geology of brushy creek impact crater, st. helena parish, la

31353rd Annual Convention � Baton Rouge, Louisiana

ORIGIN OF A CIRCULAR DEPRESSION ANDASSOCIATED FRACTURED AND SHOCKED QUARTZ,

ST. HELENA PARISH, LAPaul V. Heinrich

Louisiana Geological Survey, Louisiana State University, Baton Rouge, LA 70803,[email protected]

ABSTRACT

In 1996, geologic mapping of the Amite 1:100,000 quadrangle revealed an anomalous circular

depression, now called the “Brushy Creek feature” within southwestern St. Helena Parish,

Louisiana. The Brushy Creek feature consists of a circular depression about two kilometers in

diameter with a low and dissected rim. Petrographic study of sand from this feature revealed the

presence of both highly fractured and shocked quartz, not found in adjacent outcrops of the

Citronelle Formation.

A review of the regional geology of the area found no evidence of tectonic processes, e.g.,

volcanism and salt diapirism, which could account for the development of this depression. In

addition, the geomorphic setting of the Brushy Creek feature is incompatible with the develop-

ment of siliciclastic karst that has created similar depressions, e.g., the Carolina Bays. At this

time, the Brushy Creek feature is hypothesized to be a dissected late, possibly terminal, Pleis-

tocene meteorite impact crater.

INTRODUCTION

Between 1996 to 1997, R. P. McCulloh, the author, and J. Snead of the Louisiana GeologicalSurvey compiled a draft geologic map of the Amite 1:100,000 quadrangle (McCulloh et al., 1997).The preparation of this geologic map revealed an anomalous, 2-kilometer diameter, circular struc-ture within the Greensburg 7.5-minute quadrangle, southwestern St. Helena Parish, Louisiana.Because of resource constraints, this feature was not investigated further and was mapped as asingle polygon of “Quaternary undifferentiated.” For this study, the feature is named the “BrushyCreek feature” for Brushy Creek, which has its headwaters within this circular depression.

Ridge and ravine topography, as defined by Hack (1960), characterizes the landscape of theCitronelle Formation within the region of the Brushy Creek feature. The ridge and ravine topogra-phy consists of alternating ridges and deeply incised valleys, which, except for the larger streamsand rivers, lack significant floodplains. Drainages within the region of the Brushy Creek featureexhibit a rectilinear pattern that in many places consist of prominent lineaments. Regional relief ofthe ridge and ravine topography is about 90 to 110 ft (27 to 34 m).

This is an erosionally graded, humid-climate landscape that is in dynamic equilibrium with the

GCAGS/GCSSEPM Transactions � Volume 53 � 2003314

erosional processes that formed and are modifying it. Except for the concordant summits of thecrests of major interfluves, all pre-existing constructional topography has been destroyed. Instead,the variable resistance of local structure and lithology of the underlying bedrock control the loca-tion and trend of drainages and ridges within ridge and ravine topography (Hack, 1960). Within theregion of the Brushy Creek feature, McCulloh (2002, 2003) discussed the presence of numerous,prominent drainage alignments which he argued to be controlled by cryptic systematic fracturingof the Citronelle Formation.

The Brushy Creek feature occurs as a noticeable circular “hole” within the ridge and ravinetopography that characterizes the surface of the Citronelle Formation within southeast Louisiana.This feature is roughly circular with a relief of about 50 ft (15 m) and a diameter of about 1.2 miles(2.0 km) (Fig. 1). The rim of the Brushy Creek feature exhibits a slight polygonal shape. The head-waters of Brushy Creek have breached the southeast rim of the feature and the northern rim isalmost breached by a ravine tributary of Chandler Branch. The center of this feature lies at about3405760N, 717870E, Zone 15, and 7 miles (11 km) southwest of Greensburg within St. Helena Parish,Louisiana.

Erosion has sculpted regional ridge and ravine topography from fluvial sand and gravel of theCitronelle Formation (Campbell, 1971; Mossa and Autin, 1989). In general, the Citronelle Formationconsists primarily of varegated and mottled, poorly sorted, fine to very coarse, sandy gravel, grav-

Figure 1. Portion of the Greensburg 7.5 topographic quadrangle illustrating topographic expression of BrushyCreek feature. Open circles show location of samples associated with the Brushy Creek feature. “16SH”hasbeen dropped from sample numbers. For example, “”PD” is sample locality 16SHPD.

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elly sand, and sand containing beds of silt, clay, and mud. Typically, the beds are of limited verticaland lateral extent. According to Campbell (1971), the thickness of the Citronelle Formation is about300 to 350 ft (91 to 107 m) within the vicinity of the Brushy Creek feature.

Locally, the Citronelle Formation consists of cross-bedded, massive, poorly sorted fine to coarsesand underlain by laminated clay and silt. Field investigations indicate that the sand consists of 30to 40 ft (9 to 12 m) of deeply weathered, reddish brown, fine to coarse, poorly sorted sand. In out-crops, the sand is typically massive. However, in an exposure on the edge of the Brushy Creekfeature, locality 16SHPC, the Citronelle Formation is cross-bedded. As exposed in a Kentwood Brickand Tile Company brick pit immediately east of the Brushy Creek feature, at least 20 ft (6 m) oflaminated silts and clays underlie these sands. These silts and clays consist of cyclic beds of meter-thick laminated silt that grades upward into laminated clay. Discussions with the staff of theKentwood Brick and Tile Company indicate that in their explorations for brick clay, they found thelaminated clays and silts to be absent in holes drilled within the interior of the Brushy Creek featurebut present within holes drilled outside of its rim. Very little is known about the sediments underly-ing the laminated silt and clay beds.

As classified by Folk (1980), the sand fraction of the Citronelle Formation varies regionally incomposition from quartzarenites to sublitharenites. The sand-size fraction consists of 90 to 97percent quartz. The remaining 3 to 10 percent consists of chert, quartzite, iron oxide, and heavyminerals. Feldspar is absent from both the sand and gravel fractions. The gravel consists largely ofchert with lesser amounts of quartz, quartzite, and ironstone (Campbell, 1971).

Underlying the Citronelle Formation are 6 to 7 miles (about 10 to 11 km) of Cenozoic to Meso-zoic sedimentary strata overlying continental crust stretched by the opening of the Gulf of Mexico(Sawyer et al., 1991). Within the area of the Brushy Creek feature, the upper 11,000 to 12,000 ft (3,350to 3,660 m) consist of Cenozoic sediments of the Midway, Wilcox, Claiborne, Jackson, and Vicksburggroups and undifferentiated, largely siliciclastic, Neogene strata. Within St. Helena Parish, thesestrata dip homoclinally to the southwest lacking indication of any major faulting or salt tectonics inthe vicinity of the Brushy Creek feature (Howe, 1962; Bebout and Gutiérrez, 1983).

METHODOLOGY

An examination was made of the entire Brushy Creek feature with emphasis on the northernthird of the feature. The examination of the feature consisted of the description of exposures, exami-nation of gravel found in streams draining the feature, and collection of samples from such loca-tions.

Only one exposure, the Gehee Section, locality 16SHPC, revealed a complete section of the distalrim of this feature. This exposure consists of an upper bed of 7 to 10 ft (2 to 3 m) of massive siltysand and sandy silt. At about 5 ft (1.5 m) below the surface, a zone about 8 in (20 cm) thick containsrounded, dime-size clasts of purple silty clay floating within a silty sand matrix. The purple color ofthe silty clay clasts indicates that they came from the Citronelle Formation. Developed within theupper part of the silty sand is the profile of the modern soil with pronounced A and B horizons. Atthe base of the sandy silt and lying directly on the underlying Citronelle Formation, the exposurecontains a 5 to 12 in (13 to 30 cm) thick gravelly mud containing abundant rounded clasts com-posed of mud, clay, and ironstone nodules. At this time, it is difficult to determine the origin of thisbed. Within the Gehee Section, the gravelly mud bed overlies highly fractured and cross-beddedsand of the Citronelle Formation exposed within a ditch. It is deeply weathered saprolite. This unitis highly oxidized and shows well-developed gleying of the sand along abundant fractures and rootmolds. The Gehee Section is the only known exposure in which the Citronelle Formation is highlyfractured.

Samples were collected from locations within the Brushy Creek feature. North of and adjacentto Louisiana State Highway 37, samples of the sediment composing the rim of the Brushy Creekfeature were collected from surface exposures at localities 16SHPC, 16SHPD, and 16SHPT, and froman auger hole, locality 16SHPL. Within this auger hole, samples were taken at depths of 1.5, 3, and

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4.5 ft (0.5, 0.9, and 1.4 m). Within the Gehee Section, samples were taken from sandy silt, about 5 ft(1.5 m) below the surface; from the gravelly mud; and from the underlying Citronelle Formation 1.5ft (0.5 m) below its top. Finally, other samples were collected from within the Brushy Creek featureat three localities, 16SHPO, 16SHPP, and 16SHPQ.

These samples were processed for petrographic study. First, the samples were disaggregated bycomplete air drying and then immersion in a water-filled beaker for about an hour. The resultingmud was gently washed through a 200 mesh (3.75 phi) screen to remove the fine fraction. After thefine fraction had been removed, the sample was again air-dried in an open tray exposed to directsunlight. Once dried, the sand sample was dry sieved into 18 to 60 mesh (0.0 to 2.0 phi), and 60 to200 mesh (2.0 to 3.75 phi), fractions. Finally, thin sections of each fraction and unprocessed, butimpregnated, pieces of specific samples were prepared for petrographic examination. Several ofthese thin sections were stained for both plagioclase and orthoclase feldspars.

Because the interior of the Brushy Creek feature was inaccessible for examination when theinitial research was conducted, samples of sand originating from the interior and from the deeplycut rim were collected off site. Sand was collected from the modern alluvium of Brushy Creek atlocality 16SHPA about 160 ft (50 m) upstream of it’s intersection with Louisiana State Highway 449north of Jack, Louisiana. At this locality, samples were collected from the floodplain and a channelbar in Brushy Creek.

Six control samples, localities 16SHCY, 16SHG5, 16SHG6, 16SHG7, 16SHG8, and 16SHG9, werecollected from nearby outcrops of Citronelle Formation within a radius of 1.5 to 4.5 miles (2.4 to 7.2km) of the Brushy Creek feature. Locality 16SHCY is the same as Stop 4 of Mossa and Autin (1989).Two additional control samples were collected from an outcrop near Easleyville, Louisiana, locality16SHFF, and from a gravel pit at Kentwood, Louisiana, locality 16TAKP, which is Stop 5 of Mossaand Autin (1989). All of these samples were processed in the same manner as samples from theBrushy Creek feature. Separate sets of screens were used for sieving the control samples andsamples from the Brushy Creek feature. Thin sections of the 18 to 60 mesh (0.0 to 2.0 phi), and 60 to220 mesh (2.0 to 3.75 phi), fractions were prepared for each sample.

Abundant ironstone nodules were collected from local streams draining the Brushy Creekfeature and the gravelly mud exposed in the Gehee Section, locality 16SHPC. Several dozen nod-ules from locality 16SHPC and the streambeds were cut on a trim saw. Two petrographic thinsections were made from nodules from locality 16SHPC. Representative specimens of these noduleswere tested for the presence of significant concentrations of nickel using dimethylglyoxime.

RESULTS

All of the samples consisted of quartzarenite to sublitharenite sand containing about 90 to 95percent quartz. The remaining percentage of sand grains consisted of chert, quartzite, iron oxide,and heavy minerals. Feldspar and mica were not noted in any of these samples, except samplesfrom locality 16SHPD and the gravelly mud of 1ocality 16SHPC. All of the samples consisted ofsubangular to well-rounded grains of sand. Unlike control samples, grains of sand in samples takenwithin the Brushy Creek feature often exhibited ragged edges in thin section from the disintegrationof sand grains during processing.

Intensely fractured quartz was found in all of the samples collected from the rim and interior ofthe Brushy Creek feature. The proportion of intensely fractured quartz varied from less than tenpercent to almost 100 percent of the quartz and chert sand. The sample with the least amount ofintensely fractured quartz came from the Citronelle Formation exposed at the Gehee Section, local-ity 16SHPC. The sand from localities 16SHPD, 16SHPO, 16SHPP, 16SHPQ, and the gravelly mud inthe Gehee Section, consisted either entirely or almost entirely of intensely fractured grains. Incontrast, none of the control samples possessed the intensely fractured grains observed in samplesassociated with the Brushy Creek feature.

Examination of thin sections revealed an abundance of intensely fractured quartz of types notobserved within the control samples collected from outside the feature. One type consists of quartz

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grains exhibiting an irregular, interlocking network of fractures (Fig. 2). Another type consists of quartzgrains with blocky, rectilinear fracture sets (Fig. 3). The former type of fracturing has been illustratedand described by Kieffer (1971) and Shoemaker and Kieffer (1979) from shocked Coconino Sandstonecollected from Barringer (Meteor) Crater in Arizona. Dr. W. Feathergale Wilson (2002, per. commun.)reported having seen identical intensely fractured quartz in thin sections from the Bee Bluff ImpactStructure of Texas. The accumulation of iron oxides within many of these fractures clearly demonstratesthat they are not artifacts of the thin section preparrtion. The intensely fractured nature of the sandmade the preparation of thin sections from this sand noticeably difficult.

A unique and significant feature was noted in samples from two locations. Thin sections madefrom sand collected from locality 16SHPA revealed quartz grains with sets of planar features.Stephen Benoist (2003, per. commun.) noted that several of the grains exhibited closely spaced setsof planar features in only one direction. In addition, a few of the quartz grains exhibited sets ofplanar features in two different intersecting directions (Fig. 4). Examination of these grains byStephen Benoist (2003, per. commun.) revealed that the planar features averaged 45 degrees and 33degrees, which respectively represent the {1012} and {1122} orientations. Planar features along bothorientations, as noted by Koerbel (1997) and Stoffler and Langenhorst (1994), are characteristic ofplanar deformation features (PDF) created by shock metamorphism. The abundance of planarfeatures and shocked quartz within the sand from 16SHPA argue against their having been re-worked from distant sources, e.g., Cretaceous - Tertiary boundary deposits, but rather suggest theyare derived from a nearby primary source, i.e., the Brushy Creek feature. Finally, the gravelly mudoverlying the Citronelle Formation in the Gehee Section contains an abundance of quartz withsimilar planar features and fractures (Fig. 5) which are currently being study. In sharp contrast,none of the sand in the eight samples of Citronelle Formation collected from outcrops within thevicinity of the Brushy Creek feature contained such planar features or fractures.

Figure 2. Photomicrograph of thin section showing coarse, intensely fractured quartz sand from quartz fromgravelly mud in the Gehee Section, locality 16SHPC. Photomicrograph taken with cross-polarized light.

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Figure 3. Photomicrograph of thin section showing rectilinear fractures within coarse intensely fractured,quartz sand from locality 16SHPD. “i” indicates fractures coated with iron oxides. Photomicrograph takenwith cross-polarized light.

Figure 4. Photomicrograph of thin section showing a coarse grain of shocked quartz from locality 16SHPA.“i” indicates fractures and small cavities associated with PDFs filled with iron oxides. “A” indicates {1122}oriented PDFs and “B” indicates {1012} oriented PDFs. Photomicrograph taken with cross-polarized light.

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In addition to their orientation and parallelism, the planar features show preferential etching asa result of intensive weathering of the quartz grains. This etching has created cavities along theseplanar features that have become filled with pedogenic iron oxides (Figs 4 and 5). Just as amor-phous silica associated with PDFs is revealed by etching with hydrofluoric acid, weathering hasnaturally etched these grains along planar features and filled the resulting cavities with iron oxides.The natural etching of these planar features strongly suggests that they are PDFs from whichweathering has removed amorphous silica.

Small percentages of orthoclase, plagioclase, and mica were found in thin sections of unproc-essed samples from 16SHPD and gravelly mud of locality 16SHPC. The feldspar and micas weredeeply weathered and corroded from post-depositional weathering In contrast, all of the controlsamples taken from the Citronelle Formation lacked any feldspar or mica.

One material, which was searched for and not found, was highly weathered, “terrestrialized,”fragments of meteorites called “iron shale.” Despite studying dozens of pieces of ironstone gravelfrom streams draining the Brushy Creek feature and the gravelly mud layer, none of this materialcould be identified as “iron shale.”

DISCUSSION

The mechanisms capable of producing a circular feature like the Brushy Creek feature arediapirism, volcanism, siliciclastic karst, and meteorite impact cratering. Any relationship to saltdiapirism can be dismissed, because the Brushy Creek feature lies within a portion of the LouisianaCoastal Plain underlain by extremely thin Louann Salt (Sawyer et al. 1991). Within this area, the

Figure 5. Photomicrograph of thin section showing planar features and fractures in a coarse grain of quartzfrom gravelly mud in the Gehee Section, locality 16SHPC. “i” indicates iron oxide along planar features andfractures. “p” indicates other planar fractures. Photomicrograph taken with cross-polarized light.

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Louann Salt is much too thin to have created salt diapirs of sufficient size to have penetrated closeenough to the surface to have produced any surface expression. The lack of salt diapirs is furtherconfirmed by oil and gas fields in the surrounding region which lie south and north of the BrushyCreek feature. North of the Brushy Creek feature, the structure associated with the Joseph Branchand Greensburg fields consists of very low relief structural nosing lacking anticlinal closure greaterthan 20 ft (6 m) at a depth of about 12,700 ft (3,900 m) (Corcoran, 1993; Corcoran et al., 1993). Similarstructural nosing with less than 100 ft (30 m) of closure at a depth of 14,400 to 14,600 ft (4390 to 4450m) created the Baywood and Beaver Dam Creek fields, which lie south of the Brushy Creek feature.Thus, none of the local oil and gas fields provide any evidence of structures associated with saltdiapirism. Also, James D. Coleman (2002, pers. commun.) observed that the Brushy Creek area hasbeen explored in such great detail that it is highly improbable, if not impossible, that any saltdiapirs of the size capable of producing the Brushy Creek feature remain undiscovered.

Volcanism can produce isolated rimmed circular depressions called “maars.” The Brushy Creekfeature can be discounted as being a maar on the basis of field observations and regional geology.None of the samples from auger holes, surface exposures, or stream alluvium from the bed ofBrushy Creek contained any material of clear volcanic origins, e.g., volcanic sediments or clasts. Thelack of any volcanic materials within the Brushy Creek feature together with its passive marginsetting hundreds of kilometers from known Holocene age volcanism, e.g., Byerly (1991), makes ithighly unlikely that the Brushy Creek feature is volcanic in origin.

One plausible hypothesis for the origin of the Brushy Creek feature is that it is due to siliciclastickarst. The upper 5,000 to 6,000 ft (1,520 to 1,830 m) of the underlying Cenozoic strata lack anysignificant beds of carbonates (Howe, 1962; Bebout and Gutiérrez, 1983). This precludes formationof the feature by the dissolution of carbonates. However, as discussed by May and Warne (1999), thedissolution of siliciclastic sediments has created large circular and oval landforms, including nu-merous circular and oval enclosed depressions within the Gulf Coastal Plain of Mississippi andAlabama, and countless Carolina Bays of the Atlantic Coastal Plain. These landforms have a resem-blance to the Brushy Creek feature on a superficial level.

However, there are some notable differences between siliciclastic karst, such as the CarolinaBays of the Atlantic Coastal Plain and the enclosed depressions of the Mississippi and Alabamacoastal plains, and the Brushy Creek feature. First, unlike the Brushy Creek feature, which occurswithin very well-developed ridge and ravine topography, siliciclastic karst typically develops onflat, poorly drained, and undissected geomorphic surfaces lacking well-defined drainage systems.Well-developed drainage systems would cause lateral flow of surface and near-surface water, anderosion, which would greatly inhibit the vertical-drainage weathering needed to create siliciclastickarst (May and Warne, 1999). Second, siliciclastic karst characteristically occurs not as isolateddepressions, such as the Brushy Creek feature, but as large fields of multiple depressions thatpockmark large. Third, the Brushy Creek feature is considerably larger than the Gulf Coast depres-sions in Mississippi and Alabama which range from 150 to 2,600 ft (45 to 790 m) in diameter andfrom 3 to 40 ft (0.9 to 12 m)(Otvos, 1997). Fourth, unlike the Brushy Creek feature, the Gulf Coastdepressions lack any associated ridges or raised rims as noted by Otvos (1997). Finally, thesiliciclastic karst hypothesis fails to explain the direct association of shocked and intensely fracturedquartz with the Brushy Creek feature. Given these differences, a siliclastic origin of the Brushy creekfeature is discounted.

The most logical hypothesis for the origin of the Brushy Creek feature is that it was created by alate Pleistocene meteorite impact and that in fact, it is the Brushy Creek Impact Crater. Its roughlycircular to slightly polygonal shape is consistent with known meteorite impact craters, e.g. theBarringer (Meteor) Crater, in which local fractures have influenced the deformation of the targetmaterial. The Brushy Creek feature creates a well defined anomalous “hole” within the local ridgeand ravine topography that is unique to this part of the Louisiana Coastal Plain. As noted previ-ously discussions with the staff of the Kentwood Brick and Tile Company revealed that holesdrilled in the search for clay resources revealed a large stratigraphic “hole” in the local distributionof laminated silts and clays corresponding to the Brushy Creek feature. The presence of feldspars

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and mica in samples from 16SHPC and 16SHPD indicates that sediments from strata underlying theCitronelle Formation have been brought to the surface from hundreds of feet below. All of theseobservations are consistent with the formation of the Brushy Creek feature by impact processes.

The presence of shocked and intensely fractured quartz within sediment samples collected fromthe Brushy Creek feature provides additional direct evidence of impact processes having modifiedthese sediments. The fractured quartz indicates that the sediments, although having never beenburied more than beneath 60 ft (18 m) of overburden, were subjected to intense pressures such thatin some cases almost a hundred percent of the grains were completely shattered. The presence ofactual shocked quartz with PDFs provides direct proof of shock metamorphism resulting from animpact.

CONCLUSIONS

The preliminary study of a rimmed, circular depression, which is about 1.2 miles (2 km) indiameter, within St. Helena Parish, Louisiana, resulted in the hypothesis that it is a crater resultingfrom a hypervelocity meteorite or comet impact. The Brushy Creek feature is a regionally uniquelandform superimposed upon the local ridge and ravine topography. This circular feature occurs inan area devoid of volcanic activity, salt diapirism, and carbonate sinkholes that might explain itsorigin. Furthermore, its location is incompatible with the formation of siliciclastic karst. Limitedsubsurface data indicate the presence of a “hole” in the underlying stratigraphic units consistentwith the origin of the Brushy Creek feature by impact processes. Finally, the presence of shockedquartz provides direct evidence of impact metamorphism. Impact processes are further suggestedby abundant intensely fractured quartz within the sediments comprising its rim.

At this time, the Louisiana Geological survey is preparing for further research of the BrushyCreek feature. The author, Douglas Carlson, and Richard P. McCulloh are considering studying theinternal structure of the Brushy Creek feature using various geophysical techniques, includingmagnetic and seismic surveys. Plans to use a Giddings soil probe to acquire cores and prepare ashallow cross-section across this feature are under consideration. The silty and sandy compositionof rim of the Brushy Creek feature indicate that Ground Penetrating Radar might produce usefulcross-sections of the rim when ground truthed by cores from a Giddings rig. Finally, StephenBenoist (Louisiana State University) and the author are planning to examine the petrographiccharacteristics of shock metamorphosed sediment associated with this feature.

ACKNOWLEDGEMENTSThe initial discovery of the Brushy Creek feature was made in the course of geologic mapping

funded by the United States Geological Survey STATEMAP program under cooperative agreement1434-HQ-96-AG-01490. Additional work on this study was conducted with the encouragement andsupport of Chacko John, Director of the Louisiana Geological Survey (LGS). I thank David T. King,Jr., Donald R. Lowe, Don Johnson, W. Feathergale Wilson, and Richard P. McCulloh for their discus-sions, advice, and encouragement. I also thank Christian Koerbel, Scott Harris, and Stephen Benoistfor technical advice regarding shocked quartz. I greatly appreciate the comments made by the twogeologists who reviewed this paper for publication.

I thank Xiaogang Xie for use of the photomicroscope at the LSU Department of Geology andGeophysics SEM and Electron Microprobe Laboratory. I thank Rick Young for the preparation ofseveral thin sections. I am grateful to National Petrographic Services, Inc for high-quality thinsections that proved to be essential to my research. Finally, I am very grateful to the KentwoodBrick and Tile Company, William A. Gehee of Greensburg, Louisiana, and Soterra LLC, inc., Jack-son, Mississippi for access to their property.

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Corcoran. M. K., C. P. Cameron, and M. A. Meylan, 1993, The lower Tuscaloosa Formation in the GreensburgField and Joseph Branch Field areas, St. Helena Parish, Louisiana: Gulf Coast Association of GeologicalSocieties Transactions. v. 43, p. 87-96.

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Koerbel, C., 1997, Impact cratering: the mineralogical and geochemical evidence. , in K. s. Johnson and J. A.Campbell, eds., Ames Structure in Northwest Oklahoma and Similar Features: origin and Petroleum Produc-tion (1995 Symposium): Oklahoma Geological Survey Circular, no. 100, p. 30-54.

McCulloh, R. P., 2002, Patterns of streams of Louisiana: Louisiana Geological Survey News. v. 12, no. 1, p. 1-2.McCulloh, R. P., 2003, The stream net as an indicator of cryptic systematic fracturing in Louisiana: Southeastern

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(preliminary): Prepared in cooperation with U.S. Geological Survey, STATEMAP program, under cooperativeagreement no. 1434-HQ-96-AG-01490, 1:100,000-scale map plus explanation and notes.

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Sawyer, D. S., R. T. Buffler, and R. H. Pilger, Jr., 1991, The crust under the Gulf of Mexico Basin, in A. Salvador, ed.,The Gulf of Mexico Basin: Geological Society of America Decade of North American Geology, Geology ofNorth America, v. J, p. 53-73.

Shoemaker, E. M., and S. W. Kieffer, 1979, Guidebook to the Geology of Meteor Crater, Arizona: Publication No.17, Center for Meteorite Studies, Arizona State University, Tempe, Arizona, 65 p.

Stoffler, D., and F. Langenhorst. 1994. Shock metamorphism of quartz in nature and experiment. 1. Basic observa-tion and theory: Meteoritics. v. 29, p. 155-181.

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geology of brushy creek impact crater, st. helena parish, la

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