Abstract The Vaqueros, Sespe, and Alegria Formations are important oil and gas reservoirs in the area of the Santa Barbara Channel. At Hondo field, which is located about 40 km west of Santa Barbara and 9 km offshore, ten exploratory wells and five development wells have been drilled into these formations. The interpretation of data obtained from these wells, including a cored section from the OCS 190-2 well, suggests that Sespe and Alegria strata in the Hondo area were deposited in a deltaic environment during the Oligocene Epoch and that Vaqueros strata formed in an inner shelf setting during a late Oligocene and early Miocene transgression. Vaqueros deposition ended abruptly when rapid subsidence and deepening related to incipient basin formation terminated shallow marine sedimentation in the area. Gradation of the Vaqueros Formation into deep-water deposits of the overlying Rincon Formation, as interpreted from well logs, occurs vertically over just a few meters.

Microfossils recovered from the Roseburg, Lookingglass, and Flournoy Formations (Umpqua Group) of southwest Oregon include 33 species and subspecies of planktonic foraminifers. Some of the faunas from these units include as many as 14 species per sample, and specimen preservation is moderate to good. The assemblages are similar to those from central and southern California but differ in that they are dominated by long-ranging, high-latitude species. The Roseburg and Lookingglass Formations were deposited during early Eocene time and are assigned on the basis of planktonic foraminifers to Zone P7-8 of the standard tropical zonation. The planktonic foraminiferal faunas of these units are indistinguishable. The Flournoy Formation is assigned to lower middle Eocene Zone P10. Planktonic foraminifers and calcareous nannofossils indicate that the unconformity between the Lookingglass and Flournoy Formations represents an interval of approximately 1 to 2 m.y. Despite reports of Paleocene microfossils occurring in the Roseburg and Lookingglass Formations, no microfossils definitely of that age were found during this study. Cretaceous microfossils were recovered from a small outlier currently included in the Roseburg Formation. Although the rocks containing these fossils closely resemble those of the Roseburg Formation, they probably belong to a separate unit.