Rocks and Geology in the San Francisco Bay Region ������� ����� ��������� �������� ���� ������ ��� ��������� �������� ��������� �������� ������ ������ ������ ����� ����� ������ ����������� ����� ����������� ����� ��������� ������������ ���� ���� ������� ���� ������ ���� ������� �������� ������ ��� ����� �� ����� ������ ��� ����� ������ Bulletin 2195 U.S. Department of the Interior U.S. Geological Survey This page intentionally left blank Rocks and Geology in the San Francisco Bay Region By Philip Stoffer Bulletin 2195 U.S. Department of the Interior U.S. Geological Survey 1 U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. U.S. Geological Survey, Reston, Virginia: 2002 Electronic copies of this publication are available online at http://geopubs.wr.usgs.gov/bulletin/b2195/ Additional USGS publications can be found online at http://geology.usgs.gov/products.html For more information about the USGS and its products: Telephone: 1-888-ASK-USGS (1–888–275–8747) World Wide Web: http://www.usgs.gov/ Published in the Western Region, Menlo Park, California Manuscript approved for publication, September 30, 2002 Text and illustrations edited by James W. Hendley II Production and design by Susan Mayfield III Contents Introduction............................................................................................................................................................... 1 Geologic time....................................................................................................................................................... 2 The rock cycle..................................................................................................................................................... 3 Plate tectonics.....................................................................................................................................................4 Active versus passive continental margins....................................................................................................7 Evolution of the San Andreas Fault system.................................................................................................... 9 Generalized geologic map of the San Francisco Bay region.....................................................................12 Igneous rocks and processes.............................................................................................................................. 13 Classification of igneous rocks.......................................................................................................................14 Intrusive igneous rocks....................................................................................................................................15 Granite........................................................................................................................................................... 15 Granodiorite..................................................................................................................................................15 Why are there felsic intrusive igneous rocks in the San Francisco Bay region?.................................. 16 Diorite............................................................................................................................................................ 17 Gabbro........................................................................................................................................................... 17 Rocks from the mantle?....................................................................................................................................18 Pyroxenite..................................................................................................................................................... 19 Peridotite.......................................................................................................................................................19 Chromite ore................................................................................................................................................. 19 Anorthosite................................................................................................................................................... 20 Jade............................................................................................................................................................... 20 Extrusive igneous rocks................................................................................................................................... 20 Basalt.............................................................................................................................................................20 Andesite........................................................................................................................................................ 21 Rhyolite..........................................................................................................................................................21 Pillow basalt—how does it form?.................................................................................................................. 21 Volcanic rocks in the San Francisco Bay region.........................................................................................22 Pinnacles volcanic area—a national monument for half a volcano........................................................23 Hydrothermal deposits..................................................................................................................................... 23 Travertine...................................................................................................................................................... 23 Hydrothermally altered basalt................................................................................................................... 24 Calc-silicate rock.........................................................................................................................................24 Springs, warm springs, and hot springs and their deposits.......................................................................24 Cinnabar (mercury ore) and the New Almaden Mining District................................................................25 Sedimentary rocks and processes......................................................................................................................26 Soil....................................................................................................................................................................... 28 Layers in sedimentary rock............................................................................................................................. 29 What strata can reveal about earth history..................................................................................................30 Sediments and sedimentary deposits............................................................................................................31 Sedimentary rocks............................................................................................................................................ 32 Conglomerate............................................................................................................................................... 32 Sandstone..................................................................................................................................................... 34 Graywacke—an old sedimentary rock term with a new meaning..................................................... 36 Mudstone and siltstone.............................................................................................................................. 37 Marlstone......................................................................................................................................................37 Shale.............................................................................................................................................................. 38 Limestone and dolostone........................................................................................................................... 39 Concretions—nature’s time capsules......................................................................................................40 Chert...............................................................................................................................................................41 Gypsum..........................................................................................................................................................42 2 III Rock salt........................................................................................................................................................42 Limestone versus chert and the carbonate compensation depth (CCD).................................................43 Organic residues and rocks (petroleum and coal)......................................................................................44 Asphalt...........................................................................................................................................................45