The Fillmore Formation is about 1,800 feet thick in its type area. It is a mixed carbonate-siliciclastic unit that accumulated on a rapidly subsiding miogeoclinal platform. Sedimentologic and lithologic features indicate that the depositional environment was generally between normal wave base and storm wave base, but was punctuated by rare peritidal conditions. We interpret the Fillmore Formation largely as a product of sea-level lowstand sedimentation.

The formation consists of three major lithologic packages, 400-600 feet thick, each composed of a basal recessive interval followed by a resistant, ledgy interval. The base of the formation is a ledgy lime mudstone, similar in lithology to the underlying House Limestone. Likewise, the uppermost ledgy interval grades into the more distinct ledges of the overlying Wah Wah Limestone. We tentatively identify at least four sequence boundary intervals in the formation:

1) At twelve feet above the base of the formation, within the basal ledgy interval is a major flat-pebble conglomerate complex and friable fine sandstone interbedded in a ledgy lime mudstone succession. This horizon is coincident with a major change in conodont fauna, and occurs a few feet above major change in trilobite fauna;

2) At roughly 485-505 feet above the base of the unit, just above the top of the lower recessive-resistant package is an influx of silt and red-pebble conglomerate;

3) At 840-850 feet above the base of the formation, within the resistant part of the middle recessive-resistant package is a truncation surface and possible vadose silt; and

4) 240-255 feet below the top of the formation, within the resistant part of the upper recessive-resistant package is a succession of red-pebble conglomerate beds capped by herringbone cross-laminae.