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Show Abstract submitted for Poster Session at Great Basin Symposium, Sept. 17 to 20, Unviersity Park Hotel, Salt Lake City Contact Savona Anderson, Department of Geology, Idaho State University, Pocatello, Idaho 83209 208- 236- 3365 Lake level oscillations during the Bonneville Lake Cycle recorded with the Bear River Delta, Cache Valley, Idaho. Anderson, Savona, Link, Paul Karl, Dept. of Geology, Idaho State University, Campus Box 8072, Pocatello, Idaho 83209- 8072. The Pleistocene Bear River delta is located in northeastern Cache Valley, Idaho, the northeastern arm of the Bonneville Basin. The exposed delta sediments consist of approximately 205 m of fine- grained prodelta, delta front, and distributary mouth bar and channel sediments, 120 m of which are exposed along the cliff faces of the Bear River. It has been assumed by past authors that the Bear River delta formed during the Bonneville Lake Cycle (- 28 ka - 14.5 ka). Prodelta deposits consist of interbedded, planar- laminated couplets of silt and clay, deposited by alternating silt and clay transport and settleout. Delta front deposits contain rippled fine sand and silt beds, and interbedded sand, silt, and clay couplets deposited by seasonal channelized floods and commonly reworked by waves along their upper surfaces. The distributary channel and mouth bar deposits have scoured bases and consist of planar and trough cross- stratified, current- rippled and planar- laminated sand. Wave- influenced shoreface deposits consist of planar laminated silt, sand, and clast- supported, weakly imbricated gravel. The abundance of dewatering structures and lack of organic material within the deltaic sediments suggests rapid deposition and oscillation ripples indicate periodic wave- reworking. Detailed analysis of stratigraphic sections reveal several lake deepening cycles, or flooding surfaces, that are interpreted to be the result of both autocyclic and allocyclic processes and therefore reflect both lobe switches within the delta and lake level fluctuations on the scale of tens of meters within Lake Bonneville. Flooding surfaces interpreted to be the result of lake level fluctuations consist of sharp contacts between shallow- water facies and overlying deeper- water lacustrine facies, and are continuous within the stratigraphic sections. Since the Bear River delta only records lake fluctuations on the scale of tens of meters and not a drop of several hundred meters, these results suggest that the Bear River delta only represents the Bonneville Lake Cycle ( 30- 10 ka), even though Sr isotopic evidence of Bouchard et al. ( 1996) suggests that the Bear River entered the Bonneville Basin before Little Valley Cycle. If delta deposits of that age were deposited, they are not exposed today, either now concealed, or having been largely eroded by dissection adjusted to a lower base level between the Little Valley and Bonneville cycles. |