--> Abstract: Basin Formation as a Modifier of a Eustatically-Driven Depositional Sequence: Lower to Middle Miocene Stratigraphy, Pt. Mugo, California, by C. S. Lerch; #90987 (1993).

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LERCH, CHRISTOPHER S., Shell Offshore, Inc., New Orleans, LA

ABSTRACT: Basin Formation as a Modifier of a Eustatically-Driven Depositional Sequence: Lower to Middle Miocene Stratigraphy, Pt. Mugo, California

The succession of lower Miocene Vaqueros to middle Miocene Rincon/Topanga Formations exposed at Pt. Mugu is a eustatically-driven depositional sequence. This sequence was modified, however, by lower to middle Miocene basin formation due to changes in regional tectonism. Relative movement at the west edge of the North American plate changed from subduction to transtension upon subduction of the East Pacific Rise. Basin formation during transtension resulted in a regional deepening of many paleoenvironments in southern California.

Three locally-mapped stratigraphic units span the Vaqueros to Rincon/Topanga transition at Pt. Mugu. The lower shelly sandstone unit represents aggraded to retrograded coastal shoreface and transition zone deposits: the transgressive systems tract (TST). The middle shale/silty shale unit represents coastal transition zone to continental slope deposits: the condensed zone (CZ) and highstand systems tract (HST). Finally, the upper sandstone to alternating sandstone/silty shale unit represents continental slope to submarine-fan deposits: the lowstand systems tract (LST), completing the eustatic cycle. However, plotted on a chronostratigraphic cross section this succession exhibits a basinward shift between the TST and HST, correlating to the rapid stratigraphic transition from neritic to bathyal environments. This shift was induced by the regional tectonic deepening due to basin formation in the lower to middle Miocene.

Transtension in the region also resulted in continental terrane rotations up to 90 degrees (clockwise) during and after the middle Miocene. Paleocurrent and paleomagnetic data from Pt. Mugu support this rotation. In particular, paleocurrent data indicate an east-west trending strandline lying to the south; a 90 degree counterclockwise rotation restores this strandline to a north-south orientation and infers a source area to the east, complying with the trend of the late Oligocene to early Miocene continental margin elsewhere in this region.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.