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Alamo event, Nevada; crater stratigraphy and impact breccia realms

Jesus A. Pinto and John E. Warme
Alamo event, Nevada; crater stratigraphy and impact breccia realms (in The sedimentary record of meteorite impacts, Kevin R. Evans (editor), J. Wright Horton (editor), David T. King (editor) and Jared R. Morrow (editor))
Special Paper - Geological Society of America (2008) 437: 99-137


Based on evaluation of past results and new research, we have partitioned the distribution of the Alamo Breccia in southeastern Nevada and western Utah into six genetic Realms that provide a working model for the marine Late Devonian Alamo Impact Event. Each Realm exhibits discrete impact processes and stratigraphic products that are enumerated here. The first five form roughly concentric semicircular bands across the Devonian shallow-water carbonate platform. These are: (1) Rim Realm, where a newly defined impact stratigraphy includes both autogenic and allogenic breccias associated with the crater rim; (2) Ring Realm, where breccias are now interpreted to have formed sequentially by seismic shock, passage of the ejecta curtain, tsunami waves or surge, and runoff that accumulated over tilted terrace(s) bounded by syn-Event, ring-forming, listric faults; (3) Runup Realm, where graded breccias were stranded by tsunami surge or waves; (4) Runoff Realm, where sheet-floods carried traces of impact debris across the distal platform beds and channels filled with impact debris; (5) Seismite Realm, where near-surface beds far across the platform were uniquely deformed; and (6) Runout/Resurge Realm, where offshore channels of thick off-platform Alamo Breccia, together with large-scale olistolith(s), signal contemporaneous massive collapse of the platform margin, possibly into the central crater. Five breccia Units characterize the newly interpreted Rim Realm, in ascending order: (1) deformed target rocks, (2) injected dikes and sills, (3) chaotic fallback, (4) smeared fallback, and (5) resurge. This succession is covered by deepwater limestones deposited inside the crater rim, or across a new slope created after platform margin collapse. Unit 1 exhibits shatter-cone-like structures interpreted as impact products. Newly discovered Ordovician and probable older meter-scale clasts in Unit 3 confirm a minimum excavation depth of 1.5 km. Microscopic components in Units 3 and 4 indicate high pressures (>10 GPa), probable quenched carbonate melt, and accreted particles that may be new kinds of impact products. Postimpact tectonics and other factors obscure the full panorama, including the location and character of the missing central crater, but the assemblage of Realms offers a working model to compare with expected impact paradigms.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 437
Title: Alamo event, Nevada; crater stratigraphy and impact breccia realms
Title: The sedimentary record of meteorite impacts
Author(s): Pinto, Jesus A.Warme, John E.
Author(s): Evans, Kevin R.editor
Author(s): Horton, J. Wright, Jr.editor
Author(s): King, David T., Jr.editor
Author(s): Morrow, Jared R.editor
Affiliation: INTEVEP-PDVSA, Los Teques, Venezuela
Affiliation: Missouri State University, Department of Geography, Geology and Planning, Sprinfield, MO, United States
Pages: 99-137
Published: 2008
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 978-0-8134-2437-9
References: 114
Accession Number: 2008-110041
Categories: StratigraphyGeomorphology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., strat. col., sketch maps
N36°00'00" - N40°00'00", W117°00'00" - W113°00'00"
Secondary Affiliation: U. S. Geological Survey, USA, United StatesAuburn University, USA, United StatesSan Diego State University, USA, United StatesColorado School of Mines, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200840
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