Conglomeratic Lower Member of Lospe Formation (Lower Miocene) n sts in the Lower Miocene Lo Geophysical section offshore Santa Maria basin Geologic section onshore Santa Maria basin AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the current- year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior to the current year are listed in the most recent annual "Price and Availability List." Publications that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability List" are no longer available. 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Maps Maps Only For maps, address mail orders to Maps may be purchased over the counter at the U.S. Geologi­ U.S. Geological Survey, Map Distribution cal Survey offices: Box 25286, Bldg. 810, Federal Center Denver, CO 80225 FAIRBANKS, Alaska New Federal Building, 101 Twelfth Ave. Residents of Alaska may order maps from ROLLA, Missouri 1400 Independence Rd. U.S. Geological Survey, Earth Science Information Center 101 Twelfth Ave., Box 12 Fairbanks, AK 99701 STENNIS SPACE CENTER, Mississippi Bldg. 3101 Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California By SAMUEL Y. JOHNSON and RICHARD G. STANLEY Provenance of Sandstone Clasts in the Lower Miocene Lospe Formation Near Point Sal, California By HUGH MCLEAN and RICHARD G. STANLEY Chapters D and E are issued as a single volume and are not available separately U.S. GEOLOGICAL SURVEY BULLETIN 1995 EVOLUTION OF SEDIMENTARY BASINS/ONSHORE OIL AND GAS INVESTIGATIONS- SANTA MARIA PROVINCE Edited by Margaret A. Keller U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Robert M. Hirsch, Acting 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 Text and illustrations edited by James W. Hendley II UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994 For sale by U.S. Geological Survey, Map Distribution Box 25286, MS 306, Federal Center, Denver, CO 80225 Library of Congress Cataloging in Publication Data Johnson, Samuel Y. Sedimentology of the conglomeratic lower member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California / by Samuel Y. Johnson and Richard G. Stanley. Provenance of sandstone clasts in the Lower Miocene Lospe Formation near Point Sal, California / by Hugh McLean and Richard G. Stanley. p. cm. (Evolution of sedimentary basins/onshore oil and gas investigations Santa Maria Province ; ch. D, E) (U.S. Geological Survey bulletin ; 1995) "Chapters D and E are issued as a single volume and are not available separately." Includes bibliographical references. Supt. of Docs, no.: I 19.3: 1995-D, E 1. Geology, Stratagraphic Miocene. 2. Sedimentation and deposition California. 3. Sandstone California. 4. Lospe Formation (Calif.) I. Stanley, Richard G. II. McLean, Hugh, 1939- Provenance of sandstone clasts in the Lower Miocene Lospe Formation near Point Sal, California. 1994. III. Title. IV. Title: Provenance of sandstone clasts in the Lower Miocene Lospe Formation near Point Sal, California. V. Series. VI. Series: U.S. Geological Survey bulletin ; 1995. QE75.B9 no. 1995-D-E [QE694] 557.3 s dc20 93-39079 [551.7'87'09794 CIP Chapter D Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California By SAMUEL Y. JOHNSON and RICHARD G. STANLEY U.S. GEOLOGICAL SURVEY BULLETIN 1995 EVOLUTION OF SEDIMENTARY BASINS/ONSHORE OIL AND GAS INVESTIGATIONS- SANTA MARIA PROVINCE Edited by Margaret A. Keller CONTENTS Abstract Dl Introduction Dl Acknowledgments Dl The Lospe Formation Dl Sedimentology of the lower member of the Lospe Formation D3 Unit 1:0 to 47m D4 Unit 2: 47 to 68 m D7 Unit 3: 68 to 112m D9 Unit 4: 112 to 142m D16 Rocks overlying unit 4 in the North Beach section D16 Discussion D19 Conclusions D20 References cited D20 1. Index map of Santa Maria area, showing Santa Maria basin, major faults, and dis­ tribution of the Lospe Formation D2 2. Location map of Point Sal area, showing outcrops of Lospe Formation D3 3. Stratigraphic chart showing Lospe Formation and bounding units D4 4. Photograph of the unconformity at the base of the Lospe Formation D5 5. Rose diagrams showing paleocurrent data for Lospe Formation lower member D5 6. Schematic Stratigraphic section of Lospe Formation lower member at North Beach D6 7. Photographs and line drawing of sedimentologic features of unit 1 of Lospe For­ mation lower member D8 8. Photographs and line drawing of sedimentologic features of unit 2 of Lospe For­ mation lower member DIG 9. Photograph and line drawing of stratification style in unit 3 of Lospe Formation lower member D12 10. Photograph and line drawing of stratification style in unit 3 of Lospe Formation lower member D13 11. Photographs showing sedimentologic features of unit 3 of Lospe Formation lower member D14 12. Photograph and line drawing of stratification style in unit 4 of Lospe Formation lower member D17 13. Photographs showing sedimentologic features of unit 4 of Lospe Formation lower member D18 Contents IV Sedimentology of the Conglomeratic Lower Member of the Lospe Formation (Lower Miocene), Santa Maria Basin, California By Samuel Y. Johnson and Richard C. Stanley Abstract billion ft3 of associated gas (California Division of Oil and Gas, 199la). Subsidence of the Santa Maria basin began The lower Miocene Lospe Formation records the initial in the early Miocene during an episode of local crustal subsidenceof the Santa Maria basin, a major petroleum province in central California. The lower member of the Lospe is best extension driven by right-lateral shear along the boundary exposed in a 142-m-thick section at the North Beach (informal between the North American and Pacific plates (Hornafius, name) locality on Vandenberg Air Force Base. These North 1985; Luyendyk and Hornafius, 1987; Luyendyk, 1991; Beach exposures are divided into four units on the basis of Stanley and others, 1992). The earliest phase of basin evo­ distinctive sedimentary textures and structures. Unit 1 (0 to 47 lution is recorded by the lower Miocene Lospe Formation, m) consists mostly of crudely stratified lenticular and channel- an 800-m-thick sequence of nonmarine and shallow ma­ form pebbletoboulderconglomerate interpreted as mid-alluvial- rine sedimentary rocks and interbedded tuffs. This paper fan streamflow and debris-flow deposits. Unit 2 (47 to 68 m) describes the sedimentology of the lower member of the comprises mainly flat- to low-angle-bedded and channel-form Lospe, composed of conglomerate and sandstone. These pebble conglomerate interpreted as sheetflood and streamflow strata, interpreted herein as alluvial fan and lacustrine fan- sediments deposited in a slightly more distal alluvial fan setting. Unit 3 (68 to 112 m) is a sedimentologically heterogeneous delta deposits, provide important information for recon­ sequence of fine-grained to granular sandstone and pebble- structing paleogeography during the earliest phase of cobble conglomerate. Flat-bedded sandstone and poorly sorted evolution of the Santa Maria basin and for understanding lenticular conglomerate beds are interpreted as the subaqueous the depositional processes and facies that characterize the sediment-gravity-flow deposits of a lacustrine fan delta. Inter- initial development of transtensional basins. bedded channel-form conglomerate beds are interpreted as mainlysubaerialstreamflowanddebris-flowsedimentsdeposited