Green River, the Boar’s Tusk and Killpecker Dunes

On Saturday we drove eastwards on Interstate 80 into Wyoming to visit the Killpecker Dunes. Along the way we had the opportunity to take a brief look at the Green River Formation near the town of Green River. These laminated and trona-rich sediments were deposited in Lake Gosiute, interpreted by Whitmore – here, here and here – as an early post-Flood lake. Stratigraphically above the laminated sediments we observed the Tower Sandstone, probably deposited as an advancing delta encroaching into the lake. Massive ball-and-pillow structures in the sandstone that have distorted the underlying varves testify to the rapidity of deposition. Once off the interstate, our route took us right past the Boar’s Tusk, a well known landmark that can be seen for miles around. It’s an eroded remnant of a volcanic neck composed of lamproite. We reached the Killpecker Dunes about 5 pm and were able to sample the dune sand, including a large barchan. Tomorrow we head to the Sheep Creek Geological Loop back in Utah. The photographs show the Boar’s Tusk (above right) and wind ripples on the lee face of the barchan we sampled (bottom left).