Cryptoexplosive Structure Near Wetumpka, Alabama - Key Findings on Meteorite-Impact Geology

Sep 29, 2026

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A cryptoexplosive structure likely formed by a meteorite impact lies near Wetumpka, Alabama, along the boundary between the Gulf Coastal Plain and the Piedmont provinces. The report describes an approximately 6.5 km concentric structural system, a surrounding metamorphic rim in Cretaceous sedimentary rocks, an arcuate schist ridge, chaotic central orientations, and concentric marginal faults with 240–300 m displacement. Shock effects occur in feldspar and quartz. The post-Mooreville (Late Cretaceous) to pre-terrace (Pleistocene?) age supports the proposed name “Wetumpka Astrobleme.”

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CategoryResearch & Report

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ABSTRACT

INTRODUCTION

GENERAL GEOLOGY OF THE AREA

81158x ieop.Per RYPrOE×PLosle STRCTUE NE1R H GEOLOGICAL SURVEY OF ALABAMA REPRINT SERIES 44 cE Dpipr Uaiver Nor fBah Dakat0 Digitized by the Internet Archivein 2022 with funding fromKahle/Austin Foundation https://archive.org/details/cryproexplosives0000unse GEOLOGICAL SURVEY-OF ALABAMA Philip E.LaMoreauxState Geologist MINERAL RESOURCES DIVISION Thornton L.NeatheryChief Geologist REPRINT SERIES 44 CRYPTOEXPLOSIVE STRUCTURE NEARWETUMPKA,ALABAMA By Thornton L.Neathery,Robert D.Bentley,and Gregory C.Lines Reprinted fromGeological Society of America Bulletin,v.87,p.567-573 THORNTON L.NEATHERY Geological Survey of Alabama,University,Alabama 35486ROBERT D.BENTLEY Central Washington State College,Ellensburg,Washington 98926GREGORY C.LINES U.S.GeologicalSurvey,Cheyenne,Wyoming 82001 # ABSTRACT A cryptoexplosive structure that proba-bly resulted from a meteorite impact is lo-cated near Wetumpka,Alabama(long32°31'42"N,lat 86°14'12"W),along theboundary between the Gulf Coastal Plainand the Piedmont physiographic provinces.Its main features are(1)an approximatelyconcentric structural system about 6.5 kmin diameter,its rim formed by metamorphicrocks and surrounded by Cretaceoussedimentary rocks;(2)an arcuate ridge ofschist extending two-thirds of the wayaround the structure,which stands 60 to150m above the adjacent Piedmont pene-plain;(3)the chaotic orientation of Creta-ceous and metamorphic units in the centerof the structure,perhaps corresponding tothe central rebound area;(4)concentricmarginal faults with an estimated dis-placement of 240 to 300m;and (5)shockeffects in feldspar and quartz crystals inrock units that form the rim. A southward-sloping metamorphic rocksurface and a northward-thinning Creta-ceous cover afforded a unique geologic set-ting for the formation and possible preser-vation of the structure.A shallow marinesetting may best explain the shape and dis-tribution of rocks.Nowhere else along the 1,600 km of Coastal Plain-Piedmontboundary is there any faulting of the mag-nitude described in this report.The age ofthe Wetumpka cryptoexplosive structure ispost-Mooreville (Late Cretaceous)andpre-terrace(Pleistocene?).The name“Wetumpka Astrobleme”is proposed forthis structure.Key words:structural geol-ogy;Wettmpka Astrobleme;cryptoexplo-sive structure;Wetumpka,Alabama. # INTRODUCTION A circular area of intensely deformedmetamorphic rock mixed with slices ofsiliceous clastic and carbonate units of Cre-taceous age is astride the CoastalPlain-Piedmont boundary at long32°31'42"N,lat 86°14'12"W,approxi-mately 24 km northeast of Montgomery,Alabama(Fig.1).The deformation proba-bly resulted from the impact of a meteorite.The structure,herein called the WetumpkaAstrobleme,is the only cryptoexplosive Cryptoexplosive structure near Wetumpka,Alabama structure known in the Piedmont and is oneof the few known that involves crystallineand younger Cretaceous rocks.A possiblecryptoexplosive structure in the CoastalPlain province is near Kilmichael,Missis-sippi(Priddy,1943,p.42-46).The Kil-michael structure exposes and involves onlyCoastal Plain sedimentary rocks.A reviewof the index to terrestrial impact structures(Freeberg,1966,1969)indicated that nocryptoexplosive or impact structure hasbeen described from a geologic setting simi-lar to the Wetumpka Astrobleme.The con-trast of chaotically oriented sedimentaryunits inside the structure and flat-lying,undisturbed sedimentary units outside issimilar to the structural relations at Kil-michael,Mississippi(Priddy,1943);FlynnCreek,Tennessee(Roddy,1968);andJeptha Knob,Kentucky (Seegar,1968).Thegeometric configuration and structural styleis comparable to the cryptoexplosive struc-tures at Wells Creek,Tennessee(Wilsonand Stearns,1968). The unusual nature of the structure nearWetumpka was referred to in the field notes Figure 1.Location of mapped area,east-central Alabama at boundary between GulfCoastal Plain and Piedmont province.Outcroplimit

FAQs

Where is the Wetumpka cryptoexplosive structure located?

It is near Wetumpka, Alabama, along the boundary between the Gulf Coastal Plain and the Piedmont physiographic provinces.

What are the main geological features described in the structure?

The report identifies a ~6.5 km concentric structural system with a metamorphic rim, an arcuate schist ridge, chaotic central orientations, concentric marginal faults with 240–300 m displacement, and shock effects in feldspar and quartz.

How old is the Wetumpka astrobleme and what name is proposed?

Its age is described as post-Mooreville (Late Cretaceous) and pre-terrace (Pleistocene?). The name “Wetumpka Astrobleme” is proposed.

Sep 29, 2026

Upload by Mabel