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FLUVIO-GLACIAL TERRACES OF THE UPPER GREEN RIVER BASIN SPANNING THE PAST ~2 MA: NEW DETRITAL-SANIDINE, COSMOGENIC-BURIAL, AND LUMINESCENCE AGE CONSTRAINTS
The Peru Bench area is represented by 9 Green River strath terraces that range in elevation from 5 to 175 m above the modern river, and 3 to 5 m in thickness. Deposits consist of sandy pebble-cobble, quartzite-rich gravel and where preserved, fine-grained cover units have Stage II to III calcic soils. Six of the nine terraces have numeric age estimates. The oldest terrace (175 m above the modern river) produced a DS weighted-mean maximum depositional age of 1.30 ±0.01 Ma (n=5) and the youngest terrace produced a luminescence age of 35 ±2.1 ka. Ages of the Peru Bench terraces broadly correlate with eccentricity-driven glacial-interglacial cycles (past ca. 700 ky), whereas the oldest two terraces correlate more closely with the preceding Mid-Pleistocene Transition (~1200-700 ky).
The Browns Park area contains 9 Green River strath and fill terraces that range in elevation from 5 to 210 m above the modern river, and 5 to ~10 m in thickness. Deposits consist of sandy pebble-boulder gravel; tributary gravel locally veneers mainstem gravel. The two oldest terraces have numerical age estimates. The oldest (210 m) terrace has a DS maximum depositional age of 1.96 ±0.03 based on the age of the youngest single sanidine grain. The next oldest terrace (130 m) has a DS weighted-mean maximum depositional age of 1.28 ±0.01 Ma (n=4). Older Green River deposits are represented by small, isolated terrace remnants whereas younger terraces cover larger areas. However, the caliber of all the terrace deposits is broadly similar.
Age estimates in conjunction with broad similarities in the thickness and caliber of gravelly Green River terrace deposits from Peru Bench and Browns Park suggest that they represent a record of glacial-interglacial cycles, and that glaciation in the Rocky Mountain region extends as far back as ~2 Ma.