--> Abstract: Review of Growth Fault-Bend Folding at SE Lost Hills, by D. Medwedeff; #90911 (2000)

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Abstract: Review of Growth Fault-Bend Folding at SE Lost Hills

MEDWEDEFF, DONALD, ARCO Exploration Technology and Operational Services, Plano, TX

I review my 1989 study of SE Lost Hills, CA in light of the 1998 Lost Hills deep gas discovery.

OBSERVATIONS: Lost Hills anticline (LHA) is located in the western San Joaquin Valley, CA. Plio-Pleistocene sediments onlap a time-transgressive unconformity on the NE limb, but have uniform dip and thickness on the SW limb of the LHA. This geometry indicates basinal sediments rolled through an axial surface onto the SW limb, whereas contemporaneous sediments deposited above the NE limb are un deformed. Onlap indicates folding began in early Etchegoin Time (~8 Ma) and continued to the late Pleistocene or Recent. Nearly continuous burial of the fold causes material points to be near their maximum burial.

INTERRPRETATIONS: I interpreted the growth strata pattern as a consequence of fault-bend folding. In this model the LHA is caused by a thrust which steps up to the NE at 27 degrees. The step connects a lower detachment near the base of the Tertiary to an upper detachment in the Reef Ridge Formation. Forward- and back-limb dips of 42 degrees and 27 degrees are consistent with fault-bend fold theory. The lower and upper detachments are at 6 km and 4 km depths. Displacement on the lower detachment is about 6.5 km, folding consumes 2 km, and the remaining 4.5 km continues northeast beneath the San Joaquin Valley.

NEW MODEL?: The tri-shear model (Hardy and Ford, 1997) may be able to explain the observed growth patterns with out requiring large fault slip beneath the San Joaquin Valley.

 

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California