TERRACE FORMATION ALONG THE LARAMIE RANGE, WYOMING: EVIDENCE FOR INCREASED INCISION RATES DURING LATE PLEISTOCENE FLOODING EVENTS

For this study, terrace suites of the Laramie River and Sybille Creek found along Wyoming’s Laramie Range were studied to better understand the timing of late Quaternary incision events. In the study area at least ten terrace levels are preserved in these valleys, with terrace treads ranging from 1 to 70 meters above the present floodplains. The five youngest terrace fills, ranging from 1-18 meters above the present river levels, were sampled for optical dating, and the average age estimates for these terrace fills are ~ 16.6, 22.0, 26.2, 41.6, and 58 ka respectively. These ages, which are supported by carbonate stage morphologies, indicate average incision rates of 0.29-0.34 m/kyr over the last 60 kyr, and suggest that several significant incision events occurred during the late Pleistocene. In other studies from nearby basins, the formation of terraces has been attributed to changes in the sediment loads supplied to the rivers during the late Quaternary. These studies suggest that the glacial and periglacial conditions of the late Pleistocene resulted in increases in bedload and lowered incision rates. But at the onset of Holocene warming, the sediment supplied to the river systems decreased, allowing the streams to incise through the easily eroded bedrock of the basins. This model cannot explain terrace formation along the Laramies because incision after ~ 16.6 ka is limited to 1-2 meters, and compared to the 3-10 meters of relief between the other adjacent terrace treads in the study area, this data suggests that incision rates are higher prior to Holocene warming. The alternative model proposed here suggests that the relatively cool conditions of the late Pleistocene resulted in larger flood magnitudes, and that during these flooding events incision rates also increased dramatically. This model is supported by the fact that there were at least three significant incision events during the cold conditions of Oxygen Isotope Stage 2, but relatively little incision during the Holocene.