ABSTRACT The effects of bolide impacts on carbonate platform sedimentation and stacking patterns are poorly understood, partly because the geological evidence for marine impact sites is typically unavailable. Givetian–Frasnian carbonates in southern Nevada contain a continuous record of sedimentation before, during, and after the Devonian (Frasnian) Alamo impact event (382 Ma), evidenced mainly by the regional Alamo Breccia Member of the Guilmette Formation. Two transects arranged from seven stratigraphic sections measured through the lower ~300 m of the Guilmette Formation record environmental lithofacies deposited from peritidal to deep subtidal zones. Stacking patterns of peritidal and subtidal cycles indicate four relatively high-frequency sequences superimposed on the larger-magnitude eustatic Taghanic onlap of the Kaskaskia sequence. Sequences are interpreted based on facies proportions and cycle stacking trends because of a lack of prominent erosional surfaces developed on the Frasnian greenhouse shelf. Lateral correlation of facies and cycle stacking indicates that the Alamo impact took place during the late phase of sedimentation during deposition of “Sequence 3” in the Guilmette Formation. Underlying facies and surfaces were obliterated and excavated during the impact, resulting in truncated terminations of sequence boundary and maximum flooding zones. Eustatic sea-level rise during the late Frasnian resulted in an overarching shoreline backstep and deepening of vertical facies associations prior to the Alamo impact. Additional accommodation was gained instantaneously as a result of the Alamo impact, which formed a local, steep-sided basin and shifted the slope break of the platform margin. Postimpact sedimentation within the Alamo crater is characterized by condensed sections of continuously deposited thin-bedded mudstones with pelagic (tentaculites) fauna. Thick shoreface sandstones were deposited in a lowstand clastic wedge as the last phase of crater fill in the study area. While accommodation and depositional environment changed dramatically at the impact site, long-term sedimentation trends immediately outside of the impact site were unaffected by the Alamo event, demonstrating that the forces that control overall carbonate platform growth and evolution (tectonics, climate, oceanography, biology) are of far greater importance than even regional-scale physical perturbations such as meteor impacts.

Abstract 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. This field guide provides excursions to Devonian outcrops easily reached from the settlement of Alamo, Nevada, ~100 mi (~160 km) north of Las Vegas. Emphasis is on carbonate-platform lithostratigraphy, but includes overviews of the conodont biochronology that is crucial for regional and global correlations. Field stops include traverses in several local ranges to study these formations and some of their equivalents, in ascending order: Lower Devonian Sevy Dolostone and cherty argillaceous unit, Lower and Middle Devonian Oxyoke Canyon Sandstone, Middle Devonian Simonson Dolostone and Fox Mountain Formation, Middle and Upper Devonian Guilmette Formation, and Upper Devonian West Range Limestone. Together, 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 Sevy and Simonson appears to be linked to exposure surfaces and related underlying karst intervals. The less-altered Guilmette exhibits characteristic shallowing-upward limestone-to-dolostone cycles that contain typical carbonate-platform fossil- and ichnofossil-assemblages, displays stacked biostromes and bioherms of flourishing stromatoporoids and sparse corals, and is punctuated by channeled quartzose sandstones. The Guilmette 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 the Alamo Impact Event, is as thick as 100 m and may be the best exposed proven bolide impact breccia on Earth. It contains widespread intervals generated by the seismic shock, ejecta curtain, tsunami surge, and runoff generated by a major marine impact. Newly interpreted crater-rim impact stratigraphy at Tempiute Mountain contains an even thicker stack of impact breccias that are interpreted as parautochthonous, injected, fallback, partial melt, resurge, and possibly post-Event crater fill.