THE GREAT WALL OF MIDWAY-SUNSET

A structure contour map of the top Antelope Shale Member of the Miocene Monterey Formation in Midway-Sunset Oil Field reveals a vertical- to overturned northwest trending “wall” that separates the Midway Valley syncline from shallower Antelope equivalent productive oil pools. The northwest trending “wall” reaches a height of more than one-mile for a distance of nearly twenty miles. At each end, the “wall” flattens to more moderate dips between 400-600 and then gradually blends into the Midway Valley basin rim.

Most likely, early topographic forms of the “wall” controlled Miocene sand deposition. Although erosional events since Middle Miocene time have removed depositional units, the “wall” was eventually buried. Present-day topographic relief above the wall is generally less than five degrees.

A three-dimensional structural model of Middle Miocene and younger sediments constructed primarily from well marker picks, leaves the viewer with a grand appreciation of the spectacular forces involved that produced this high-relief feature within the San Joaquin Valley. The paucity of deep wells, coupled with the lack of good quality seismic or diagnostic gravity-magnetic data, allows the interpreter to make several possible structural models of the stratigraphic units below the Antelope Shale. The authors speculate on some of the possible deep structural models creating the “wall” and briefly describe some of the tightly folded structures west of the “wall” and east of the San Andreas Fault.