Monday, June 8, 2009

Geology Map of the San Diego 30' x 60' Quadrangle

Geology pf the San Diego 30' x 60' Quadrangle

Preliminary Geologic Map at 1:100,000 Scale to provide the public timely access to digital geology, prepared by the California Geological Survey Regional Geologic Mapping Project.

The San Diego 30' X 60' quadrangle was prepared by the Department of Conservation, California Geological Survey pursuant to a U.S. Geological Survey STATEMAP cooperative mapping award (# 1434-94-A-1224). It is a product of the Southern California Areal Mapping Project (SCAMP), a cooperative U.S. Geological Survey-California Geological Survey mapping project, This map is a compilation of published geological mapping (Fig. 1). The published mapping has been modified only to the extent necessary to integrate variables in nomenclature and scale. The onshore part of the map was digitized at a scale of 1:24,000 and the offshore part has been enlarged from 1:250,000. The quadrangle is between 32.5° and 33.0° N. latitude and 117.0° and 118.0° W. longitude. It encompasses the greater San Diego area, the second largest metropolitan area of California.
The area is tectonically active and is dissected by four major northwest-trending, oblique right slip faults that lie within the western part of the Pacific/North American Plate boundary. They include the Rose Canyon-Newport-Inglewood Fault Zone along the coastal margin, the Palos Verdes-Coronado Bank Fault Zone on the inner shelf, the San Diego Trough Fault Zone (origin of the 1986, ML=5.3, Oceanside earthquake) in the central offshore and the San Clemente Fault Zone on the outer offshore margin. Within the greater San Diego metropolitan area, the Rose Canyon Fault Zone as depicted by Kennedy and others (1975), Moore and Kennedy (1975), Kennedy and Welday (1980), Clarke and others (1987), Treiman (1993) and Kennedy and Clarke (2001) includes the Mount Soledad, Old Town, Point Loma, Silver Strand, Coronado and Spanish Bight faults. The Rose Canyon Fault Zone displaces Holocene sediment in Rose Canyon 7 km north of San Diego Bay where a late Pleistocene slip rate of 1-2 mm/yr has been estimated (Lindvall and Rockwell, 1995). A study of the recency and character of faulting in the greater San Diego metropolitan area suggests a long-term Tertiary slip rate for the Rose Canyon Fault Zone of about 1-2 mm/yr (Kennedy and others, 1975). Although there is significant late Quaternary deformation in the San Diego region the seismicity is relatively low (Simons, 1977).
The San Diego quadrangle is underlain by a thick sequence (>5 km) of Mesozoic fore-arc and fore-arc basin andesitic flows and coarse-grained volcaniclastic breccias that have been in large part metamorphosed to low-grade greenshist facies and are pervasively penetratively deformed. However, in the upper part of the section these rocks are not metamorphosed and are only moderately deformed. Marine sedimentary interbeds in Penasquitos Canyon, near Del Mar, contain the fossil Buchia piochii, which indicates a Late Jurassic (Tithonian) age for these strata (Fife and others, 1967; Jones and Miller, 1982). Zircon U/Pb ages from the metavolcanic rocks are reported to range from 137 Ma to 119 Ma (Anderson, 1991) indicating that they are coeval with the surrounding plutonic rocks of the western Peninsular Ranges batholith. The batholithic rocks are mostly granodiorite and tonalite and based on U-Pb isotopic ages range from 140 Ma to 105 Ma (Silver and Chappell, 1988). Much of the basement rock has been deeply weathered and altered. The weathered bedrock and Quaternary alluvial deposits derived from them contain expansible clays, mostly smectite.
The western part of the quadrangle is underlain by a relatively thick (>1,000 m) succession of Upper Cretaceous, Tertiary and Quaternary sedimentary rocks that unconformably overlie basement rocks. They consist of marine, paralic, and continental claystone, siltstone, sandstone and conglomerate. The Upper Cretaceous rocks are composed of marine turbidites and continental fan deposits assigned to the Rosario Group (Kennedy and Moore, 1971). The Lusardi Formation, the basal formation of the Rosario Group is a nonmarine boulder fanglomerate deposited along the western margin of a tectonic highland upon a deeply weathered surface of the older Cretaceous and Jurassic plutonic and metamorphic basement rocks. Clasts within the Lusardi Formation are composed exclusively of these weathered basement rocks. The Lusardi Formation is overlain by the Point Loma Formation, the middle part of the Rosario Group. It is composed mostly of marine sandstone, siltstone and conglomerate sequences that together form massive turbidite deposits. The Point Loma Formation is Campanian and Maestrichtian in age (Sliter, 1968; Bukry and Kennedy, 1969) and underlies most of the Point Loma Peninsula and the hills southeast of La Jolla. It is conformably overlain by the uppermost part of the Rosario Group, marine sandstone and conglomerate of the Maestrichtian (Sliter, 1968; Bukry and Kennedy, 1969) Cabrillo Formation. Following the deposition of the Rosario Group, the San Diego coastal margin underwent uplift and erosion until the middle Eocene when nine partially intertonguing middle and upper Eocene sequences composed of siltstone, sandstone, and conglomerate were deposited during several major transgressive-regressive cycles. The succession is over 700 meters thick and grades from nonmarine fan and dune deposits on the east through lagoonal and nearshore beach and beach-bar deposits to marine continental shelf deposits on the west near the present-day coastline. The age and environmental interpretation of the Eocene sequence is based on the mapped distribution of lithofacies coupled with the presence of a pelagic fossil calcareous nannoplankton flora in the continental shelf facies (e.g., Bukry and Kennedy, 1969), a shallow water molluscan fauna in the nearshore facies (e.g., Givens and Kennedy, 1979), and a fossil terrestrial vertebrate mammal fauna in the paralic facies (e.g., Golz, 1973). Cross bedding, cobble imbrications, paleo-stream gradients and clast petrology indicate a local eastern source for these rocks. The nonmarine facies of the Eocene formations are typically well indurated and cemented whereas the lagoonal facies are soft and friable. The nearshore facies are well indurated, well sorted, and locally concretionary. The marine deposits are typically fine-grained, indurated, and cemented. Following the deposition of Eocene rocks the San Diego margin was again elevated and eroded. During the Oligocene, continental and shallow water lagoonal deposits of the Otay Formation, were deposited. The Otay Formation is light-gray and light- brown, medium- and coarse-grained, arkosic sandstone intertongued with light-brown siltstone and light-gray claystone. Much of the claystone is composed of lightgray bentonite in beds up to 1 m in thickness. Following Oligocene time the San Diego coastal margin underwent uplift and extensive erosion. The next major marine transgression did not occur until Pliocene time when the strata of the San Diego Formation were deposited. The San Diego Formation rests unconformably upon Oligocene, Eocene and Upper Cretaceous beds across its outcrop from Pacific Beach to the International border with Mexico. The San Diego Formation is late Pliocene in age and contains a rich molluscan fauna (e.g., Arnold, 1903; Demere, 1983). It consists mostly of yellowish-brown and gray, fine- to mediumgrained, marine sandstone and reddish-brown, transitional marine and nonmarine pebble and cobble conglomerate. Following the deposition of the San Diego Formation and continuing to the present time, the San Diego coastal margin has undergone relatively steady uplift (Fig. 2). A series of continually evolving marine abrasion platforms have been carved and uplifted during this time and are manifest in the marine terraces and their deposits that are ubiquitous to the San Diego coastal region. The deposits consist of nearshore marine, beach, estuarine, lagoonal and continental dune facies that were deposited across a marine/nonmarine transition zone and along a coastal strandline. Changes in sea level coupled with regional uplift give rise to the preservation and/or obliteration of both the abrasion platforms and their overlying deposits (e.g. Lajoie, and others, 1991; Kern and Rockwell, 1992; Kern, 1996a, 1996b).
The authors appreciate very helpful reviews by Victoria R.Todd and J. Philip Kern

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