e-mail: benjamin.e.rendall@exxonmobil.comand

Meteor impacts have long been a subject of geologic interest because of their cataclysmic nature and potential link to mass extinction events (Alvarez et al. 1980). Most well-studied impact sites are continental, allowing for preservation of circular and ellipsoid craters with conspicuous features and deposits. The marine Alamo impact event occurred during the Late Devonian (Frasnian) in present-day Nevada (Fig. 1) on the westward-deepening carbonate platform that covered much of the western United States. A bolide impact on a carbonate shelf is an interesting sedimentological scenario because of two main reasons: (1) The carbonate factory organisms that produce sediment on carbonate platforms would have been completely killed off instantaneously during the impact, begging questions of recovery times for carbonate-producing communities; and (2) the bolide would have created a local intrashelf basin (crater) upon impact, increasing accommodation space and excavating through preexisting rocks—this would presumably result in dramatic alterations of stratal geometries depending on proximity to the target rocks. So, how was regional accommodation/sedimentation affected away from the target rocks? Were the physical or biological effects great enough to alter carbonate sedimentation patterns far from ground zero?

The complex crater that the bolide left behind has since been obscured by Mesozoic–Holocene tectonic shortening and extension, most notably, during the compressional Sevier and Laramide orogenies, followed by Basin and Range extension. As such, the impact event is evidenced mainly by a regional carbonate megabreccia, which makes up the Alamo Breccia Member of the Guilmette Formation (Fig. 2). A detailed description of the Alamo Breccia units, depositional processes, and spatial distribution was provided by Pinto and Warme (2008, and references therein).

The Alamo crater had a transient diameter estimated between 37 and 65 km (Morrow et al. 2005, Retzler et al. 2015), which collapsed inward to form a final crater no less than 111 km wide. This impact crater is significant because of its large size, placement within a verdant carbonate platform, and a modern exposure that provides multiple outcrop perspectives of the platform rocks before, during, and after the impact event. No other known impact site has a comparable set of outcrops, making this region an unparalleled natural laboratory for studying the interaction of marine impacts and carbonate systems. We present new interpretations of the stratigraphic architecture of the Devonian platform in the wake of a meteor impact.

Several studies have addressed the Guilmette Formation from a sequence stratigraphic perspective. Chamberlain and Warme (1996) provided a review of the lithostratigraphic nomenclature throughout the region, and that paper is a good reference for the regional geology and stratigraphy, but it focuses at a much larger scale than this study. They cited an average cycle thickness of 22 ft (~7 m) over 5000 ft (1524 m) of section, spanning two states, while our study focuses on more local-scale observations with average cycle thicknesses of 6 to 9 ft (2–3 m) confined to two counties in southern Nevada. The Chamberlain and Warme (1996) study used a surface gamma-ray detector to create gamma logs that corresponded to measured sections and used the combination to pick what they termed "sequences." Chamberlain and Warme (1996) stated upfront that their sequences generally correspond to formation members. As a result, some of their "sequences" are purely lithostratigraphic.

LaMaskin and Elrick (1997) provided a detailed analysis of the overarching Guilmette Formation sequence stratigraphy, but their work was conducted entirely outside of the impacted area (Schell Creek and Egan ranges), thus not addressing the accommodation questions associated with the Alamo impact. The sequences interpreted herein correspond to Sequences 1 to 4 of LaMaskin and Elrick (1997).

Retzler et al. (2015) used environmental facies associations immediately above the Alamo Breccia Member as a proxy for crater size and geometry. The sequences they identified are an extension of those proposed in an M.S. thesis by Rendall (2013) that were based on facies associations and Fisher plots in and around the Alamo impact region. What Retzler et al. (2015) did not do, and what is novel here, is compare the pre-impact with postimpact sequence stratigraphy couched in terms of accommodation and crater proximity.

The Middle to Late Devonian greenhouse had little polar ice, resulting in low-amplitude fluctuations in global sea level (De Vleeschouwer et al. 2014). Flooded continental shelves provided abundant shallow, well-lit ecospace for the proliferation of diverse stromatoporoid, coral, and brachiopod communities. High sea levels and favorable ocean conditions allowed many groups of organisms to achieve global or near-global distributions (McGhee 1997).

Southern Nevada was located ~15°S of the paleoequator (Witzke and Heckel 1989, Scotese and McKerrow 1990) on the shallow epicontinental margin of the eastern Panthalassic Ocean. Cyclic subtidal–peritidal shelf carbonates and intermittent sandstones were deposited on top of a 4- to 7.5-km-thick succession of passive-margin strata as old as Neoproterozoic (Stewart and Poole 1974, Bond and Kominz 1984). The Guilmette Formation was deposited during Givetian–Famennian time (Johnson et al. 1985, Johnson and Sandberg 1989). The eustatic Taghanic onlap corresponds to transgressive-regressive (T–R) cycle IIa sea-level rise of Johnson et al. (1985, 1996) and was coincident with deposition of the middle varcus conodont zone and upper Fox Mountain Formation, which underlies the Guilmette Formation. Sequences interpreted here are superimposed on a larger-magnitude backstepping Late Devonian shoreline.

During the early–middle Frasnian (ca. 382 Ma), the Nevada carbonate platform was struck by a large bolide during the Alamo impact event (Sandberg et al. 1997, Warme and Kuehner 1998, Pinto and Warme 2008, Warme 2008). The impact resulted in catastrophic resedimentation of platform carbonates, which form the regional Alamo Breccia Member (Sandberg et al. 1997, Pinto and Warme 2008). Over 25 years of research supports the interpretation of an impact deposit, primarily evidenced by mapping of breccia units and depositional zones (Warme and Kuehner 1998, Pinto and Warme 2008) (Fig. 2), an anomalous spike of iridium (Warme and Sandberg 1995, Koeberl et al. 2003), shocked quartz (Leroux et al. 1995, Morrow et al. 1998, Sandberg et al. 2002), carbonate accretionary lapilli (Warme et al. 2002), and reworked Ordovician and Late Cambrian conodonts from within the Devonian breccia matrix (Warme and Sandberg 1995, Morrow et al. 2005).

The Guilmette Formation has been traditionally subdivided based on its lithostratigraphic components and weathering profiles, which are summarized below. Our study references these names occasionally, only to place our work in the context of the many past publications that have used this terminology.

The Lower Guilmette Formation is informally divided into two intervals based on weathering profile and depositional interpretation (Fig. 2). The YSF is the lowest stratigraphic subdivision of the Guilmette Formation and is commonly 30 to 80 m thick in southern Nevada (Sheffield 2011). It has been reported as far east as central Utah (Sandberg 2009). It consists of thinly bedded dolomite and limestone overlying the entirely dolomitic Fox Mountain Formation. The Fox Mountain Formation is easily distinguished from the Lower Guilmette because it contains abundant characteristic Stringocephalus brachiopods, which are absent from the overlying YSF. The base of the YSF is marked by two beds (~1.5 m thick) of digitate and laterally linked hemispheroid stromatolites. Basal YSF stromatolitic boundstones are lithologically correlative across the study area and form the datum to which all measured sections presented herein are pinned. The platy, finely bedded YSF is the most conspicuous unit of the Guilmette Formation because of its recessive weathering profile and yellow hue.

The LFI (informal) conformably and gradationally overlies the YSF. The base of the LFI is delineated in the field by the first prominent, ledge-forming bed representing a shift in lithofacies from the YSF. This boundary is subjective and renders the transition difficult to determine where talus is abundant. Resistant basal LFI beds are composed of stacked subtidal micritic limestone that has been locally dolomitized and contain varying degrees of Thalassinoides ichnofabric. The LFI is composed of shallowing-upward peritidal-to-subtidal cycles.

The Alamo Breccia is an impactogenic carbonate megabreccia located mostly in Lincoln County, Nevada. The breccia was formally designated as the middle ‘‘Alamo Breccia Member’’ of the Guilmette Formation by Sandberg et al. (1997). Pinto and Warme (2008) delineated five ‘‘realms’’ of breccia deposition associated with the impact. That paper described stratigraphy associated with the ‘‘rim realm,’’ located within the paleocrater, and the ‘‘runup realm,’’ outside of the paleocrater, where thinner graded breccia was deposited. Alamo Breccia stratigraphic subdivisions are briefly described here (Fig. 2); for more thorough descriptions of breccia units and realm model delineation, the reader is referred to Warme and Kuehner (1998) and Pinto and Warme (2008).

The D unit of the Alamo Breccia is a generally <3-m-thick monomict limestone breccia that defines the spatial extent of the collapsed portion of the impact crater and underlies megaclasts of the C unit. This unit is interpreted as once-fluidized limestone bedrock that has been preserved along a detachment surface between preexisting limestone beds of the LFI (Pinto and Warme 2008).

The C unit is composed of discontinuous megaclasts up to ~500 m long and ~80 m thick (Pinto and Warme 2008). If the D unit is not identified, it can be difficult or impossible to differentiate C unit megaclasts from underlying LFI beds. In other instances, C blocks have been ‘‘peeled up’’ and folded into synimpact deformational structures (Warme and Sandberg 1995, Pinto and Warme 2008).

The Alamo Breccia B unit is similar in character to debris-flow deposits, often containing very poorly sorted, nonoriented clasts in a carbonate mud matrix (Dunn 1979, Pinto and Warme 2008). The B unit contains abundant mobilized stromatoporoid heads and is composed of reworked LFI carbonates. Large lithoclasts are common near the base, and it fines into the overlying A unit. This unit is extremely variable in thickness and ranges from >80 m thick near the crater center to 0 m thick in ‘‘runup realm’’ deposits far from target rocks (Fig. 1).

The uppermost A unit is a normally graded, clast-supported polymict breccia. This unit was originally interpreted as gravity-flow deposits because of a turbidite-like appearance (Dunn 1979). Sedimentary styles of the terminal Alamo breccia beds can be divided into pristine graded, reworked/burrowed, dolomitized, and erosional, corresponding to accommodation trends following the impact event (Tapanila et al. 2014).

Conodonts from beds below, within, and above the Alamo Breccia Member constrain the age of the Alamo impact to depositional ages corresponding to the Frasnian punctata zone. This has placed the timing of the impact ~6 million years before the end-Frasnian biodiversity crisis and has not been shown to have contributed to that episode of faunal turnover (Sandberg and Warme 1993, Sandberg and Morrow 1998, Sandberg et al. 2002).

The Upper Guilmette contains an array of depositional environments deposited after the Alamo impact and is composed of a mixed carbonate–siliciclastic succession that generally deepens to the west. Sandberg et al. (1997) proposed that Upper Guilmette carbonates span the punctata-hassi zone based on conodont specimens collected at Hancock Summit, Nevada. Upper Guilmette limestones deepen dramatically past the paleo slope break formed by the Alamo crater.

The lower part of the Upper Guilmette Formation is made up primarily of burrowed nodular mudstone and skeletal brachiopod–crinoid wackestone–packstone that form the middle and base of subtidal cycles, which are capped by stromatoporoid floatstone–boundstones or Amphipora wackestone–packstone.

In measured sections located just westward of the location of the annular trough rim, the Upper Guilmette Formation is made almost entirely of quartz arenite sandstone (>100 m thick). Cross-bedded and laminated sandstones are often punctuated by <20-m-thick intervals of limestone and siltstone interbeds. Initial deposition of sandstone is presumed to correlate with the paleokarst surface at Mt. Irish, which is infilled by red quartz silt.

Seven detailed sections were measured through the lower ~300 m of the Guilmette Formation in Lincoln and Nye Counties, Nevada. Sections contain detailed bed-by-bed logs that record lithology, Dunham texture, sedimentary structures, ichnofacies, grain size and type, matrix composition, and fossil content.

Facies types were determined from lithology, fossil content, and Dunham texture and then assigned depositional environments (interpreted). Peritidal cycle tops were picked in laminated dolomudstones. Associated sedimentary structures include desiccation cracks, evaporite pseudomorphs, and thin monomict brecciation surfaces. Subtidal cycle tops were determined by breaks in Waltherian shallowing-upward successions that are overlain by deeper-water facies that break the shallowing-upward trend.

In addition to the seven measured sections presented herein, over 100 measured sections measured by LT and students from Idaho State University were available to test interpretations and are publicly available in appendices to a series of master’s theses (Anderson 2008, Thomason 2010, Myers 2011, Sheffield 2011, Retzler 2013).

Sequence stratigraphic interpretations are primarily based on cycle thickness trends and facies proportions due to the low-amplitude sea level fluctuations during greenhouse periods. Gradual sequence boundary zones were picked within stacks of peritidal cycles that did not display prominent erosional surfaces. Erosional sequence boundaries are uncommon, but they are present locally at the top of the Mt. Irish (MS1) locality.

Six major carbonate environmental lithofacies dominate the study interval (Table 1; Fig. 3). Deposition spanned a period of ~4 million years, based on conodont ages from Kaufmann (2006). Facies in the Upper Guilmette are consistent with those described by Retzler et al. (2015), and those from the Lower Guilmette are consistent with those described by LaMaskin and Elrick (1997).

Deeper subtidal facies are typically gray to dark brown and thin bedded with light brown silt stringers or laminations. Mudstones are nodular and locally burrowed. Allochems include peloids, quartz silt, and occasional bioclasts. Indicative fauna include whole, unabraded Productella brachiopods with excellent preservation of spine bases and other delicate features, indicating a very low energy environment. Hardgrounds and firmgrounds were noted at several horizons and are indicative of slow sedimentation and starved basin conditions. Deeper subtidal facies are interpreted to have been deposited below fair weather wave base (Fig. 3A).

Intermediate/shallow subtidal facies contain diverse allochems that include disarticulated brachiopods, alveolitid and thamnoporid corals, and a variety of stromatoporoid morphologies (mostly domical and tabular forms). Digitate stromatoporoids common in this facies include transported Stachyoides and Amphipora genera. Microbial oncoids are locally common in laterally continuous beds (Fig. 3B, C).

Stromatoporoid floatstone facies are mud-to-clast supported. Allochems are dominated by domical and bulbous stromatoporoids that are commonly perforated by spar-filled Entobia devonica borings (Tapanila 2006). The matrix component is often burrowed and contains peloids, brachiopods, crinoid columnals, and rugose and tabulate corals. The digitate stromatoporoid Stachyoides is common (Fig. 3D).

Semirestricted shallow subtidal facies are dominated by in situ digitate stromatoporoid Amphipora specimens, which are typically recrystallized, but are easily identified by their characteristic central lumen. Gastropods are locally common, and a generally high abundance/low diversity faunal composition is characteristic of this facies (Fig. 3E).

Restricted subtidal facies are dominated by lime mudstone and dolomudstone with rare Amphipora and gastropods. Thalassinoides burrows are locally common. This facies was likely formed in a low energy restricted lagoonal setting deposited just seaward of tidal flats (Fig. 3F).

Tidal flat facies are made of white–gray to brown laminated crystalline dolomudstone. Evaporite pseudomorphs and fenestrae are locally common. Tidal flat facies include both crinkly and planar-laminated dolomudstone that was deposited in an intertidal to supratidal setting. Desiccation cracks and thin monomict breccias are common (Fig. 3F).

The study interval contains peritidal and subtidal cycles, which were determined by lithology, facies composition, sedimentary structures, and the nature of cycle tops (Fig. 4A–C). Cycles generally shallow upwards, with an average thickness of 2 to 3 m, and they are bounded by flooding surfaces.

Two types of peritidal cycles are recognized (Fig. 4A). One group contains subtidal bases, and the other contains entirely peritidal facies. In the first group, Amphipora packstone and stromatoporoid floatstone–boundstone are common subtidal bases (Fig. 5). In the second group, peritidal cycles are made up of tidal flat laminites and barren dolomudstones. Color changes from brown bases to white tops and downward-tapering desiccation cracks mark cycles within dolomitized peritidal successions. Thin monomict breccias are common in brown dolostone bases that formed when flooding reworked sediments freed during subaerial exposure.

Subtidal cycles grade from burrowed nodular lime mudstone to an assortment of shallower facies (Fig. 4B). Subtidal cycle tops include skeletal gastropod- and stromatoporoid-rich wackestone–packstone, skeletal oncoid-dominated wackestone and packstone, and Amphipora packstone.

Subtidal cycles vary in terms of facies proportions and thicknesses depending on the position along the shelf. The deepest subtidal cycles contain muddy facies and abundant silt stringers. In these facies, cyclicity can be difficult to determine because of noncyclic mudstone deposition. Intermediate subtidal cycles shallow from burrowed mudstone bases to skeletal, brachiopod-dominated wackestone and packstone tops. Shallow subtidal facies shallow from either burrowed mudstone or skeletal wackestone to stromatoporoid-dominated packstones, boundstones, and floatstones. The shallowest subtidal facies are capped by either dolomitized Amphipora packstone or sparsely burrowed to barren dolomudstone.

This study identified four genetically related sequences bounded by unconformities or correlative conformities within the Guilmette Formation. Sequence boundaries on greenhouse ramps and rimmed platforms are subtle due to low-amplitude fluctuations in sea level. Prominent erosional surfaces are uncommon, and gradual changes in cycle thicknesses and facies proportions were used to identify "turnarounds." Sequence boundaries formed on the middle platform are diffuse over tens of meters of stratigraphic thickness and are better characterized as sequence boundary zones (Montañez and Oslager 1993, LaMaskin and Elrick 1997). Sequence boundary zones are correlative conformities where relative sea-level fall was not sufficient in amplitude or duration to expose the carbonate platform long enough to develop extensive subaerial disconformities. A notable exception to this is the terrigenous silt-filled paleokarst surface that marks the end of Sequence 4 at the top of the Mt. Irish reef (Fig. 6).

Sequence boundary zones in midshelf outcrops are expressed as shifts from facies indicative of progradation (thick tidal flat successions) dominated by peritidal cycles to those indicative of aggradation and retrogradation dominated by subtidal cycles. Shifts from highstand systems tracts (HST) to transgressive systems tracts (TST) were picked in facies stacking patterns either where peritidal cycles began to thicken or in zones with significant increases in subtidal cycles.

Deposits associated with lowstand systems tracts are absent at Guilmette Formation localities deposited on the midshelf (LaMaskin and Elrick 1997); however, the thick Upper Guilmette sandstone is interpreted to be a progradational lowstand wedge deposited distal to the break in slope associated with the annular trough of the Alamo impact crater (Morrow and Sandberg 2008, Retzler et al. 2015).

Maximum flooding zones (MFZs) occur as thick successions of thinly bedded nodular mudstone (Fig. 7) with poorly preserved cyclicity or zones of shallow subtidal facies within peritidal-dominated intervals. Whole, unabraded atrypid brachiopods were associated with thick mudstone successions at several localities (Fig. 7). As with sequence boundary zones, maximum flooding zones are spread over tens of meters and record gradual changes in facies proportions rather than discrete surfaces.

Two regional cross sections (Figs. 8, 9) were used to interpret the sequence stratigraphic architecture across the Devonian shelf adjacent to the Alamo impact. Our new field measurements taken within and outside the impacted region are leveraged with results from previous studies (Larsen et al. 1989, Ackman 1991, Chamberlain and Warme 1996, Elrick 1996, LaMaskin and Elrick 1997) to discriminate local effects of the Alamo impact from regional depositional trends.

Sequence 1 spans the upper units of the Fox Mountain Formation through the platy, silty, dolomitic YSF beds in the Lower Guilmette. The initiation of Sequence 1 is recorded within the 75- to 175-m-thick Fox Mountain Formation and is coincident with the beginning of the second-order Taghanic onlap (Sandberg et al. 1997). TST1 was not systematically described for this study because our measured sections begin at the base of the YSF (basal Guilmette Formation), which is in the overlying highstand systems tract (HST1). HST1 is dominated by slope-forming, platy laminated silty dolostones with occasional digitate and hemispheroidal stromatolites. HST1 is interpreted to represent widespread progradation of tidal flats corresponding to the T-R cycle IIa-1 regression (LaMaskin and Elrick 1997). The upper boundary of Sequence 1 is a zone that represents deposition on a periodically inundated shelf that was not exposed long enough to undergo widespread erosion or nondeposition.

TST2 is interpreted within an ~25-m-thick transition from dolomitic peritidal cycles to peritidal cycles with subtidal limestone bases in the lowest units of the LFI. Limestone cycle bases were deposited during pulses in relative sea-level rise that flooded the platform to shallow subtidal depths. Laminated dolomite cycle tops suggest filled accommodation. Downdip localities are dominated by subtidal cycles that filled only to shallow subtidal depths. MFZ2 occurs as a suite of <10 m of subtidal cycles of barren and Thalassinoides-burrowed mudstone and wackestone at almost all localities. At Mail Summit, there is no abrupt facies shift at the end of TST2, and the MFZ is interpreted in an interval of relatively thick peritidal cycles. The absence of subtidal facies at this locality suggests lower accommodation than surrounding localities, and it may have been positioned on a slight structural high.

HST2 is marked by a return to peritidal cycles at most localities and increase in frequency of laminated dolomite cycle tops (74% at FM1 and 100% at MS1). Western localities contain a relatively high proportion of subtidal facies because of their downdip position. The upper boundary of HST2 is recognized as a sequence boundary zone, but it commonly contains monomict exposure breccias at the tops of peritidal cycles in updip localities.

Depositional Sequence 3 is significant because it spans the interval containing the Alamo breccia. It contains two domains in which sedimentation was affected by bathymetry and energy conditions created by the impact. At localities far from target rocks, Sequence 3 deposition is continuous because local accommodation remained unchanged after the impact event. At localities close to target rocks, the Alamo impact created a major disconformity by scouring through pre-impact shelf carbonates.

The Fox Mountain (FM1) measured section is located outside of the paleocrater and contains only a thin (0.5–1.5 m) section of A unit of the Alamo breccia. It displays only depositional breccia with no basal erosional surface, meaning that the entirety of Sequence 3 is recorded. The Alamo Breccia pinches out updip and is absent in the Schell Creek range (SHP1), where the initiation of TST3 is interpreted from several peritidal cycles with subtidal (stromatoporoid boundstone) bases that transition up section to fully subtidal cycles. Subtidal cycles of TST3 have nodular mudstone bases overlain by stromatoporoid boundstone or skeletal (brachiopod–crinoidal) wackestone tops. The shallowest subtidal cycle tops in this succession are semirestricted Amphipora packstone facies.

The top of TST3 outside of the crater is marked by a succession of ~10 m of nodular mudstone containing abundant in situ atrypid brachiopods and silt stringers. MFZ3 mudstone is overlain by skeletal wackestone (subtidal cycle top), marking the first cycle recorded in early deposits of HST3. This zone appears to have been removed during the impact from localities farther to the west that experienced significant erosion. Where preserved, HST3 is characterized by shallow subtidal and peritidal cycles that are in turn dominated by bulbous stromatoporoid biostromes and Amphipora wackestones.

Sequence 3 is obscured in localities that were close to target rocks because much of the upper part of TST3 and the lower part of HST3 were excavated or dislocated during impact reworking. This resulted in removal of MFZ3 facies within the Alamo crater. At Mail Summit (MS1) and Mt. Irish (MI1), TST3 is interpreted from a thickening of peritidal cycles that contain stromatoporoid boundstone and Amphipora packstone bases. At near-crater localities, TST3 is truncated by the B breccia disconformity, which was formed during the scouring and removal of variable amounts of TST3 and HST3 cycles throughout the study area.

Comparisons of cycles and Alamo Breccia thicknesses between pristine sections measured outside of the crater to sections measured within the crater suggest that ~25 cycles were removed during the impact. Within the crater, the end of HST3 is represented by up to five peritidal cycles capped by laminated dolomite immediately overlying the Alamo Breccia. At midplatform localities (e.g., Mail Summit and Mt. Irish), HST3 ends in a sequence bounding zone dominated by <15 m of peritidal cycles above the top of the Alamo Breccia A unit, where peritidal cycles transition gradually to subtidal cycles composed of barren/burrowed mudstone with skeletal wackestone, stromatoporoid floatstone–boundstone, and Amphipora packstone tops. In western downdip localities (e.g., Hancock Summit and Monte Mountain), the top of HST3 is a correlative conformity that occurs within a zone of <10-m-thick barren/burrowed lime mudstone to skeletal wackestone.

A complete representation of Sequence 4 is only recorded in localities where Upper Guilmette deposits are fairly thick (>100 m), or in downdip localities, where it is composed entirely of subtidal mud-dominated facies. At Sixmile Flat (SMFN1), TST4 is expressed as a shift from peritidal cycles to subtidal cycles dominated by bulbous stromatoporoid boundstone–floatstone and open-marine skeletal wackestone ~15 m above the Alamo Breccia. At Schell Creek (SHP1), TST4 is characterized by thick successions of noncyclic, thinly bedded burrowed mudstone, which also makes up MFZ4.

HST4 contains a shift back to shallow subtidal facies and is characterized by the dominance of shallow subtidal, thickly bedded stromatoporoid boundstone and Amphipora packstone facies. Shallow subtidal limestone cycle bases are commonly capped by dolomitized shallow subtidal facies or laminated dolomite. The entirety of Sequence 4 is expressed uniquely at Mt. Irish, where massive stromatoporoid reef facies make cycle determination difficult.

The upper boundary of HST4 is expressed as either a sequence boundary or as a correlative conformity depending on locality. The most notable is expression of this sequence boundary is a paleokarst surface at the top of the Mt. Irish reef, which contains large cavities filled with red quartz siltstone (Fig. 6). As the shelf became exposed, continentally sourced sand was channelized (Morrow and Sandberg 2008) and accumulated in offshore localities. In downdip localities where the shelf did not become exposed during relative sea-level fall, the uppermost beds of HST4 are composed of thinly bedded, pelagic tentaculitid-rich mudstone overlain by shoreface sandstone. This is interpreted to represent a correlative conformity to SB4 and the lowstand clastics deposited on the seaward side of the crater margin.

Exposure features within peritidal facies are present in the upper bounding zone of Sequence 4, where laminated dolomites are truncated along irregular surfaces and overlain by monomict breccias composed of tidal flat facies.

Sequences 1 and 2 are dominated by tidal flats and low-energy peritidal muds across the platform. This type of sedimentation represents filled accommodation, and low-angle progradation is inferred during highstands, while aggradation dominated during transgressions. In such low-relief settings, only a few meters of relative sea-level rise would be sufficient to inundate large portions of the shelf. Downdip localities (e.g., Hancock Summit) are dominated by tidal flats through Sequence 2 and transition to shallow-marine facies during the early transgressive phase of Sequence 3. At the same time as shallow-marine facies were being deposited in early Sequence 3 at Hancock Summit, localities updip continued to be dominated by peritidal and supratidal sedimentation. A significant backstep occurs in Sequence 3, culminating at the MFZ, which is only recorded outside of the Alamo crater (Fox Mountain and Schell Creek). This interval is dominated by very thinly bedded noncyclic mudstones and a diverse brachiopod fauna. A shift to aggradation and subsequently progradation resulted in the widespread deposition of stromatoporoid boundstone facies in the upper part of Sequence 3. The upper part of Sequence 3 corresponds to the timing of the Alamo impact event. Excavation by the bolide is why the MFZ from Sequence 3 is only recorded outside of the crater, and it illustrates the importance of carrying correlations to localities outside of the breccia realms, because it suggests significant deepening had already occurred prior to the impact event. The end of Sequence 3 resulted in tidal flat deposition in updip localities transitioning to shallow-marine and eventually deep-marine facies downdip. At Hancock Summit, tidal flat facies are absent above the breccia, and the correlative conformity is interpreted in open-marine wackestones a few meters above the breccia. Sequence 4 contains a second major backstep and deepening coincident with deposition of open-marine limestones, stromatoporoid floatstones–boundstones, and fossiliferous burrowed mudstone. Sequence 4 culminates in tidal flat, barren peritidal mudstone, or thin sandstone facies across most of the eastern localities. The karsted upper surface of the Mt. Irish reef is indicative of substantial sea-level fall, prolonged exposure of the reef, and ultimately forced regression of the coeval sediments. Associated basinal deposition is represented by thick successions of progradational shoreface sandstone only present seaward of the crater margin.

Sequence 3 is unique in that it incorporates the Alamo Breccia as well as its disconformable basal surface within the crater. The presence of a local disconformity within Sequence 3, which is otherwise genetically related to low-amplitude sea-level fluctuations, could justify splitting it into two entirely separate, spatially delineated sequences. Outside of the impact basin, Sequence 3 represents accommodation trends typical of a Devonian greenhouse shelf, including low-angle depositional geometries, broad facies belts, low-amplitude sea-level fluctuations, and lack of prominent erosional surfaces. Within the impact basin, Sequence 3 is dramatically different and contains a major erosional surface that has removed millions of years of depositional history. Whether Sequence 3 is divided based on the impact zone or not, the scenario illustrates the importance of context. If observed from a single core or measured section without the spatial and temporal context of the regional accommodation history, one might interpret the prominent disconformity to represent a major forced regression and would need to invoke unrealistic tectonic or climatic scenarios to bring upon such an exposure in the middle of the undeformed Frasnian greenhouse shelf. Within the regional context, it would be acceptable to split Sequence 3 while understanding that the zone with the major disconformity is the result of a local phenomenon (Alamo impact) rather than part of a plate- or planet-wide sea-level signature.

While accommodation trends outside of the crater were not influenced by the Alamo impact, the effect on the biosphere is unclear. Meteor impacts are often regarded as catalysts for mass extinction events. The three largest Phanerozoic impact craters in transient diameter (Dt)—Chicxulub (150 km), Popigai (90 km), and Manicouagan (85 km)—are each implicated in faunal turnover events (Alvarez et al. 1980, Schulte et al. 2010, Renne et al. 2013), but they represent only the upper tail of discovered impact craters. All others are less <1/2 Dt and delivered <1/100 of the energy (Dence et al. 1977). Mid-sized (Alamo) impact craters (30–65 km) occur four to five times per 100 million years and have yet to be correlated with faunal turnover and extinction; however, this has not been systematically studied at a regional scale. The biological influence on carbonate sedimentation is well established, so there should be a visible effect on facies compositions and carbonate factory production associated with biological events. Having occurred ~6 million years prior to the Frasnian–Famennian biodiversity crisis, it is unlikely that the Alamo event had any contribution to that major episode of faunal turnover. However, at a regional scale, was it large enough to affect the composition of carbonate factory communities? Two paleoecological studies have been published since the recognition of the Alamo breccia as an impactogenic unit (Casier et al. 2006, Tapanila 2006). Tapanila (2006) examined complex burrow networks penetrating the terminal breccia units and suggested that recovery of infauna was rapid. Casier et al. (2006) focused on the increase in ostracod diversity in postimpact deposits, but the study was limited in scope and did not include any of the groups that make up the majority of the skeletal rock volume. Certainly, the local distribution of fauna would have been affected by postimpact bathymetry. In pre-impact deposits, a nearly flat shelf existed, while after the impact, a series of mesoenvironments would have resulted from the undulating postimpact seafloor. One example of this is the spectacular stromatoporoid reef developed at Mt. Irish, which was able to grow to more than 50 m thick. Further study in this area could help to address questions related to threshold sizes that physical events must attain before altering carbonate factory community composition or production, and how that may result in particular sedimentological or stratigraphic trends. The sequence development proposed herein provides the accommodation framework necessary for comparisons of carbonate factory faunal associations in pre-, syn-, and postimpact strata.

Four sequences are interpreted from stacking patterns identified in stratigraphic sections measured from outside to within the Alamo crater. The four sequences record the change from a nearly flat carbonate shelf to a nearly flat carbonate shelf with a local intrashelf basin within the Alamo crater. Sequence 1 is dominated by dolomitic tidal flat and peritidal mudstone deposits. Sequence 2 contains an increase in shallow subtidal cycles and incorporates the informal LFI beds of the lithostratigraphic Lower Guilmette Formation. Sequence 3 contains two domains that could be subdivided into discrete sequences based on proximity to the Alamo crater. Outside of the Alamo crater, Sequence 3 records the regional accommodation signature that was not affected by the Alamo impact. Within the crater, Sequence 3 contains a major disconformity at the base of the Alamo breccia that omits significant thicknesses of preexisting platform carbonates. Sequence 4 records postimpact sedimentation trends, including condensed sections at downdip western localities (within crater) and shallow subtidal to peritidal shelf cyclicity in eastern, updip localities (outside crater). At a local scale, the Alamo impact created significant accommodation through the removal of preexisting shelf carbonates; however, at a regional scale, the impact had no discernible effect on the greater accommodation signature. The magnitude of the Alamo impact was not large enough to alter regional accommodation trends outside of the immediate vicinity of the target area.

This work was funded by National Science Foundation (NSF)–Sedimentary Geology and Paleobiology (SGP) grant 102484 (LT) and a Geological Society of America Graduate Student Research Grant (BR). We would like to thank A. Retzler for assistance with data collection and analysis, I. MacPherson and B. Govonni for field assistance, and S. Holland and P. Link for helpful manuscript revisions and data discussions. We would also like to acknowledge external reviewers Maya Elrick and John Warme for their thoughtful evaluations that greatly improved the manuscript. Extra thanks go to the editor, Ted Playton, whose extra efforts provided helpful insight and many constructive suggestions.