Geology Of Brushy Creek Impact Crater, St. Helena Parish, La

ORIGIN OF A CIRCULAR DEPRESSION AND ASSOCIATED FRACTURED AND SHOCKED QUARTZ, ST. HELENA PARISH, LA Paul 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 development 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, Pleistocene meteorite impact crater.

INTRODUCTION Between 1996 to 1997, R. P. McCulloh, the author, and J. Snead of the Louisiana Geological Survey 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 structure 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 a single polygon of “Quaternary undifferentiated.” For this study, the feature is named the “Brushy Creek 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 the Citronelle Formation within the region of the Brushy Creek feature. The ridge and ravine topography consists of alternating ridges and deeply incised valleys, which, except for the larger streams and rivers, lack significant floodplains. Drainages within the region of the Brushy Creek feature exhibit a rectilinear pattern that in many places consist of prominent lineaments. Regional relief of the 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 53rd Annual Convention
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Heinrich erosional processes that formed and are modifying it. Except for the concordant summits of the crests 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 location and trend of drainages and ridges within ridge and ravine topography (Hack, 1960). Within the region 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 fracturing of the Citronelle Formation. The Brushy Creek feature occurs as a noticeable circular “hole” within the ridge and ravine topography 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 headwaters of Brushy Creek have breached the southeast rim of the feature and the northern rim is almost breached by a ravine tributary of Chandler Branch. The center of this feature lies at about 3405760N, 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 the Citronelle Formation (Campbell, 1971; Mossa and Autin, 1989). In general, the Citronelle Formation consists 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 Brushy Creek feature. Open circles show location of samples associated with the Brushy Creek feature. “16SH”has been dropped from sample numbers. For example, “”PD” is sample locality 16SHPD. 314

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Heinrich elly sand, and sand containing beds of silt, clay, and mud. Typically, the beds are of limited vertical and lateral extent. According to Campbell (1971), the thickness of the Citronelle Formation is about 300 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 coarse sand underlain by laminated clay and silt. Field investigations indicate that the sand consists of 30 to 40 ft (9 to 12 m) of deeply weathered, reddish brown, fine to coarse, poorly sorted sand. In outcrops, the sand is typically massive. However, in an exposure on the edge of the Brushy Creek feature, locality 16SHPC, the Citronelle Formation is cross-bedded. As exposed in a Kentwood Brick and Tile Company brick pit immediately east of the Brushy Creek feature, at least 20 ft (6 m) of laminated silts and clays underlie these sands. These silts and clays consist of cyclic beds of meterthick laminated silt that grades upward into laminated clay. Discussions with the staff of the Kentwood Brick and Tile Company indicate that in their explorations for brick clay, they found the laminated clays and silts to be absent in holes drilled within the interior of the Brushy Creek feature but present within holes drilled outside of its rim. Very little is known about the sediments underlying the laminated silt and clay beds. As classified by Folk (1980), the sand fraction of the Citronelle Formation varies regionally in composition from quartzarenites to sublitharenites. The sand-size fraction consists of 90 to 97 percent quartz. The remaining 3 to 10 percent consists of chert, quartzite, iron oxide, and heavy minerals. Feldspar is absent from both the sand and gravel fractions. The gravel consists largely of chert 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 Mesozoic 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 (3,350 to 3,660 m) consist of Cenozoic sediments of the Midway, Wilcox, Claiborne, Jackson, and Vicksburg groups and undifferentiated, largely siliciclastic, Neogene strata. Within St. Helena Parish, these strata dip homoclinally to the southwest lacking indication of any major faulting or salt tectonics in the 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 northern third of the feature. The examination of the feature consisted of the description of exposures, examination of gravel found in streams draining the feature, and collection of samples from such locations. Only one exposure, the Gehee Section, locality 16SHPC, revealed a complete section of the distal rim of this feature. This exposure consists of an upper bed of 7 to 10 ft (2 to 3 m) of massive silty sand and sandy silt. At about 5 ft (1.5 m) below the surface, a zone about 8 in (20 cm) thick contains rounded, dime-size clasts of purple silty clay floating within a silty sand matrix. The purple color of the silty clay clasts indicates that they came from the Citronelle Formation. Developed within the upper part of the silty sand is the profile of the modern soil with pronounced A and B horizons. At the base of the sandy silt and lying directly on the underlying Citronelle Formation, the exposure contains a 5 to 12 in (13 to 30 cm) thick gravelly mud containing abundant rounded clasts composed of mud, clay, and ironstone nodules. At this time, it is difficult to determine the origin of this bed. Within the Gehee Section, the gravelly mud bed overlies highly fractured and cross-bedded sand of the Citronelle Formation exposed within a ditch. It is deeply weathered saprolite. This unit is highly oxidized and shows well-developed gleying of the sand along abundant fractures and root molds. The Gehee Section is the only known exposure in which the Citronelle Formation is highly fractured. Samples were collected from locations within the Brushy Creek feature. North of and adjacent to Louisiana State Highway 37, samples of the sediment composing the rim of the Brushy Creek feature were collected from surface exposures at localities 16SHPC, 16SHPD, and 16SHPT, and from an auger hole, locality 16SHPL. Within this auger hole, samples were taken at depths of 1.5, 3, and 53rd Annual Convention
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Heinrich 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 1.5 ft (0.5 m) below its top. Finally, other samples were collected from within the Brushy Creek feature at three localities, 16SHPO, 16SHPP, and 16SHPQ. These samples were processed for petrographic study. First, the samples were disaggregated by complete air drying and then immersion in a water-filled beaker for about an hour. The resulting mud was gently washed through a 200 mesh (3.75 phi) screen to remove the fine fraction. After the fine fraction had been removed, the sample was again air-dried in an open tray exposed to direct sunlight. Once dried, the sand sample was dry sieved into 18 to 60 mesh (0.0 to 2.0 phi), and 60 to 200 mesh (2.0 to 3.75 phi), fractions. Finally, thin sections of each fraction and unprocessed, but impregnated, pieces of specific samples were prepared for petrographic examination. Several of these thin sections were stained for both plagioclase and orthoclase feldspars. Because the interior of the Brushy Creek feature was inaccessible for examination when the initial research was conducted, samples of sand originating from the interior and from the deeply cut rim were collected off site. Sand was collected from the modern alluvium of Brushy Creek at locality 16SHPA about 160 ft (50 m) upstream of it’s intersection with Louisiana State Highway 449 north of Jack, Louisiana. At this locality, samples were collected from the floodplain and a channel bar in Brushy Creek. Six control samples, localities 16SHCY, 16SHG5, 16SHG6, 16SHG7, 16SHG8, and 16SHG9, were collected from nearby outcrops of Citronelle Formation within a radius of 1.5 to 4.5 miles (2.4 to 7.2 km) 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, locality 16SHFF, and from a gravel pit at Kentwood, Louisiana, locality 16TAKP, which is Stop 5 of Mossa and Autin (1989). All of these samples were processed in the same manner as samples from the Brushy Creek feature. Separate sets of screens were used for sieving the control samples and samples from the Brushy Creek feature. Thin sections of the 18 to 60 mesh (0.0 to 2.0 phi), and 60 to 220 mesh (2.0 to 3.75 phi), fractions were prepared for each sample. Abundant ironstone nodules were collected from local streams draining the Brushy Creek feature and the gravelly mud exposed in the Gehee Section, locality 16SHPC. Several dozen nodules from locality 16SHPC and the streambeds were cut on a trim saw. Two petrographic thin sections were made from nodules from locality 16SHPC. Representative specimens of these nodules were 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 95 percent 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 samples from locality 16SHPD and the gravelly mud of 1ocality 16SHPC. All of the samples consisted of subangular to well-rounded grains of sand. Unlike control samples, grains of sand in samples taken within the Brushy Creek feature often exhibited ragged edges in thin section from the disintegration of sand grains during processing. Intensely fractured quartz was found in all of the samples collected from the rim and interior of the Brushy Creek feature. The proportion of intensely fractured quartz varied from less than ten percent to almost 100 percent of the quartz and chert sand. The sample with the least amount of intensely fractured quartz came from the Citronelle Formation exposed at the Gehee Section, locality 16SHPC. The sand from localities 16SHPD, 16SHPO, 16SHPP, 16SHPQ, and the gravelly mud in the Gehee Section, consisted either entirely or almost entirely of intensely fractured grains. In contrast, none of the control samples possessed the intensely fractured grains observed in samples associated with the Brushy Creek feature. Examination of thin sections revealed an abundance of intensely fractured quartz of types not observed within the control samples collected from outside the feature. One type consists of quartz 316

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Heinrich grains exhibiting an irregular, interlocking network of fractures (Fig. 2). Another type consists of quartz grains with blocky, rectilinear fracture sets (Fig. 3). The former type of fracturing has been illustrated and described by Kieffer (1971) and Shoemaker and Kieffer (1979) from shocked Coconino Sandstone collected 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 Impact Structure of Texas. The accumulation of iron oxides within many of these fractures clearly demonstrates that they are not artifacts of the thin section preparrtion. The intensely fractured nature of the sand made the preparation of thin sections from this sand noticeably difficult.

Figure 2. Photomicrograph of thin section showing coarse, intensely fractured quartz sand from quartz from gravelly mud in the Gehee Section, locality 16SHPC. Photomicrograph taken with cross-polarized light. A unique and significant feature was noted in samples from two locations. Thin sections made from 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 sets of planar features in only one direction. In addition, a few of the quartz grains exhibited sets of planar features in two different intersecting directions (Fig. 4). Examination of these grains by Stephen Benoist (2003, per. commun.) revealed that the planar features averaged 45 degrees and 33 degrees, which respectively represent the {1012} and {1122} orientations. Planar features along both orientations, as noted by Koerbel (1997) and Stoffler and Langenhorst (1994), are characteristic of planar deformation features (PDF) created by shock metamorphism. The abundance of planar features and shocked quartz within the sand from 16SHPA argue against their having been reworked from distant sources, e.g., Cretaceous - Tertiary boundary deposits, but rather suggest they are derived from a nearby primary source, i.e., the Brushy Creek feature. Finally, the gravelly mud overlying the Citronelle Formation in the Gehee Section contains an abundance of quartz with similar 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 the vicinity of the Brushy Creek feature contained such planar features or fractures. 53rd Annual Convention
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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 taken with 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. 318

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

In addition to their orientation and parallelism, the planar features show preferential etching as a result of intensive weathering of the quartz grains. This etching has created cavities along these planar features that have become filled with pedogenic iron oxides (Figs 4 and 5). Just as amorphous silica associated with PDFs is revealed by etching with hydrofluoric acid, weathering has naturally 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 which weathering has removed amorphous silica. Small percentages of orthoclase, plagioclase, and mica were found in thin sections of unprocessed samples from 16SHPD and gravelly mud of locality 16SHPC. The feldspar and micas were deeply weathered and corroded from post-depositional weathering In contrast, all of the control samples 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 gravel from streams draining the Brushy Creek feature and the gravelly mud layer, none of this material could be identified as “iron shale.”

DISCUSSION The mechanisms capable of producing a circular feature like the Brushy Creek feature are diapirism, volcanism, siliciclastic karst, and meteorite impact cratering. Any relationship to salt diapirism can be dismissed, because the Brushy Creek feature lies within a portion of the Louisiana Coastal Plain underlain by extremely thin Louann Salt (Sawyer et al. 1991). Within this area, the 53rd Annual Convention
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Heinrich Louann Salt is much too thin to have created salt diapirs of sufficient size to have penetrated close enough to the surface to have produced any surface expression. The lack of salt diapirs is further confirmed by oil and gas fields in the surrounding region which lie south and north of the Brushy Creek feature. North of the Brushy Creek feature, the structure associated with the Joseph Branch and Greensburg fields consists of very low relief structural nosing lacking anticlinal closure greater than 20 ft (6 m) at a depth of about 12,700 ft (3,900 m) (Corcoran, 1993; Corcoran et al., 1993). Similar structural nosing with less than 100 ft (30 m) of closure at a depth of 14,400 to 14,600 ft (4390 to 4450 m) 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 salt diapirism. Also, James D. Coleman (2002, pers. commun.) observed that the Brushy Creek area has been explored in such great detail that it is highly improbable, if not impossible, that any salt diapirs of the size capable of producing the Brushy Creek feature remain undiscovered. Volcanism can produce isolated rimmed circular depressions called “maars.” The Brushy Creek feature 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 of Brushy Creek contained any material of clear volcanic origins, e.g., volcanic sediments or clasts. The lack of any volcanic materials within the Brushy Creek feature together with its passive margin setting hundreds of kilometers from known Holocene age volcanism, e.g., Byerly (1991), makes it highly 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 siliciclastic karst. The upper 5,000 to 6,000 ft (1,520 to 1,830 m) of the underlying Cenozoic strata lack any significant beds of carbonates (Howe, 1962; Bebout and Gutiérrez, 1983). This precludes formation of the feature by the dissolution of carbonates. However, as discussed by May and Warne (1999), the dissolution of siliciclastic sediments has created large circular and oval landforms, including numerous circular and oval enclosed depressions within the Gulf Coastal Plain of Mississippi and Alabama, and countless Carolina Bays of the Atlantic Coastal Plain. These landforms have a resemblance to the Brushy Creek feature on a superficial level. However, there are some notable differences between siliciclastic karst, such as the Carolina Bays of the Atlantic Coastal Plain and the enclosed depressions of the Mississippi and Alabama coastal plains, and the Brushy Creek feature. First, unlike the Brushy Creek feature, which occurs within very well-developed ridge and ravine topography, siliciclastic karst typically develops on flat, 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, and erosion, which would greatly inhibit the vertical-drainage weathering needed to create siliciclastic karst (May and Warne, 1999). Second, siliciclastic karst characteristically occurs not as isolated depressions, such as the Brushy Creek feature, but as large fields of multiple depressions that pockmark large. Third, the Brushy Creek feature is considerably larger than the Gulf Coast depressions in Mississippi and Alabama which range from 150 to 2,600 ft (45 to 790 m) in diameter and from 3 to 40 ft (0.9 to 12 m)(Otvos, 1997). Fourth, unlike the Brushy Creek feature, the Gulf Coast depressions lack any associated ridges or raised rims as noted by Otvos (1997). Finally, the siliciclastic karst hypothesis fails to explain the direct association of shocked and intensely fractured quartz with the Brushy Creek feature. Given these differences, a siliclastic origin of the Brushy creek feature is discounted. The most logical hypothesis for the origin of the Brushy Creek feature is that it was created by a late Pleistocene meteorite impact and that in fact, it is the Brushy Creek Impact Crater. Its roughly circular to slightly polygonal shape is consistent with known meteorite impact craters, e.g. the Barringer (Meteor) Crater, in which local fractures have influenced the deformation of the target material. The Brushy Creek feature creates a well defined anomalous “hole” within the local ridge and ravine topography that is unique to this part of the Louisiana Coastal Plain. As noted previously discussions with the staff of the Kentwood Brick and Tile Company revealed that holes drilled in the search for clay resources revealed a large stratigraphic “hole” in the local distribution of laminated silts and clays corresponding to the Brushy Creek feature. The presence of feldspars 320

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Heinrich and mica in samples from 16SHPC and 16SHPD indicates that sediments from strata underlying the Citronelle Formation have been brought to the surface from hundreds of feet below. All of these observations 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 from the Brushy Creek feature provides additional direct evidence of impact processes having modified these sediments. The fractured quartz indicates that the sediments, although having never been buried more than beneath 60 ft (18 m) of overburden, were subjected to intense pressures such that in some cases almost a hundred percent of the grains were completely shattered. The presence of actual shocked quartz with PDFs provides direct proof of shock metamorphism resulting from an impact.

CONCLUSIONS The preliminary study of a rimmed, circular depression, which is about 1.2 miles (2 km) in diameter, within St. Helena Parish, Louisiana, resulted in the hypothesis that it is a crater resulting from a hypervelocity meteorite or comet impact. The Brushy Creek feature is a regionally unique landform superimposed upon the local ridge and ravine topography. This circular feature occurs in an area devoid of volcanic activity, salt diapirism, and carbonate sinkholes that might explain its origin. Furthermore, its location is incompatible with the formation of siliciclastic karst. Limited subsurface data indicate the presence of a “hole” in the underlying stratigraphic units consistent with the origin of the Brushy Creek feature by impact processes. Finally, the presence of shocked quartz provides direct evidence of impact metamorphism. Impact processes are further suggested by abundant intensely fractured quartz within the sediments comprising its rim. At this time, the Louisiana Geological survey is preparing for further research of the Brushy Creek feature. The author, Douglas Carlson, and Richard P. McCulloh are considering studying the internal structure of the Brushy Creek feature using various geophysical techniques, including magnetic and seismic surveys. Plans to use a Giddings soil probe to acquire cores and prepare a shallow cross-section across this feature are under consideration. The silty and sandy composition of rim of the Brushy Creek feature indicate that Ground Penetrating Radar might produce useful cross-sections of the rim when ground truthed by cores from a Giddings rig. Finally, Stephen Benoist (Louisiana State University) and the author are planning to examine the petrographic characteristics of shock metamorphosed sediment associated with this feature.

ACKNOWLEDGEMENTS The 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 agreement 1434-HQ-96-AG-01490. Additional work on this study was conducted with the encouragement and support 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 discussions, advice, and encouragement. I also thank Christian Koerbel, Scott Harris, and Stephen Benoist for technical advice regarding shocked quartz. I greatly appreciate the comments made by the two geologists who reviewed this paper for publication. I thank Xiaogang Xie for use of the photomicroscope at the LSU Department of Geology and Geophysics SEM and Electron Microprobe Laboratory. I thank Rick Young for the preparation of several thin sections. I am grateful to National Petrographic Services, Inc for high-quality thin sections that proved to be essential to my research. Finally, I am very grateful to the Kentwood Brick and Tile Company, William A. Gehee of Greensburg, Louisiana, and Soterra LLC, inc., Jackson, Mississippi for access to their property.

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REFERENCES Bebout, D. G., and D. R. Gutierrez, 1983, Regional cross sections Louisiana Gulf Coast {eastern part}: Louisiana Geological Survey Folio series. no. 6., 11 p. Byerly, G. R., 1991, Igneous activity, in A. Salvador, ed., The Gulf of Mexico Basin: Geological Society of America Decade of North American Geology, Geology of North America, v. J, p. 91-108. Campbell, C. L., 1971, The gravel deposits of St. Helena and Tangipahoa parishes: Ph.D. Dissertation. Department of Geology, Tulane University, New Orleans, LA, 295 p. Corcoran, M. K., 1993, The lower Tuscaloosa Formation in the Greensburg Field and Joseph Branch Field areas, St. Helena Parish, Louisiana: Master’s thesis. Department of Geology, University of Southern Mississippi, Hattiesburg, MS, 83 p. Corcoran. M. K., C. P. Cameron, and M. A. Meylan, 1993, The lower Tuscaloosa Formation in the Greensburg Field and Joseph Branch Field areas, St. Helena Parish, Louisiana: Gulf Coast Association of Geological Societies Transactions. v. 43, p. 87-96. Folk, R. L., 1980, Petrology of Sedimentary Rocks. Hemphill Publishing Company. 184 p. Hack, J. T., 1960, Interpretation of erosional topography in humid temperate regions: American Journal of Science. v. 258-A, p. 80-97. Howe, H. J., 1962, Subsurface geology of St. Helena, Tangipahoa, Washington and St. Tammany parishes, Louisiana. Transactions of the Gulf Coast Association of Geological Societies. v. 7, pp. 121-135. Kieffer , S. W., 1971, I. Shock metamorphism of the Coconino Sandstone at Meteor Crater, Arizona. II. Specific heat of solids of geophysical interest: Ph.D. Dissertation. California Institute of Technology, Pasadena, California, p. 191. 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 Production (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 Geology, in press. McCulloh, R. P., Heinrich, P., and Snead, J., compilers, 1997, Amite, Louisiana 30 x 60 minute geologic quadrangle (preliminary): Prepared in cooperation with U.S. Geological Survey, STATEMAP program, under cooperative agreement no. 1434-HQ-96-AG-01490, 1:100,000-scale map plus explanation and notes. May, J. H., and A. G. Warne, 1999, Hydrologic and geochemical factors required for the development of Carolina Bays along the Atlantic and Gulf Coastal Plain, U.S.A. Engineering Geology. v. 5, p. 261-270. Mossa, J., and W. J. Autin, 1989, Quaternary geomorphology and stratigraphy of the Florida parishes, southeastern Louisiana. Louisiana Geological Survey Guidebook Series no. 5, 98 p. Otvos, I. G., Jr., 1976, “Pseudokarst” and “ pseudokarst terrains”: problems of terminology. Geological Society of America Bulletin. v. 87, p. 1021-1027. Otvos, I. G., Jr., 1997, Northeastern Gulf of Mexico Plain revisited Neogene and Quaternary units and events - old and new concepts. New Orleans Geological Society, New Orleans, Louisiana, 143 p. 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 of North 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 observation and theory: Meteoritics. v. 29, p. 155-181.

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