Depositional Environments and Controls of Juncal Formation, Southern San Rafael Mountains, California

Timothy J. Thompson

The lower to middle Eocene Juncal Formation, north of the Santa Ynez fault in easternmost Santa Barbara County, represents the onset of marine sedimentation following an approximately 20-m.y. hiatus. The Juncal paraconformably overlies the upper Campanian "Unnamed" sandstone and is gradationally overlain by the middle Eocene Matilija Formation. Access to the Juncal exposures of this area was considerably enhanced by the Wheeler fire of July 1985.

Facies analysis of both the vertical and lateral exposures of the Juncal Formation (~ 1,400 m thick) indicate that the Juncal represents coalescing outer-fan depositional lobes. This interpretation is based on the lateral continuity of sandstone beds, presence of thickening-upward cycles, and high-concentration sediment gravity flows, bathyal fauna, and regional associations. Local channelization (to 10 m deep) represents the extension of a mid-fan channel over its associated depositional lobe. Outcrops are subparallel to depositional strike in the eastern part of the study area and form a broad syncline in the western part. Together, these exposures allow documentation of the depositional lobes vertically, laterally, and longitudinally.

Outer-fan deposits of the Juncal Formation are part of a progradational basin-filling episode. The Juncal grades upward into the outer- to mid-fan deposits of the lower Matilija Formation. The upper Matilija Formation shoals upward into deltaic facies. This regressive sedimentary sequence was probably initiated by the major sea level fall which occurred near the early/middle Eocene boundary and coincides with the onset of fan sedimentation elsewhere along the California margin. Within the study area, depositional lobe activity was probably governed by either minor fluctuations in relative sea level or channel switching up-system.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.