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SEQUENCE STRATIGRAPHY ACROSS AN EVENT DEPOSIT: PRE-, SYN-, AND POSTIMPACT ACCOMMODATION TRENDS AND SEQUENCE DEVELOPMENT SURROUNDING THE ALAMO IMPACT BRECCIA

By
Benjamin E. Rendall
Benjamin E. Rendall
ExxonMobil Exploration Company, 22777 Springwood Village Parkway, Spring, Texas 77389, USA
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Leif Tapanila
Leif Tapanila
Division of Earth Science, Idaho Museum of Natural History, 921 S. 8th Avenue, Pocatello, Idaho 83209, USA
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Published:
January 01, 2017

ABSTRACT

The effects of bolide impacts on carbonate platform sedimentation and stacking patterns are poorly understood, partly because the geological evidence for marine impact sites is typically unavailable. Givetian–Frasnian carbonates in southern Nevada contain a continuous record of sedimentation before, during, and after the Devonian (Frasnian) Alamo impact event (382 Ma), evidenced mainly by the regional Alamo Breccia Member of the Guilmette Formation. Two transects arranged from seven stratigraphic sections measured through the lower ~300 m of the Guilmette Formation record environmental lithofacies deposited from peritidal to deep subtidal zones. Stacking patterns of peritidal and subtidal cycles indicate four relatively high-frequency sequences superimposed on the larger-magnitude eustatic Taghanic onlap of the Kaskaskia sequence. Sequences are interpreted based on facies proportions and cycle stacking trends because of a lack of prominent erosional surfaces developed on the Frasnian greenhouse shelf. Lateral correlation of facies and cycle stacking indicates that the Alamo impact took place during the late phase of sedimentation during deposition of “Sequence 3” in the Guilmette Formation. Underlying facies and surfaces were obliterated and excavated during the impact, resulting in truncated terminations of sequence boundary and maximum flooding zones. Eustatic sea-level rise during the late Frasnian resulted in an overarching shoreline backstep and deepening of vertical facies associations prior to the Alamo impact. Additional accommodation was gained instantaneously as a result of the Alamo impact, which formed a local, steep-sided basin and shifted the slope break of the platform margin. Postimpact sedimentation within the Alamo crater is characterized by condensed sections of continuously deposited thin-bedded mudstones with pelagic (tentaculites) fauna. Thick shoreface sandstones were deposited in a lowstand clastic wedge as the last phase of crater fill in the study area. While accommodation and depositional environment changed dramatically at the impact site, long-term sedimentation trends immediately outside of the impact site were unaffected by the Alamo event, demonstrating that the forces that control overall carbonate platform growth and evolution (tectonics, climate, oceanography, biology) are of far greater importance than even regional-scale physical perturbations such as meteor impacts.

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Contents

Society for Sedimentary Geology

NEWADVANCES IN DEVONIAN CARBONATES: OUTCROP ANALOGS, RESERVOIRS AND CHRONOSTRATIGRAPHY

Society for Sedimentary Geology
Volume
107
ISBN electronic:
9781565763456
Publication date:
January 01, 2017

GeoRef

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