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AAPG Rocky Mountain Section Meeting

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Exhumed Impact Craters, Sheep Mountain, Douglas, Wyoming, USA, Evidence from Field Sampling, Satellite, and Drone Imagery

Abstract

Fifteen 280 Ma impact craters have been studied on the northeast flank of Sheep Mountain near Douglas, Wyoming, USA. The craters are in the quartz-cemented sandstone top of the Casper Formation. The largest most studied crater is SM1 measuring 63 m length and 53 m width. Three resistivity lines using a 56 electrode (5 m between electrodes) AGI SuperSting R8/IP/SP system were acquired across the crater in 2018. The initial N-S Line 1 shows a crater form expressed as higher resistivity, presumably caused by shock metamorphism. It is a relatively shallow structure 180 m long and 25 m deep with a smaller 12 m deep central crater depression filled with more conductive rock, Opeche red siltstones. Two more resistive ring fractures match resistant quartzite ledges of Casper Formation at the surface. Line 2 was shot perpendicular to Line 1 along an east-west orientation across 65 m of relief. Again, the inner crater showed higher conductivity sedimentary clayrich siltstones 12 m deep. An anomalous deeper high conductivity (200 times background) area was imaged on this line from 25 to 60 m depth and 30 m length. The anomaly is interpreted as a metal rich aureole where an ironnickel meteorite landed after creating the SM1 crater. Since impact most of the iron would have leached away by ground water, but less soluble nickel, palladium, platinum, chrome and other metals typical of this meteorite type would not move very far and could create this conductive aureole anomaly. Line 3 verified the location, depth, and size of this highly conductive impact aureole. The location of this anomaly relative to the surface crater indicates an impactor arriving from an east-northeast direction. Future drilling of SM1 crater and the conductive anomaly located 40 m west of the crater center will test these hypotheses. A magnetic survey using a Geometrics G858G Cesium vapor gradient magnetometer was also conducted across the surface area of SM1. The magnetic results faintly outline the crater rim, but do not see a large magnetic anomaly at the high conductivity anomaly. We interpret that all of the original elemental magnetic iron in the meteorite converted into nonmagnetic iron oxides (hematite). We thank EPSCoR, WYCEGH, and FINSE programs at University of Wyoming, especially Brad Carr and Steven Holbrook for equipment, training, and some processing.