GMEG - Geology and Geophysics
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TASK 2 - Earth surface geologic studiesStatement of Problem: Surface geologic mapping is one of the primary inputs into a 3-D geologic map, but modern digital geologic maps do not exist for much of the study area. Furthermore, a section of the study area along the Peninsula San Andreas section has been identified as requiring additional geologic map studies beyond those integrated into the presently available digital geologic map (Brabb and others, 2000, USGS MF-2332). The tectonic events that transformed the coastal California plate boundary from a convergent margin to a transpressive margin, and the effects of the San Andreas Fault system offsets, are recorded in the sediments and basins along the San Andreas Fault system. A detailed paleontologic framework can reveal both time and position data required for the 4-D reconstruction. However, the long-standing problem of mixing lithostratigraphic and time-stratigraphic units in California has influenced nearly every geologic study done in the past 150 years. Furthermore, the microfossil correlations which were developed in California are time transgressive and tend to have a local or regional focus. Focused studies of faunal assemblages can reveal details in timing of deposition, including the detection of time-transgressive units that may be related to deposition across a moving fault. Paleoenvironmental data revealed by faunal assemblages can provide information about latitude at time of deposition, as well as proximity to the coastline and depth below sea level for marine sediments. And matching faunal assemblages in now offset rock bodies may provide piercing "points" that control timing and amount of strike-slip offset.
Objectives: New digital geologic maps and map databases are required for four areas in the study region, broken down by 30' X 60' quadrangle:
1. The Point Arena 30'X60' quadrangle, especially that region north of Sonoma County (Sonoma County is presently available at 1:62,500 scale in Blake and others, 2002, USGS MF-2402)
2. The Point Sur and Coalinga 30'X60' quadrangles, especially the area including and surrounding the San Andreas Fault zone. This mapping would bridge the gap between the existing map of the Monterey 30'X60' quadrangle (Wagner and others, 2002, CGS CD 2002-04) and the work of the Salinas Valley Task of the 3-D Geologic Mapping and Visualization Project presently finishing up.
3. The San Luis Obispo and Taft 30'X60' quadrangles, especially the area including and surrounding the San Andreas Fault zone. This mapping would extend the coverage of modern digital geologic mapping to the southern extent of the study area.
4. The Healdsburg 30'X60' quadrangle (see subtask below).
We also recognize the need for additional geologic mapping along the Peninsula San Andreas Fault in the Santa Cruz Mountains. We have been granted permission to solicit a Mendenhall Postdoctoral Fellow to pursue these studies. The objectives of the paleontological investigations will be focused on expanding the paleontological database, focusing on rocks deposited during the development of the San Andreas Fault system (<30 Ma) in the following 3 areas:
1. Age control. Improve the available age control by reexamining existing fossil collections and recollecting existing localities as required, and developing new fossil localities in collaboration with geologic mappers. The age-transgressive nature of California microfossil zonation demands tie-ins with the global microfossil zonation via an interpretation that separates local, regional, global, and ecological influences.
2. Cross fault correlation. Analyze existing and new fossil collections in comparison to known faunal assemblages in cross-fault units.
3. Paleoenvironmental control for 4-D reconstruction. Analyze existing and new fossil collections for faunal evidence of latitude and depth of deposition.
Methodology: For the production of new geologic maps, we will follow the standard procedures for geologic field data gathering and
interpretation. We will especially emphasize collecting data on structural orientation that is important for projecting surface
geology into the 3-D geologic map and collecting any fossils/tephra that could yield improved time-stratigraphic
information. We will also work with our potential field geophysics colleagues to gather data that could enhance their
interpretation of the regional gravity and magnetics. Past experience has suggested that previously erected stratigraphic
frameworks will prove to be overly simplified, so we will need to develop new stratigraphic interpretations that take into
account the juxtaposition of originally widely separated depositional basins by large fault offsets and the potential for
TASK 2 - Surface geologic studies
Sub Task 1: Healdsburg 30'X60' quadrangle with CGS (Robert McLaughlin)