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.

Scattered remnants of highly diverse stratigraphic sections of Tertiary lacustrine limestone, andesite flows, and 23.8–18.2 Ma regional ash-flow tuffs on the north flank of the Mormon Mountains record previously unrecognized deformation, which we interpret as pre–17 Ma uplift and possibly weak extension on the north flank of a growing dome. Directly to the north of the Mormon dome, 17–14 Ma ash-flow tuffs and rhyolite are interstratified with landslides, debris avalanches, debris flows, and alluvial-fan deposits that accumulated to a thickness of more than 2 km in an extension-parallel basin. The source for the landslides and debris avalanche deposits is unknown, but it was probably an adjacent scarp along a transverse fault bounding an early part of the Mormon dome. An average 45° of easterly tilt of the entire Tertiary basin-fill succession represents the major post–14 Ma deformation event in the region. We question the basis for the published estimate of 22 km of westerly displacement on the Mormon Peak detachment fault and, on the basis of landslides in the upper plate having a probable source in the adjacent Mormon dome, constrain the heave to ~4 km. We interpret the dome and basin as coupled strains similar to others in the region and suggest that these strains reflect a waveform pattern of extension-normal lateral midcrustal ductile flow. Previously, doming was interpreted as an isostatic response to tectonic unloading by large-displacement detachment faults or as pseudo-structural highs stranded by removal of middle crust from adjacent areas. Moreover, we argue that the strong thinning of upper-plate rock successions throughout the Mormon Mountains and Tule Springs Hills resulted from a loss of rock volume by protracted fluid flow, dissolution, and collapse, seriously limiting the usefulness of upper-plate strain in evaluating extension magnitude. We present a geohydrologic model that couples uplift driven by ductile inflow with dissolution driven by fluid infiltration, possibly augmented by mantle-derived CO 2 -rich fluids. Karsting in the uplands led to carbonate sedimentation in adjacent lowlands. Whether or not our downward revision of extension in the Mormon Mountains is valid, extension at that latitude is isolated from extension in the Lake Mead area by a low-strain corridor between the two areas. Recognition of the isolated and potentially diminished strain impacts estimates of maximum finite elongation of the Basin and Range Province because one of three vector paths used in those estimates passes through the Mormon Mountains.

Dissolution has removed large volumes of rock at low-angle normal faults, i.e., detachment faults, in the Mormon Mountains and the Tule Springs Hills in the eastern Basin and Range Province, southeastern Nevada. Evidence for major dissolution includes widespread solution-collapse breccias, meter-scale stylolite structures, and high-angle accommodation faults that terminate at or merge with dissolution seams. Chemically reactive fluids moving along the fault zones led to a strong depletion of 18 O in the detachment fault breccias (e.g., a δ 18 O decrease of 8‰ relative to the unaltered rocks). These strong chemical shifts, demonstrated by (1) negative oxygen isotope values and (2) steep compositional gradients marked by metal enrichment in elements such as Au, Ag, Ti, Pb, Zn, and Cu, are generally restricted to the narrow (<1 m to 8 m) microbreccia zones. Extensional faulting and fracturing, accompanying regional uplift, opened conduits for the influx of meteoric waters from above and hydrothermal fluids from below. As the largest, most permeable structures that formed during uplift, detachment faults focused the fluid flow. In this deformation and hydrogeologic model, dissolution-caused stratal thinning is a major complement to detachment faulting and is an important process that resolves void space issues in the reconstruction of cross sections.