D. Development of the Absaroka thrust fault: faulting
E. Development of the Absaroka thrust fault: faulting
F. Post-Absaroka Thrust Deformation
G. Simplified structure section through present-day Fossil Basin
(Oyster Ridge is off the diagram to the East)
Structural interpretation by Dr. D.L. Blackstone
In early Eocene times, the effects of continued downwarping allowed Fossil Lake to form in the Fossil Basin. By the late Eocene, however, basinal subsidence could no longer keep pace with deposition and the dominantly lacustrine (lake) sedimentation was replaced by a fluviatile (riverine) sedimentary regime. Deposition of fluviatile sediments probably continued into the later Tertiary when, in the late Pliocene, regional uplift of the Rocky Mountain interior reversed the sedimentary cycle from depositional to erosional. The last 3 or 4 million years of geologic history have witnessed the excavation of much of the Tertiary sedimentary fill from the Fossil Basin. Fossil Butte and the Ham’s Fork Plateau to the northeast of the butte are high erosional remnants of this early Tertiary basin fill. The traces of the Absaroka and other thrust faults are buried beneath these remaining Tertiary deposits in the Fossil Basin.
While most of this complex structural history took place before the Tertiary sediments were deposited, some deformation continued into the early Eocene. Near Prow Point, for example, is a fault that developed during early Eocene time. This can be noted in the very northwest corner of the monument, just to the west of Prow Point ([Fig. 6]). Here, the Lower Member of the Wasatch Formation on the west is faulted against the Main Body on the east (USDI 1964). This fault also is responsible for elevating the Thaynes Limestone so it is exposed ([Fig. 10]).