Isopach maps of the major zones of the East Wilmington field shed significant light on the structural evolution of the Wilmington anticline.

A minor ancestral basement high marked the location of the Wilmington anticline at the close of the early Mohnian, but the structure was essentially a syncline on the north flank of the Catalina uplift. Uninterrupted sedimentation occurred throughout the late Mohnian and early Delmontian. No evidence of the present Wilmington anticline is seen prior to the deposition of the lower part of the Ranger. The source of sediment supply was on the northeast and the direction of transport was southwest.

Growth of the Wilmington anticline was initiated in late Miocene time, early in Ranger deposition. Most Ranger subzone isopachs reflect Ranger structure, and demonstrate continuous anticlinal growth during lower Repetto deposition. Movement on the Long Beach Unit fault began during Ranger X-sand deposition and culminated at the close of F-sand deposition. The F sand was eroded and channeled by current action along and across the anticlinal axis prior to F_o-sand deposition.

The F_o sand was deposited as a lens-shaped body on the eroded F, and is present over a minimum 48-sq-mi area. No evidence of structural growth is seen during the deposition of this sand, in contrast to the other Ranger sands. The F_o is thought to be primarily a tractionite.

Renewed uplift and additional movement on earlier faults occurred during the Pasadenan orogeny and completed Wilmington structural development.

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