Luncheon – August 7, 2009

Title: Devonian Carbonate Platform of Nevada: Facies, Surfaces, Cycles, Sequences, Reefs, and Cataclysmic Alamo Impact Breccia

Speaker: John E. Warme, Professor Emeritus, Department of Geology and Geological Engineering, Colorado School of Mines

Date: August 7, 2009

Publication: The Outcrop, August 2009, p. 24-25

The Devonian shallow-water carbonate platform of southeastern Nevada exhibits a gallery of features that can be studied for their significance related to programs of global mineral exploration and development both for hydrocarbons and for important metallic and other commodities. Examples include dolomitized porous platform beds, platform margin and mid-platform organic reefs, regional lowstand karst systems, and transgressive or highstand seals. In addition, this specific platform contains the partial crater and thick impact breccias from the major Late Devonian extraterrestrial Alamo Impact Event; impact craters also produce such commodities.

The carbonate platform is comprised of the familiar Lower Devonian Sevy and Middle Devonian Simonson dolostone formations and the Upper Devonian Guilmette mainly limestone formation; recent work on these units has broken out some new formations and members. These rocks are superbly exposed in numerous mountain ranges, centered about 150 km (100 mi) north of Las Vegas, where they total as much as -1500 m (5000 ft) in thickness. They exhibit continuous exposures of a classic, long-lived, shallow-water carbonate platform, comprised of a hierarchy of hundreds of partial to complete shallowing-upward Milankovitch-scale cycles, each representing tens to hundreds of thousands of years. The cycles are grouped into sequences bounded by regionally signicant exposure surfaces or flooded intervals. The cycles and sequences show up on surface gamma-ray profiles, which can also be interpreted as sea-level cycles.

Dolomitization in the Sevy and Simonson is likely linked to long-term exposure and related deep underlying karstified intervals. The less-altered Guilmette exhibits characteristic shallowing­upward limestone-to-dolostone cycles that contain typical carbonate-platform fossil assemblages, display stacked biostromes and bioherms of nourishing and diverse calcareous sponges (stromatoporoids) and sparse corals, and are punctuated by channeled quartz sandstones. The Guilmette also contains completely exposed reefs. One is -50-m (165 ft) in thickness, constructed mainly of diverse stromatoporoids, and later exposed and karstified across the crest. These buildups exemplify such Devonian structures known from surface and hydrocarbon-bearing subsurface locations worldwide, notably in the Alberta basin of Canada and the Middle East.

Of special interest is the stratigraphically anomalous Alamo Breccia that now represents the formal middle member of the Guilmette Formation. This spectacular cataclysmic megabreccia, produced by the Alamo Impact Event, is as thick as 100 m and may be the best exposed, proven bolide impact breccia and impact debris field on earth. It contains widespread intervals generated by the seismic shock, ejecta curtain, tsunami surge, and runoff from a major marine impact. Different facies of the Alamo Breccia are placed into regional genetic Realms, labeled Crater Rim, Ring, Runup, Runoff, Seismic, and Runout/Resurge. Each Realm exhibits a specific combination of processes and products that help interpret the varied facies of the Alamo Breccia, as well as the depth of excavation and direction to the target zone.