SUDDEN DESICCATION OF LAKE GOSIUTE AT ~49 MA: A DOWNSTREAM RECORD OF HEART MOUNTAIN FAULTING?

The catastrophic emplacement the upper plate of Heart Mountain Detachment (HMD) during the early middle Eocene is supported by the recognition of a major desiccation event and subsequent volcaniclastic sedimentation in the time-equivalent lacustrine Green River Formation, deposited by Lake Gosiute within the Washakie Basin.

The damming of drainage systems by debris avalanche deposits is known to occur in modern volcanic successions. The Deer Creek Member (DCM) of the Wapiti Formation, a large debris-avalanche deposit that is interpreted as the distal facies of the HMD, blanketed the Eocene paleotopography, and filled a number of large paleovalleys.

Downstream, in sediments deposited by Lake Gosiute, mudcracks that were 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 the sudden and intense desiccation. As Lake Gosiute had a history of major drainages entering the basin from the north, we propose that emplacement of the upper plate of the HMD caused this desiccation through blockage of south-flowing rivers. Overlying, stratified epiclastic volcanic strata may represent the deposits of a short-lived lake that was impounded upstream of this debris-avalanche dam. In the Washakie Basin, dolomitic and volcaniclastic mudstone, siltstone and sandstone (known as the "buff marker bed") was deposited above the mudcrack horizon. These fluvial deposits mark the first appearance of volcaniclastic sediments in the Green River Formation, 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 (occurring over more than 1,000,000 years) of the upper plate of the HMD, and highlights the importance of lacustrine strata as archives of continental tectonics and paleogeomorphology.