Abstract: Neogene Evolution of the Livermore Basin in the California Coast Ranges

David W. Andersen

Miocene and younger sedimentary rocks in and around the Livermore Valley record the paleogeographic evolution of this part of the northern Diablo Range. The section consists mostly of fluvial strata with some lacustrine and shallow-marine rocks. Both sand and gravel clasts can be used to document uplift of distinctive source regions that supplied sediment to the basin.

Coast Range uplift in the vicinity of Mt. Hamilton, south of the Livermore basin, began as early as about 10 Ma. Subsequently, Miocene andesitic sediment derived from active volcanoes in the Sierra Nevada was deposited upon older Tertiary and Mesozoic rocks throughout the region. These include the youngest marine rocks in the Livermore basin. Renewed uplift within the Coast Ranges produced pulses of sediment first from the Mt. Hamilton block at about 7 Ma, next from the Altamont Hills area to the east by about 6 Ma, and finally from the Mt. Diablo block on the north by 5 Ma. Sierran sediment continued to be transported north of Mt. Diablo and south of Mt. Hamilton, but it is not recognized within the Livermore basin in rocks younger than about 7 Ma. Within the basin, grain size and pr portion of high-gradient fluvial facies are greatest near the basin margins and also increase dramatically in rocks younger than about 3 Ma.

The area around the Livermore basin has experienced deformation since the passage of the Mendocino Triple Junction about 10 Ma. Uplift of the Altamont Hills by about 6 Ma suggests that the Coast Ranges in this area extended as far east in the Miocene as they do today. Increase in clast size and inferred stream gradients in deposits younger than about 3 Ma suggest increasing intensity of deformation after Pliocene reorganization of Pacific-North America plate motion.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California