EPISODIC DEPOSITION IN CLOSED DEPRESSIONS: PROXY EVIDENCE OF HOLOCENE PALEOSEISMIC EVENTS, PROVO SEGMENT OF THE WASATCH FAULT ZONE, UTAH

We emplaced a ~56-m long and 3-m deep trench between two westward-dipping splays of the Provo Segment of the Wasatch fault zone (WFZ) near Provo, Utah (~150 m south of Rock Canyon). The trench exposed ~14-18 ka old, Bonneville lacustrine gravels overlain by 3, discrete clay and silty clay deposits that fill a ~170-m long and 52-m wide, north-south elongate topographic depression. The Bonneville beds are tilted ~6 degrees to the east; the overlying pond sediments have progressively less tilt, consistent with their relative age, and generally thicken eastward along the axis of the depression. The pond deposits abut an ~9-m high escarpment formed by vertical displacement along 4 faults that comprise a portion of the Provo segment of the WFZ.

The depression (pond)-filling units are ~1-1.5 m thick: the upper (youngest) is the most extensive; the middle is the least extensive; and the lower is of intermediate extent. We infer that sediment extent reflects pond size and, therefore, the relative height of tectonically produced scarps that form the eastern boundary.

Trenching and radiocarbon dating along the Provo segment of the WFZ by others indicate that 3 major scarp-forming events took place ~ 0.6, 2.6, and 5.3 ka ago. We therefore suggest that the three depression-filling deposits at our trench site may well record the last three paleoseismic events. Accordingly, we propose that these episodic, depression-filling deposits are similar to strike-slip produced sag-pond sediments, and thus may prove to be excellent proxy indicators of paleoseismic tilting and scarp-formation along the WFZ.