SOFT SEDIMENT DEFORMATION AND THE PROSPECT FAULT, TACONIC CONVERGENCE, MOHAWK VALLEY, CENTRAL NEW YORK STATE

Late Ordovician faulting in the central Mohawk Valley region, attributed to plate flexure during convergence between the Laurentian plate and a Taconic island arc, has been identified on the basis of stratigraphic relationships including growth fault geometries, unconformities, and paleoflow. However, questions remain as a source of debate. For example, were actual scarps associated with these faults? Recent detailed structural and sedimentological analysis in the region of one of the most western of the Mohawk Valley faults, the Prospect Fault, provides insights concerning the importance of these faults in the depositional and tectonic history of the area.

Soft-sediment deformation in carbonates occurs in the middle Rust Formation (Upper Trenton Group) in exposures along West Canada Creek (and adjacent exposures) about 20 km north of Utica, NY. A spectacular outcrop at Prospect, N.Y., displays a Prospect Fault-parallel scarp over which a slump (slide) mass apparently translated. The slide transforms in character from intact strata on the high, northwest side of the scarp to "broken formation" where the slide bent over the lip of the scarp and to debris flow below the top of the scarp. South of the Prospect Fault (on its downthrown side) folded slump blocks, debris flow material, soft-sediment thrust faults, and diamicton injections indicate massive sediment slides translated northward toward the fault. The inferred paleoslope suggests that the fault block back-rotated, consistent with a listric fault. At Trenton Falls deformation at the same horizon is characterized by debrites with large olistoliths that flowed down a narrow, shallow, curving slide scar/channel. Debris flow material was injected into sedimentary layers at the margin of the slide scar. In all locations, deformed beds are cross cut by the overlying non-deformed grainstone. These exposures demonstrate that 1) the fault activity was syndepositional, 2) the faults did result in seafloor scarps, as do similar faults in modern convergence zones, 3) the fault may have been a listric fault, and 4) that, consistent with previous interpretations, the sense of offset at the time of deposition (down on the south) was opposite to the reverse sense-of-motion presently displayed.