Recent tectonic reinterpretations of the Late Paleozoic Southwest Laurentian margins recognize widespread Late Paleozoic deformation as a critical component in the boundary region development. Overprinted late Paleozoic structures record repeated shortening events in both northern and southern Nevada, but spatial and temporal data are currently lacking to resolve the evolution of this margin. The Timpahute Range, south-central Nevada, bridges part of the spatial gap between previous detailed studies of Late Paleozoic deformation. The purpose here is to (1) evaluate structures in the area that do not appear to fit with recognized Sevier hinterland structures (the Central Nevada thrust belt [CNTB]) and (2) consider whether these contractional structures may be Late Paleozoic and possibly link, or not, structures to the north and south. New mapping in the Timpahute Range documents four geometrically or kinematically distinct sets of structures: Tempiute Ridge folds, Schofield Pass fault zone (SPFZ), structures of the CNTB, and Cenozoic extensional faults. The first three are interpreted to represent separate shortening events based on cross-cutting relations and differences in orientations of the Tempiute Ridge folds and SPFZ (north [N]), and structures of the CNTB (northwest [NW]). The Tempiute Ridge folds represent the oldest event, D 1 . These folds are large, trend N and verge east (E). The SPFZ is west (W)-vergent, cuts across the limb of a D 1 fold and represents D 2 . The SPFZ is interpreted to be older than the CNTB structures, D 3 , based on positions of fault cut offs, and differences in footwall and hangingwall facies. All of the shortening events predate the newly dated 102.9 ± 3.2 Ma Lincoln stock and its contact metamorphic aureole. New and previous correlations suggest that a belt of Permian deformation extends from southeast (SE) California northward at least to the Timpahute Range. The Tempiute Ridge folds and SPFZ have the same distinctive geometries, styles, and kinematics as structures in the Nevada National Security Site. The mountain-size, E-vergent Tempiute Ridge folds and the W-vergent SPFZ correlate to structures associated with the Belted Range thrust and the W-vergent CP thrust, respectively. The Belted Range thrust previously has been correlated southward into the Death Valley region. Thus, convergence created large-amplitude folds and thrusts for ~200 km along strike. Structures of this age are exposed in northern Nevada but are smaller. These new relations fill a data gap and suggest differences in the size and structural style of Permian structures along strike and corresponding variations in the plate boundary configuration.

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.

Based on evaluation of past results and new research, we have partitioned the distribution of the Alamo Breccia in southeastern Nevada and western Utah into six genetic Realms that provide a working model for the marine Late Devonian Alamo Impact Event. Each Realm exhibits discrete impact processes and stratigraphic products that are enumerated here. The first five form roughly concentric semicircular bands across the Devonian shallow-water carbonate platform. These are: (1) Rim Realm, where a newly defined impact stratigraphy includes both autogenic and allogenic breccias associated with the crater rim; (2) Ring Realm, where breccias are now interpreted to have formed sequentially by seismic shock, passage of the ejecta curtain, tsunami waves or surge, and runoff that accumulated over tilted terrace(s) bounded by syn-Event, ring-forming, listric faults; (3) Runup Realm, where graded breccias were stranded by tsunami surge or waves; (4) Runoff Realm, where sheet-floods carried traces of impact debris across the distal platform beds and channels filled with impact debris; (5) Seismite Realm, where near-surface beds far across the platform were uniquely deformed; and (6) Runout/Resurge Realm, where offshore channels of thick off-platform Alamo Breccia, together with large-scale olistolith(s), signal contemporaneous massive collapse of the platform margin, possibly into the central crater. Five breccia Units characterize the newly interpreted Rim Realm, in ascending order: (1) deformed target rocks, (2) injected dikes and sills, (3) chaotic fallback, (4) smeared fallback, and (5) resurge. This succession is covered by deepwater limestones deposited inside the crater rim, or across a new slope created after platform margin collapse. Unit 1 exhibits shatter-cone-like structures interpreted as impact products. Newly discovered Ordovician and probable older meter-scale clasts in Unit 3 confirm a minimum excavation depth of 1.5 km. Microscopic components in Units 3 and 4 indicate high pressures (>10 GPa), probable quenched carbonate melt, and accreted particles that may be new kinds of impact products. Postimpact tectonics and other factors obscure the full panorama, including the location and character of the missing central crater, but the assemblage of Realms offers a working model to compare with expected impact paradigms.