Abstract: Sedimentology and Stratigraphy of the Sacate-Sespe Formations, Central Santa Ynez Mountains, Santa Barbara County, CA

PIETRAS, JEFFREY T., Department of Geology and Geophysics, Univ. Wisconsin-Madison;KIRT M. CAMPION, Exxon Production Research;J. A. (TONI) SIMO, Department of Geology and Geophysics, Univ. Wisconsin-Madison

The late Eocene to Oligocene (Refugian-Zemorrian California stages; Priabonian-Chattian stages) Sacate, Gaviota, Alegria, and Sespe formations of the Central Santa Ynez Mountains have been measured and correlated both using field relations and aerial photographs. A sequence stratigraphic framework and facies partitioning can be extrapolated to offshore subsurface data.

These formations form a succession ~4,000 feet thick that starts with an angular unconformity at the base of the Gaviota and ends with the incision of the Vaqueros Formation. Within this large-scale facies evolution four surfaces can be correlated defining higher frequency events: a regional bed with large articulated oysters separating the Gaviota from the Alegria/Sespe, a dissolution-enhanced surface with abundant nodules and root traces capping the Alegria/Sespe, and two significant erosional events within the Sespe. The Alegria/Sespe package represents the in-facies relationship of the Sespe in the east with the Alegria to the west.

The Gaviota Formation shows coarsening-upward parasequences of tabular siltstones grading into sandstones of increasing grain size and oyster content. The oyster-rich base of the Alegria/Sespe is interpreted as a flooding event. The Alegria/Sespe consists of fining-upward parasequences with sandstone channels containing mud rip-ups and burrows. These grade into supratidal red siltstones with plant root casts. The Alegria/Sespe upper boundary is interpreted as a regolith. The Sespe is marked by numerous fining-upward parasequences that are grouped into three sequences. Each contains a succession of sandstones and siltstones with the proportion of siltstone increasing upwards. The upper two sequences have a basal conglomerate with Franciscan clasts.

The nested hierarchy of sequences discussed will affect reservoir compartmentalization.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California