Bowersox, J. Richard¹
(1) University of South Florida, Tampa, FL

ABSTRACT: Late Neogene Paleobathymetry, Relative Sea Level, and Basin Margin Subsidence, Northwest San Joaquin Basin, California

The northwestern San Joaquin Basin remained near sea level throughout the late Neogene despite lying on a tectonically active basin margin. What may be inferred is that from latest Miocene through Late Pliocene deposition kept pace with basin subsidence. The late Neogene San Joaquin Basin was 175 km long, 100 km wide, and bounded by mountains to the east, south, and west. It connected to the Pacific Ocean at the northwest through a narrow and shallow strait ~13 km wide and <50 m deep. Paleobathymetry determined from benthic foraminifera faunas from the subsurface of southeastern Lost Hills oil field shows water depth in the San Joaquin Basin >200 m in the latest Miocene, becoming progressively shallower to ~125 m by middle Pliocene, then to ~25 m by middle Late Pliocene where it remained until the Pacific Ocean connection was tectonically closed at 2.2 Ma. I constructed a relative sea level curve for the late Neogene northwest San Joaquin Basin by assigning appropriate water depths to the succession of latest Miocene through latest Pliocene molluscan communities based on their similarity to published modern communities in San Francisco Bay then smoothed it to remove most tectonic “noise.” When compared to the 3rd order Gulf of Mexico eustatic curve, this relative sealevel curve shows very close correlation. Using the relative sea level curve to refine stratigraphic timing, I developed a time-thickness diagram indicative of relative basin margin subsidence. Latest Miocene basin margin subsidence averaged ~25 cm/kyr then accelerated to a peak of 140 cm/kyr in the middle Early Pliocene coincident with Coast Range uplift. By Late Pliocene subsidence slowed to 11 cm/kyr then again peaked in latest Pliocene at 86 cm/kyr immediately preceding closure of the connection to the Pacific Ocean.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.