Crater morphology of the Late Devonian Alamo impact, Nevada Post-impact depositional environments as a proxy for crater morphology, Late Devonian Alamo impact, Nevada

Andrew J. Retzler1,†,*, Leif Tapanila1,2,*, Julia R. Steenberg3,*, Carrie J. Johnson4,*, and Reed A. Myers5,* 1Department of Geosciences, Idaho State University, 921 South 8th Avenue, Pocatello, Idaho 83209-8072, USA 2Division of Earth Science, Idaho Museum of Natural History, 921 South 8th Avenue, Pocatello, Idaho 83209-8096, USA 3Minnesota Geological Survey, 2642 University Avenue W., St. Paul, Minnesota 55114-1032, USA 4Chesapeake Energy Corporation, Building 05, Offi ce 249, 6001 North Classen Boulevard, Oklahoma City, Oklahoma 73118, USA 5Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Alberta T6G2E3, Canada

ABSTRACT INTRODUCTION as expected for marine bolide impacts (Dypvik and Jansa, 2003; Dypvik and Kalleson, 2010). Marine facies of carbonate and siliciclas- Marine bolide impact events are under- Estimates of the fi nal crater diameter have relied tic sediments deposited on top of the upper represented in the rock record due to their low exclusively on the extent and composition of the Devonian Alamo Breccia Member identify preservation potential. Consequently, few stud- Alamo Breccia Member and not on geomorphic the shape and size of the Alamo impact cra- ies exist documenting marine impact crater size, features specifi c to marine impact craters. ter in south-central Nevada (western USA). morphology, and effects on sedimentation pat- The aim of this paper is to interpret crater There are 13 measured sections that record terns; of the 27 known marine impact craters on morphology based on post-impact deposi- peritidal to deep-subtidal deposition across Earth, 20 of them are currently located on land tional environments in the context of a regional the impacted platform, and these are corre- (Dypvik and Jansa, 2003). The Late Devonian sequence stratigraphic framework. By identi- lated to three regional depositional sequences Alamo impact of south-central Nevada (western fying key boundaries of the crater margin, we above the Alamo Breccia Member. Facies and USA) is one such case, providing a rare oppor- make new size estimates of the Alamo cra- accommodation patterns identify a concave tunity to study a marine impact in outcrop at the ter based on linear scaling relationships from seafl oor that we interpret as the post-impact regional scale. well-studied seismically imaged marine impact legacy of the Alamo crater. Together with The Alamo impact occurred on a carbon- craters (Melosh, 1989; Dypvik and Kalleson, isopach and lithostratigraphic trends in the ate platform along the western margin of 2010). These methods could prove useful in underlying Alamo Breccia Member, a new North America. This catastrophic event is now estimating the size of other marine impact cra- map of the Alamo crater is presented show- expressed in the Guilmette Formation (Late ters that lack seismic data, or that are associated ing the eastern outer rim fault and the annu- Devonian, Frasnian) across present-day south- with post-impact tectonism that has obscured lar trough. Size estimates were made using central Nevada and western Utah (Fig. 1). The the original crater morphology. the newly defi ned crater features and linear resultant impact stratum, known as the Alamo scaling relationships from other marine- Breccia Member, covers an area of ~28,000 km2 BACKGROUND target complex craters. Revised dimensions and is one of the largest and best-exposed marine of the Alamo crater place its transient diam- impact deposits on Earth (Pinto and Warme, Stratigraphy eter between 37 and 65 km, and its apparent 2008). Evidence supporting an impact origin diameter between 111 and 150 km. These for the Alamo event includes melt breccia, shat- The Alamo impact occurred ca. 382 Ma on a estimates are more than double previous ter cone–like structures, carbonate accretionary shallow-marine, west-facing carbonate platform estimates based on the biostratigraphy of lapilli, iridium anomalies, and shocked quartz during deposition of the Guilmette Formation the Alamo Breccia Member. If correct, these (Pinto and Warme, 2008). (Sandberg and Morrow, 1998). Three members new estimates place the Alamo crater as one Post-impact tectonism throughout the region compose the Guilmette Formation: the lower of the largest marine impacts of the Phanero- has obscured the original crater morphology and member, the Alamo Breccia Member, and the zoic, and conservatively larger than the well- buried important strata, making it diffi cult to upper member (Fig. 2) (Ackman, 1991). studied Eocene Chesapeake Bay crater. correlate between sections and characterize the The lower member consists of a basal yellow impact crater (Pinto and Warme, 2008). Prior slope-forming interval capped by a ledge-form- †Corresponding author: Present address: Minne- descriptions of the impact crater are based on ing interval (Fig. 2) (Ackman, 1991; Sandberg sota Geological Survey, 2642 University Avenue W., the lithostratigraphy and features of the Alamo et al., 1997). The yellow slope-forming interval St. Paul, Minnesota 55114-1032, USA. Breccia Member (Warme and Sandberg, 1995; comprises thinly bedded silty dolostone, and *Emails: Retzler: aretzler@ umn .edu; Tapanila: tapaleif@isu .edu; Steenberg: and01006@umn .edu; Pinto and Warme, 2008). While useful for its base is marked by beds of digitate stromato- Johnson: carrie.johnson@ chk .com; Myers: ramyers@ regional correlation, this terminology does not lites (Sandberg et al., 1997). The ledge-forming ualberta .ca. relate the deposits to a complex crater model interval consists of ~100 m of intertidal and sub-

Geosphere; February 2015; v. 11; no. 1; p. 123–143; doi:10.1130/GES00964.1; 15 fi gures; 4 tables; 1 supplemental fi le. Received 10 July Month 2013 ♦ Revision received 9 October 2014 ♦ Accepted 4 December 2014 ♦ Published online 14 January 2015

For permission Geosphere, to copy, contact February [email protected] 2015 123 © 2015 Geological Society of America

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115°30′00″W 115°15′00″W +

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NEVADA+

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! Tertiary

@ @ Cretaceous @ !! 37°45′00″N !

+

+ B′ ! !!! ! MAILMAIL ! + SMFN2

+ !!! Permian/Penn. + !

+ + B + SUMMITSUMMIT ! + ! !

+ HHN1 + ! @ ! ! MMN4 ! + !! +

+ +

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MONTEMONTE + Mississippian + + ! + ! !! + !

+ + !!

+ MOUNTAINMOUNTAIN !! +

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+ +

+ ! Hiko /" !!! 375 Ordovician

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+ " Town PAHRANAGATP RANGE ! PTN0 !! A Transect Locality H + Cambrian *lies off transects, R 37°30′00″N but referenced in text A N + ! A Other Locality + G

A + Major Road !! T !! +

!! + + ! 93 + Known Thrust Fault HN5

R + Known Fault ! A+ ! + DDB1 ! N ! ! + Concealed Fault G

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Figure 1. Geologic map of the Alamo impact region, Lincoln County, southeastern Nevada, showing the location of prominent measured sections referred to in this study. Interpretations are focused along the A-A′ and B-B′ transects shown. Localities included along these tran- sects are labeled (modifi ed from Crafford, 2007). DDB—Hancock Summit down-dropped block; DMP—Hiko Hills south dump; GGS— Golden Gate south; HCE—Hiko Hills east-central; HE—Hancock east; HHN—Hiko Hills north; HN—Hancock north; MI—Mount Irish; MIN—Mount Irish north; MMN—Monte Mountain north; MMS—Monte Mountain south; PTN—Pahranagat north; SMFN—Six Mile Flat north; TMP—Tempiute Mountain.

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Stratigraphy Brief Description & Interpretation Series Stage T-R Cycle T-R ~Ma Conodont biozone Dep. Seq. quartzarenite common in most outcrops 5

hassi upper 381.5 member 4 (lower part) biostromes, bioherms present at various localities Figure 2. Generalized stratig- platform carbonates, supratidal to subtidal depending on location raphy of the Guilmette Forma- This Study tion and matching conodont 382 A well-graded polymict gravel to sandy breccia

biozones. Corresponding depo- punctata B sitional sequences (Dep. Seq.) Alamo poorly graded/sorted, megablock to boulder polymict breccia Breccia brittle/ductile deformed, tilted LFI rocks, crosscut by B-unit LaMaskin and Elrick, 1997; 3 Member C Frasnian Rendall, 2013) and Devonian irregular thickness, monomict pebble breccia transgressive-regressive (T-R) D cycles (Johnson et al., 1996) 382.5 are shown on the far right Guilmette Formation (adapted from Sandberg et al., transitans ledge forming peritidal to subtidal cyclical platform carbonates 1997; Kaufmann, 2006; Mor- 2 interval row et al., 2009). LFI—ledge- 383 forming interval.

lower member yellow yellow to gray dolomite with supratidal cyanobacterial laminites slope- 25 m forming

Givetian interval stromatolites common in lower 6 m of unit

disparilis1 falsiovalis 385 IIa-1 IIa-2 IIb IIc Middle DevonianFox Upper Devonian Mountain Stringocephalus brachiopods common near top Formation (upper part) gray fossiliferous subtidal dolomite and limestones

tidal mudstones that transition eastward to thin, The upper member was deposited after the the impact region (Dunn, 1979; Tapanila and silty dolostone and microbial laminite (Sand- Alamo impact. It is composed of subtidal lime- Ekdale, 2004; Tapanila et al., 2014). Quartz- berg et al., 1997). The Alamo bolide struck the stone, dolomite, and quartz sandstone (Fig. 2) arenite is present ~40–100 m upsection from the ledge-forming interval of the lower member. (Sandberg et al., 1997; Chamberlain, 1999). Stro- top of the Alamo Breccia Member and represents The Alamo Breccia Member is a carbonate matoporoid reefs and mud mounds are locally a shift from carbonate- to siliciclastic-dominated megabreccia that forms the regional middle present within the upper member throughout deposition. member of the Guilmette Formation (Fig. 2) (Pinto and Warme, 2008). It unconformably overlies the ledge-forming interval of the lower member (Warme et al., 1991). The base of the LFI Alamo Breccia Member (D unit; Warme and Sandberg, 1995) is marked by a monomict, fl u- A YSFI idized detachment breccia typically <3 m thick. Above this are tilted megaclasts (to 80 m thick) Detached LFI of pre-impact carbonate platform rocks (C unit; Detachment Surface Warme and Sandberg, 1995) from the lower LFI member of the Guilmette Formation; these are B YSFI overlain by 1–30-m-thick surge and resurge breccia deposits (B and A units; Warme and AB Sandberg, 1995). These deposits grade normally upsection into sand- and mud-sized particles, LFI marking the top of strata deposited during the C YSFI Alamo impact. The top of the Alamo Breccia West East Member is here interpreted as an isochronous datum. Excavation of the lower member of the Figure 3. Conceptual diagram showing a net gain or loss of thickness due to differential Guilmette Formation and emplacement of the deposition of the Alamo Breccia Member. (A) Pre-impact deposition of the lower member Alamo Breccia Member within the impact cra- of the Guilmette Formation. (B) Syn-impact detachment surface within the LFI (ledge- ter may have resulted in a net loss or gain of forming interval; lower member, Guilmette Formation) unit. (C) Differential deposition sediment thickness, affecting regional accom- of impact breccia atop this detachment surface. AB—Alamo Breccia Member (Guilmette modation (Fig. 3). Formation); YSFI—yellow slope-forming interval (lower member, Guilmette Formation).

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Post-Impact Tectonic History 38°40′N Pancake Range Lineament The deformation history of the Alamo crater during Paleozoic time is unknown. No faulting

with appreciable displacement is associated 115°W with the Antler orogeny (Late Devonian to Mis- SB sissippian). However, during Permian through Cretaceous time, the central Nevada thrust belt developed in Lincoln County within the hinter- Silver King Lineament land of the Sevier orogenic belt (Misch, 1960). This produced parallel sets of north-striking R ! thrust faults as part of the Garden Valley thrust system (Tschanz and Pampeyan, 1970; Bartley and Gleason, 1990; Armstrong, 1991; Taylor Figure 4. Map of structural fea- et al., 2000). To the east, the Mount Irish thrust F ! includes three correlated faults (Golden Gate– tures that signifi cantly affected ! G ! Mount Irish–East Pahranagat), and to the west, the location of Devonian out- !! the Rimrock, Freiberg, and Lincoln thrusts crops examined in this study. E— !!!! !!! Timpahute S ! ! ! ! ! East Pahranagat fault; F—Frei- ! ! !! Transverse Zone are correlated southward out of the fi eld area. ! ! I !! berg thrust; G—Golden Gate ! ! All localities in this study, except Tempiute L ! ! thrust; I—Mount Irish thrust; M ! Mountain, are present within the hanging wall ! or footwall of the Mount Irish thrust (Fig. 4). L—Lincoln thrust; M—Monte !!! E Mountain thrust; P—Pahrana- !! East-directed compression on the Mount Irish !! !! Alamo thrust is likely <10 km, with an overall increase gat shear zone (modified from ! in magnitude southward to the East Pahranagat Sheffield, 2011); R—Rimrock; fault (Taylor et al., 2000). S—Schofield Pass fault; SB— P The Tempiute Mountain section is located on Seaman breakaway. the west side of the Monte Mountain, Lincoln, ! and Schofi eld Pass thrusts, suggesting that it may have moved signifi cantly eastward by as 01020305Km much as 24–30 km (Taylor et al., 1994). The N Pahranagat shear zone accounts for 9–16 km of sinistral slip (Tschanz and Pampeyan, 1970; Lincoln Co. Liggett and Ehrenspeck, 1974; Ekren et al., 1977). Other faulting events in the region are Symbols Mt. Irish Hanging Wall of a much smaller scale, including normal and Mt. Irish Footwall Strike-slip fault oblique faulting (<1 km slip) associated with the ! Transect Locality Thrust fault passage of Tertiary volcanic centers (e.g., Tay- ! Normal fault Other Locality lor and Switzer, 2001); the east-dipping Seaman Town breakaway fault (2 km slip; Taylor and Bartley, 1992); and high-angle normal faulting of Basin and Range extension. Greater magnitudes of Basin and Range extension can be found to the north and south of the Timpahute transverse 2004; Poag et al., 2004; Horton et al., 2006). It the outer rim fault and the peak ring (Poag et al., zone, but these differences can be accounted is assumed here that the Alamo crater has simi- 2004). This portion of the crater forms during for by correlating the thrust segments across the lar complex crater morphology. Terminology the modifi cation stage, as the platform collapses fi eld area (Taylor et al., 2000). used by Poag et al. (2004) to describe features toward the transient crater (Melosh, 1989; Turtle of the Chesapeake Bay complex crater is used et al., 2005). Often the annular trough makes up Complex Crater Models here to describe the Alamo crater. This model >50% of the overall fi nal diameter of marine defi nes two large-scale features: the transient impact craters (Dypvik and Kalleson, 2010). Marine impacts, unlike their subaerial crater formed directly from the impactor, and The boundary between the apparent crater and counter parts, involve a water column of vary- the apparent crater formed from the inward col- transient crater is defi ned by the peak ring and ing depth, the presence of saturated unlithifi ed lapse of the carbonate platform (Fig. 5). is typically truncated in marine impact craters sediments, and often the presence of basement The apparent crater contains the outer rim (Dypvik and Jansa, 2003; Turtle et al., 2005). rocks that contribute to a complex morphology fault and the annular trough from its outermost Inward of this boundary is the inner basin, the (Dypvik and Kalleson, 2010). Detailed analyses extent inward. The outer rim fault denotes the deepest excavated portion of a complex crater of other marine impact sites (e.g., Chesapeake outermost margin of the apparent crater, signi- (Turtle et al., 2005). The center of a complex Bay, Mjølnir, Siljan) suggest a specifi c com- fying the transition from inside to outside the crater is known as the central uplift and is occu- plex crater morphology (Gudlaugsson, 1993; impact crater (Poag et al., 2004). The annular pied by uplifted, deep-seated strata deformed Kenkmann and Dalwigk, 2000; Dypvik et al., trough is a broad portion of the crater between during rebound (Turtle et al., 2005).

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Apparent Crater (Da) These depositional environments and their stacking patterns are used to infer changes in Transient Crater (Dt) accommodation along the platform, and to rec- ognize sequence stratigraphic surfaces, systems tracts, and depositional sequences. Outer RimAnnular Two facies comprise the peritidal environ- Trough Peak RingInner Central Uplift Basin ment. Laminated dolomitic mudstone (Fig. 7A) is interpreted to represent a wave-agitated, Faults hypersaline depositional environment with occasional subaerial exposure, such as a tidal- fl at setting, given its suite of sedimentary struc- Figure 5. Idealized complex crater model and its various parts. tures. Fenestral lime or dolomitic mudstone Da—apparent crater diameter; Dt—transient crater diameter. Ter- (Fig. 7B) is interpreted as peritidal due to the minology is from Poag et al. (2004). Schematic, approximate vertical presence of planar laminae, chert nodules, and exaggeration = 12:1. fenestrae (cf. Mazzullo and Birdwell, 1989). Two facies compose the shoreface environ- METHODS trending paleoshoreline (Morrow and Sandberg, ment. Quartzarenite (Fig. 7C) is interpreted to 2008). Additional nearby sections (n = 73; Table represent an upper shoreface, marginal-marine Four detailed sections were measured from A1 in the Supplemental File [see footnote 1]) setting, indicated by the presence of planar lami- the top of the Alamo Breccia Member to the top were also considered (Anderson, 2008; Thoma- nations, low-angle cross-bedding, Taenidium of the upper member of the Guilmette Forma- son, 2010; Myers, 2011; Retzler, 2013). burrows, and stratigraphic relationship between tion (Figs. A1–A6 in the Supplemental File1). We used a regional structural data set to recon- peritidal and subtidal facies. Previous studies in In addition, nine previously measured sec- struct modern outcrops to their relative Devonian the Guilmette Formation interpreted this facies tions were reinterpreted and included in this positions (e.g., Tschanz and Pampeyan, 1970; as either peritidal or subtidal (Estes-Jackson, study (Figs. A7–A17 in the Supplemental File Bartley et al., 1988; Jayko, 1990; Ackman, 1991; 1996; LaMaskin and Elrick, 1997); however, a [see footnote 1]) (Anderson, 2008; Thomason, Taylor et al., 1994, 2000; Switzer, 1996; Cham- shoreface environment is not recognized within 2010; Myers, 2011). Each measured section berlain, 1999; Bidgoli, 2005; Bidgoli and Taylor, those depositional models. Estes-Jackson (1996) log includes the lithology, grain size, matrix, 2008) using a Python-based geographic infor- described six distinct types of quartzarenite in Dunham rock type, sedimentary structures, and mation system script developed by Sheffi eld the Pahranagat Range, distinguished by differ- fossil content. We interpreted depositional envi- (2011). This script generates new x-y values for ent sedimentary structures and the presence or ronments from facies and facies associations. localities within fault-bound polygons according absence of burrows. All quartzarenite deposits Notable sedimentary structures (i.e., desiccation to direction and magnitude of fault offset (Fig. 6) examined in this study fi t within a shoreface cracks, rip-up clasts) and facies stacking patterns (Tschanz and Pampeyan, 1970; Liggett and environment, eliminating the need for numer- were then used to interpret cyclicity. Deposi- Ehrenspeck, 1974; Ekren et al., 1976; Taylor ous, more specifi c facies types. Siltstone and/or tional sequences and system tracts in this study and Bartley, 1992; Taylor et al., 1994, 2000). silty mudstone (Fig. 7D) is interpreted as a lower are identifi ed from facies and cycle stacking Recognition of the outer rim fault is impor- shoreface, marginal-marine setting based on the patterns, as well as the correlation of mappable tant for making reasonable size estimates of presence of Taenidium and Teichichnus burrows, sequence stratigraphic surfaces. Sections were crater diameter (Turtle et al., 2005). We indi- low-angle cross-bedding, planar laminae, and correlated along transects using the top of the rectly identifi ed the outer rim fault by facies stratigraphic relationship to the quartzarenite Alamo Breccia Member as a datum, and tied to and thickness patterns of syn-impact and post- facies. LaMaskin and Elrick (1997) described sequence stratigraphic terminology established impact sediments, providing the means to esti- this facies as a tidal-fl at setting; a shoreface for the Guilmette Formation and underlying Fox mate transient and apparent crater diameter. environment was not included within their depo- Mountain Formation of nearby Egan and Schell Direct evidence of syn-impact faulting within sitional model and may explain the discrepancy Creek Ranges (depositional sequences 1–11 of the Alamo crater was diffi cult to discern because between depositional interpretations. LaMaskin and Elrick, 1997; Devonian global of the patchy distribution of localities and the Two facies represent the channel environ- transgressive-regressive cycles IIa–IIe of John- overprint of post-impact tectonic events. Several ment. Lithic-dominated breccia or conglom- son et al., 1996). The Alamo impact occurred factors (i.e., impactor angle, velocity, target rock erate (Fig. 7E) is interpreted as a high-energy during the highstand systems tract (HST) of rheology) could have affected the fi nal diame- marine bypass channel, supported by a channel depo si tional sequence 3 (HST3), based on an ter and symmetry of the Alamo crater, but are morphology, imbricated clasts, and erosional analysis of pre-impact, syn-impact, and post- unknown and are not considered here. base. Bioclastic-dominated breccia or conglom- impact sedimentation across the impacted region erate (Fig. 7F) is often found directly atop the using Fischer plots and vertical facies stacking RESULTS lithic-dominated breccia or conglomerate and is patterns (Rendall, 2013). Transects are roughly also interpreted as a high-energy marine bypass parallel and perpendicular (Fig. 1) to the north- Depositional Environments channel. A channel morphology is apparent at two stratigraphic sections that are off the tran- 1Supplemental File. Detailed measured sections We grouped 18 facies into 7 depositional sect lines (HE1 and HE1.5; Table A1 in the (Figures A1–A17) and compiled localities with GPS environments ranging from peritidal to deep Supplemental File [see footnote 1]). coordinates (Table A1). If you are viewing the PDF of this paper or reading it offl ine, please visit http:// dx subtidal, based on lithology, sedimentary struc- Three facies compose the semirestricted to .doi .org /10 .1130 /GES00964 .S1 or the full-text article tures, paleontology, fi eld associations, and pre- restricted shallow-subtidal environment. Bar- on www .gsapubs .org to view the Supplemental File. vious interpretations of similar facies (Table 1). ren dolomitic mudstone (Fig. 7G) was likely

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the presence of open-marine biota, and the AB38°40′N interpretations of previous regional studies (see Table 1). Burrowed fossiliferous mudstone to wackestone (Fig. 7M) is sometimes associated with iron-oxidized fi rmground surfaces and was 115°W likely deposited within an open, subtidal set- ting due to its abundant bioturbation and open marine biota. Siltstone and/or silty mudstone 2km† with interbedded mudstone (Fig. 7N) is char- ! ! acterized by alternating beds of yellow to gray siltstone or silty mudstone and dark gray mud- stone. Its diverse biota and trace fossils place it 1km‡! !! within an open, subtidal setting. Other regional ! !! 0–10 km,*! ! studies describe a similarly named facies that 17.8° !! !! differs from our environmental interpretation !!! !!! !!! !!(! ! ! ! ! ! !( ! ! ! ! ! !! !! !! !! (Elrick, 1986; Chamberlain and Warme, 1996); ! ! !! ! ! ! (!! ! ! ! ! !( ! ! ! ! however, their facies are devoid of fossils and 30 km* ! !( ! !( ! sometimes include turbidites, unlike the facies !! ! !! !! ! !! in this study. The lower contact of the skeletal !! ! !! !! !! !! ! !! ! packstone to grainstone (Fig. 7O) facies is often ! an irregular eroded surface infi lling preexisting 10–20 km,* burrow structures. This facies may have been 17.8° 9–16 km* deposited during storm events, given its graded ! ! laminae and rare low-angle cross-bedding. ! Three facies compose the deep-subtidal envi- 01020305Km N ronment (Table 1; Figs. 7P, 7R). Tentaculitid silty wackestone (Fig. 7P) is recorded only at one stratigraphic section (MMN4). The tentaculitids Symbols Faults are scattered in no preferred orientation, sug- 3km* Block Movement Direction, gesting that they were pelagic and settled out of ! Present-day Locality Amount (km), and Reference (*) the water column below storm wave base. Silty- ! Reconstructed Locality No Movement sandy barren mudstone (Fig. 7Q) is interpreted ! Reconstructed Transect Locality Cenozoic Movement as deep subtidal, given its normal grading, lack Late Paleozoic - Mesozoic Movement of biota, and stratigraphic relationship with other deep-subtidal facies. Mudstone with interbedded Figure 6. Map view of the palinspastic reconstruction. (A) Summary of kinematics and tim- siltstone (Fig. 7R) is characterized by thin- to ing of signifi cant fault block movement throughout the region (reference symbols: aster- medium-bedded, light to dark gray lime mud- isks—Taylor et al., 2000; dagger—Taylor and Bartley, 1992; double dagger—Chamberlain, stone with very thin interbedded siltstone. The 1999). (B) Position of localities before and after palinspastic reconstruction (modifi ed from siltstone was likely shed from the shallow plat- Sheffi eld, 2011). form during episodic storm events or changes in fl uvial input, and deposited as thin layers atop the lime mudstone (cf. Elrick et al., 1991). deposited in a saline or evaporitic tidal-fl at bioherm facies, possibly within a back-reef set- setting. This interpretation is based on its lack ting (cf. Machel and Hunter, 1994; Ballantine Cycles of biota, light color, dolomitization, and asso- et al., 2000). Stromatoporoid framestone (Fig. ciation with other peritidal facies. Skeletal 7K) is the most common facies within the bio- Both peritidal and subtidal cycles are repre- mudstone to packstone (Fig. 7H) is interpreted herm environment and is likely representative of sented within the two transects. Some succes- to represent a semirestricted shallow-subtidal a reef-core setting (cf. Hladil, 1986; Machel and sions include massive siliciclastic or carbonate setting with increased salinities, given its biota Hunter, 1994). Similar Guilmette Formation deposits, and appear to be noncyclic. These and previous regional studies (see Table 1). facies were interpreted as shallow to intermedi- cases only represent 5% of all successions in Amphipora and stromatoporoid mudstone (Fig. ate subtidal deposition (LaMaskin and Elrick, this study. Several covered intervals are present 7I) was deposited within a restricted shallow- 1997; Rendall, 2013); however, a bioherm at sections MMN4 and MMS2, assumed here as subtidal setting, delineated by its preserved environment was not distinguished within their deeper, more recessively weathered units based laminae, hypersaline biota, and association with depositional models and may account for this on the occurrence of barren mudstone fl oat. other restricted facies. difference. Two types of peritidal cycles are recognized. Two facies defi ne the bioherm environment. Four facies compose the open, shallow to The fi rst type is defi ned by semirestricted to Rhodolith grainstone (Fig. 7J) is only found intermediate subtidal environment. Stromato- restricted shallow-subtidal or shallow to inter- at the Mount Irish reef section (MI1) and was poroid boundstone (Fig. 7L) represents an mediate subtidal facies at its base, which shal- likely deposited as part of the reef complex open, shallow-subtidal environment based on low upward into peritidal or shoreface facies. given its stratigraphic relationship with other the tabular morphology of the stromatoporoids, The second type is composed solely of peritidal

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TABLE 1. COMPILED FACIES BY DEPOSITIONAL ENVIRONMENT PERITIDAL LAMINATED DOLOMITIC MUDSTONE FENESTRAL LIME OR DOLOMITIC MUDSTONE Lithology and Bedding: Gray to tan color; recessive, laminated to thinly bedded, Lithology and Bedding: Dark brown-gray to light gray color; limestone/dolomitic silty dolomitic mudstone; thin to medium bedding mudstone with abundant fenestrae; thin to medium bedding Sedimentary Structures: Planar laminations, desiccation cracks, rip-up clasts, Sedimentary Structures: Fine laminations, burrow mottling, and chert nodules and chert nodules Biota: Algal fenestrae Biota: Rare Amphipora Comparable Interpretation: Laporte, 1967; Dorobek and Read, 1986; Collins and Comparable Interpretation: Laporte, 1967; Read, 1985; Holland, 1993; Chen Lake, 1989; Giles et al., 1999; Chen et al., 2001 et al., 2001 Guilmette Formation Reference: Larsen et al., 1988 Guilmette Formation Reference: Elrick, 1986; Larsen et al., 1988; Chamberlain and Warme, 1996; Estes-Jackson, 1996; LaMaskin and Elrick, 1997; Rendall, 2013 SHOREFACE QUARTZARENITE SILTSTONE AND/OR SILTY MUDSTONE Lithology and Bedding: Tan, orange-brown, or gray color; quartzarenite Lithology and Bedding: Tan or brown color; siltstone with dolomitic cement or sandstone; medium sand; calcite or dolomite cement; medium to massive silty dolomudstone; thin bedded bedding Sedimentary Structures: Massive, low-angle cross-bedding, planar laminations Sedimentary Structures: Planar laminations and occasional low-angle cross- Biota: Occasional Taenidium and Teichichnus burrows bedding Comparable Interpretation: N/A Biota: Rare Taenidium burrows Guilmette Formation Reference: LaMaskin and Elrick, 1997 Comparable Interpretation: Giles et al., 1999; Colquhoun, 1995 Guilmette Formation Reference: Elrick, 1986; Estes-Jackson, 1996; LaMaskin and Elrick, 1997 CHANNEL LITHIC-DOMINATED BRECCIA OR CONGLOMERATE BIOCLASTIC-DOMINATED BRECCIA OR CONGLOMERATE Lithology and Bedding: Gray to yellow color; sandstone and mudstone lithoclasts Lithology and Bedding: Gray color; coralline bioclasts with minor lithoclasts of in a poorly sorted fi ne to coarse, well-rounded quartz arenite; medium to thick limestone and sandstone; medium to thick ledges ledges Sedimentary Structures: Erosional base, imbricated clasts Sedimentary Structures: Erosional base, imbricated clasts Biota: Fragmented alveolitids, stromatoporoids, rugosans, Thamnopora, Biota: Fragmented alveolitids, rugosans, brachiopods, crinoids and fi sh bone brachiopods, crinoids, bryozoans and fi sh bone Comparable Interpretation: N/A Comparable Interpretation: N/A Guilmette Formation Reference: N/A Guilmette Formation Reference: N/A SEMIRESTRICTED OR RESTRICTED SHALLOW SUBTIDAL BARREN DOLOMITIC MUDSTONE AMPHIPORA AND STROMATOPOROID MUDSTONE Lithology and Bedding: Light gray color; dolomitic mudstone; coarse crystalline Lithology and Bedding: Light to dark gray color; platy beds of lime mudstone; texture; thin to medium bedding thin bedding Sedimentary Structures: None Sedimentary Structures: Planar laminations Biota: None Biota: Rare Amphipora and stromatoporoids Comparable Interpretation: Chen et al., 2001 Comparable Interpretation: Collins and Lake, 1989; Chen et al., 2001 Guilmette Formation Reference: Rendall, 2013 Guilmette Formation Reference: Rendall, 2013

SKELETAL MUDSTONE TO PACKSTONE Lithology and Bedding: Light to dark gray color; lime or dolomitic mudstones, wackestones, and packstones Sedimentary Structures: Burrow mottling Biota: Abundant Amphipora, bryozoans, and gastropods; rare brachiopods and stromatoporoids Comparable Interpretation: Chen et al., 2001; Chow et al., 2013 Guilmette Formation Reference: Elrick, 1986; Chamberlain and Warme, 1996; LaMaskin and Elrick, 1997; Rendall, 2013 (continued)

facies and is recognized via changes in sedimen- surfaces that are recognized by an oxidized bed- ure 8. The north-south transect (A-A′) and east- tary structures and/or the presence or absence ding plane with pre-omission Thalassinoides west transect (B′-B) are each composed of 7 of biota. The base of this cycle type is often and Paleophycos burrows. Cycle tops are char- sections over a reconstructed distance of 55 km a fenestral lime or dolomitic mudstone that acterized by semirestricted to restricted shallow- and 37 km, respectively (Figs. 9 and 10). is capped by laminated dolomitic mudstone. subtidal, bioherm, or shallow to intermediate Cycle tops and bottoms for both types are often subtidal facies. Deepening-upward subtidal Depositional Sequence 3 marked by desiccation cracks or transgressive cycles are less common throughout the tran- Along the north-south transect, HST3 is lags, respectively. sect lines. They generally correspond to fi ning- defi ned by a series of ~1-m-thick shallowing- Subtidal cycles are devoid of peritidal and upward trends into deep-subtidal facies that are upward cycles (Fig. 9). In the northernmost shoreface facies. Unlike the peritidal cycles, overlain by a coarser, shallow to intermediate section (GGS3), these cycles are capped by transgressive lags and exposure surfaces are subtidal facies. peritidal or semirestricted to restricted shal- uncommon. Subtidal cyclicity is largely repre- low-subtidal facies. HST3 is thicker at GGS3 sented by shallowing-upward successions that Sequence Stratigraphy relative to the southern sections, possibly coincide with coarsening-upward trends. Cycle due to an increase in accommodation space bases are often marked by shallow to interme- A compilation of sequence stratigraphic generated during and after the impact. To the diate subtidal and deep-subtidal facies. In some abbreviations, symbols, and facies patterns used south (MMN4 to HE6), HST3 is thinner and cases, cycle bases are denoted via fi rmground to describe the two transects is displayed in Fig- shallowing-upward cycles are capped by deep-

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TABLE 1. COMPILED FACIES BY DEPOSITIONAL ENVIRONMENT (continued) BIOHERM RHODOLITH GRAINSTONE STROMATOPOROID FRAMESTONE Lithology and Bedding: Light gray color; lime bioclastic grainstone; ledge- Lithology and Bedding: Light gray color; lime or dolomitic framestone; ledge- forming; thin bedding forming; massive bedding Sedimentary Structures: None Sedimentary Structures: Bioturbation Biota: Rhodoliths with minor gastropods Biota: Stromatoporoids, brachiopods, rugosans, tabulate corals, and crinoid debris Comparable Interpretation: Machel and Hunter, 1994; Ballantine et al., 2000 Comparable Interpretation: Laporte, 1967; Hladil, 1986; Machel and Hunter, Guilmette Formation Reference: N/A 1994; Chow et al., 2013 Guilmette Formation Reference: LaMaskin and Elrick, 1997; Rendall, 2013 OPEN SHALLOW TO INTERMEDIATE SUBTIDAL STROMATOPOROID BOUNDSTONE SILTSTONE AND/OR SILTY MUDSTONE WITH INTERBEDDED MUDSTONE Lithology and Bedding: Light to dark gray color; boundstone with lime or Lithology and Bedding: Yellow to light/dark gray color; alternating beds of dolomite mud matrix; ledge-forming; medium to thick bedding siltstone/silty mudstone and lime mudstone; thin to medium bedding Sedimentary Structures: Bioturbation Sedimentary Structures: Lime mudstone can appear in nodular or tabular lenses; Biota: Dominated by tabular and lenticular stromatoporoids; less common occasional planar laminations Amphipora, gastropods, brachiopods, and solitary rugosans Biota: Fragmented brachiopods, crinoids, alveolitids, gastropods, rugosans, Comparable Interpretation: Laporte, 1967; Chen et al., 2001; Chow et al., 2013 bryozoans, and fi sh bones; Zoophycos, Teichichnus, and Thalassinoides Guilmette Formation Reference: Larsen et al., 1988; Chamberlain and Warme, burrows 1996; LaMaskin and Elrick, 1997; Rendall, 2013 Comparable Interpretation: N/A Guilmette Formation Reference: Elrick, 1986; Chamberlain and Warme, 1996

BURROWED FOSSILIFEROUS MUDSTONE TO WACKESTONE SKELETAL PACKSTONE TO GRAINSTONE Lithology and Bedding: Light to dark gray color; lime or dolomitic mudstone and Lithology and Bedding: Gray to bluish-gray color; bioclastic packstone or wackestone; thin to medium bedding grainstone; ledge-forming; thin to medium bedding Sedimenatry Structures: Iron-oxidized fi rmground surfaces at top, planar Sedimentary Structures: Often an erosional base, graded laminae, and rare low- laminations, and bioturbation angle cross-bedding Biota: Teichichnus and Thalassinoides burrows, brachiopods, and rugosans Biota: Fragmented crinoid ossicles Comparable Interpretation: Giles et al., 1999; Colquhoun, 1995 Comparable Interpretation: Giles et al., 1999; Holland, 1993 Guilmette Formation Reference: Elrick, 1986; Larsen et al., 1988; Chamberlain Guilmette Formation Reference: Elrick, 1986; LaMaskin and Elrick, 1997 and Warme, 1996; LaMaskin and Elrick, 1997; Rendall, 2013 DEEP SUBTIDAL TENTACULITID SILTY WACKESTONE MUDSTONE WITH INTERBEDDED SILTSTONE Lithology and Bedding: Brown to gray color; platy, silty wackestone; recessive, Lithology and Bedding: Light to dark gray color; lime mudstone with very thin thin bedding interbeds of siltstone; siltstone weathers recessively; ledge-forming; thin to Sedimentary Structures: None medium bedding Biota: Tentaculitids Sedimentary Structures: Rare planar laminations Comparable Interpretation: Chen et al., 2001 Biota: Lime mudstone contains Thalassinoides burrows and fragments of crinoids, Guilmette Formation Reference: Larsen et al., 1988; LaMaskin and Elrick, 1997; brachiopods, gastropods, and tentaculitids; siltstone contains fragments of Rendall, 2013 crinoids and brachiopods Comparable Interpretation: Markello and Read, 1981 Guilmette Formation Reference: Elrick et al., 1991; Chamberlain and Warme, 1996; LaMaskin and Elrick, 1997

SILTY-SANDY BARREN MUDSTONE Lithology and Bedding: Light to dark gray color; silty or sandy mudstone; ledge- forming; thin to medium bedding Sedimentary Structures: Occasionally graded; rare low-angle cross-bedding Biota: None Comparable Interpretation: Chow et al., 2013 Guilmette Formation Reference: Chamberlain and Warme, 1996; Rendall, 2013 Note: N/A indicates not applicable.

subtidal and open, shallow to intermediate sub- cycles (Fig. 10). Cycles at the easternmost tive conformity surface (SB-CC-3) inferred from tidal facies. Many of the cycle tops were inter- section (SMFN2) are exclusively peritidal, facies and cycles that signify low accommoda- preted from fi rmground surfaces that represent whereas cycles in the central part of the tran- tion space across the platform (Figs. 9 and 10). periods of slow sedimentation, and may indi- sect (HCE1 to DMP1) are both peritidal and cate a more restricted environment; however, subtidal. Farther west (MIN2 and MI1), depos- Depositional Sequence 4 at PTN0 in the central part of the transect, a its consist of one shallowing-upward subtidal Along the north-south transect, deepening- bed of reworked Alamo Breccia Member con- cycle capped by bioherm facies, signifying reef upward subtidal cycles, massive noncyclic taining brachiopod and crinoid fragments is growth along the platform. The westernmost deposits, and shallowing-upward subtidal cycles present above pristine Alamo Breccia Member section (MMN4) is composed of two shal- are interpreted to represent a platform-wide deposits. This unit was likely reworked during lowing-upward subtidal cycles with one deep- increase in accommodation space, signaling a storm event and is interpreted to represent an ening-upward subtidal cycle between them. transgressive systems tract (TST) TST4 (Fig. 9). open, shallow to intermediate subtidal envi- Shallowing-upward subtidal cycles are capped In the northernmost section (GGS3), TST4 is ronment. As a whole, HST3 is dominated by by open shallow-intermediate or deep-subtidal composed of an ~30-m-thick, noncyclic shal- shallow-subtidal facies in the north and south, facies. Overall, HST3 is dominated by peritidal low-subtidal deposit. To the south (MMN4 and while central portions are dominated by deep- and shallow-subtidal facies in the east and by MMS2), deepening-upward subtidal cycles subtidal facies (Fig. 11A). deep-subtidal facies in the west (Fig. 11A). grade into covered intervals believed to repre- Across the east-west transect, HST3 is char- The end of HST3 along both transects is inter- sent recessively weathered deep-subtidal facies. acterized by a series of shallowing-upward preted as a Type 2 sequence boundary–correla- TST4 deposits at PNT0 are mostly covered;

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A B

C D

E F

Figure 7 (on this and following two pages). Photograph of each facies described in this study. (A) Peritidal: laminated dolomitic mudstone from HHN1. (B) Peritidal: fenestral dolomitic mudstone from SMFN3.5. (C) Shoreface: quartzarenite with Taenidium burrows from HCC3. (D) Shoreface: siltstone and/or silty mudstone from HEF1. (E) Channel: lithic-domi- nated breccia to conglomerate from HEF1.5. (F) Channel: bioclastic-dominated breccia or conglomerate from HE1.5.

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G H

1 mm I J

1 mm K L

1 mm

Figure 7 (continued). (G) Semirestricted to restricted shallow subtidal: barren dolomitic mudstone from SMFN2. (H) Semirestricted to restricted shallow subtidal: photomicrograph of skeletal mudstone to packstone from HHS3. (I) Semirestricted to restricted shallow subtidal: photomicrograph of Amphipora and stromatoporoid mudstone from HHS3 showing Amphipora, peloids, calcispheres, and a crinoid fragment. (J) Bioherm: hand sample of rhodolith grain- stone from MI1. (K) Bioherm: photo micro graph of stromatoporoid framestone matrix from MI1 showing a brachiopod spine in cross section. (L) Open, shallow to intermediate subtidal: stromatoporoid boundstone from HHS2.

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M N

O P

2 mm Q R

Figure 7 (continued). (M) Open, shallow to intermediate subtidal: burrowed fossiliferous mudstone to wackestone from DDB1. (N) Open, shallow to intermediate subtidal: siltstone and/or silty mudstone with interbedded mudstone from HE1 showing nodular mud infi lling burrows. (O) Open, shallow to intermediate subtidal: skeletal packstone to grainstone from HE2 showing an erosional and burrowed base. (P) Deep subtidal: tentaculitid silty wackestone from MMN2. (Q) Deep subtidal: silty-sandy barren mudstone from MMS2. (R) Deep subtidal: mudstone with interbedded siltstone from HN4. DDB—Hancock Summit down-dropped block; HCC—Hiko Hills central; HE—Hancock east; HEF—Hancock east footwall; HHN—Hiko Hills north; HHS—Hiko Hills south; MI—Mount Irish; MMN—Monte Mountain north; MMS—Monte Mountain south; SMFN—Six Mile Flat north.

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Depositional Environments: Lithologies: Cycles: Surfaces: subtidal facies, while MMS2 is composed of Sequence Boundary/ shallowing-upward subtidal cycles interpreted Peritidal Limestone Deepening Correlative Conformity upward from recessively weathered deep-subtidal cov- Maximum Flooding Surface Shoreface Silty limestone ered intervals. At PTN0, HST4 deposits are Shallowing Transgressive Surface characterized by shallowing-upward subtidal upward Channel Dolomite Datum cycles capped by open, shallow to intermedi- (Top of Alamo Breccia Member) Semirestricted to ate subtidal facies. Cycle tops at PTN0 are restricted Silty dolomite Noncyclic marked by fi rmground surfaces. HST4 at HN5 shallow-subtidal and DDB1 is composed of shallowing-upward Dolomitic Bioherm sandstone cycles capped by open, shallow to intermediate Abbreviations: subtidal facies. The southernmost section (HE6) Open, shallow to Sandstone M = mudstone LST = lowstand systems tract intermediate-subtidal (quartzarenite) records an ~8-m-thick subtidal cycle capped by W = wackestone TST = transgressive systems tract open, shallow to intermediate subtidal facies, Deep Siltstone P = packstone HST = highstand systems tract subtidal followed by an ~30-m-thick peritidal cycle. G = grainstone Alamo Breccia Overall, HST4 records shallow-subtidal deposi- Breccia Br = breccia Member tion (both restricted and open) in the north and south, and intermediate- to deep-subtidal depo- Figure 8. Compilation of sequence stratigraphic abbreviations, symbols, and patterns used sition in central regions (Fig. 11C). to describe the two transects in Figures 9 and 10. Along the east-west transect, HST4 is marked by the reoccurrence of shallowing-upward peri- tidal and subtidal cyclicity (Fig. 10). The east- however, sparse deep-subtidal outcrops are facies. At MI1, TST4 deposits are mainly rep- ernmost sections (SMFN2 to HHN1) are char- interpreted to mark the maximum fl ooding resented by an ~25-m-thick noncyclic deposit acterized by shallowing-upward cycles capped surface (MFS-4). By defi nition, the covered and overlying ~5-m-thick deposit, both of the by peritidal and semirestricted to restricted unit between SB-CC-3 and MFS-4 belongs bioherm facies. MMN4 contains several deep- shallow-subtidal facies. DMP1 is composed of within TST4. Farther south (HN5 and DDB1), ening-upward subtidal cycles capped by reces- shallowing-upward subtidal cycles dominated deposition of TST4 is indicated by ~8-m-thick sively weathered deep-subtidal facies. TST4 by shoreface and semirestricted to restricted deepening-upward subtidal cycles composed largely records subtidal deposition, with an shallow-subtidal facies. At MIN2, HST4 is of open, shallow to intermediate subtidal intermediate- to deep-subtidal setting present at composed of one open, shallow to interme- and deep-subtidal facies. At the southern- MMN4 (Fig. 11B). diate subtidal facies. The upper boundary of most section (HE6), TST4 is represented by The end of TST4 is marked by MFS-4, HST4 is not well constrained here due to the an ~10-m-thick subtidal cycle composed of interpreted through three observations: (1) the lack of identifi able cycles within this deposit. deep-subtidal and open, shallow to intermedi- end of noncyclic or deepening-upward domi- At MI1, a single bioherm deposit that is above ate subtidal facies. TST4 deposits thin toward nated cycle deposition, (2) the top of the last the transgressive rhodolith deposit is interpreted the center of the north-south transect (MMS2 shallowing-upward subtidal cycle prior to peri- to represent HST4. At MMN4, HST4 consists and PTN0) and thicken away from the center. tidal cyclicity, and (3) the deepest-water facies of shallowing-upward subtidal cycles capped by The only exception to this pattern is the TST4 recorded at a locality (Figs. 9 and 10). An excep- open, shallow to intermediate subtidal facies. deposit at the southernmost section (HE6), tion to this is found along the east-west transect HST4 predominantly records deeper conditions which is noticeably thinner. This may represent at MI1, where MFS-4 has been placed at the in the west, and shallower conditions in the east sediment starvation of the offshore subtidal set- top of a rhodolith grainstone deposit (Fig. 10). (Fig. 11C). ting as an increase in accommodation genera- This deposit is the only recognizable change SB-CC-4 is inferred to occur at a shift from tion outpaced sedimentation. in lithology within an otherwise noncyclic bio- carbonate- to siliciclastic-dominated deposi- TST4 within the east-west transect is defi ned herm interval. Rhodolith-bearing limestones tion (Figs. 9 and 10). This surface is interpreted by a shift into deepening-upward cycles, have been interpreted as transgressive marker as a Type 1 sequence boundary, as indicated noncyclic deposits, and subtidal shallowing- beds in a study on Cenozoic deposits of New by paleokarst fi lled with terra rossa atop the upward cycles, similar to those observed in the Zealand (Nalin et al., 2008); although many of bioherm deposit at MI1 (east-west transect; north-south transect (Fig. 10). The easternmost these rhodolith beds are clast supported, contain Fig. 10). section (SMFN2) consists of an ~7-m-thick numerous other fossil fragments, and/or occur deepening-upward subtidal cycle of open, shal- directly atop ravinement surfaces, all of which Depositional Sequence 5 low to intermediate subtidal facies. At HCE1 do not apply here. Deposits above SB-CC-4 are characterized and HHN1, the majority of TST4 is represented HST4 within the north-south transect is indi- by shoreface deposition interpreted to represent by thick (>20 m), noncyclic, siliciclastic- and cated by a return to shallow-subtidal cyclic- lowstand systems tract (LST) LST5 (Figs. 9 and carbonate-rich shoreface deposits. These may ity with minor peritidal conditions, interpreted 10). The sequence stratigraphic framework of represent the formation of a barrier bar along to represent a loss in accommodation space LaMaskin and Elrick (1997) does not include the carbonate platform (Fig. 11B). Farther west (Fig. 9). At GGS3, this is recorded in two a lowstand systems tract within depositional at DMP1, TST4 is composed of an ~12-m-thick shallowing-upward subtidal cycles capped by sequence 5; however, their locations are in a deposit of open, shallow to intermediate sub- semirestricted to restricted shallow-subtidal midshelf position closer to the Late Devonian tidal and channel facies. MIN2 includes two facies. Southward, MMN4 deposits record sev- paleoshoreline (Morrow and Sandberg, 2008). shallowing-upward subtidal cycles dominated eral shallowing-upward subtidal cycles capped We suggest that LST5 deposits bypassed those by open, shallow to intermediate subtidal by shoreface and open, shallow to intermediate midshelf locations and were deposited as a low-

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Crater morphology of the Late Devonian Alamo impact, Nevada

SEQUENCE 4 SEQUENCE 5 SEQUENCE 4 SEQUENCE SEQUENCE 3 SEQUENCE MFS-4 SB/CC-4 SB/CC-3 (A′) LST5 HST4 TST4 HST3 S MWPGBr m 0 10 40 30 20 ~5.5 km MWPGBr m 0 40 30 20 10 ~4.2 km HST3 HST4 TST4 MWPGBr m 0 10 40 30 20 ) through post-impact deposits across the Alamo impact region. Alamo impact region. the post-impact deposits across ) through ′ ~7.2 km MWPGBr PTN0 HN5 DDB1 HE6 m 0 30 20 10 ~13.8 km HST4 HST3 MWPGBr MMS2 Figure 9. North-south transect (A-A Figure Cycles are included to the right of each measured section. Three depositional sequences are recognized recognized depositional sequences are Three section. included to the right of each measured Cycles are and labeled. Distance between localities is based colored systems tracts are corresponding (3–5) and their 8. DDB— shown in Figure are Symbols, patterns, and abbreviations on the palinspastic reconstruction. block; GGS—Golden Gate south; HE—Hancock east; HN—Hancock Hancock Summit down-dropped north; MMN—Monte Mountain MMS—Monte south; PTN—Pahranagat north. m 0 20 10 ~7.1 km TST4 MWPGBr m 0 40 30 10 20 ~20.9 km MWPGBr GGS3 MMN4 m 0 70 60 50 40 30 20 10 N

TST4

HST3 HST4

MFS-4 (A) SEQUENCE 3 SEQUENCE SEQUENCE 4 SEQUENCE SB/CC-4 SB/CC-3 Datum

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Retzler et al. SEQUENCE 3 SEQUENCE 4 SEQUENCE 5 SEQUENCE 4 SEQUENCE 3 SEQUENCE 5 - (B′) S Datum T SB/CC-4 MFS-4 SB/CC-3 E LST5 HST4 TST4 HST3 TST5 ? MWPGBr SMFN2 m 0 40 30 20 10 ~5.7 km MWPGBr HCE1 m 0 80 70 60 50 40 30 20 10 ~4.3 km MWPGBr HHN1 m 0 50 40 30 20 10 ? ~12.4 km HST4 HST3 TST4 LST5 MWPGBr TST5 m 0 40 30 20 10 ? ~2.8 km ? MWPGBr MIN2 DMP1 m 0 40 30 20 10 ~4.7 km MWPGBr MI1 m 0 70 60 50 40 30 20 10 ) through post-impact deposits across the Alamo impact region. Cycles are included to Cycles are Alamo impact region. the post-impact deposits across ) through ′ ~21.4 km MWPGBr ? MMN4 m 0 40 30 10 20 W 5 -

TST5 LST5 HST4 HST3 TST4 S

T MFS-4 SEQUENCE 4 SEQUENCE 5 SEQUENCE 4 SEQUENCE (B) SB/CC-3 SB/CC-4 Figure 10. West-east transect (B-B West-east 10. Figure the right of each measured section. Three depositional sequences are recognized (3–5) and their corresponding systems corresponding (3–5) and their recognized depositional sequences are Three section. the right of each measured Symbols, patterns, and labeled. Distance between localities is based on the palinspastic reconstruction. colored tracts are 8. DMP—Hiko Hills south dump; HCE—Hiko east-central; HHN—Hiko shown in Figure are and abbreviations Hills north; MI—Mount Irish; MIN—Mount Irish MMN—Monte Mountain SMFN—Six Mile Flat north.

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A Depositional Sequence 3 (late HST3) B Depositional Sequence 4 (late TST4)

SUBTIDAL PERITIDAL SUBTIDAL Deep Shallow Deep Shallow

GGS3 PA A BH P RS RS R Deepening Mail P SMFN2P Mail PB B Tempiute BO Summit HHN1 Tempiute Summit ′ MMN4 R R B′ Mountain DMP1 P Mountain Monte Monte D Mountain MIN2O HCE1 Mountain SF MMS2 MI1 Hiko O BH BH Hiko P Hills Hills R BH RS PTN0 O

Deepening O HN5 DDB1 Transect Locality O O Other Locality HE6O A′ A′ 0510 km N

C Depositional Sequence 4 (late HST4) D Depositional Sequence 5 (early LST5)

SUBTIDAL PERITIDAL SUBTIDAL PERITIDAL Deep Shallow Deep Shallow

A A

RS

Mail B Tempiute Mail B Tempiute B′ Summit Mountain B Summit Mountain ′ Monte Monte Mountain Mountain BH Hiko PK Hiko Hills Hills TURBIDITES WITH THIN SANDS SF LOWSTAND WEDGE LOWSTAND

A′ A′

Figure 11. Map view of depositional environments during all three sequences across the impacted region. Yellow (in D) indicates thin shoreface deposits within a channelized system, and orange indicates a thick sand wedge from amalgamated fan lobes. BH—bioherm setting; DDB—Hancock Summit down-dropped block; DMP—Hiko Hills south dump; GGS—Golden Gate south; HCE—Hiko Hills east-central; HE—Hancock east; HHN—Hiko Hills north; HN—Hancock north; HST—highstand systems tract; LST—lowstand systems tract; MI—Mount Irish; MIN—Mount Irish north; MMN—Monte Mountain north; MMS—Monte Mountain south; PK—paleo- karst feature; PTN—Pahranagat north; RS—Semirestricted to restricted shallow-subtidal setting; SF—shoreface setting; SMFN—Six Mile Flat north; TST—transgressive systems tract. Reconstructed locality positions.

stand wedge farther offshore (west). Subaerial ment has been documented in the upper Devo- its and channel deposits precede quartzarenite unconformities are not recognized in our tran- nian carbonate platform of western Alberta deposition at PTN0, HN5, and DDB1. At GGS3, sects between depositional sequences 4 and (Whalen et al., 2000). LST5 is only represented by an ~1-m-thick silty 5, except at MI1, suggesting that bypass was Along the north-south transect, the major- mudstone deposit. LST5 is primarily repre- submarine (cf. Rendall, 2013). Comparable ity of LST5 consists of quartzarenite shoreface sented by a lowstand wedge deposit within the submarine bypassing across a midshelf environ- deposits (Fig. 9). Finer grained shoreface depos- north-south transect (Fig. 11D).

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Within the east-west transect, LST5 is recog- to deep-subtidal facies in the west geographi- In the north-south transect, the outer rim fault nized by thin shoreface deposits in the east that cally across depositional sequences 3 and 4 position has been suggested to exist north of thicken westward into massive quartzarenite at (Fig. 10). We interpret this pattern to refl ect a the GGS3 section (Warme and Kuehner, 1998; MMN4 (Fig. 10). The eastern sections (SMFN2 concave seafl oor that is progressively shal- Pinto and Warme, 2008). At GGS3, depositional and HCE1) include several shallowing-upward lower eastward of Tempiute Mountain (Fig. 11). sequences 3 and 4 are nearly three times as thick peritidal cycles that we assign to LST5. Below This concavity is consistent with isopach and as sections toward the south. We suggest that the quartzarenite unit at DMP1 is an ~5-m-thick lithostratigraphic trends in the Alamo Breccia thicker post-impact deposits here are the result channel deposit possibly derived from the bio- Member seen throughout the study area, and of increased accommodation generated by slip herm deposits in the west, as indicated by its farther to the north and south within the Golden along the outer rim fault to the north (Fig. 13). stromatoporoid- and coral-rich bioclasts. LST5 Gate Range and Delamar Mountains (Pinto and Our interpretation of the outer rim fault near deposits are not recorded at MI1 because the Warme, 2008; Sheffi eld 2011). We interpret this the Hiko Hills and Golden Gate Range indi- bioherm was subaerially exposed during this concave pattern as the post-impact legacy of the cates that the majority of the transect sections duration, forming a Type 1 sequence bound- Alamo crater. are within the annular trough (Figs. 12 and 13). ary (SB-CC-4). Instead, deposition above this These locations also share similar Alamo Brec- boundary consists of several shallowing-upward Alamo Crater Features cia Member lithostratigraphic characteristics subtidal cycles capped by bioherm deposits, (Warme and Kuehner, 1998; Pinto and Warme, interpreted to represent TST5. Along the east-west transect, peritidal and 2008; Sheffi eld, 2011), implying they were subtidal deposits at SMFN2 transition later- formed in a similar portion of the impact crater. DISCUSSION ally into an ~30-m-thick quartzarenite unit at Variations of facies and bathymetry along HCE1, coincident with an abrupt thickness transects can be explained by faulting within Seafl oor Topography change in the Alamo Breccia Member (Warme the annular trough, such as between MIN2 and and Kuehner , 1998; Pinto and Warme, 2008; MI1 (Figs. 11 and 12). These variations corre- The north-south transect records a transition Sheffi eld, 2011). We interpret this change in spond to thickness changes in the underlying from semirestricted to restricted shallow-sub- facies and accommodation space as the surfi - Alamo Breccia Member and lower member tidal, to deep-subtidal, to open, shallow-subtidal cial expression of the outer rim fault (Fig. 12). of the Guilmette Formation (Figs. 12 and 13) facies geographically across depositional Localities east of SMFN2 record similar thin (Sheffi eld, 2011). Thickness changes in the sequences 3 and 4 (Fig. 9). In addition, the east- Alamo Breccia Member deposits (Kuehner, Alamo Breccia Member may also be the result west transect grades from peritidal in the east 1997; Pinto and Warme, 2008). of one or more of the following: (1) incised

W (B) E (B′) ANNULAR TROUGH MMN4 MI1 MIN2 DMP1 HHN1 HCE1 SMFN2

SL SL

? T

? L

U

A

F

.

? R ? . O

? ? 0100 m 0100 0 5 km

FM YSFI LFI ABHST3 TST4 HST4

Figure 12. Cross section along the west-east (B-B′) transect showing post-impact depositional sequences 3 and 4. Reconstructed locality posi- tions. Thicknesses of AB (Alamo Breccia Member, Guilmette Formation) and underlying units are from Sheffi eld (2011) and Rendall (2013). Cross section was drawn using a top-down thickness approach based on the approximate water depth represented in HST4 deposition at each locality. Approximate water depths are as follows: SMFN2, HCE1, HHN1, DMP1, and MIN2 = ~15 m, shallow ramp environment between sea level and fair-weather wave base (Tucker and Wright, 1990); MI1 = 0 m, exposure and karsted reef top representing sequence boundary–correlative conformity surface SB-CC-4; MMN4 = ~50 m, deep ramp between fair-weather wave base and storm wave base (Tucker and Wright, 1990). DMP—Hiko Hills south dump; FM—Fox Mountain Formation; HCE—Hiko Hills east-central; HHN—Hiko Hills north; HST—highstand systems tract; LFI—ledge-forming interval (lower member, Guilmette Formation; LST—lowstand systems tract; MI—Mount Irish; MIN—Mount Irish north; MMN—Monte Mountain north; O.R. fault—outer rim fault; SL—sea level; SMFN— Six Mile Flat north; TST—transgressive systems tract; YSFI—yellow slope-forming interval (lower member, Guilmette Formation).

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valleys formed within the annular trough dur- exist farther west of Tempiute Mountain and actual values (Fig. 14). Values for the Wetumpka ing resurge events (Dalwigk and Ormö, 2001; coincides with the presumed point of impact crater (Da/Dt ratios < 2.3 and Da/AT ratios > Dypvik and Jansa, 2003; Dypvik et al., 2004), near the Quinn Canyon and Reveille Ranges 3.6) were excluded because this impact was (2) differential deposition of the Alamo Breccia (Morrow et al., 2005; Pinto and Warme, 2008). unusually small and basement involved, sharing Member within the crater, and (3) post-impact little with the presumed target area of the Alamo structural modifi cation of the crater (Tsikalas Crater Size Estimates impact (Morrow et al., 2005; Dypvik and Kalle- and Faleide, 2007). son, 2010). From the plot, fi ve calculated (calc) Currently, no evidence of the peak ring or Approach 1. Diameter from pairs (Table 3) were chosen that represent the central uplift has been documented in Nevada. Scaling Relationships highest and lowest possible ratios, as well as the However, the section west of our transects at Linear scaling relationships have been previ- mean (Fig. 14). These pairs were used to esti- Tempiute Mountain (TMP in Fig. 1) is the ously documented between the apparent crater mate the size range of the Alamo crater (Da and only locality to include evidence of melted and diameter (Da) and transient crater diameter Dt) utilizing the AT width and the following two marbleized Alamo Breccia Member (Pinto and (Dt) in marine impacts (see Fig. 5) (Dypvik and equations: Warme, 2008), an indication of proximity to Jansa, 2003; Dypvik and Kalleson, 2010). This

the inner basin (Grieve et al., 1981; Kenkmann relationship, as well as the relationship between DaAlamo = (Da/AT)calc × ATAlamo. (1) et al., 2014). Pinto and Warme (2008) believed the Da and the width of the annular trough (AT),

this to represent the inner slope of the crater is examined for six well-studied marine impact DtAlamo = (Da/Dt)calc × DaAlamo. (2) rim that would correspond to the inner basin in structures (Table 2). Among these six, Da/Dt the complex crater terminology of Poag et al. values range from 1.9 to 3.0 (mean of 2.5) and The east-west transect in this study only (2004). Furthermore, the upper member of the Da/AT values range from 3.0 to 4.2 (mean of covers ~37 km (reconstructed) from the outer rim Guilmette Formation at Tempiute Mountain is 3.5). This empirical relationship can be used to fault to MMN4 within the annular trough. If the a mixture of carbonate slope deposits and grav- estimate the size of the Alamo crater (Da and boundary of the annular trough (marked by the ity-fl ow deposits (Pinto and Warme, 2008) that Dt) based solely on the width of the annular peak ring) were hypothetically located at MMN4,

may have been generated along the transition trough (AT). AT Alamo would equal ~37 km. This generates

from the annular trough, across the peak ring, Data from the six well-studied marine an apparent crater diameter (DaAlamo) between and into the inner basin. If this interpretation is impacts were used to calculate known Da/Dt 111 and 133 km and a transient crater diameter

correct, then the peak ring boundary is located and Da/AT ratios for marine impacts (Dypvik (DtAlamo) between 37 and 58 km (Table 4). Since between Tempiute Mountain (TMP) and Monte and Kalleson, 2010). These values were the boundary of the annular trough is not appar- Mountain (MMN4) and would be absent from obtained through a stepwise pairing of Da/Dt ent at MMN4, these calculations represent the our transects. The central uplift structure would ratios with each Da/AT ratio, and plotted among minimum size of the Alamo crater.

N (A) S (A′) ANNULAR TROUGH GGS3 MMN4 MMS2 PTN0 HN5 DDB1 HE6

SL SL

O ? . R . F A U ? L ? T ? 0100 m 0100 0 5 km

YSFI LFI ABHST3 TST4 HST4

Figure 13. Cross section along the north-south (A-A′) transect showing post-impact sequences 3 and 4. Reconstructed locality positions. Thicknesses of Alamo Breccia Member (AB) and underlying units are from Sheffi eld (2011) and Rendall (2013). Cross section was drawn using the typical bottom-up thickness approach. Approximate water depth representing HST4 deposition at locality GGS3 was used to determine sea level across the entire transect: ~15 m, shallow ramp environment between sea level and fair-weather wave base (Tucker and Wright, 1990). DDB—Hancock Summit down-dropped block; GGS—Golden Gate south; HE—Hancock east; HN—Hancock north; HST—highstand systems tract; LFI—ledge-forming interval (lower member, Guilmette Formation); LST—lowstand systems tract; MMN—Monte Mountain north; MMS—Monte Mountain south; O.R. fault—outer rim fault; PTN—Pahranagat north; SL—sea level; TST—transgressive systems tract; YSFI—yellow slope-forming interval (lower member, Guilmette Formation).

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Based on the interpretation that the Tempiute Range of Actual and Possible Marine Impact Scaling Ratios Mountain section (TMP) existed within the 3.1 inner basin of the Alamo crater, the boundary of the annular trough would be between TMP 2.9 and MMN4. Using the midpoint between TMP Plausible Alamo Impact Ratios and MMN4, ATAlamo would equal ~55 km. This 2.7 generates an apparent crater diameter between

165 and 198 km and a transient crater diameter 2.5 between 55 and 86 km (Table 4).

2.3 Approach 2. Diameter of Outer Rim Da/Dt

The apparent diameter of the Alamo crater can 2.1 also be estimated based on the diameter of the outer rim fault, using interpretations discussed 1.9 earlier and those made previously (Warme and Kuehner, 1998; Pinto and Warme, 2008). Three 1.7 control points defi ne the location of the Alamo outer rim fault: (1) between the southern and 1.5 central Golden Gate Range (GGS and GGC), 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 (2) between HCE1 and SMFN2 in the Hiko Hills Da/AT

Range, and (3) between the Delamar Mountains Actual Ratios Plausible Ratios Calculated Ratio Pairs and the Arrow Canyon Range (ACR) (Fig. 15). A conservative estimate of ~100 km for the appar- Figure 14. Plot of actual and possible marine impact scaling ratios based on the six well- ent diameter of the Alamo crater was calculated studied marine impacts listed in Table 2. The dashed box represents the plausible ratios from these points (Fig. 15). As this measurement that would be associated with the Alamo impact. AT—annular trough width; Da—apparent only captures a chord, not diameter, of the crater, crater diameter; Dt—transient crater diameter. it represents a minimum estimate.

Proposed Crater Diameter and Depth on Earth larger than 80 km in apparent diameter TABLE 3. CALCULATED RATIO PAIRS Morrow et al. (2005) calculated that the (Poag et al., 2004; Dypvik and Kalleson, 2010). (Da/AT)calc (Da/Dt)calc fi nal crater could be no greater than 150 km By estimating the diameter of the Alamo Pair 1 3.0 3.0 Pair 2 3.0 2.3 wide based on the lack of crystalline basement crater, the excavation depth (Hexc) can be cal- Pair 3 3.3 2.6 clasts within the Alamo Breccia Member and a culated using the following equation given by Pair 4 3.6 3.0 regional stratigraphic thickness of 6 km of sedi- Melosh (1989): Pair 5 3.6 2.3 Note: Da—apparent crater diameter; Dt—transient mentary strata above basement. This calculation crater diameter; AT—annular trough width; calc— used a mean Da/Dt ratio of 2.5. It is assumed Hexc = 1/10(Dt). (3) calculated. here that the Alamo impact did not involve crys- talline basement and a maximum crater size of As shown in Table 4, the minimum possible 150 km is accepted. Therefore, an apparent cra- transient crater diameter is 37 km, while the Group, established by regional stratigraphic ter diameter of 111–150 km and a transient cra- maximum is 65 km. This range yields a mini- thicknesses. This contrasts with excavation esti- ter diameter of 37–65 km are proposed for the mum and maximum crater excavation depth mates by Morrow et al. (2005) of 1.7–2.5 km Alamo crater (Table 4). This estimate is more of 3.7–6.5 km. However, a maximum depth of based on the presence of upper Cambrian than double the Morrow et al. (2005) estimates 6.0 km is assumed, based on available strata microfossil elements within the Alamo Brec- of 44–65 km for the apparent diameter. Further- atop crystalline basement (Morrow et al., 2005). cia Member. Had the crater excavated into the more, this yields an Alamo crater size that is Consequently, the Alamo bolide would have Prospect Mountain Quartzite or McCoy Creek conservatively larger than the Chesapeake Bay excavated at least into the lower Cambrian Group, composed mainly of quartzite and argil- marine impact crater (Da = 90 km; Dt = 35 km), Prospect Mountain Quartzite and possibly into lite, it would not have left any fossil signatures one of only eight impact structures discovered the underlying Neoproterozoic McCoy Creek (cf. Kellogg, 1963; Misch and Hazzard, 1962) within the Alamo Breccia Member. Further- more, it is unlikely for deeply excavated rocks TABLE 2. SCALING RATIOS OF WELL-STUDIED MARINE IMPACTS to be deposited within the annular trough Da Dt AT impact breccia (Grieve et al., 1981; Kenk- Name Location (km) (km) (km) Da/Dt Da/AT Chesapeake Chesapeake Bay, Virginia, USA 90 35 27.5 2.6 3.3 mann et al., 2014), meaning diagnostic lithic Mjolnir Barents Sea near Norway 40 16 12 2.5 3.3 fragments containing distinctive or potentially Montagnais South of Nova Scotia, Canada 45 20 12.5 2.3 3.6 distinctive detrital zircons would not be found Neugrund Southern coast of Gulf of Finland, Estonia 20 8 6 2.5 3.3 Kardla Kardla, Estonia 12 4 4 3 3.0 within the known Alamo-related deposits. It is Wetumpka Wetumpka, Alabama, USA 7.6 4 1.8 1.9 4.2 possible that these clasts were confi ned within Mean: 2.5 3.5 the inner basin of the transient crater. The best Note: Data from Dypvik and Kalleson (2010). Da—apparent crater diameter; Dt—transient crater diameter; chance of fi nding such clasts would be within AT—annular trough width. the Tempiute Mountain section or other Alamo-

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TABLE 4. ALAMO IMPACT CRATER (NEVADA, USA) SIZE ESTIMATES 2. Transects parallel and perpendicular to the ATAlamo DaAlamo DtAlamo paleoshoreline depict a concave seafl oor pat- (km) (Da/AT) (Da/Dt) (km) (km) calc calc tern that circumscribes the eastern half of the Pair 1 37 3.0 3.0 111 37 exposed Alamo impact crater. Signifi cant shifts Pair 2 37 3.0 2.3 111 48 in accommodation space of impact breccia and Pair 3 37 3.3 2.6 122 47 post-impact facies reveal the eastern and north- = ~37 km ern margins of the outer rim fault in the Hiko

Alamo Pair 4 37 3.6 3.0 133 44 (This study) Hills and Golden Gate Ranges. AT Pair 5 37 3.6 2.3 133 58 3. Identifi cation of the apparent crater margin allows for new size estimations based on scal- Pair 1 55 3.0 3.0 165 55 ing relationships observed in other marine com- Pair 2 55 3.0 2.3 165 72 plex craters. Pair 3 55 3.3 2.6 182 70

= ~55 km 4. Conservative estimates indicate a transient crater diameter between 37 and 65 km, and an

Alamo Pair 4 55 3.6 3.0 198 66 (This study)

AT apparent crater diameter between 111 and Pair 5 55 3.6 2.3 198 86 150 km. These values more than double previ- Pair 1 N/A N/A 3.0 150 50 ous estimates based on the biostratigraphy of the impact breccia. If correct, the new estimates rank Pair 2 N/A N/A 2.3 150 65

max the Alamo crater as one of the largest Phanero- Pair 3 N/A N/A 2.6 150 58 zoic craters on Earth. Alamo Pair 4 N/A N/A 3.0 150 50

Da 5. Bolide impacts have a longstanding infl u- Pair 5 N/A N/A 2.3 150 65 ence on sedimentation patterns within a marine (Morrow et al., 2005) setting, shown here across three depositional Minimum 111 37 Maximum* 150 65 sequences. Note: Data from Dypvik and Kalleson (2010). Da—apparent crater diameter; Dt—transient crater diameter; 6. The stratigraphic approach of this study AT—annular trough width; calc—calculated. can be applied to other marine impact craters to *Maximum based on Morrow et al. (2005) maximum calculation. estimate their size, especially those dissected by complex tectonic histories or otherwise lacking seismic data. related deposits at or west of Tempiute Moun- CONCLUSIONS ACKNOWLEDGMENTS tain. Morrow et al. (2005) reported fi nding poly- crystalline quartz grains in an Alamo-related 1. The Alamo impact occurred at the edge of This work was made possible by fi nancial support deposit to the far northwest of Tempiute Moun- a carbonate platform during highstand deposi- from the National Science Foundation (grant SGP tain near Eureka, Nevada, that may have been tion; 18 facies of post-Alamo impact deposits 102484 to Tapanila), the Nevada Petroleum Society (Myers), Idaho State University Graduate Student derived from the Prospect Mountain Quartzite record peritidal to subtidal cyclicity in the Research and Scholarship Committee grants (Steen- or even lower. impacted region. berg and Myers), and Geological Society of America graduate student grant 8819-08 (Johnson). We thank Ben Rendall for help in data collection and interpre- tation, and Jesse Davenport for fi eld assistance. We also thank Todd LaMaskin, Henning Dypvik, and Figure 15. Map view of the an anonymous reviewer for helpful comments that ? GGC impact region outlining the greatly improved the clarity of the manuscript. Alamo impact crater and the GGS REFERENCES CITED possible location (dotted lines ?

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