Jared R. Morrow (October 8, 1959–October 7, 2010)
No abstract available.
Meteoritics & Planetary Science (MAPS)↗
Geology topics
John E. Warme
Publications and source records attributed to John E. Warme.
Devonian carbonate platform of eastern Nevada: Facies, surfaces, cycles, sequences, reefs, and cataclysmic Alamo Impact Breccia
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.
Nevada↗
Alamo megabreccia: Record of a late devonian impact in southern Nevada
The Alamo breccia is probably the most voluminous known outcropping carbonate megabreccia. It occupies ~4000 km2 across 11 mountain ranges in southern Nevada, has an average thickness of ~70 m, and contains a volume of 250+ km3. The breccia is a single bed, of early Frasnian (early Late Devonian) age, that formed in the wake of a giant slide that deposited a lower chaotic debrite, containing clasts as large as 80 × 500 m, and an upper exquisitely graded turbidite. It is anomalously intercalated with cyclic shallow-water platform carbonates of the Guilmette Formation. The Alamo breccia is interpreted as a product of the Alamo event, a nearby marine impact of an extraterrestrial object, whereby impact-generated crustal shock waves and/or marine superwaves detached the upper ~60 m of platform along a horizontal surface. Loosened bedrock slid seaward across the platform, and some of it accumulated as the lower debrite. Rock-water exchange induced landward-propagated tsunami(s), whose uprush and/or backwash deposited the upper turbidite, partly above sea level. Evidence for impact includes shockedquartz grains, an iridium anomaly, and reworked conodonts, all found only within the breccia. Because the Alamo breccia is not known outside of Nevada, and because the early Frasnian time of the Alamo event is not noted for accelerated extinctions, being ~3 m.y. before the Frasnian-Famennian impact(s) and biotic crisis, the impact was probably only of moderate size.
Nevada↗