Fluvial terrace formation along Wyoming's Laramie Range as a response to increased late Pleistocene flood magnitudes

Abstract

This study evaluates the potential climatic mechanisms involved in fluvial terrace genesis along Wyoming's Laramie Range. We used optical dating methods to determine depositional ages for fluvial fills, and to calculate incision rates for terrace suites along two of the region's larger rivers. Optical ages were determined for the five lowest terrace levels (T5–T1) which were deposited at ∼ 59.6, 39.2, 26.3, 22.7, and 18.5 ka, and incision rates calculated for the two rivers were ∼ 0.29–0.34 m/kyr over the last ∼ 60 kyr. The formation of fluvial terraces in the central Rocky Mountains is commonly attributed to climatically induced changes in sediment input. According to most studies, relatively low incision rates existed during the colder periods of the Pleistocene due to high stream sediment loads, but terraces were formed during warmer interglacial periods when reduced sediment availability facilitated higher incision rates. However, this conceptual model cannot explain the incision records presented here, which show that the streams incised 9–10 m in two to three events during Oxygen Isotope Stage 2, but only 1–2 m during the warmer climates of the last ∼ 18.5 ka. The stream power model we adapted to this setting suggests that late Quaternary streams operated under two basic states. During the colder conditions of the Pleistocene, higher flood magnitudes resulted in higher lateral erosion and incision rates. However, the lower stream discharge common to the warmer interglacial periods resulted in relatively inactive streams when both lateral erosion and incision rates were lowered. This model can explain the high incision rates during the cold conditions of Oxygen Isotope Stage 2, the occurrence of terrace fill ages near cold to warm transitions, and the apparent acceleration in incision rates toward the end of the last glacial cycle. Finally, this study suggests that terrace fill ages and incision rates are similar for the distally glaciated Laramie River, nonglaciated Sybille Creek, and the proximally glaciated streams that drain the Wind River Range. This implies that rivers responded synchronously to late Quaternary climate changes regardless of their influence by alpine glaciation.