A GEOLOGICAL HISTORY OF MARIN COUNTY, EAST OF THE SAN ANDREAS FAULT

This page is dedicated to Salem Rice who, when I came to Marin, shared with me (and many others in Marin) his knowledge of the geology of Marin. His relentless efforts to wrest the story from its rocks and his tireless, and gentle, efforts to share with us that knowledge have made Marin a better place and us better people. Jim Locke, 1995

The geologic history of Marin County east of the San Andreas Fault includes production of sea floor crust in a divergent plate boundary setting, a long period of transit and marine sedimentation, a voluminous accumulation of terrigenously derived, turbidity current transported materials, and profound evidence of convergent boundary actions in the folding, faulting and low grade to blueshist metamorphism found in the county. The convergent boundary action may have culminated in, 10-12 million years ago, volcanics which are typical of convergent boundary types. And finally Marin County has running through it, the most famous slice of the well known San Andreas transform fault system. This transform fault creates a dramatic contrast in rocks east and west of the fracture. This document only considers the rocks east of this impressive crack, while a companion series being developed on Pt. Reyes discusses the geologic history of the western portion of Marin County.

EAST MARIN'S GEOLOGICAL STORY

Pillow Basalt origins.

The formation of these rocks are clearly associated with submarine volcanic activity. They were likely generated at a divergent plate tectonic boundary like those of areas known to be experiencing sea-floor spreading at the present time. Magnetic information in associated rocks indicates formation in the southern hemisphere. Fresh exposures are dark green, while weathered (rusted) exposures are bright red. Point Bonita on the Marin Headlands side of the Golden Gate channel is a notable occurance of unweathered pillowed lava. Red Hill in San Anselmo is a good example of highly weathered pillow lava. Other good exposures occur on Hwy 101 immediately north of the Waldo Tunnel and at the base of Nicasio Dam in West Marin.

Deposition of radiolarian siliceous oozes

Modern radiolarian oozes are being formed in the equatorial regions of the ocean. The Marin Headlands, Twin Peaks in San Francisco, San Rafael Hill behind the mission in San Rafael and Red Rock Island adjacent to the Richmond-San Rafael Bridge contain notable exposures of these sedimentary sequences.

Submarine Hydrothermal Activity

Hydrothermal alteration of pillow basalts to "greenstone" and radiolarian oozes to jasper, along with the deposition of secondary mineral (quartz and calcite) veins and manganese crusts. Tectonically this association is likely similar to activities associated with the "hot smokers" found in today's East Pacific Rise. The Marin Headlands and Red Rock Island have notable exposures of these features.

Marine Pelagic Deposition Changing to Turbidite Deposition

Continued deposition of radiolarian oozes and conversion to "ribbon" cherts. Lower (older) units of the chert are more altered by heat than upper (younger) units. Microfossils recovered from the cherts date from the Age of Dinosaur times of Early Jurassic (200 mya) to Middle Cretaceous (98 mya). In the Hwy 101 roadcut immediately north of the Golden Gate Bridge and along the Alexander Ave. exit a transition from quiet pelagic marine sedimentation to the deposition of the greywacke and shale sequence of the Franciscan Formation is well recorded. This association of sedimentary rocks likely represent ancient turbidity currents or submarine landslides. Perhaps originally deposited as deep sea fans at the mouth of an ancient submarine canyon, rare fossils recorded from these rocks usually date from the Cretaceous Period. Noteworthy occurances of clams of Cretaceous age have been found in the greywackes of Alcatraz Island. Fresh exposures as in the San Rafael, McNear's Quarry (1997 aerial) are grey, while weathered exposures are a yellow brown.

The Compressive Period

Deformation of the seafloor rocks with consequent folding, faulting, and large scale deformation or "melanging" of the units. High pressure metamorphism of the old seafloor rocks (pressures required occur 10 to 15 miles within the earth) and the inclusion of former mantle materials (serpentine). Fort Point and Angel Island contain notable occurances of the melange material in association with serpentine. The Nature Conservancy's Ring Mountain Preserve on the Tiburon Ridge has extensive exposures of serpentine and the rare high pressure minerals. This process was associated with the plate tectonic convergence of old sections of the Pacific Ocean floor and the western edge of North America.

The More Recent History

Tectonic uplift of surrounding highlands and depression of valleys combined with volcanic activity. Burdell Mountain north of Novato has exposures of 12 million year old volcanic rocks and there was evidence in the old Hutchinson Quarry (where the Courtyard Hotel now stands) of rhyolitic dikes transecting the greywackes. The processes of erosion (especially during lower stands of sealevel) have produced the current bedrock topography. Running water and landsliding have been, the dominant erosional processes involved in the shaping of the bedrock surfaces, and the dominant deposition processes in creating Ice Age (Pleistocene) and modern alluvial deposits.

I might suggest that you go back to the geology home and follow the geology pick "up section" to discussions of each of these parts of the Marin County geologic story. Enjoy the work of the TSAW contributors. 


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