Abstract

The Conception submarine fan is located in the northwestern corner of the Santa Barbara basin, within the tectonically active western Transverse Ranges Province of southern California. This small radial fan heads off a series of small submarine canyons and slope gullies that were fed by the modern and ancestral Santa Ynez River, intermittent streams and longshore drift.

A unique data set of high resolution analog to deep penetration, CDP seismic reflection profiles and records, collected along north-south/east-west tracklines was used for this study. These data were collected for or by the energy industry primarily between 1960-1985, and were augmented by boring logs and near-surface core and grab samples. Surface and deep-tow side scan sonar records over critical features (e.g. fan channels, faults, etc.) complete the data set.

East-west trending oblique-slip faults and related folds control the position of shelf break along the northern margin of the basin. Barriers or tectonic dams along the shelf were formed by growing anticlinal folds and active faults and breached by streams incised along shear zones such as the south Santa Ynez fault and minor cross faults. At these points the small submarine canyons that fed the fan during low-stands of sea-level were established.

Inactive today, this Quaternary fan extends from shelf break (-90 m) to the basin floor (-550 m), a distance of 25 km. In width the middle fan is 20 to 25 km across, while the lower fan is 35 km across where it is deflected eastwards along the southern or island margin of the basin. The fan is about 150 m thick and internally consists of four major, 30-40 m thick sand-thin mudstone sequences that represent low and high sea level stands.

Uplift along the northern and western margins of the basin and the southern or island margin restricted the fan’s radial growth and progressively shifted its activity to the southeast toward the center of the basin. The southern edge of the fan laps onto the basin apron of the island or southern margin of the basin. The eastern edge of the fan is marked by an abrupt southwesterly deflection of the east-west trending isobaths that are typical of the northern slope. Internally this boundary is evidenced by a rapid change in seismic facies as slope mudstones interfinger with sand-rich fan deposits.

Isopachs of the four seismic sequences of the fan document the offset or horizontal stacking of these sequences. The maximum thickness of each overlying sequence is offset horizontally from the maximum thickness of the underlying sequence. As a potential reservoir the Conception Fan offers four 30-45 m (maximum) thick sand-rich sequences separated by mud seals (3± m thick). Each sand sequence is topographically high along its axis of maximum thickness. The entire fan is encased in mud-as it overlies mudstones of the Pico Formation and is covered by a high-stand mud deposit.

An offset stacking model of depositional sequences or “fan lobes” provides a valuable tool for exploration. Examples of offset stacking include the Lathrop and the Winters Formation fans of the northern Great Valley and Neogene fans in southern California.