PROVENANCE AND IMPLICATIONS OF STROMATOPOROIDS REDEPOSITED IN UPPER DEVONIAN ALAMO IMPACT BRECCIA, SE NEVADA

Bulbous stromatoporoids that occur as clasts are by far the most abundant macrofossils in the coarse part of graded Unit B of the Upper Devonian (Frasnian, punctata conodont Zone) of the Alamo Impact Breccia. Our findings are based on Breccia outcrops, situated within the Guilmette Formation, at two localities in southeastern Nevada: (1) Hancock Summit, western Lincoln County, the type locality of the Alamo Breccia; and (2) Golden Gate North, southern Nye County. Six species were identified from the Breccia. Hammatostroma albertense is known from the Frasnian of Nevada, Iowa, and Alberta, and Habrostroma turbinatum is known from the Givetian of Missouri and Frasnian of Iowa. Although Actinostroma sp., one of three species left in open nomenclature, represents a genus that occurs in the Lochkovian through Frasnian, its robust skeletal structure is most typical of the Givetian and Frasnian. Atelodictyon sp. represents a genus that ranges through the Devonian, and Hermatoporella sp. represents an Eifelian to Frasnian genus; however, these species also occur in the Guilmette below the Alamo Breccia, and thus should be considered Frasnian. The most definitive occurrence is that of Atopostroma distans, the only other reported occurrence of which is in the Emsian of the Canadian Arctic. The presence of this species in the Breccia is enigmatic because it is preserved as limestone clasts. As Emsian rocks on the carbonate platform were pervasively dolomitized before the Frasnian, a possible explanation is that the clasts had a western provenance and were transported eastward by megatsunami waves. Thus, the deepest bolide excavation that can be documented by stromatoporoids is Emsian. However, conodonts from small lithic clasts and lapillistone clasts in the Breccia are as old as Ordovician and originated deep within the crater. Our finding of Emsian limestone clasts supports the interpretation of its more westerly original location.