PRE-BONNEVILLE LAKES IN CACHE VALLEY AND EXCAVATION OF CUTLER NARROWS ACROSS THE CACHE BUTTE DIVIDE IN THE WEST, NORTH-CENTRAL UTAH AND SOUTHEASTERN IDAHO

Cache Valley is a graben E of the Cache Butte Divide, a horst upfaulted between the Wasatch (W) and West Cache (E) fault zones. Cutler Narrows is the canyon of the Bear River across the Cache Butte Divide, cut in hard Paleozoic bedrock 392 m deep on its S side. Pre-Bonneville pluvial lakes between 1314 - 1517 m high would connect exclusively there. At higher levels, water could connect across this horst at 3 additional saddles between 1517 - 1577 m (local Bonneville highstand). These saddles are shallow, and underlain by weak Neogene Salt Lake Formation. Ongoing uplift has raised the highest bedrock at Cutler Narrows (1706 m on the S side), far above the Bonneville highstand. Thus, initial cutting at Cutler Narrows may be quite old, perhaps along an unmapped WSW-trending fault.

Shoreline deposits of pre-Bonneville pluvial lakes show that the penultimate Cutler Dam lake (~60 ka, OSL) rose ~102 m higher in Cache Valley (1443 m) than westward in the main Bonneville basin (1341 m). The higher level likely resulted from diversion of the Bear River into Cache Valley from Gem Valley, in the NE, where final incision of Oneida Narrows was ~55 ka (OSL) (Pederson and others, 2016). The earlier Little Valley lake level (~150 ka OSL) may have been similar in the Bonneville basin and in Cache Valley, but likely differences in tectonics and in rebound since render this moot. Evidence of an early rise of Lake Bonneville ~22 ka (OSL) to above the Provo level (Newton Hill gravel pit), and to ~1515 m (mouth of Green Canyon; Janecke and others, 2013), ~22 ka (14C), also indicates higher levels in Cache Valley than in the main Bonneville basin during the rising stage of Lake Bonneville, perhaps with initial westward flow across Cutler Narrows into the main Bonneville basin. However, most of the excavation of Cutler Narrows below 1443 m (~0.05 km3) was by subsequent eastward flow during the Bonneville flood.

Cutting of the upper part of Cutler Narrows could predate and also be coeval with the oldest known pluvial deposits in the main Bonneville basin, the Lava Creek B (~620 ka) and the Pokes Point (~420 ka) lake cycles. Bedrock is lower elsewhere along the Cache Butte Divide, so the upper part of this canyon probably was superimposed from overlying Salt Lake Formation as the horst rose. The volume removed pre-Little Valley was >1.02 km3, and the moot part below, <0.35 km3.