Sedimentology and Stratigraphy
Depositional History and Coastal Environments of the Merced Formation
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.
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
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
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 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.
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.
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.
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.
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
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-
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
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.
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.