Abstract

The general chronology of deformation on the south flank of the Wind River Basin has been established by Thompson and White (1954), Keefer (1960, 1965, 1970), Paape (1961), Love, McGrew and Thomas (1963), and Love (1970). Recent detailed geologic mapping and stratigraphic studies on the south flank of the basin permit more precise reconstruction of the sites, times, and effects of deformation. Five distinct episodes of deformation affected the southwest arm of the basin from late Campanian (Cretaceous) through early Eocene time and subsequent structural movement deformed rocks as young as Miocene. Deformation by uplift and southwest movement of the Granite Mountains block along the Emigrant Trail thrust fault produced marked stratigraphic and structural discontinuities in rocks exposed from Conant Creek west to the Alkali Butte, Sand Draw, and Beaver Creek areas. The discontinuities include (1) unconformities that truncate older rocks most deeply at the site of, or adjacent to, the Emigrant Trail thrust fault; (2) folds adjacent to that fault which change either in degree of appression of limbs or in trends of axial traces across unconformities; (3) secondary faults which are truncated by erosional surfaces at unconformities; and (4) tilting, folding, and faulting of all outcropping strata.

Deformation in late Campanian time is represented by an unconformity at the base of the Teapot Sandstone Member of the Mesaverde Formation (Gill and Cobban, 1966). The unconformity bevels progressively older rocks of the Mesaverde from east to west across the basin. Deformation was dominantly by northeast tilting of the region. During late Maestrichtian time additional northeast tilting of the southwest margin of the Basin is evident from relations along an unconformity at the base of the Lance Formation, which truncates successively lower parts of the Mesaverde Formation from northeast to southwest. Faults that displace older rocks near Alkali Butte are truncated at the unconformity and are buried by overlying strata.

During Paleocene time two strong pulses of deformation associated with uplift of the Granite Mountains block affected the southwest arm of the Wind River Basin. An unconformity overlain by lower Paleocene rocks truncates the Lance Formation and part of the Mesaverde Formation at Alkali Butte, Sand Draw, and Beaver Creek. Stratigraphic relations along a major unconformity at the base of a sequence of middle and upper Paleocene beds define local arching east of Alkali butte and along the trend of the Sand Draw and Beaver Creek anticlines. That trend seems to have been structurally higher than Alkali Butte because Paleocene beds there rest on stratigraphically lower beds of the Mesaverde Formation and Cody Shale than at Alkali Butte. Northward into the basin both Paleocene unconformities die out into a conformable succession of Paleocene rocks (Keefer, 1960, 1965).

The lower Eocene Wind River Formation unconformably overlies rocks as old as Triassic at the surface along the east edge of the southwest arm of the basin and lies on Precambrian granite where concealed by younger rocks on that edge. The pattern of onlap is evidence of local uplift and erosion of the west flank of the Alkali Butte anticline and the Sand Draw area at the site of the Emigrant Trail thrust fault. The fault subsequently offset the Wind River Formation near Alkali Butte. At the west end of the Granite Mountains and east of Sand Draw along the trace of the thrust fault, rocks of late Eocene, Oligocene, and Miocene age are gently folded and faulted (Van Houten, 1964; Love, 1970).

During Paleocene time regional dips as great as 15 degrees and local dips of 65 degrees developed at and just east of the present trace of the Emigrant Trail thrust fault; the higher dips resulted from folding near the edge of the subsequently upthrown block of the fault. Such dips may have influenced the south and southeast migration of petroleum from deeper parts of the Wind River Basin, causing the petroleum to be entrapped in anticlinal structures along the front of the advancing fault, but lost from reservoir rocks truncated up plunge on the upper plate of the fault. The distribution of unconformities suggests that potential reservoir rocks of late Mesozoic age were truncated by erosion across the site of the developing fault and that these rocks are absent beneath the upper plate. Potential reservoir rocks of Paleozoic age should be present beneath the upper plate and could contain petroleum along the trends of early developed folds; but the Paleozoic rocks were likely breached by erosion at sites now located in eastern-most subthrust positions. Potential reservoir rocks of all ages east of the fault trace on the south flank of the basin have likely had a long history of fluid communication up plunge to the Granite Mountains and thus seem to be poor exploration objectives.