Abstract: Timing and Geometry of Left-Slip Faulting and Compressional Folding in Hondo Field, Western Santa Barbara Channel

Michael McGroder, Chip Millson, Dave Gardner

Hondo field is located within the Santa Ynez offshore lease unit, approximately 5 mi south of Gaviota, California. Production is from Monterey fractured shales and deeper sandstone horizons. Two types of structures are well imaged on 3-D reflection data from the east end of the field, which has undergone 3-D prestack time migration. The most conspicuous structure is an east-west compressional fold with a gently to moderately dipping southern limb and a shorter and more steeply dipping north limb. The shape of the fold suggests that uplift was related to southward transport up a north dipping thrust ramp at (subseismic) depth. The northern limb of this structure is widest at the level of the top Sisquoc Formation and narrowest in sediments 2-3 cycles beneath the sea floor. This triangu ar panel of north dip indicates growth of the structure commenced during deposition of the lower Pico Formation in the Pliocene, and continued through the Pleistocene.

The second-order structures that control trap distribution within the field include high-angle faults that trend roughly east-west. The northernmost of these faults, the north bounding fault (NBF) is known from well data to have 1500 feet of throw and to dip steeply to the south. It would restore to a vertical or steep north dip upon removal of the effects of Pliocene-Pleistocene folding. The southernmost of these faults (SF) appears to change displacement from down-to-the-south in the east to down-to-the-north in the west and also dips steeply. A high-angle fault in the center of the field (CF) also has a vaiable sense of offset across it. Several prominent northeast-southwest faults lie between the CF and SF and clearly exhibit normal separation on vertical time sections and left se aration on time slices. Variable thicknesses of Sisquoc Formation and roughly uniform thicknesses of Monterey Formation indicate that these faults were active in latest Miocene to early Pliocene.

We have used visualization technology to create 3-D images of several horizons in Hondo field. These images and the observations described above lead us to conclude that the high-angle faults in this part of the western channel formed during a short-lived but intense episode of transtensional left slip at around 5-4 Ma. This event was followed shortly thereafter by the onset of regional north-south compression on deep thrusts within the channel and Transverse Ranges. We speculate that the location of ramps in the deep thrust system and hence the geometry of overlying folds was controlled to some extent by the presence of the previously active high-angle faults. The undeformed sea floor suggests that the deep thrusts and overlying folds may not be active today.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994