| Description |
The Deep Creek Mountains, located on the border of Utah and Nevada, are part of Late Pleistocene Lake Bonneville's western boundary and are relatively undocumented in terms of Quaternary stratigraphy and geomorphology. This mountain range offers a unique opportunity to study the interactions of alluvial systems before, during, and after the presence of Lake Bonneville. Preserved along the range, at the base of Reilly Wash, is a large barrier complex with spit-bar elements, as well as "intermediate shorelines" or embankments, as described and puzzled over by G.K. Gilbert more than 120 years ago. This study focuses on mapping one of the largest barrier complexes in the Bonneville basin and reevaluates Gilbert's brief geomorphologic and sedimentologic assessments using modern techniques. The significance of these intermediate shoreline features, which can be found primarily between the Stansbury, Bonneville, and Provo shorelines, are described by reconstructing the transgressive and regressive states of Lake Bonneville in this portion of the basin through observing a number of geomorphic features and sediment exposures. These include bedded marls, well-preserved regressive Provo sediments representing waning post-Bonneville flood stages, and a boulder strandline marking the transition from alluvial fan material to Bonneville sediments. Nine new radiocarbon accelerator mass spectrometry analyses from in situ gastropods provide dates to bracket the depositional chronology of the complex, from ~20,000 14C yr B.P. to the Bonneville Flood (~14,500 14C yr B.P.), and support the hypothesis that links intermediate shoreline deposition to lake oscillations (Unnamed 2 and Unnamed 3). Fluvial modeling and tectonic geomorphic analyses were used to determine the approximate age of the alluvial fan deposition from which the spit complex extends and to estimate the volume of the complex. Tectonic geomorphic indices, mountain front sinuosity (Smf), valley floor width-to-height ratio (Vf) and hypsometric integral (HI) highlight the watersheds in the vicinity of the spit complex as being the least developed in the range in terms of watershed maturity. This immaturity is believed to be cause by previously undocumented tectonic activity near these watersheds, depositing two large alluvial fans, in the mid-late Quaternary. Volumetric estimates conclude that the spit complex is ~75 million cubic meters, which is 2 to 3 times larger than the Stockton Bar, a landform that was previously thought to be one of the largest barrier bars formed in Lake Bonneville. This study is the first modern exploration of the interaction of Lake Bonneville and the Deep Creek Mountains. Future work documenting the depositional chronology of other landforms along the range is needed before a correlation of intermediate shorelines can be made. |
