TILL GENESIS AND MORAINE DEPOSITION DURING PLEISTOCENE GLACIATIONS IN THE UINTA MOUNTAINS, UTAH

The record of Pleistocene glaciations in the western Uinta Mountains features well-preserved sequences of continuous moraines at the mouths of several valleys. Recent mapping of surficial geology in this area, cosmogenic-exposure dating of moraine boulders and glacier reconstructions provide insight to the timing and climate of glaciations in the Uintas, but processes of till genesis and moraine deposition during these events remain poorly understood. To address this issue, we studied three outcrops of moraine sediments in the Blacks Fork and Lake Fork valleys where Pleistocene glaciers deposited latero-frontal moraines on the piedmont, and in the Hades Creek valley where a glacier deposited a continuous lateral moraine. The moraines in these three areas are sharp-crested, narrow, continuous ridges with 30 – 70 m of relief and lengths of as much as 11 km.

The genesis of tills exposed in end moraines was inferred from pebble fabrics, grain-size analysis, sedimentary structures and stratigraphic observations. The most abundant till facies in all three moraines is a matrix-supported (60 – 80% matrix), light brown to reddish brown sandy loam to loamy sand diamicton, in which long axes of elongate pebbles display consistent trends, but eigenvalues suggest a weak pebble fabric (S1 = 0.47 – 0.63). We interpret this facies to represent deposition by sediment flow.

The Lake Fork moraine is composed entirely of sediment-flow deposits, whereas moraines at the Blacks Fork and Hades Creek sites are also composed of thin (~5 – 50 cm thick) lenses of sorted sand and gravel, indicating that ice-marginal melt water was partly responsible for moraine deposition. Evidence for deposition of melt-out till or lodgment till in the studied moraines is limited; only one till exposed in the Blacks Fork moraine was inferred to be deposited by lodgment based on its degree of compaction and high clay content relative to the sediment-flow deposits. The abundance of sediment-flow deposits suggests that ice surfaces in the Uinta Mountains were mantled by debris near their termini, and that moraines were constructed primarily by deposition of supraglacial sediment.