BASIN-FLOOR LAKE BONNEVILLE STRATIGRAPHIC SECTION AS REVEALED IN PALEOSEISMIC TRENCHES ON THE WEST VALLEY FAULT ZONE, SALT LAKE VALLEY, UTAH

Recent paleoseismic trenching on the Granger fault of the West Valley fault zone exposed a relatively complete section of late Pleistocene Lake Bonneville (LB) sediments on the floor of Salt Lake Valley (40° 47.4′ N., 112° 0.7′ W., ~1286 m AMSL). Clay at the base of the section contains charophyte stem encrustations and Scirpus-type (bulrush) seed fragments, and shows evidence of burrowing. The clay is overlain by a 0.8-m-thick sequence of ripple-laminated, locally cross-bedded silty sand that generally fines upward into a 0.5-m-thick massive gray clay. The gray clay is overlain by 1.3 m of massive red clay containing 2–20-cm-thick interbeds of silt and fine sand and the ostracodes Limnocythere ceriotuberosa, Candona caudata(?), and Candona adunca. The laterally continuous interbeds have abrupt bounding contacts and contain clay rip-up clasts; we interpret them as turbidites resulting from episodic influx of coarser sediment associated with destabilization of lake-margin deposits, possibly caused by lake-level fluctuations or earthquakes. The planar upper surface of the red beds is marked by vertical, infilled fractures (desiccation cracks?). Finely laminated marl ~0.6 m thick that overlies the red beds contains an ostracode assemblage that includes Cytherissa lacustris. A layer of tufa overlies an unconformity in the middle part of the marl and consists of broken fragments and intact pods of tufa several tens of centimeters thick. Our preliminary interpretation of the stratigraphic sequence is as follows: clay at the base of the section represents pre-LB wetland/fluvial-marsh sedimentation; the ripple-laminated sand represents the initial rise of LB above this altitude; the gray and red clay with silt and sand interbeds represents the mid- to late-transgressive phase of LB; marl overlying the red clay may represent the regressive phase of LB or perhaps post-LB wetland deposition; and the unconformity, tufa layer, and overlying marl represent lake transgression to the Gilbert shoreline. We see no evidence for a late Holocene transgression of Great Salt Lake at this site. Radiocarbon and luminescence dating (in progress) will provide temporal control for our interpretation of the lacustrine sequence as well as the timing of surface-faulting earthquakes.