A UNIQUE BOULDER DEPOSIT IN A SMALL, HIGH-ELEVATION, TOPOGRAPHICALLY ENCLOSED BASIN: NORTH CRATER LAKE, FISH LAKE PLATEAU, UTAH

The Crater Lakes occupy an internally-drained, 3-km long NE-SW trending graben in the Mytoge Mountains near Fish Lake, Utah. Displacement along graben-bounding faults ranges from 60 to 140 m and transverse faults cut the north and south ends of the graben. Prominent boulder rings encircle both Crater Lakes, however north Crater Lake has the most continuous and well developed ring. The top of the boulder ring around north Crater Lake is at a consistent elevation of ~2908±1 m and the ring has a prominent edge, or drop, of 1­­–3 m of relief as it slopes towards the center of north Crater Lake. The ring is composed of sub angular to sub rounded volcanic boulders that are up to 3 m in diameter (b-axis). These clasts are colluvium derived from the nearby steep hill slopes of volcanic bedrock that crop out along the walls of the Crater Lakes graben. We hypothesize that this boulder ring may have accumulated around the margin of a small, topographically constrained ice or firn “pond” that was present in north Crater Lake during the penultimate glacial period (~Bull Lake; ~MIS 6; ~150 kyr ago). We suspect deposition during the penultimate glacial period because of four ³He exposure ages of boulder ring boulders that range from 119±3 to151±4 ka (ages include snow shielding corrections but not boulder surface weathering corrections) and the possible extended duration and severity of the penultimate glacial period compared to the LGM. Local LGM ELAs estimated from nearby Fish Lake Plateau glacial deposits range from ~3000–3150 m and were likely not low enough to “glaciate” the ~2900 m floor of north Crater Lake. Bull Lake age moraines around the Fish Lake Plateau are significantly lower in elevation than corresponding LGM moraines, suggesting that the ELA during Bull Lake time may have approached the ~2900 m level.