Strath terraces on the western High Plains indicate climatically-driven variations in sediment supply from source basins in the Colorado Front Range
Abstract
Large strath terraces adjacent to the Colorado Front Range record the local history of fluvial planation and incision into the erodible rocks of the Denver basin over the last 2 million years. Terrace surfaces have been correlated into ~6 alluvial units using elevation and soil development; each alluvial unit was thought to represent a fairly consistent elevation of the Denver basin during various stages of exhumation, driven by base-level fall of the South Platte River. Here we show instead that (1) strath terraces in the western High Plains cannot be correlated based on elevation alone and (2) exhumation of the Denver basin is likely spatially and temporally variable due to climatically-driven variations in sediment supply from the source basins. We collected samples for cosmogenic radionuclide (CRN) profiles (10Be and 26Al) and a soil chronosequence from three strath terraces adjacent to Lefthand Creek near Boulder, CO. 10Be profile data on the upper- and middle-elevation terraces yield dates of 95 ka and 91 ka; these dates are much younger than the correlative alluvial units to the south of Boulder, which date to 1.5 Ma and 250 ka, respectively. Soils on the upper and middle terraces are similar in soil development and clast weathering, consistent with the narrow time window obtained from CRN dating of the two units. 10Be-derived rates for catchment-wide paleo-denudation are ~8.0 cm/ka from the flat and broad upper-terrace gravels and ~3.5 cm/ka from the steeper and narrower middle-terrace gravels. Young terraces at Lefthand Canyon are more consistent with a model of fluvial incision and aggradation driven by climate-controlled variations in sediment production from source basins in the Front Range. High catchment-wide denudation rates generate a high sediment supply, leading to aggradation and lateral planation. Terrace sediments are likely deposited and eroded multiple times during periods of lateral planation; the most recent occupation is preserved in the gravel caps on strath terraces. Lower catchment-wide denudation rates, and thus low sediment supply, lead to vertical bedrock incision and abandonment of terrace surfaces. Our data indicate that Front Range rivers experience a complex and basin-specific history where long periods of aggradation and lateral planation are punctuated by brief episodes of rapid incision through the soft Cretaceous shale underlying the Boulder area.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP23B..08F
- Keywords:
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- 1150 GEOCHRONOLOGY Cosmogenic-nuclide exposure dating;
- 1130 GEOCHRONOLOGY Geomorphological geochronology;
- 1105 GEOCHRONOLOGY Quaternary geochronology;
- 0486 BIOGEOSCIENCES Soils/pedology