Growth Fault-Bend Folding at Southeast Lost Hills, San Joaquin Valley, California

Abstract

Synsedimentary deformation has created complex structural and stratigraphic relationships along the west side of the San Joaquin Valley, California. This case study of Lost Hills anticline documents how a simple structural process--fault-bend folding--results in complex growth stratigraphy. Lost Hills anticline is a fault-bend fold caused by a step in the Lost Hills thrust. The thrust steps up to the northeast at 27° from a detachment at the base of the Tertiary to an upper detachment in the Reef Ridge formation. The lower and upper detachments are at 20,000 ft (6 km) and 13,000 ft (4 km). Displacement on the lower detachment is approximately 4 mi (6.5 km); folding consumes 1.2 mi (2 km) and the remaining 2.8 mi (4.5 km) continues northeast beneath the San Joaquin Va ley. Forward- and back-limb dips of 42° and 27°, respectively, are consistent with fault-bend fold theory. Pliocene-Pleistocene sediments onlap a time-transgressive unconformity on the northeast (front) limb, but have uniform dip and thickness on the southwest (back) limb. This characteristic growth fault-bend fold geometry indicates basinal sediments rolled through an axial surface onto the back limb, whereas contemporaneous sediments deposited above the front limb are undeformed. Basinal sediments onlapping the fold crest indicates fault slip is greater than ramp length. Onlapping growth sediments indicate folding began during deposition of the Etchegoin Formation (~8 Ma) and continued to the late Pleistocene or Holocene, corresponding to an average displacement rate on the Lost Hills thrust of 2,700 ft/m.y. (820 m/m.y.). Continuous burial of the fold during growth caused each material ( .e., physical) point in the fold to be at its maximum depth of burial. Contours of equal density and seismic velocity are relatively flat, so that the anticline causes little or no gravity anomaly and less-than-expected pullup in seismic time sections. The geometry of growth sediments imaged on seismic lines indicates Lost Hills and adjacent anticlines grew by uniform displacement to the northeast with no significant rotation. Oblique convergence between the Pacific and North American plates is manifested as synchronous, but spatially separate, transcurrent and compressive displacements. Thus, Lost Hills anticline and perhaps other folds in the area are more appropriately described as compressive folds in a transpressive region than as transpressive folds. Duplicated lower Tertiary strata beneath the Lost Hills-Kettleman Hills trend are potential targets for petroleum exploration.