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East-Trending Sespe-Vaqueros Transition in the Eastern Los Angeles Basin: Structural Implications

T. H. McCulloh1 and L. A. Beyer2
1Petroleum Geosciences Consulting, Dallas, TX
2U.S. Geol Survey, Menlo Park, CA

Distribution and thickness of undifferentiated upper Eocene-lower Miocene Sespe-Vaqueros Formations were mapped from outcrops in the Santa Ana Mountains and San Joaquin Hills and from subsurface penetrations beneath the Los Angeles basin east of 118o W. Maximum thickness of the combined conglomerate-sandstone-siltstone units ranges from >1.2 km northwest of Laguna Beach to about 0.9 km beneath the west edge of the northern Santa Ana Mountains. The zero-thickness line results from depositional pinch out along the basin margin and from erosional stripping along uplifted margins and local positive structures.

Marine interbeds (Vaqueros facies) are absent throughout the northern third of the basin and along much of its eastern fringe. Conspicuous marine beds in the south and southwest are separately mappable in the San Joaquin Hills. Northernmost marine fossil occurrences in drill holes and outcrops define a nearly linear easttrending facies boundary close to 33.8o N. If the facies boundary is projected about 10 km easterly along trend from the northeasternmost fossil locality southwest of the Chino-Elsinore fault zone (at 33.831o N and 117.747o W), it intersects the southwesternmost break of the Whittier-Elsinore fault. This intersection point is 10–11 km northwest along the fault from the only Vaqueros fossil locality known northeast of the Chino-Elsinore fault (at 33.800o N and 117.521o W). Using the Vaqueros facies boundary as a piercing line, Chino fault right slip of <1–2 km is suggested in light of 9 km of Whittier fault right slip.

The east-trending facies boundary is incompatible with published palinspastic reconstructions of the Los Angeles basin and with >13– 14 km of left slip on the Santa Monica-Raymond-Sierra Madre- Cucamonga fault zone. It is compatible with large post-18 Ma clockwise rotation of the Santa Monica Mountains and entire western Transverse Ranges. Our data offer promise of improved understanding of the Miocene Los Angeles basin opening.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California