2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 13
Presentation Time: 4:45 PM


ANDERSON, David and SUNDELL, Kent A., Geology, Casper College, 125 College Drive, Casper, WY 82601, climbingfrog@bresnan.net

Large boulder deposits form prominent ridges (moraines) and terraces along Garden Creek, emanating from the north slope of Casper Mountain. Shadeline glaciers and climatically related flooding are the cause and processes involved in distributing these deposits 1-5km from the closest Paleozoic and Precambrian bedrock sources. Rapid climate change from cold, water-accumulating, glacial stages to warm, melting, interglacial stages is the likely environmental shift juxtaposing these perplexing boulder deposits. The east-west orientation of Casper Mountain and a steep asymmetrically folded and faulted north-facing slope are dominant factors in episodic accumulation and ablation of snowfields and glaciers by direct sunlight.

Field evidence of short U-shaped valleys, exhumed moraines, exhumed erratics, paleotopography, shape and orientation of diamictons, striated and facetted stones, and long distance transport of very large boulders suggest ice movement as shadeline glaciers. Subsequent flooding is supported by reworking of moraines, multiple large boulder terraces, steeper boulder terrace gradients, wide flood plains, subrounded clasts, terrace sedimentology, and redistribution of finer clasts (not the largest boulders) downslope from direct glacial deposits.

At least three episodes of glacial advance are documented by field mapping of exhumed and washed out moraines. A washed out moraine forms a continuous band of large boulders (glacially transported) protruding from a terrace deposit, composed of generally finer grained sediments reworked by outwash streams. Caliche deposits, up to 3cm thick, occur on boulders and cobbles within the terraces and moraines. The caliche is thickest on old (high elevation) terraces and decreases thickness with decreasing relief of terrace height above the present floodplain of Garden Creek. Precise dating of these deposits will be critical in determining if they match the end of the coldest glacial stages (exhumed terminal moraines) and rapid switching to warm interglacial stages (terrace and caliche deposits).