Geology of Point Reyes National Seashore and vicinity, California: A digital database

Abstract

The Point Reyes Peninsula is a roughly triangular projection of the Marin County coast with an apex at Point Reyes and a base along Tomales Bay and the rift valley of the San Andreas fault zone. This fault zone forms the active tectonic boundary between the Pacific and North American lithospheric plates and here separates Upper Cretaceous granitic and older metamorphic rocks of the Salinian block to the west from Upper Jurassic to Lower Cretaceous graywacke sandstone and melange of the Franciscan complex to the east. Resting nonconformably upon the crystalline basement of the Point Reyes Peninsula is a composite section of as much as 4,370 m (14,340 ft) of moderately deformed Tertiary marine sedimentary rocks that comprise three sequences, separated by unconformities. The oldest sequence consists of as much as 210 m (700 ft) of the Point Reyes Conglomerate of Galloway (1977) of early Eocene age that rests depositionally upon the porphyritic granodiorite of Point Reyes. A middle and upper Miocene sedimentary sequence is as much as 1,610 m (5,300 ft.) thick and consists of a basal sandstone unit, the Laird Sandstone, and an overlying porcelanite unit, the Monterey Formation. This Neogene sequence crops out on the southwest slope of Inverness Ridge, where it rests nonconformably upon the granodiorite and granite of Inverness Ridge. Resting unconformably on all older rocks of the Point Reyes Peninsula are the widely transgressive beds of the upper Miocene and Pliocene sedimentary sequence. This youngest sequence is as much as 2,550 m (8,500 ft.) thick and consists of a basal glauconitic sandstone unit, the Santa Margarita Sandstone, a siliceous mudstone unit, the Santa Cruz Mudstone, and an upper siltstone, mudstone, and sandstone unit, the Purisima Formation. Clark and others (1984) applied these formational names from the Santa Cruz Mountains to the correlative units at Point Reyes because of their very similar stratigraphic relationships, lithologies, and fossil faunas and floras. A complete superpositional section is nowhere exposed on the Point Reyes Peninsula, and good rock exposures are generally restricted to the sea cliffs. Three deep exploration wells (wells 4, 5, and 6, sheet 1) penetrated thick sections of the upper Miocene and Pliocene sequence and of the middle and upper Miocene sequence (see Oil Test Wells , sheet 1). Additionally, the Chevron Tevis no. 1 well drilled near Double Point (well 4, sheet 1) penetrated 5 m (16 ft) of altered olivine pyroxene basalt beneath the middle and upper Miocene sequence and above as much as 502 m (1,646 ft) of dark shale of early Eocene age (Kris McDougall, written commun., 1997). The close similarity of the Upper Cretaceous porphyritic granodiorites and of the lower Eocene and middle and upper Miocene sequences of the Point Reyes and Monterey Peninsulas suggests as much as 150 km (94 mi) of total right slip displacement on the San Gregorio fault in the last 11 Ma. Similarities of the glauconitic Santa Margarita and of the thick Santa Cruz Mudstone sections of the up...

The search for geologic evidence of distant-source tsunamis using new field data in California

“…We positioned our core transect as distant as possible from a slough channel cutting through the marsh, as well as from the steep hillslopes on the northeast side of the marsh. The hillslopes bordering the marsh are composed of highly erodible late Tertiary mudstones, siltstones, and sandstones of the Purisima Formation (Clark and Brabb, 1997), as evidenced by pale yellowish brown alluvial deposits at the base of slope in some locations, particularly below the bluffs at the seaward edge of the marsh. In addition, a large alluvial fan is present at the termination of drainages at the landward edge of the marsh.…”

California State Waters Map Series — Offshore of Tomales Point, California

“…Folds axes trend west to northwest and folds have common wavelengths of 0.5 to 2 kilometers. The folding occurred during the late Pliocene or Pleistocene; it does not obviously involve post-LGM sediments, and the upper Miocene and Pliocene Purisima Formation is apparently the youngest folded unit (Clark and Brabb, 1997). Folding may be synchronous with activity on the northwest-trending Point Reyes Thrust Fault which lies about 16 km offshore of the map area (McCulloch, 1987;Heck and others, 1990; also see sheet 10, fig.…”

“…In map area, both backscatter data and direct camera observations show small intensity contrasts, suggesting that depressions are filled with sediment that is coarser than intervening elevated sandy shelf deposits; general area in which unit is found is not likely to change substantially, but boundaries of unit(s) and locations of individual depressions (and intervening flat sheets) likely are ephemeral, changing during significant storm events Qstb Sediments of central Tomales Bay (late Holocene)-Mixed fine sand and silt on the mostly flat floor of Tomales Bay. Anima and others (2008) report that sediment in the south-central part of the bay is fine sand to coarse silt; they sampled fine to very fine sand at the mouths of several small coastal watersheds Qsw Sand at the mouth of Tomales Bay (late Holocene)- Anima and others (2008) (Clark and Brabb, 1997); occurs on east shore of Tomales Point…”

“…Onshore bedrock mapping is compiled from Galloway (1977), Clark and Brabb (1997), and Wagner and Gutierrez (2010). Onshore Quaternary mapping is compiled from Witter and others (2006) and Wagner and Gutierrez (2010) The morphology and the geology of the offshore part of the Offshore of Tomales Point map area result from the interplay between tectonics, sea-level rise, local sedimentary processes, and oceanography.…”

California State Waters Map Series — Offshore of Bodega Head, California

“…2, 3, 8, 9, 10, 11 on sheet 8). Nearby outcrops onshore (Clark and Brabb, 1997), and also offshore wells (Heck and others, 1990), reveal that these granitic rocks are overlain to the west and north by folded Neogene sedimentary rocks, which are imaged by folded, high-frequency seismic reflections (see southwest end of fig. 3 on sheet 8).…”