Abstract

Mudcracks originally more than 2 m deep are directly superimposed on profundal lacustrine mudstone of the lower LaClede Bed of the Green River Formation in the Washakie Basin, recording sudden and intense desiccation of Eocene Lake Gosiute. We propose that this desiccation was caused by a giant rockslide/debris avalanche that blocked a major southward-flowing river to Lake Gosiute in response to the catastrophic emplacement of the upper plate of the Heart Mountain Detachment (HMD). In the Washakie Basin, dolomitic mudstone, siltstone, and sandstone (known as the “buff marker bed”) were deposited above the mudcrack horizon. These fluvial deposits mark the first appearance of volcaniclastic sediments in the Washakie Basin, and together with overlying lacustrine mudstone record the reestablishment of regional drainages after the debris avalanche. Sudden desiccation of Lake Gosiute contradicts models that require slow emplacement of the upper plate of the HMD and highlights the importance of lacustrine strata as archives of continental tectonics.