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
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
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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