SEDIMENTARY FINGERPRINT OF A THRUST FAULT

The timing of movement on faults is often difficult to ascertain. Although the relative ages of the lithologies involved can often be ascertained it can be difficult in sedimentary successions to pinpoint the point in which movement actually occurred. This is particularly true in thrust fault settings wherein uplift on a basin’s margins commonly results in erosion of the sedimentary successions involved.

Sediments within the Cathedral Bluffs Tongue of the Wasatch Formation preserve an unusual association of lithofacies that provide a snap shot of movement on the Wind River Thrust Fault in the southern Wind River Mountains. The Cathedral Bluffs Tongue is dominated by variegated silty to sandy paleosol units and laterally restricted coarse-grained sandstone and conglomerate units. It has been interpreted to record deposition in a meandering fluvial system although several thin lacustrine intervals, indicated by bioclastic to crystalline limestone lithologies, stromatolites and kerogen-rich shale beds, also occur. One of these lacustrine interludes consists of a standing forest of trees, covered in a thick (4 to 20 cm) sheath of cyanobacterial stromatolite. These erect trees are preserved in a matrix of coarse-grained pebble-rich sandstone and sand-matrix conglomerate. The conglomerate-bound, cyanobacterially-sheathed trees overlie, and are overlain by, variegated paleosol mudstone. This sedimentary succession is herein interpreted to record an episode of movement by the Wind River Thrust Fault, which runs through the margin of the study area.

The episode of thrusting preserved in the study interval caused several meters of rapid subsidence that caused an alluvial plain forest to be inundated by the waters of Lake Gosiute. This was followed by an interval of very low sedimentation allowing stromatolites to grow on the trunks and branches of the still-standing trees. The coarse lithologies that surround the stromatolites reflects erosion of the thrust block in the adjacent montane highlands adjacent to the study area and rapid progradation of a braided fluvial / alluvial fan delta burying the stromatolite-covered forest and preserving the full succession. This unique sedimentary association provides clear evidence of syntectonic depositon and provides a snapshot of sedimentary system response to faulting.