The Sheeprock Mountains are part of a horst of Proterozoic, Paleozoic and Cenozoic sedimentary and igneous rocks located in the transitional region between the Cordilleran fold-thrust belt and the hinterland in the Basin-Range province of west-central Utah. Prominent structural elements in the Sheeprock Mountains are the Sheeprock Thrust, juxtaposing Proterozoic rocks above Paleozoic ones with a stratigraphic separation exceeding 10 km; the Pole Canyon Thrust, thought to be an upper plate imbrication of the Sheeprock Thrust; the Pole Canyon Anticline, a recumbent fold vergent to the northeast and cut by the Pole Canyon Thrust; the east-northeast-striking Indian Springs (tear) Fault; and two low-angle normal faults (the Harker and Lion Hill Faults) which together account for stratigraphic omission of several kilometres. The Pole Canyon Anticline is thought to have developed in the late Mesozoic during propagation of the thrusts parallel to the Indian Springs Fault, and this transport direction is corroborated by minor structures. Fault geometry suggests that the Harker and Lion Hill Faults are younger than the thrusts and probably of late Cenozoic age, although some mid-Cenozoic or even earlier displacement cannot be entirely ruled out. My preferred interpretation of the structural history of the Sheeprock Mountains is consistent with minimal regional extension before the mid-Cenozoic and with the view that crustal shortening in the fold-thrust belt is for the most part unrelated to hinterland extension.

ABSTRACT The rootless Ventura Avenue, San Miguelito, and Rincon anticlines (Ventura fold belt) in Pliocene-Pleistocene turbidites are related to south-dipping reverse faults rising from a decollement in underlying Miocene shale. To the east, Sulphur Mountain anticlinorium comprises the upper plate of a south-dipping set of thrusts (Sisar, Big Canyon, Lion) that merge downward into a decollement in mudstone of the Rincon Formation, named here the Sisar decollement. Shortening of the Miocene and younger sequence is ~3 km greater than that of the underlying competent Vaqueros, Sespe, and marine Eocene sequence in the Ventura fold belt and ~7 km greater farther east at Sulphur Mountain. Cross section balancing requires that this difference be taken up by the Paleogene sequence at the Oak Ridge fault to the south. Convergence is NE to NNE based on earthquake focal mechanisms and piercing-point offset of the South Mountain seaknoll by the Oak Ridge fault. A NE-trending line connecting the west end of Oak Ridge and the east end of the Sisar fault separates an eastern domain where late Quaternary displacement is taken up entirely on the Oak Ridge fault and a western domain where displacement is transferred to the Sisar decollement and its overlying rootless folds. This implies that (1) the Oak Ridge fault near the coast presents as much seismic risk as it does farther east, despite negligible shallow post-0.2 Ma movement, (2) ground-rupture hazard is high for the Sisar fault set in the upper Ojai Valley, and (3) the decollement could fail on a low-angle thrust at depth and produce an earthquake analogous to the 1987 Whittier Narrows event in Los Angeles.

This field trip is an overview and reappraisal of the prolific oil basins of southern California (Fig. 1A) using exploration methods now commonly used in international exploration. As a result of the dramatic decline in oil and gas exploration in California during the last decade these mature and well known basins have received limited modern hydrocarbon research and it is hoped that our field trip and guidebook will outline some of the important aspects and questions of these intriguing petroleum systems. We have used balanced cross sections and other types of structural analyses integrated with basin modeling, geochemical and geophysical data to gain new insights into the structure, trapping mechanisms, and petroleum systems (Magoon and Dow, 1994) in a setting combining strike-slip and convergence (transpression). Southern California geology also has the scientific advantage, but societal disadvantage, of earthquakes (Fig. IB) which provide useful data about the deeper structure which will be presented during the trip. Our field examples are in the eastern Ventura basin, Ridge Basin, southern San Joaquin basin, Cuyama basin and western Ventura basin as well as a transect of the western Transverse Ranges (Fig. 1A).