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

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Alamo Megabreccia: Record of a Late Devonian Impact in Southern Nevada John E 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 accel- erated 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. Copyright not claimed on content prepared wholly by U.S. Government employ- Washington Report .................... 14 Calendar .............................. 31 ees within the scope of their employment. Permission is SAGE Remarks ......................... 16 Classifieds ............................. 32 granted to individuals to photocopy freely all items other than the science articles to further science and PEP Talk ............................... 16 Award Nominations Deadlines .......... 34 education. Individual scientists are hereby granted per- mission, 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 edu- cation 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 member- ship 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
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