SEA-LEVEL CONTROL OF MOLLUSCAN DIVERSITY AND EXTINCTION IN THE PLIOCENE SAN JOAQUIN BASIN, CENTRAL CALIFORNIA

The Pliocene San Joaquin Basin was a silled marginal basin 175 km long, 100 km wide, and ~125m deep connected to the Pacific Ocean through the Priest Valley Strait, a seaway 30 km long, 13 km wide, and 10-50 m deep. Paleogeography determined that the environment inside the basin differed substantially from those of the Pacific Coast. Peak molluscan diversity corresponds with highest sea-levels. This suggests that immigrant species entering the basin from the Pacific became established during periods of warm climate and normal marine conditions, corresponding to maximum eustatic levels and basin flooding, where they adapted to the local environment. Upon sea-level fall during colder climates the basin became cooler and brackish and relict faunas from warmer and normal marine conditions, and nearly all immigrants from during the highest eustatic levels, became extinct. Low diversity faunas characterize the periods of low and rising sea-level. This pattern repeated during each of the major eustatic cycles affecting the San Joaquin Basin. During low and rising sea-level circulation from the Pacific Ocean through the Priest Valley Strait was insufficient to maintain normal marine conditions and the San Joaquin Basin remained too brackish for most immigrants from coastal faunas to become established. There were 7 extinction events affecting the molluscan faunas in the San Joaquin Basin: 2 extinctions in the upper Etchegoin Formation and 5 extinctions in the upper San Joaquin Formation including the end-San Joaquin extinction coincident with the final tectonic closure of the Priest Valley Strait. During the end-Etchegoin extinction diversity progressively declined as species adapted to warmer and more normal marine conditions became extinct leaving species tolerant of cooler brackish water. Diversity rebounded but remained low throughout the lower San Joaquin during which brackish-water conditions prevailed and did not recover until marine flooding peaked in the basal upper San Joaquin. Latest San Joaquin faunas are dominated by species characteristic of brackish conditions. Thus the Priest Valley Strait was not a only a corridor between the Pacific and the San Joaquin Basin but also a filter and trapdoor: Species unable to adapt to conditions inside basin were filtered-out while those species that adapted and established became trapped in the basin.