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Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 8:00 AM-12:00 PM

QUATERNARY SEDIMENTARY STRUCTURE OF THE SOUTHWESTERN LOS ANGELES BASIN REGION


EDWARDS, Brian D.1, CATCHINGS, Rufus D.2, HILDENBRAND, Thomas G.3, MILLER, Meghan S.3, PONTI, Daniel J.4 and WOLF, Stephen C.5, (1)US Geol Survey, 345 Middlefield Road, Menlo Park, CA 94025, (2)US Geol Survey, 345 Middlefield Rd, MS-977, Menlo Park, CA 94025-3591, (3)U.S. Geol Survey, Mail Stop 989, 345 Middlefield Road, Menlo Park, CA 94025, (4)U.S. Geol Survey, 345 Middlefield Rd, MS-977, Menlo Park, CA 94025, (5)US Geol Survey, 345 Middlefield Rd, MS-999, Menlo Park, CA 94025-3591, bedwards@usgs.gov

Geophysical and borehole data at two sites (LWEB and LBPC) were collected from the Los Angeles and Long Beach coastal plain and adjacent mainland shelf as part of a U.S. Geological Survey study designed to develop a regional stratigraphic model of the southwestern LA basin. The study, done in cooperation with local water agencies, employs gravity, magnetic, seismic-reflection, and borehole log data to extend stratigraphic interpretations from detailed reference sections, developed at the boreholes, throughout the basin.

Existing gravity stations, supplemented by 600 new measurements, provide a regional perspective of lateral density variations in the near surface. Filtering techniques used to enhance shallow sources identify the Wilmington anticlinorium extending NW from offshore Long Beach to offshore Redondo Beach. The anticlinorium is flanked to the SW by a narrow (3-km wide) subbasin inboard of the Palos Verdes peninsula (here named the Palos Verdes basin) and to the NE by the broad LA basin. A preliminary 3-D inversion supports a structural relief of about 1 km from the crest of the anticline to the floor of the Palos Verdes basin. Aeromagnetic data, strongly influenced by cultural activity, support the general northwest structural grain observed in the gravity data. Formation Micro Imager (FMI) data obtained within the LWEB borehole, located on the NE flank of the anticline, identify over 950 boundary picks that have a consistent 6° dip to the NE between the base of the Holocene at 30 m and the bottom of the borehole at 425 m. On-shore seismic-reflection data support this interpretation. In the vicinity of the LBPC borehole, marine seismic-reflection data show this anticlinorium to crest about 70 ms (~60 m) below the seafloor. The SW flank of the fold provides an on-lap surface for clinoform beds infilling the Palos Verdes basin. A flat-lying, broad channel complex overlies the clinoform section. Deformations observed in key reflectors lapping onto the fold imply structural growth into the Holocene.

Session No. 46--Booth# 6

T29. Quaternary Framework of the Los Angeles Basin (Posters)
Wednesday, 11 April 2001: 8:00 AM-12:00 PM
Geological Society of America Abstracts with Programs. Vol. 33, No. 3, March 2001, p.75
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