FIELD TRIP OVERVIEW
PS-SEPM Fall Field Trip: October 4-6, 2013
DEVONIAN CARBONATE PLATFORM OF EASTERN NEVADA: FACIES, SURFACES, CYCLES, SEQUENCES, REEFS, AND THE CATACLYSMIC ALAMO IMPACT BRECCIA.
Trip Leaders: John E. Warme (Evergreen, Colorado) and Mara E. Brady (Fresno State, firstname.lastname@example.org)
Stratigraphic column showing ~250 m of carbonate platform mostly cyclic limestones and dolostones from the upper part of the newly named Fox Mountain Formation, the informal yellow slope-forming member and shallow carbonate platform facies of the lower Guilmette Formation, the newly named Alamo Breccia Member of the middle Guilmette, and the lower beds of post-Breccia limestones and sandstones of the informal upper Guilmette (from Jared R. Morrow, after Sandberg, Morrow and Warme, 1997).
Devonian limestone and dolostone formations are superbly exposed in numerous mountain ranges of southeastern Nevada. The Devonian is as thick as 1500 m there and reveals continuous exposures of a classic, long-lived, shallow-water carbonate platform. Our two-day field excursion will be to Devonian outcrops easily reached from the settlement of Alamo, Nevada, ~100 mi (~160 km) north of Las Vegas. Emphasis will be on carbonate-platform lithostratigraphy, but includes overviews of the conodont biochronology that is crucial for regional and global correlations. These formations are mainly composed of hundreds of partial to complete shallowing-upward Milankovitch-scale cycles and are grouped into sequences bounded by regionally significant surfaces. Dolomitization in the Lower Devonian Sevy Dolostone and Middle Devonian Simonson Dolostone appears to be linked to exposure surfaces and related underlying karst intervals. The less-altered Upper Devonian Guilmette Formation exhibits characteristic shallowing-upward limestone-to-dolostone cycles that contain typical carbonate-platform fossil- and ichnofossil-assemblages, displays stacked biostromes and bioherms of flourishing stromatoporoid sponges and sparse corals, and is punctuated by channeled quartzose sandstones. The Guilmette Formation also contains a completely exposed ~50-m-thick buildup that is constructed mainly of stromatoporoids, with an exposed and karstified crest. This buildup exemplifies such Devonian structures known from surface and hydrocarbon-bearing subsurface locations worldwide.
Of special interest is the stratigraphically anomalous Alamo Breccia that represents the Middle Member of the Guilmette. This spectacular cataclysmic megabreccia, produced by a bolide strike ~382 million years ago, is as thick as 100 m where we will study it. The Breccia may be the best exposed proven impact debris field on Earth. It contains widespread intervals created by the seismic shock, ejecta curtain, tsunami surge, runoff and probable continental margin collapse generated by a major marine impact.
Travel & Logistics: Participants should plan to arrive Friday night for a Saturday 8am start of the two-day field excursion. Registration (details to be posted shortly) will include a complete printed field guide.
On Saturday, the group will visit Hancock Summit, where participants can observe and discuss (1) carbonate facies, cycles, and surfaces and (2) the Alamo Breccia and its bolide impact signatures.
On Sunday, we will visit Silver Canyon, where participants will investigate (1) the upper interval of the Alamo Breccia, which contains large clasts of platform carbonates and clasts of impact lapillistone; and (2) exceptional exposure of a carbonate buildup (Reso’s Reef), lithologies that interfinger with the buildup, and the karstified crest at the top of the buildup.