P. 0. Box 21087, Oakland, CA, 94620-1087 (katb@ pangea.stanford.edu)
Abstract: A Multidisciplinary Approach to Conglomerate Analysis Suggests 180 (+/- 5 km) post-Eocene Dextral Offset of the San Gregorio-Northern San Andreas Fault
This research focuses on a search for clasts of unique rock varieties that appear to have been derived from the same parent rocks and that are now found in different conglomerates at distant sites. The basic premise of the study is that durable clasts derived from a specific parent rock will be found throughout the downstream depositional area, and therefore suites of matching clasts can be found in different segments of sedimentary systems that have been subsequently offset by strike-slip faulting. Conglomerates are a useful basis for paleogeographic reconstruction because, unlike finer-grained sedimentary rocks, their clasts preserve more complete information, thus revealing their provenance more specifically, and because the stronger force needed to transport larger clasts usually results in deposits of limited extent.
Correlation of unique rock varieties offers a more accurate basis for correlation of faultoffset pairs than does comparison of the percentages of general rock types within clast populations. Microscopic petrography is essential in determination of whether (or not) those clasts which are virtually identical in hand sample could have been derived from the same specific parent rock. Sedimentological and structural analysis, petrologic and petrographic analysis of conglomerate matrix, microprobe geochemistry, and radiometric dating add further points of comparison. So far as I am aware, my multidisciplinary method of employing unique clast types has not been used previously to compare conglomerates to other conglomerates.
This method has demonstrated correlation of conglomerates at two pairs of localities: Pt. Reyes with Pt. Lobos and Anchor Bay with the Pilarcitos block. Each pair is separated by ~180 km.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California