GEOCHRONOLOGIC AND GEOMORPHIC EVIDENCE FOR STREAM TERRACE REOCCUPATION ALONG THE LARAMIE RANGE, WYOMING

The reoccupation of a terrace tread by a trunk stream has significant implications for deciphering short-term stream incision rates. In this study two episodes of stream terrace reoccupation are identified from geomorphic relationships and geochronologic data in strath terraces found along the Laramie River and Sybille Creek on the eastern edge of Wyoming's Laramie Range. A T1 terrace tread with a fill having optical age estimates ranging from 22-15 ka lies 1-2 m above the present Laramie River floodplain. Its surface morphology shows multiple episodes of braided channel activity on its tread, and one optical age suggests the trunk channel reoccupied the tread at ~ 8 ka. A similar trunk channel reoccupation deposit is found on the T4 terrace tread that lies ~ 17 m above Sybille Creek's floodplain. Here the T4 tread, which doesn't show any intact alluvial morphology through the rest of the study stretch, has a channel with a distinct braided morphology that lies 1-2 m below the T4 tread. Three optical ages within the channel feature range from 25-19 ka overlapping optical age estimates for the T3-T1 terraces, and those outside the channel range from 40-32 ka, consistent with the age range from other dated T4 terrace fills. In theory, periods of greater stream terrace reoccupation should correspond to periods of lower stream incision rates. However, based on geomorphic and geochronologic data, the reoccupation of the T1 tread appears to occur when incision rates were low while the reoccupation surface on the T4 tread appears to occur during a period of relatively high incision rates.