EAST-TRENDING SESPE-VAQUEROS TRANSITION IN THE EASTERN LOS ANGELES BASIN: STRUCTURAL IMPLICATIONS

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 118° 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 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 fossils occurrences in drill holes and outcrops define a nearly linear east-trending facies boundary close to 33.8° 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 33.831° N and 117.747° 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 33.800° N and 117.521° 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 previously published palinspastic reconstructions of the Los Angeles basin and with the concept of >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 improving understanding of the locus and mechanics of the Miocene opening of the Los Angeles basin.