Facies relations in the Eocene-Oligocene in the Santa Ynez Mountains, California

Abstract

Facies relationships between deep and shallow marine to continental deposits in the Eocene-Oligocene sequence of the Santa Ynez Mountains, California, have been studied in detail to (i) determine interrelationships between shallower and deeper marine sands (coastal and turbidite); (2) establish criteria for the recognition of genetic sand types; (3) determine factors influencing facies distribution; and (4) propose hypotheses of marine sand deposition. The rocks studied include the Anita, Sierra Blanca, Juncal (with the Camino Cielo Member), Matilija, Cozy Dell, and “Coldwater” formations but not the topmost unit of the Eocene-Oligocene sequence, the non-marine Sespe formation. In landward sequence, the facies recognized include: Turbidites and marine lutites, proximal turbidites, shallow marine, coastal and continental facies. These occur in two major regressive sequences. In the first, the Juncal-Matilija sequence, thin-bedded turbidites and marine lutites are overlain by, and are laterally equivalent to, very thick proximal turbidites which pass upward into shallow marine and coastal sands. The major sand accumulations are in the basin margin shallow marine, coastal, and proximal turbidite facies. The second regression, the Cozy Dell-Sespe sequence, lacks significant proximal turbidite deposits but has extensive shallow marine and coastal deposits. Facies distribution and stratigraphical sequence are explained as responses to the interplay of depositional and structural processes. Detailed stratigraphical mapping has clarified correlation in the Eocene sequence to the Santa Ynez Mountains. Micropaleontology is used in support of correlations and bathy-metric interpretations.