Mission Creek dissects 5 geologic units including the Briones Formation (Miocene), the Tice Shale (Miocene), the chert and siliceous shale member of the Claremont Formation (Miocene), older alluvial fan deposits (Pleistocene), and flood basin deposits (Holocene). A “formation” is a geologic term that includes rocks that were deposited under similar conditions over a large area. Erosion occurs predominantly in the hills where the Briones Formation, Claremont Formation, and the Tice shale are exposed, but erosion also occurs in the older alluvial fan deposits.  Sedimentation occurs further down the creek in the flood plain deposits and in Lake Elizabeth.

Mission Creek is located within the area covered by the Niles and La Costa 7.5-Minutes Quadrangles, which are maps produced by the U.S. Geological Survey. Thomas Dibblee completed the first preliminary geologic maps in 1980.   Graymer, et al (1994), in his preliminary geologic map of the Niles 7.5 Minutes Quadrangle did not assign a name to the Cretaceous sediments,  but  correlated the Miocene sediments with those found further north in the Berkeley area and assigned them the unit and formation names common in the Berkeley Hills. The aerial extent of the geologic units in Graymer’s map differs little from Dibblee’s in the Mission Creek area.  His descriptions of the map units are more detailed and show some differences to Dibblee’s. The interpretation of the structure in the area is also interpreted differently. While Dibblee recognizes a syncline (Niles Syncline) and an anticline in the Cretaceous and Miocene sandstone units,  Graymer sees them as faults, the Sheridan Creek fault and the Dresser Fault/Mill Creek Fault respectively. Dibblee interprets the layers of Miocene sandstones between Mission fault and the anticline northeast of Mission Fault as being overturned and dipping northeast. Graymer sees these layers as just dipping to the northeast between Mission Fault and Mill Creek Fault. Below is a summary of the units of Graymer, et al (1994).  The codes can help you find them on the simplified geologic map in this book.

UNNAMED (Holocene) (QTig, Qhaf, Qhb, Qhf, Qhsc, Qls)
Organic-rich clay to very fine silty-clay deposits occupying the lowest topographic position either between the Holocene levee deposits or Holocene flood plain deposits.

UNNAMED  (Pleistocene) (Qpaf)
Tan to reddish brown, dense, gravel to clay sands or clayey gravel that grades upward to sandy clay.

BRIONES FORMATION  (late Miocene) (Tbu)
The basal part of this formation consists of distinctly bedded, gray to white fine-grained sandstone and siltstone. Bedding is parallel and cross-beds are not evident. Sandstone beds are as thin as 5 to 10 cm with 2 to 10 cm thick shaly interbeds. These are interbedded with massive fine-grained sandstone beds as much as 5 meters thick. The middle part of the formation (shell beds) consists of indistinctly-bedded, white, fine to coarse-grained sandstone, conglomeratic sandstone, and massive, shell-hash conglomerate. Shell-hash conglomerate is made up of marine mollusk shells in a white calcareous sandstone matrix. Pebble and cobble conglomerate beds are present in a few places. Conglomeratic clasts include black chert, red chert, quartzite, andesite, argillite, siltstone, basalt, felsic tuff, and vein quartz. The shell beds and conglomerates are hard and resistant and form prominent ledges, ridges and peaks such as Mission Peak. The upper portion of the formation consists of distinctly indistinctly-bedded, massive to cross-bedded, fine to coarse-grained light colored sandstone. Sand grains are predominantly quartz and feldspar.

CLAREMONT FORMATION (middle to late Miocene) (Tcc, Tcs)
Chert occurs as distinct, massive, gray beds as much as 10 cm thick with thin (about 1 to 2 mm) shale partings. Chert forms about 30 % of the member in the Niles quadrangle. Siliceous shale is dark brown to gray, finely laminated, with grains ranging from clay to silt size.

Simplified geologic map of Mission Creek Watershed


Distinctly to indistinctly - bedded black mudstone, and foraminifera - bearing, brown to tan siltstone and fine -grained sandstone.  In places these rocks have a large amount of secondary calcite.  The Oursan is distinguished from the Briones and Claremont Formations by its darker color, finer grain size, and presence of Foraminifera in siltstone, sandstone and dolomite.  In this area it is 300 to 1000 meters thick.

Distinctly to indistinctly bedded pebble to boulder conglomerate, conglomeratic sandstone, and coarse to medium - grained lithic sandstone.  At least 1500 meters thick in this area, although the base and top are not exposed in continuous space.  It is easily distinguished from other conglomerates in the area by the red and green color.

White, fine to medium grained quartz sandstone.  Occurs in discontinuous outcrops below the base of the Claremont Formation in the eastern part of the quadrangle, nowhere more than 120 meters thick, in fault contact with underlying Cretaceous rocks.

TICE SHALE (middle Miocene) (Tt)
Distinctly bedded, dark brown, gray and tan, siltstone, mudstone and siliceous shale. Tice shale weathers in place to a reddish brown color, and in places contains abundant fish scales. The shale also contains numerous lenses of massive, tan dolomite, as much as 2 m in length that weathers to a characteristic bright orange color.  Many of the dolomite lenses contain Foraminifera that are evident on weathered surfaces of the rock. Tice shale has a maximum thickness of 290 m in this area. Tice shale is similar to shale in the Claremont Formation in the Niles Quadrangle, but lacks the chert beds characteristic of the Claremont.

UNNAMED (Cretaceous)
Distinctly bedded gray to white, well lithified, massive to cross bedded, micaceous, coarse to fine grained sandstone, siltstone and shale. Sandstone varies from granitic (quartz, feldspar, and biotite grains) to lithic wacke (grains of mica, clay, quartz, feldspar and lithic fragments). Sandstone and shale beds are interfingering and range from several cm to several meters thick. In some places the shale contains small (10 cm) to large (1-2 m) limestone concretions. Poorly preserved foraminifera are present in many shale outcrops, and plant debris is common. Sandstone tends to form outcrops on the ridges and uplands, and prominent resistant outcrops in canyons, whereas the shale interbeds are largely visible only in canyons.

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