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San Andreas Fault System in Southern California (SAFSOC)

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TASK 4 - Paleontologic Studies Along the San Andreas Fault System

Task leader: Kristin Mcdougall-Reid

Statement of Problem:
The Pacific-North America plate boundary and the San Andreas Fault system have evolved in distinct phases, beginning at about 30 Ma for the former and 20 +/- 2 Ma for the latter. Sedimentary basins figure prominently in our understanding of the evolution of the San Andreas Fault system. Basin deposits in southern and central California record pre-San Andreas tectonic events associated with the pre-30-Ma convergent plate boundary, the transition from convergent to transform plate boundary, the inception of the San Andreas Fault system, and transitions between phases in the evolution of the San Andreas system. Basins that formed prior to inception of the San Andreas Fault system help to constrain overall offsets on the various faults of the system; basins that formed during or subsequent to activity on individual faults help define incremental offsets and evolutionary phases within the fault system. Detailed micropaleontologic study of basin sediments reveals both time and position data that can be used to constrain reconstruction of the fault system. These studies indicate the time of deposition and the paleoenvironmental data such as information about latitude at time of deposition, as well as proximity to the coastline and depth below sea level for marine sediments. Matching faunal assemblages in now offset rock bodies can provide piercing "points" that constrain timing and amount of strike-slip offset. This approach is yielding promising results in Eocene strata that have experienced the full cumulative displacement along the San Andreas Fault system and in Miocene and Pliocene rocks in the Salton Trough and Gulf of California that were deposited then offset during intermediate and late phases in the evolution of the San Andreas system. empty box graphic
Spring-fed pond with California Fan Palms in the Coachella Valley Preserve, southern CA
Spring-fed pond with California Fan Palms at McCallan Grove in the Coachella Valley Preserve, southern CA. This pond is located along the Mission Canyon Fault strand of the San Andreas Fault system. The pond is a refuge to desert pupfish.

Objectives:
(1) Refine the timing of the opening of the Gulf of California and deposition of the Imperial, Bouse, and related formations.
(2) Examine Eocene marine formations which can be used to correlate tectonic blocks and provide piercing points for
reconstruction.

Methodology:
The research approach is:
(1) Identify marine sedimentary rocks east and west of the San Andreas Fault from geologic maps that appear to be offset
(2) Select and sample stratigraphic and paleontologic sections that will allow analyses of geologic age and environment (depositional facies) for cross fault correlations. Where warranted, initiate collaborative magnetostraphic and geochronologic studies to complement the paleontological studies.
(3) Work with geologists to develop or modify tectonic reconstructions based on age and environmental constraints of the faunas.
(4) Compile databases that can be used for paleontologic analysis and for production of geologic maps.

Task Products:
Journal or Periodical Article, Planned: McDougall, K., 2011, Marine-nonmarine controversy for the origin of foraminiferbearing strata in the Bouse Formation, Journal to be determined
 

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