LARGE LIQUEFACTION FEATURES AND EVIDENCE FOR EARTHQUAKES INDUCED BY LAKE BONNEVILLE IN CACHE VALLEY: A PROGRESS REPORT

Little is known about the late Pleistocene behavior of the main normal faults in the NE Great Basin in response to loading and unloading by Lake Bonneville. An exposure in a gravel pit at the mouth of Green Canyon in Cache Valley, northern Utah, exposes deformed and liquefied deltaic sediment in the hanging wall of the East Cache fault, near the junction of the northern and central segments. The site is ~40 m below the Bonneville high stand and ~60 m above the Provo shoreline and preserves sediment that were deposited and seismically deformed and slumped several times under sub-aqueous conditions as Lake Bonneville rose. Three ¹⁴C ages on stagnicola shells and three OSL ages confirm that the lacustrine sediment was deposited during the transgression of Lake Bonneville. Deposition started before 22.17+ 3.65 ka and continuing after 18.74+ 3.24. An alluvial gravel that overlies the deformed beds in angular unconformity, is likely early Provo age based on its geometry in the landscape. The study site is <300 m basinward of three strands of the East Cache fault and exposes four liquefied units that are separated by intact beds, slip surfaces, and unconformities. Three liquefied units (1-5 m thick) are localized and displaced in the hanging walls of listric east- and west-dipping slip surfaces of coeval (?) lateral spreads. Evidence that seismic shaking triggered the deformation include fault-graded beds that are more than 5 m thick and the similarity with the secondary deformation generated by the 1934 Hansel Valley earthquake. Other features include fault wedges, pseudonodules, buried scarps, sand dikes, and angular unconformities. We propose that four separate earthquakes shook and liquefied the sediment between ~23 ka and the Bonneville flood (~17.4 ka). These earthquakes also triggered the lateral spreads. The temporal clustering of the deformational events, and the lack of deformation at the site since the flood, suggest episodic slip and loading-induced seismicity during transgression of Lake Bonneville. Our work indicate a decline in earthquake frequency on adjacent strands of the East Cache fault after the Bonneville flood, in contrast to the Wasatch fault, which increased its slip rate markedly at the same time (Karow and Hampel, 2010). The lake reached the site earlier than predicted by current hydrographs.