Alamo Megabreccia: Record of a Late Devonian Impact in Southern Nevada John E

Home , Pilot crater, Shocked quartz

Vol. 6, No. 1 January 1996 INSIDE • Presidential Address, p. 10 GSA TODAY • GSA Bulletin Update, p. 13 • New Editors, p. 20 A Publication of the Geological Society of America • Cordilleran Section Meeting, p. 25

Alamo Megabreccia: Record of a Late Devonian Impact in Southern Nevada John E. Warme, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401 Charles A. Sandberg, 395 South Lee Street, Lakewood, CO 80226-2749

ABSTRACT 116 o 115 o 114 o Figure l. Study area showing towns SIXMILE The Alamo breccia is probably the Currant X RANCH WHITE PINE CO. (squares), Devonian localities (x’s), and WOOD CANYON X NORTH NEVADA lateral distribution zones of Alamo brec- most voluminous known outcropping UTAH HOT cia. Breccia thicknesses: zone 1, ~130 carbonate megabreccia. It occupies CREEK X CANYON HEATH m; zone 2, ~60 m; zone 3, <1–10 m. 2 TROUGH MTN X SIDEHILL ~4000 km across 11 mountain ranges SPRING X X PASS in southern Nevada, has an average CANYON

FOX thickness of ~70 m, and contains a vol- WARM X X SPRINGS MTN Figure 2. Enlargement of inset 3 X N WEST ume of 250+ km . The breccia is a single X X RANGE STREUBEN 38o in Figure 1 showing mountain bed, of early Frasnian (early Late Devo- KNOB EAST X RIDGE Pioche ranges, outcrop localities, and nian) age, that formed in the wake of X CREST NORTH MEEKER GOLDEN X GATE RGSOUTH SEAMAN the perimeter enclosing Alamo PEAK X X WASH a giant slide that deposited a lower MAIL HIKO breccia in zones 1 and 2. SUMMITX X NARROWS 115° 30' 115° chaotic debrite, containing clasts as X MT. IRISH TEMPIUTE N X X Caliente NYE CO TO X Hiko ELY MTN S MONTE ° LINCOLN CO large as 80 × 500 m, and an upper MTN S X HIKO 38 1 HILLS exquisitely graded turbidite. It is N X W XX CARBONATE HANCOCK SW Alamo SUMMIT 3 anomalously intercalated with cyclic WASH X SEAMAN RANGE

GOLDEN GATE WORTHINGTON LIME X MTS RANGE shallow-water platform carbonates of 2 MTN 318 the Guilmette Formation. The Alamo DELAMAR Area of Fig. 2 X MTS SW NORTH PAHROC RANGE o breccia is interpreted as a product of the 37 HIKO TIMPAHUTE RG LINCOLN CO. HIKO 93 Alamo event, a nearby marine impact TO CLARK CO. MT IRISH RANGE CALIENTE S. PAHROC RG of an extraterrestrial object, whereby RANGE ASH 37° 30' GROOM RANGE LAS 375 SPRINGS impact-generated crustal shock waves Mercury VEGAS X ARROW RANGE X CANYON Indian RANGE E. PAHRANAGAT RG and/or marine superwaves detached the NEVADA Springs ALAMO CALIFORNIA YUCCA HANCOCK upper ~60 m of platform along a hori- X GAP WEST SUMMIT PAHRANAGAT RG TO 93 LAS zontal surface. Loosened bedrock slid VEGAS seaward across the platform, and some NYE CO. NV Las Vegas 010mi of it accumulated as the lower debrite. DELAMAR 0 10 20 30 Miles 01020km MOUNTAINS

o ARIZONA Rock-water exchange induced land- 36 0 10 20 30 40 50 Kilometers ward-propagated tsunami(s), whose uprush and/or backwash deposited the upper turbidite, partly above sea level. Evidence for impact includes shocked- quartz grains, an iridium anomaly, and reworked conodonts, all found only within the breccia. Because the Alamo breccia is not known outside of Nevada, and because the early Frasnian time of the Alamo event is not noted for accelerated extinctions, being ~3 m.y. before the Frasnian-Famennian impact(s) and biotic crisis, the impact was probably only of moderate size.

INTRODUCTION The Alamo breccia, a newly recognized carbonate megabreccia, is an anomalous event–stratigraphic bed of immense proportions, probably the largest megabreccia known in surface exposures. Figure 3. View northward from the Hancock Summit area (see Fig. 2) of West Pahranagat Range, It was emplaced in a geologic instant of showing the 55-m-thick Alamo breccia (AB) within the 600-m-thick Guilmette Formation. The Guilmette begins at the yellow slope-forming interval (YSF in Fig. 4) in the saddle ~150 m below the breccia, is underlain by the Simonson Dolostone at left, and is overlain by the West Range Megabreccia continued on p. 2 Limestone and/or Pilot Shale on dip slope at right. IN THIS ISSUE GSA TODAY January Vol. 6, No. 1 1996 Alamo Megabraccia: Record of GSAF Update ...... 18 a Late Devonian Impact in GSA Today Science Editor ...... 20 GSA TODAY (ISSN 1052-5173) is published Southern Nevada...... 1 monthly by The Geological Society of America, Inc., New Geology Editor ...... 20 with offices at 3300 Penrose Place, Boulder, Colorado. GSA On the Web ...... 7 About People ...... 20 Mailing address: P.O. Box 9140, Boulder, CO 80301- 9140, U.S.A. Second-class postage paid at Boulder, Col- Rock Stars: Walcott...... 8 Book Reviews ...... 21 orado, and at additional mailing offices. Postmaster: Send address changes to GSA Today, Membership Ser- Presidential Address ...... 10 Cordilleran Section Meeting ...... 25 vices, P.O. Box 9140, Boulder, CO 80301-9140. Bulletin Update ...... 13 In Memoriam ...... 31 Copyright © 1996, The Geological Society of America, GSA Division Officers ...... 13 Memorial Preprints ...... 31 Inc. (GSA). All rights reserved. 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Individual scientists are hereby granted permission, without royalties or further requests, to make GSA Division News ...... 17 GeoVentures ...... 35 unlimited photocopies of the science articles for use in Northeastern Section Research Grants . . . 17 GSA Meetings ...... 39 classrooms to further education and science, and to make up to five copies for distribution to associates in GSA Research Grant Alternates ...... 17 Coal Division Award ...... 40 the furtherance of science; permission is granted to make more than five photocopies for other noncom- mercial, nonprofit purposes furthering science and education upon payment of the appropriate fee ($0.25 per page) directly to the Copyright Clearance Center, 27 Megabreccia continued from p. 1 SUBMARINE SLIDES, Congress Street, Salem, Massachusetts 01970, phone CARBONATE MEGABRECCIAS, (508) 744-3350 (include title and ISSN when paying). AND TSUNAMITES Written permission is required from GSA for all other early Late Devonian time and is now forms of capture, reproduction, and/or distribution of preserved within the Guilmette Formation Many enormous submarine debrites any item in this journal by any means. GSA provides this and other forums for the presentation of diverse opin- of southern Nevada. The breccia, as yet and/or turbidites, comparable in extent ions and positions by scientists worldwide, regardless of informally named, crops out in 11 and volume to the Alamo breccia, are their race, citizenship, gender, religion, or political view- mountain ranges around the community known from both modern (e.g., Piper et point. Opinions presented in this publication do not reflect official positions of the Society. of Alamo, Nevada (Figs. 1–3). It spans a al., 1988; Moore et al., 1994) and ancient minimum area of 4000 km2, has an (e.g., Macdonald et al., 1993) settings. SUBSCRIPTIONS for 1996 calendar year: Society average thickness of ~70 m, and represents Catastrophic failure of the seaward Members: GSA Today is provided as part of membership dues. Contact Membership Services at (800) detachment, mobilization, and resedimen- margins of present and past carbonate 3 472-1988 or (303) 447-2020 for membership informa- tation of at least 250 km of Devonian car- platforms results in voluminous carbonate tion. Nonmembers & Institutions: Free with paid bonate rock. megabreccias that commonly contain subscription to both GSA Bulletin and Geology, otherwise $45 for U.S., Canada, and Mexico; $55 elsewhere. Con- The Alamo breccia represents an huge transported blocks (e.g., Cook and tact Subscription Services. Single copies may be extraordinary style of carbonate-platform Mullins, 1983; Hine et al., 1992). However, ordered from Publication Sales. Claims: For nonreceipt collapse and submarine slide, now all large-scale marine mass-flow deposits or for damaged copies, members contact Membership Services; all others contact Subscription Services. Claims preserved as a single bed with the described to date, whether terrigenous are honored for one year; please allow sufficient delivery characteristics of a debrite (debris-flow or carbonate, intercalate with thinner time for overseas copies. deposit) in the lower part that evolved resedimented beds and were interpreted into a capping turbidite (Figs. 4, 5). It is to accumulate in relatively deep water sea- STAFF: Prepared from contributions from the GSA staff and membership. anomalously intercalated within cyclical ward from shelf or platform edges. Executive Director: Donald M. Davidson, Jr. shallow-water carbonate-platform facies Massive shelf-edge slumping causes Science Editor: Suzanne M. Kay of the Guilmette Formation (Figs. 3–6), a water-for-rock volume exchange that Department of Geological Sciences, Cornell University, Ithaca, NY 14853 rather than occurring in deeper water as induces an onshore tsunami. Numerous Forum Editor: Bruce F. Molnia expected. Its huge magnitude, singular massive Quaternary underwater slumps U.S. Geological Survey, MS 917, National Center, epiplatform occurrence, horizontal occurred adjacent to the Hawaiian Islands, Reston, VA 22092 Managing Editor: Faith Rogers delamination from platform bedrock, and each potentially triggered destructive Production & Marketing Manager: James R. Clark and exotic components including shocked onshore waves (Moore et al., 1994). One Production Editor and Coordinator: Joan E. Manly quartz, iridium, and reworked conodonts, example is suspected of causing a prehis- Graphics Production: Joan E. Manly, Adam S. McNally all indicate genesis from the consequences toric catastrophic tsunami that formed an ADVERTISING of an impact of an extraterrestrial object uprush 375 m above sea level on Lanai, Classifieds and display: contact Ann Crawford with Earth, named the Alamo event Hawaii, leaving a deposit (tsunamite) of (303) 447-2020; fax 303-447-1133 (Warme et al., 1991; Warme, 1994). coral and other rubble in its wake (Moore Issues of this publication are available electronically, in Discovery of the Alamo breccia and Moore, 1988). However, tsunamis full color, from GSA as Acrobat “Portable Document Format” (PDF) files. These can be viewed and printed coincides with current intense interest in produced by underwater landslides would on personal computers using MSDOS or MSWindows, geologically short-lived but significant likely be dwarfed compared with those on Macintoshes, or on Unix machines. You must use the events (e.g., Clifton, 1988) and new from oceanic extraterrestrial splashdowns; appropriate Adobe Acrobat Reader, available for free download from GSA and other online services. The appreciation for the possible physical the waves could be as high as the target more powerful Adobe Exchange program, available and biological effects of extraterrestrial water depth, conceivably 1000 m or more from commercial software suppliers, may also be used. impacts (e.g., McLaren and Goodfellow, (Silver, l982), but few deposits from such Download the issues of GSA Today and/or the appropriate Readers using the Uniform Resource Locator (URL): 1990; Sharpton and Ward, 1990). This waves have been identified. Probable http://www.aescon.com/geosociety/index.html. Issues preliminary report describes the breccia, impact-related slump deposits and tsuna- of GSA Today are posted about the first of the month of discusses its genesis as a catastrophic bed, mites were described at the Cretaceous- publication. and presents evidence that it was triggered Tertiary (K-T) boundary in cores from the This publication is included on GSA’s annual CD-ROM and magnified by a nearby marine impact Gulf of Mexico as well as in circum-gulf GSA Journals on Compact Disc. Call GSA Publication Sales for details. of moderate size. outcrops (e.g., Smit et al., 1994). An Printed with pure soy inks on recyclable paper in the U.S.A. Megabreccia continued on p. 3

2 GSA TODAY, January 1996 Megabreccia continued from p. 2 the conodont biofacies scheme described selective or complete dolomitization. by Ziegler and Sandberg (1990). As shown Most clasts are recognizable components Eocene breccia in the subsurface under in Figures 4 and 5, platform cycles below of the Devonian carbonate platform that Chesapeake Bay was first regarded as the the breccia contain shallow-subtidal disintegrated during the Alamo event. tsunamite from a North Atlantic impact, biofacies and indicate water depths of 10 Quartz grains and other noncarbonate then was reinterpreted as brecciated target m or less. Collections from fossiliferous components recovered from conodont- rock (impactite) within the actual crater postbreccia beds at or above its top indi- sample insoluble residues represent ‹‹1% (Poag and Aubry, 1995). Owing to the cate landward shallowing, from 60–100 m of the breccia volume. numerous exposures and well-understood on the west to 10–20 m toward the east. stratigraphic framework of the Alamo Samples from easternmost localities (zone Clasts and Matrix breccia, it provides a useful comparison 3 in Fig. 1) were largely barren of con- The Alamo breccia contains a popula- and basis for interpreting other less acces- odonts. Samples collected within the tion of megascopic clasts ranging in size sible catastrophic deposits. lower part of the breccia, from both plat- from sand to blocks 80 × 500 m. Breccia form-derived clasts and breccia matrix, “matrix” is a relative term; matrix GEOLOGIC SETTING OF THE yielded shallow-water biofacies, as between clasts of given sizes is simply ALAMO BRECCIA expected. However, samples from the smaller fragments, regardless of their upper part contain admixed deep-water absolute scale (Fig. 8). Gravel-, sand-, and Southern Nevada falls within the contemporary Devonian and reworked smaller sized particles are ubiquitous Basin and Range physiographic province, Ordovician species, probably derived from between blocks, but become better sorted where Paleozoic rocks are exposed along deeper environments far to the west. and occupy progressively greater propor- linear mountain ranges, buried deeply in tions of the volume upward, culminating intervening valleys (Fig. 2), deformed by Age and Timing of Alamo Event in the well-graded top. post-Devonian orogenies, and covered by Conodont age determinations Cenozoic volcanic rocks. The area of the show that the Alamo event is narrowly Lateral Variations: Zones 1 to 3 Alamo breccia in Figures 1 and 2 is not bracketed within the middle part of the The Alamo breccia exhibits character- palinspastically restored. punctata conodont Zone of Ziegler and istic thicknesses, internal structures, and Sandberg (1990), as shown in Figures 4 internal variability in each of the three Stratigraphic Framework and 5, which represents ~0.5 m.y. of early zones shown in Figures 1, 2, 4 and 6. From Cambrian into Late Devonian Late Devonian (Frasnian) time. The Zone 1: Foreslope or Basin Floor. time, a westward-facing linear carbonate punctata Zone was ~3 m.y. before the Zone 1, along the west end of Tempiute platform rimmed the west side of the much-studied late Frasnian mass extinc- Mountain (Figs. 1 and 2), probably North American craton, trending tion (Sandberg et al., 1988), and has a extends much farther westward. It is 130 approximately north-south through biochronologic date of ~13 m.y. before m thick and is composed of a thick (~100 central Nevada (e.g., Poole et al., 1992). the end of Devonian time. Because the m) turbidite (unit A, described below) The Alamo breccia is within the shallow- Devonian-Carboniferous boundary has overlying a thinner debrite (unit B): the platform facies of the Guilmette Forma- been variously dated as 340 to 360 Ma, turbidite is generally finer grained than in tion, except in its westernmost known the Alamo event was between ~355 and zone 2 to the east. Maximum-sized clasts exposure at Tempiute Mountain (Figs. 1, 370 Ma. are <5 × 15 m in two dimensions. Zone 1 2) where it is overlain by ~300 m of deeper is interpreted as an area of platform fores- water facies of the correlative Devils DESCRIPTION OF ALAMO BRECCIA lope or basin floor. Gate Limestone (Fig. 4). The Guilmette The Alamo breccia is a single bed At Tempiute Mountain, twisted and comprises ~150 shallowing-upward deposited during one event. Although its fractured bedrock underlies the breccia, carbonate-platform cycles (Fig. 3) internal structure varies significantly both laced by an array of sedimentary dikes averaging ~5 m thick. The Alamo event laterally and vertically, the most obvious and sills injected with carbonate breccia slide consumed a stratigraphic interval trends are decreasing thickness landward and rich in quartz sand grains. The overly- ~60 m thick and containing ~10 to 15 (eastward) and decreasing clast and matrix ing Alamo breccia, in contrast, is relatively cycles. Cycle tops directly beneath the sizes (normal grading) upward. For undeformed and weathers as a shear cliff breccia exhibit dolomitized algal descriptive purposes, the breccia is divided ~100 m high. The deformation is inter- laminites, desiccation cracks, and fenestral into lateral zones 1 to 3, from west to east preted as listric faults that moved and fabrics that indicate upper-intertidal (Figs. 1 and 2), and vertical units A to D, rotated during the Alamo event (Fig. 6; see to supratidal carbonate platform environ- from top to base (Fig. 5). Winterer et al., 1991). Dilated fault planes ments. Lithofacies directly above the are filled with debris derived from fissure breccia are more variable and demonstrate Distribution and Lithology walls, overlying slope deposits, and the a post–Alamo event westward-dipping To date, the Alamo breccia is known bypassing breccia. Margins of the fissures ramp. at 23 localities spread across ~4000 km2 show liquefaction phenomena similar to (Figs. 1 and 2). Thickness varies from 130 that of unit D, described below, at the Conodont Biofacies m in zone 1 to <1 m in parts of zone 3. base of the Alamo breccia. Analyses of 75 conodont samples If 70 m is taken as the average thickness, Zone 2: Seaward Platform. from the lower part of the Guilmette then the minimum volume of rock dis- Across the broad area of zone 2, the Formation in the study area yielded both placed during the Alamo event is ~280 Alamo breccia averages ~60 m in thick- paleoenvironmental information, crucial km3. The total distribution and volume ness, contains discontinuous giant clasts for understanding the Alamo breccia, and may be much greater; they are not fully as much as 80 × 500 m, and commonly an exact biostratigraphic date for the bed. documented because of sparse outcrops exhibits all four vertical units A to D Conodont samples include 30 from the and restricted access to the south and described below. Alamo breccia and 45 from the Guilmette west. Zone 3: Landward Platform. The confining beds. The paleoecological The Alamo breccia is almost exclu- Alamo breccia in zone 3 (<1 to ~10 m interpretation of the lower Guilmette, sively carbonate rock (Figs. 7–10). Its thick) may exhibit only a graded turbidite including the interval of the Alamo composition is limestone, except for (unit A), with <1-m-diameter clasts, or breccia (Sandberg and Warme, 1993; minor synsedimentary supratidal Warme and Sandberg, 1995), employs dolostones and local but significant Megabreccia continued on p. 4

GSA TODAY, January 1996 3 Megabreccia continued from p. 3 Vertical Units Units D and C—Detachment The Alamo breccia is composed of Interval and Megaclasts. Units C and may be dominantly a debrite (unit B) from one to four vertical units, A to D. D are genetically related, occur together transitionally overlain by a thin turbidite, They are arranged from top to base as but discontinuously along the base of the suggesting extensive turbidity-current follows (Fig. 5): unit A: upper turbidite— breccia (Fig. 5), and illustrate the mode of bypass. The debrite contains tabular clasts present in all zones (1–3); unit B; lower epiplatform slide detachment across zone up to 30 m long, oriented (sub)parallel to debrite—best developed in zone 2; unit C: 2. Unit D is an unusual, light-gray–weath- bedding. Zone 3 breccia was stranded basal megaclasts, actually or nearly in ering, calcareous diamictite, <1 cm to ~3 above sea level and subaerially exposed original positions—zone 2 only; unit D: m thick, that developed ~60 m beneath for a significant time before deposition diamictite under megaclasts—zone 2 only. the contemporary Devonian platform of the next platform cycle. It may be selec- tively eroded at the top, bleached by Section at VE dolomitization, or karsted, in contrast A Basin Rim Lagoon Shore to its confining beds. Stromatoporoid-coral Fluctuation zone Sea level

W ALAMO BRECCIA (AB) E Zone 1 Zone 2 ~100 m MUDMOUN Slope Guilmette Formation VE ~25 SHELF Dg Yellow slope-former Ddg EDGE ~5 km Simonson Dolostone

Early hassi

AB (80 m) B Basin Rim Shore Stromatoporoid-coral Fluctuation zone Sea level

punctata punctata ? Slope Guilmette Formation ? Y S F Dg Yellow slope-former M varcus Dsi 30 km Simonson Dolostone Figure 6. Interpretation of processes that produced the Figure 4. Diagrammatic cross section of Alamo Alamo breccia: A: Rimmed breccia (AB) within the zone 2 shallow-water C Seawater replacement Devonian carbonate platform Guilmette Formation (Dg) and equivalent zone 1 Headwall at 1:1 vertical and horizontal ard deep-water Devils Gate Limestone (Ddg). The seaw scale. B: Platform at 25:1 verti- flow Upper Simonson Dolostone (Dsi) is within the Mid- is br cal exaggeration. C: Platform dle Devonian Middle varcus conodont Subzone, De Detachment surface: actual potential showing two sets of fractures overlain by the basal Guilmette yellow slope-form- along which failure occurred: Listric faults with ing interval (YSF). Overlying cyclical carbonate sedimentary dikes and sills listric at the platform edge, rocks and the Alamo breccia are within the early and nearly horizontal across Frasnian punctata Zone. Beds beginning 4–10 m the platform. D: Platform fail- above the breccia are in the Early hassi Zone. D ure: movement along listric faults created sedimentary Tsunami rd dikes and sills, which removed eawa w s punctata ZONE ALAMO flo lateral support from the plat- ris CONODONT BRECCIA eb form. As bedrock detached BIOFACIES D Reduced or vanished UNIT and slid seaward, evacuation SHALLOW AND DEEP LATERAL ZONES caused the sea surface to tilt landward. E: Tsunami caused by bedrock– sea-water E exchange crossed the platform MIXED SHALLOW A A Zone 1Zone 2 Zone 3 and overtopped the headward AND DEEP kwash slide scar, while debris flow (WITH REWORKED bac mi ORDOVICIAN) a continued seaward. F: Tsunami un Ts backwash stranded a turbidite along the periphery of the platform (zone 3), continued 10 m across the detachment area B (zone 2), and flowed into deep water beyond the platform F (zone 1). G: The final product; Zone 1Zone 2 Zone 3 B C the inset shows the giant sand- SHALLOW waves across the top of the breccia. ~100 m VE ~25

D ~5 km SHALLOW

Figure 5. Vertical units A to D, and conodont biofacies paleobathymetry. Units D and C, if 50 m present, are together: D is a zone of bedrock fluidization; C is giant clasts preserved in the process of tearing loose and incorporating into the slide. Unit B is chaotic debrite, beginning at the base of the breccia where unit C blocks were removed. Unit A, transitional upward from unit B, is a well-graded turbidite capping breccia every- where. Tufted symbols represent stromatoporoids; cones represent corals concentrated at the top.

4 GSA TODAY, January 1996 surface. The evolution of the diamictite is preserved in many localities, where within a distance of 1 m, intact bedrock changes laterally to a tight fracture- Figure 7. Characteristics of mosaic, a dilated fracture-mosaic, a unit D fluidization, showing melange of isolated and rotated fragments, nearly intact bedrock (right), and ultimately to the diamictite (Fig. 7). a mosaic of fractures (center), Unit D is preserved only where the huge dilation (left-center and blocks of unit C are actually or nearly in across base), and complete fluidization to calcareous original position over it. Where unit C diamictite (upper left). blocks were lifted, fragmented, and incorporated into the unit B debrite, unit D was unprotected and scoured away. Throughout zone 2, the Alamo breccia rests at about the same stratigraphic level, where unit D cuts indiscriminately along the same or Unit A—Graded Bed. Unit A is a - Upper Boundary adjacent shallowing-upward carbonate- well-organized turbidite that transitionally Unit A usually exhibits complete platform cycles, rarely at a bed or cycle overlies the chaotic fabric of unit B. It grading from boulder conglomerate boundary. The massive unit C blocks, ranges from roughly graded meter-sized upward to calcareous mudstone. In zone directly above unit D, are (sub)parallel clasts near the middle of the breccia to an 1, the very fine grained top of the breccia to bedding and have ragged, sharp lateral exquisitely graded 5–30 m interval (100 m merges with overlying deep-water lime- terminations, beyond which the debrite in zone 1) at the top. The sorting process stones, and in zone 3 the karsted top is of unit B extends to the basal plane of left zones rich in domal boulder- to cob- overlain by peritidal carbonate-platform the breccia (Fig. 5). ble-sized stromatoporoids and their beds. However, zone 2 is more variable. Unit B—Chaotic Debrite. A bewil- hydraulically equivalent lithoclasts, frag- Pebble-sized clasts at the top may repre- dering spectrum of clast sizes, shapes, and ments of tabular stromatoporoids and sent the broad crests of giant sediment orientations characterizes unit B. The their hydraulic equivalents, and, near the waves, hundreds of meters apart (e. g., largest clasts are tabular, tens to hundreds top, concentrations of fragmented corals, Bretz, 1969; Moore and Moore, 1988), per- of meters in longest dimension, and fully brachiopods, and their equivalents. haps stranded above sea level. They may encased in unit B matrix, in contrast to The topmost 1 m commonly exhibits have separated parallel linear lagoonal the unit C clasts along the base of the calcarenite cross-bed sets up to 30 cm depressions, evidenced at other localities breccia. Smaller unit B clasts are tabular high, representing the climbing ripples by bioturbation across the upper contact. to equidimensional (Fig. 8), and some of Bouma turbidite interval C, overlain are intricately deformed or preserved in by a thin, finer grained, horizontally lami- EXOTIC COMPONENTS the process of peripheral fragmentation nated Bouma interval D. At some localities Three significant exotic components into finer grained matrix. At some locali- the upper 1 m exhibits compound grading occur within the Alamo breccia and are not ties, multiple slabs >100 m long rode over caused by repeated scour and fill during present in overlying and underlying beds. one another toward the top of the breccia waning runoff. and formed logjamlike megafabrics. Beds The top of unit A also contains rare Shocked Quartz Grains at the base of unit B are preserved in all large clasts, as much as 10 × 30 m, that Insoluble residues of conodont sam- stages of being ripped up, injected below interrupt the graded profile. They suggest ples from the Alamo breccia concentrate by matrix, torn away, and incorporated that the underlying debris flow still moved unusual quartz grains (Fig. 10), common into the breccia. Figure 9 shows a as a viscous mass and that clasts from in zone 1 and progressively scarcer land- spectacular example. below were buoyed upward and incorporated into the accumulating turbidite. Megabreccia continued on p. 6

Figure 8. Upper part of the Alamo breccia showing large clasts (beneath the Figure 9. A spectacular example of beds deformed and preserved in the pro- person) that are transitional to the graded bed above. The small, dark clasts cess of detachment and incorporation into the Alamo breccia, West Pahrana- are mostly whole or fragmented domal stromatoporoids. West Pahranagat gat Range. A folded package, ~20 m thick, was pried up along the base of Range. the slide by a huge, chisel-shaped clast moving from the right. The massive bed over the fold is 30-m-thick graded units B and A. Total breccia thickness is 60 m.

GSA TODAY, January 1996 5 Megabreccia continued from p. 5 required to fill the space evacuated by the debris flow. Such superwaves best account for the exotic deep-water punctata Zone ward. By optical petrography, these grains conodonts, which indicate a probable exhibit one to six sets of internal parallel water depth of >300 m at the impact site, lamellae and mosaic extinction, both typi- and the reworked Ordovician conodonts cal of shock metamorphism associated recovered from the breccia. The waves or with impacts (e.g., Stöffler and Langen- associated currents may also have trans- horst, 1994). By transmission electron ported Ir and shocked quartz from the microscopy (TEM), they reveal microme- impact site. An impact could have gener- ter-scale parallel deformation structures ated immense runoff from water ejecta and fractured and rotated crystal frag- and rainfall from condensed vapor, which ments between lamellae (Leroux et al., Figure 10. Thin section of a shocked-quartz grain swept debris off large areas of the adjacent 1995). The grains display unusual periph- showing four to six directions of shock lamellae, craton and flowed across all three lateral eral studding by crystals of iron oxides trains of inclusions along lamellae, and large dis- zones of the Alamo breccia. and sulfides (Fig. 10), which may represent placive hematite crystals; the longest grain dimension is ~150 µm. diagenetic products or even impact phe- DISCUSSION nomena. The shocked grains were air- or water-borne from target rock. Distinctive attributes of the Alamo breccia separate it from all other known senting relatively high angle listric faults, Iridium catastrophic carbonate megabreccias. It removed lateral support at the platform Samples from within and above the was first recognized for its anomalous margin. Horizontal delamination along Alamo breccia were analyzed for Ir and epiplatform framework. Its shocked quartz the second set allowed seaward transport other elements that signify extraterrestrial and Ir strongly suggested an impact of overlying beds across zone 2 (Fig. 6D). material (Alvarez et al., 1980). Two sample trigger. Given an impact, seismic shock (3) The resulting debris flow created a profiles across the top of the breccia, ~100 could have induced movement on the westward-facing, flat-floored, epiplatform m apart in the Worthington Mountains platform-margin listric faults, causing depression, equal in volume to the (Figs. 1 and 2), showed similar results. loss of lateral support, and generated hori- bedrock removed, and induced gravity- Background Ir in the area is <10 parts per zontal epiplatform fractures, leading to driven landward-propagated tsunami(s). trillion (ppT). Sixteen samples spread ±0.5 platform delamination and failure. Radial By inertia the wave(s) overstepped the m from the breccia top showed slight Ir shock waves, similar to those accompany- headward slide scar and flooded the elevation, averaging 20.1 ppT, the maxi- ing thermonuclear detonations, may have adjacent platform of zone 3 (Fig. 6E). (4) mum being 39 ppT. Eight samples from delaminated the platform at a uniform Backwash stranded the thin breccia across 0.5–1.5 m below the top averaged 69.4 depth below the surface (unit D) and frac- zone 3, and dumped excess debris as a ppT, the maximum being 139 ppT. Results tured overlying beds into approximately seaward-thickening turbidite over the are not available for samples from 1.5–12 equal horizontal segments (unit C blocks). debrite of zones 2 and 1 (Fig. 6F). The m, but six samples from 12–55 m averaged Concurrently, or alternatively, abrupt debrite was still moving seaward, indi- only 8.5 ppT. The Ir may have been loading and unloading of superwaves over cated by absence of a clear debrite-tur- diagenetically mobilized a few meters platform bedrock, and/or shear from bidite contact and by the oversized downward from the breccia top. Alterna- catastrophic wave uprush and backwash, clasts that were rafted upward into the tively, the Ir was water-borne to the site caused rapid oscillation of subsurface pore accumulating turbidite. (5) Air- and/or (see Displaced Conodonts below) and pressures and induced fluidization along water-borne shocked-quartz grains, Ir- accumulated with the thick unit A graded unit D. bearing particles, and exotic conodonts bed, making the anomaly very significant Several characteristics of the Alamo were incorporated into the breccia matrix. because of the overwhelming carbonate breccia suggest that the impact was (6) As sea level equilibrated, the debris that must have greatly diluted any relatively small, located near our study breccia in zone 3 was exposed (and accumulating Ir (and shocked quartz). area, and created waves and/or currents eventually karsted), the shoreline was that brought a significant volume of near the slide scar, giant sandwaves Displaced Conodonts debris onto the platform. The reworked crossed the top and may have separated Collections from zone 1 and from conodonts and shocked quartz grains are linear lagoons, and water depths increased pebbly zones of units B and A in zone 2 progressively less abundant in landward across zone 2 into Zone 1 (Fig. 6F). The contain rare reworked Ordovician con- samples, and may have been water-borne. platform was temporarily converted from odonts, probably derived from admixed The broad Ir spike 0.5–1.5 m below the a rim to a ramp. (7) Post–Alamo event target-rock fragments, and punctata Zone top of the breccia suggests subaqueous deep-water limestones accumulated in deep-water conodonts, which were accumulation simultaneous with waning- zone 1 and outer zone 2, and shallow- probably preserved in the matrix. Both phase deposition. Most significantly, the water cyclic deposition eventually represent exotic elements, likely trans- volume of the Alamo breccia appears too resumed across inner zone 2 and zone 3. ported from the west beyond the platform great to have come only from the shallow The events shown in Figure 6 fail to margin. submarine slide shown in Figure 6. Zone 3 account for the excess volume of breccia contains significant breccia volume, but it that was spread across zone 3, approxi- DEPOSITIONAL HISTORY is landward of the slide; the depression mately filled the slide area of zone 2, across zone 2 is brim-full with breccia, but Our scenario for the origin of the and left a 130-m-thick deposit in zone 1. should have been initially almost empty Alamo breccia, shown in Figure 6, is We believe that a nearby extraterrestrial and have temporarily accumulated deeper consistent with our field observations impact intensified the processes dia- water sediments; the deposit in zone 1 is and sample data. (1) The setting for the grammed in Figure 6 and accounts for the 130 m thick, and extends westward for an breccia is the rimmed, flat-topped, Late volume observed. Superwave uprush and unknown distance. All these relations sug- Devonian carbonate platform of Nevada backwash amplified the tsunami generated gest that catastrophic waves, debris-laden (Fig. 6, A and B). (2) Movement on two by the debris-flow–sea-water exchange, sets of fractures (Fig. 6C) detached the enlarged the landward flood and with- platform. Failure along the first set, repre- drawal, and transported the sediment Megabreccia continued on p. 7

6 GSA TODAY, January 1996 Megabreccia continued from p. 6 REFERENCES CITED orogen, Conterminous U.S.: Boulder, Colorado, Alvarez, L. W., Alvarez, W., Asaro, F., and Michel, H. V., Geological Society of America, Geology of North 1980, Extraterrestrial cause for the Cretaceous/Tertiary America, v. G-3, p. 9–56. from a nearby impact, brought the exotic extinctions: Science, v. 208, p. 1095–1108. Sandberg, C. A., and Warme, J. E., 1993, Conodont breccia components and required rock Bretz, J. H., 1969, The Lake Missoula floods and the dating, biofacies, and catastrophic origin of Late Devo- volume onto the platform. channeled scabland: Journal of Geology, v. 77, nian (early Frasnian) Alamo Breccia, southern Nevada: p. 505–543. Geological Society of America Abstracts with Programs, A relatively small, local impact is also v. 25, no. 3, p. 77. implied, because accelerated biotic Clifton, H. E., editor, 1988, Sedimentologic conse- Sandberg, C. A., Ziegler, W., Dreesen, R., and Butler, J. L., extinctions are not documented within quences of convulsive geologic events: Geological Society of America Special Paper 229, 167 p. 1988, Late Frasnian mass extinction; conodont event the punctata Zone. However, the Late stratigraphy, global changes, and possible causes, in Devonian in general and the Frasnian- Cook, H. E., and Mullins, H. T., 1983, Basin margin Ziegler, W., ed., 1st International Senckenberg Confer- environment, in Scholle, P. A., et al., eds., Carbonate ence and 5th European Conodont Symposium (ECOS Famennian (F-F) boundary specifically depositional environments: American Association of V), Contribution 1: Courier Forschungsinstitut Senck- are times of extinction and rapid biotic Petroleum Geologists Memoir 33, p. 539–617. enberg, v. 102, p. 263–307. turnover perhaps unequaled in the Hine, A. C., and eight others, 1992, Megabreccia shed- Sharpton, V. L., and Ward, P. D., 1990, Global catastro- Paleozoic (e.g., Sandberg et al., 1988; ding from modern, low-relief carbonate platforms, phes in Earth history: Geological Society of America Nicaraguan Rise: Geological Society of America Bulletin, Special Paper 247, 631 p. McLaren and Goodfellow, 1990). The v. 104, p. 928–943. Alamo event occurred ~3 m.y. before the Silver, L. T., 1982, Introduction, in Silver, L. T., and Hut, P., Alvarez, W., Elder, W. P., Hansen, T., Kauffman, Schultz, P. H., eds., Geological implications of impacts F-F boundary, but may represent one of E. G., Keller, G., Shoemaker, E. M., and Weissman, P. R., of large asteroids and comets on the Earth: Geological several sequential events during the Devo- 1987, Comet showers as a cause of mass extinctions: Society of America Special Paper 190, p. xii–xix. Nature, v. 329, p. 118–126. nian (McGhee, 1994) that destabilized Smit, J., Roep, Th. B., Alvarez, W., Montanari, S., and many existing taxa and rendered them Leroux, H., Warme, J. E., and Doukhan, J. C, 1995, Claeys, P., 1994, Stratigraphy and sedimentology of KT Shocked quartz in the Alamo breccia, southern Nevada: plastic beds in the Moscow Landing (Alabama) outcrop: prone to extinction by the flux of subse- Evidence for a Devonian impact event: Geology, v. 23, Evidence for impact-related earthquakes and tsunamis quent impact, volcanic, eustatic, or other p. 1003–1006. [abs.], in New developments regarding the KT event and natural events (e.g., Hut et al., 1987). Macdonald, D. I. M., Moncrieff, A. C. M., and Butter- other catastrophes in Earth history: Lunar and Plane- worth, P. J., 1993, Giant slide deposits from a Mesozoic tary Institute Contribution 825, p. 119–120. CONCLUSIONS fore-arc basin, Alexander Island, Antarctica: Geology, Stöffler, D., and Langenhorst, F., 1994, Shock metamor- v. 21, p. 1047–1050. phism of quartz in nature and experiment: 1. Basic The Alamo breccia is one of the McGhee, G. R., Jr., 1994, Comets, asteroids, and observation, experiment, and theory: Meteoritics, v. 29, p. 155–181. largest catastrophic megabreccias known the Late Devonian mass extinction: Palaios, v. 9, p. 513–515. Warme, J. E., 1994, Catastrophic Alamo Breccia, Upper in terms of its area, thickness, volume, and Devonian, southeastern Nevada [abs.], in New develop- clast sizes. In 1990 the breccia was recog- McLaren, D. J., and Goodfellow, W. D., 1990, Geologi- cal and biological consequences of giant impacts: ments regarding the KT event and other catastrophes nized within a formation that had been Annual Review of Earth and Planetary Sciences, v. 18, in Earth history: Lunar and Planetary Science Institute well studied by previous researchers. It p. 123–171. Contribution 825, p. 127–128. commonly forms the thickest cliff within Moore, G. W., and Moore, J. G., 1988, Large-scale Warme, J. E., and Sandberg, C. A., l995, The catastrophic Alamo breccia of southern Nevada: Record Guilmette Formation outcrops (Fig. 3) but bedforms in boulder gravel produced by giant waves in Hawaii, in Clifton, H. E., ed., Sedimentologic conse- of a Late Devonian extraterrestrial impact: Courier was unnoticed or misinterpreted as reef, quences of convulsive geologic events: Geological Forchungsinstitut Senckenberg, v. 188, W. Ziegler storm, karst, or tectonic breccias, and its Society of America Special Paper 229, p. 101–110. Commemorative Volume. huge clasts were unwittingly measured Moore, J. G., Normark, W. R., and Holcomb, R. T., Warme, J. E., Chamberlain, A. K., and Ackman, B. W., and described as being in situ. 1994, Giant Hawaiian underwater landslides: Science, 1991, The Alamo Event: Devonian cataclysmic breccia v. 264, p. 46–47. in southeastern Nevada: Geological Society of America We interpret the Alamo breccia to be Abstracts with Programs, v. 23, no. 2, p. 108. one result of a nearby extraterrestrial Piper, D. J. W., Shor, A. N., and Clarke, J. E. H., 1988, The 1929 “Grand Banks” earthquake, slump, and Winterer, E. L., Metzler, C. V., and Sarti, M., 1991, impact. We have yet to understand the turbidity current, in Clifton, H. E., ed., Sedimentologic Neptunian dykes and associated breccias (Southern details of its genesis—we seek the crater. consequences of convulsive geologic events: Geological Alps, Italy and Switzerland): Role of gravity sliding Society of America Special Paper 229, p. 77–92. in open and closed systems: Sedimentology, v. 38, p. 381–404. ACKNOWLEDGMENTS Poag, C. W., and Aubry, M.-P., 1995, Upper Eocene impactites of the U.S. east coast: Depositional origins, Ziegler, W., and Sandberg, C. A., 1990, The Late biostratigraphic framework, and correlation: Palaios, Devonian standard conodont zonation: Courier Supported by grants from the Forschungsinstitut Senckenberg, v. 121, p. 1–115. National Science Foundation (EAR- v. 10, p. 16-43. Manuscript received May 1, 1995; revision received 9106324) and UNOCAL. We thank B. W. Poole, F. G., and eight others, 1992, Latest Precambrian ■ to latest Devonian time: Development of a continental August 18, 1995; accepted September 20, 1995 Ackman, J. E. Estes-Jackson, Yarmanto, margin, in Burchfiel, B. C., et al., eds., The Cordilleran H.-C. Kuehner, and A. K. Chamberlain for results of their theses research (Colorado School of Mines) on the Guilmette Forma- tion and the Alamo breccia; W. Alvarez, A. K. Chamberlain, P. Claeys, H. E. Cook, P. E. Gretener, D. O. Hurtubise, W. N. GSA ON THE WEB Kent, J. R. Morrow, W. R. Page, W. J. Perry, F. G. Poole, and many others for field What’s new on the GSA home page on the World Wide Web? assistance and discussions; E. L. Winterer If you haven’t yet connected to the Web, the Uniform Resource Locator (URL) is for interpretation of the detachment zone http://www.aescon.com/geosociety/index.html. at the base of the breccia; J. L. Butler and J. R. Morrow for processing conodont If you want to know more about The Publications section has a samples; H.-C. Kuehner for separating the GSA Employment Service or about monthly table of contents and abstracts of quartz grains; J. Skok for making thin becoming a GSA Campus Representative, articles for the GSA Bulletin and Geology. sections; M. Attrep and the late C. Orth check the Membership section, which Also in this section is a guide for authors for providing iridium analyses; H. Leroux also has information on nominating a preparing manuscripts for submission to for TEM identifications of shocked quartz; member to fellowship and on obtaining GSA publications. GSA Today issues are and R. R. Charpentier, P. Claeys, W. J. forms for applying to become a GSA posted here for downloading and viewing. Perry, and E. L. Winterer for manuscript Member or Student Associate. For Congressional Contact Informa- reviews. See the Geoscience Calendar tion, see the Administration section. ■ section for a listing of meetings of general geological interest.

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