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Post-Impact Depositional Environments As a Proxy for Crater Morphology, Late Devonian Alamo Impact, Nevada

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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 permissionGeosphere, to copy, contact February [email protected] 2015 123 © 2015 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/1/123/3333229/123.pdf by guest on 27 September 2021 Retzler et al. 115°30′00″W 115°15′00″W + @ + GOLDENGOLDEN GATEGATE @ + + RANGERANGE @ @ + + + ! + @ @ + + NEVADA+ + + + + GGS3 ! A + + + !! ! Tertiary @ @ Cretaceous @ !! 37°45′00″N ! + + B′ ! !!! ! MAILMAIL ! + SMFN2 + !!! Permian/Penn. + ! + + B + SUMMITSUMMIT ! + ! ! + HHN1 + ! @ ! ! MMN4 ! + !! + + + + + + MONTEMONTE + Mississippian + + ! + ! !! + ! + + !! + MOUNTAINMOUNTAIN !! + + DMP1 *TMP ! HCE1 + + + ! + MIN2 + ! ! + !! + + MMS2 ! HIKOHIKO TEMPIUTETEMPIUTE+ MI1 Miss./Dev. undiff. HILLSHILLS MOUNTAINMOUNTAIN + ! + ! + ! + + + ! + + Devonian + + + ! Hiko /" !!! 375 Ordovician + + " 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 ! + + E + Inferred Fault ! + + !!! + !! + Alamo " + A′+ HE6 ! + 0693Km + ± + + + 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. 124 Geosphere, February 2015 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/1/123/3333229/123.pdf by guest on 27 September 2021 Crater morphology of the Late Devonian Alamo impact, Nevada 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
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