:10/21/2012
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