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1 University of Memphis, Ground Water Institute, 300 Engineering, Memphis, Tennessee 38152, USA
2 University of Memphis, Earth Sciences, 1 Johnson Hall, Memphis, Tennessee 38152-3430, USA
3 University of Memphis, Civil Engineering, 110D Engineering Science Building, Memphis, Tennessee 38152, USA
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGY OF THE REELFOOT...FAULTING WITHIN THE REELFOOT...METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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Geologic interpretation of 5077 shallow borings in the central Mississippi River valley enabled the construction of a structure contour map of the Pliocene–Pleistocene unconformity (top of the Eocene–base of Mississippi River alluvium) that overlies most of the Reelfoot rift. This map reveals both river erosion and tectonic deformation. Deformation of the Pliocene–Pleistocene unconformity appears to be controlled by the northeast- and southeast-trending basement faults. The northeast-trending rift faults have undergone and continue to undergo Quaternary dextral transpression. This has resulted in displacement of two major rift blocks and formation of the Lake County uplift, Joiner ridge, and the southern half of Crowley's Ridge as compressional stepover zones that appear to have originated above basement fault intersections. The Lake County uplift has been tectonically active over the past 
2400 yr and corresponds with a major segment of the New Madrid seismic zone. The aseismic Joiner ridge and the southern portion of Crowley's Ridge may reflect earlier uplift, thus indicating Quaternary strain migration within the Reelfoot rift.
Keywords: Reelfoot rift • Mississippi embayment • New Madrid seismic zone • Mississippi River alluvium • geomorphology
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE REELFOOT...FAULTING WITHIN THE REELFOOT...METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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GEOLOGY OF THE REELFOOT RIFT REGION |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE REELFOOT...FAULTING WITHIN THE REELFOOT...METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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Initiation of Reelfoot rifting may have been due to mantle plume upwelling that occurred along terrane boundaries (Dart and Swolfs, 1998). Alternatively, the Reelfoot rift may be a consequence of right-lateral strike-slip motion along a northwest-oriented transform fault that formed the Paleozoic continental margin of southeastern Laurentia (Thomas, 1985, 1991). Whatever the mechanics, intracratonic extension ultimately succeeded in creating the Reelfoot rift with an estimated extension between 17% (Nelson and Zhang, 1991) and 33% (Hildenbrand, 1985).
Cambrian Reelfoot rifting occurred primarily along large normal faults that appear to become listric with depth (Howe and Thompson, 1984; Howe, 1985; Nelson and Zhang, 1991). However, the exceptionally straight margin faults suggest that these northeast-trending rift structures originated as strike-slip faults (Hildenbrand, 1985). During rifting the Reel-foot graben accumulated a maximum of 7 km of sediment, while outside the rift only 1.5 km of contemporary sediments accumulated (Howe and Thompson, 1984; Howe, 1985). Local erosion of nearby granite-rhyolite rocks provided Early Cambrian basal arkosic sediments (Crone et al., 1985; Dart and Swolfs, 1998). Howe and Thompson (1984) suggested that faulting occurred syndepositionally within and along the Rift margins up to Middle Cambrian time. After this, deposition within the rift shifted from subaerial to marine and as much as 6 km of marine Upper Cambrian Lamotte Sandstone, Bonterre Formation limestones, Elvins Group shales, and the Potsdam Supergroup were deposited (Thomas, 1985; Howe, 1985). Deposition kept pace with regional subsidence during Late Cambrian–Middle Ordovician time (Howe and Thompson, 1984; Howe, 1985; Dart and Swolfs, 1998), resulting in the thick, shallow-marine Knox (Arbuckle) carbonate Supergroup that overlies the rift (Thomas, 1985; Howe, 1985). From Middle Ordovician to Pennsylvanian time, subsidence and uplift alternated due to distal effects of the Taconic, Acadian, and Alleghanian orogenies (Howe, 1985). Structural reactivation within the Reelfoot rift began during the late Paleozoic with the assembly of Pangaea (Thomas, 1985; Howe, 1985). Middle Ordovician to mid-Cretaceous rocks are largely missing above the rift, partly due to nondeposition (Permian–Late Cretaceous) and partly due to late Paleozoic and/or mid-Cretaceous uplift and erosion, which produced a major unconformity at the top of the Paleozoic section. Many normal faults within the central United States were inverted during the Paleozoic collisional processes (Howe and Thompson, 1984; Howe, 1985; Marshak and Paulsen, 1996).
Cox and Van Arsdale (1997, 2002) proposed that regional mid-Cretaceous uplift and subsequent Late Cretaceous subsidence of the Mississippi embayment occurred because the North American plate drifted over the Bermuda hotspot. The thermally uplifted area formed a north-trending arch from which 2 km of Paleozoic strata were eroded during the mid-Cretaceous (Cox and Van Arsdale, 1997). Following the North American plate's migration off the hotspot during the Late Cretaceous, the eroded mid-Cretaceous arch subsided as it cooled, forming the Mississippi embayment trough. Late Cretaceous and Cenozoic sediments record transgressive-regressive sequences within the trough that include the McNairy, Clayton, Porters Creek, Fort Pillow, Flour Island, Claiborne Group, Upland Complex, and the Mississippi River alluvium (Howe and Thompson, 1984; Thomas, 1985).
The Mississippi River Valley within the Mississippi embayment is a broad alluvial lowland (Fig. 1). Notable in eastern Arkansas is the 320-km-long Crowley's Ridge, a northerly trending topographic high that divides the valley into the Western and Eastern Lowlands. The ridge ranges in width from 1.6 to 19 km and averages 60 m above the surrounding lowlands. It is composed in ascending order of Eocene Wilcox and Claiborne Groups (Meissner, 1984), Pliocene Upland Complex (Van Arsdale et al., 2007), and Pleistocene loess (Guccione et al., 1990).
Crowley's Ridge formed due to erosion of the Western Lowlands by the ancestral Mississippi River, erosion of the Eastern Lowlands by the ancestral Ohio River, and by Quaternary reactivation of ridge-bounding faults (Van Arsdale et al., 1995). Depositional environments within the valley were fluvial during the Pliocene. This resulted in deposition of the Upland Complex, a terrace of the ancestral Mississippi–Ohio river system (Van Arsdale et al., 2007). Repeated periods of glacial melt-water escape, sea-level change, loess deposition, and structural deformation have produced various river terraces, river courses, lakes, and areas of warping during the Quaternary (Autin et al., 1991; Schweig and Van Arsdale, 1996). These landforms are dominated by glacio-fluvial processes and sediments that produced Pliocene and Pleistocene landforms consisting mostly of terraces and valley trains (Saucier, 1974; Autin et al., 1991). Repeated changes in late Pliocene and Pleistocene base level have controlled much of the Mississippi Valley's landforms and deposits through repeated degradation and aggradation. Four processes especially influenced base level through the Quaternary: glacio-eustatic sealevel changes, variations in rates and patterns of sediment yield, climatic changes, and tectonic activity. Pleistocene rivers carrying coarse-grained sediment were dominantly braided, in contrast to the high silt and clay sediment load of the Holocene rivers that now occupy meandering channels.
Structural features affecting regional Quaternary stratigraphy within the Mississippi River valley include the Jackson Dome, Lake County uplift, Monroe uplift, Ouachita fold and thrust belt, Wiggins arch, and several fault zones (Autin et al., 1991; Schweig and Van Arsdale, 1996; Cox et al., 2000). Based on shallow alluvial cores, Mihills and Van Arsdale (1999) recognized modern topographic relief mirroring the Pliocene–Pleistocene unconformity (base of Mississippi River alluvium), reflecting deformation of the Lake County uplift, Reelfoot Lake basin, and the northeast Arkansas sunklands. Some of these structures have modified local stream gradients and sedimentary processes during the Quaternary (Holbrook et al., 2006).
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FAULTING WITHIN THE REELFOOT RIFT |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE REELFOOT...FAULTING WITHIN THE REELFOOT...METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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Several faults have been identified within the Reelfoot rift (Fig. 1). The Reelfoot reverse fault, coincident with the northwest-trending seismic zone in Figure 1, produces most of the current seismicity. Surface deformation occurred along this fault during the February 1812 earthquake. The White River fault zone is an up-to-the-south fault, based upon gravity, magnetic (Hildenbrand and Hendricks, 1995; Langenheim and Hildenbrand, 1997), and seismic reflection data (Howe, 1985), that may form a southern terminus to the Reelfoot rift. The Axial fault, a vertical fault (Howe, 1985) that trends down the center of the rift, is the locus of the northeast-trending seismicity in Figure 1.
A number of southeast-trending strike-slip faults in southeastern Missouri and northeastern Arkansas may cross the Reelfoot rift (McCracken, 1971; Guinness et al., 1982; Zietz, 1982; Hildenbrand, 1985; Howe, 1985; Simpson et al., 1986; Sims et al., 1987; Cox, 1988; Dart and Swolfs, 1998). Stark (1997) suggested that major thrusting and transpressive deformation during the Late Proterozoic Grenville orogeny modified the rift region, resulting in orthogonal basement faults with southeast and northeast strikes. Many of these southeast-trending faults displace Paleozoic strata and thus have been active during the Phanerozoic (Stark, 1997).
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METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE REELFOOT...FAULTING WITHIN THE REELFOOT...METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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The Precambrian database was maintained and interpreted in the ArcGISTM and Landmark GraphicsTM software packages. These packages each offer unique advantages for interpolation and interpretation of the data. The top of the crystalline Precambrian was interpolated using ordinary kriging. Ordinary kriging is an interpolation algorithm typically used with geologic data. This algorithm calculates any overriding trend to the data, and models these data as a polynomial. The measured data are interpolated following removal of the trend. After interpolation, the random errors are assessed and the trend polynomial is added back into the interpolated surface to yield a meaningful result. Both LandmarkTM and ArcGISTM have this capability, yet only ArcGISTM offers a visual inspection of the semi-variogram and an interactive means of minimizing error through adjustment of the semi-variogram parameters, while only Landmark offers the capability to incorporate fault and break-line information into the interpolation. The semi-variogram model parameters determined in ArcGISTM were entered into Landmark's ZmapTM package. Unfaulted interpolation schemes were built in tandem within ArcGISTM and Landmark ZmapTM for comparison.
Faults were subsequently inserted into the ZmapTM model as break lines and polygons to the interpolated Precambrian surface. These faults were obtained from published seismic reflection lines (Howe and Thompson, 1984; Howe, 1985; Nelson and Zhang, 1991; Parrish and Van Arsdale, 2004), gravity and magnetic lineaments (Hildenbrand, 1985; Hildenbrand and Hendricks, 1995; Langenheim and Hildenbrand, 1997), mapped faults (McCracken, 1971; Zietz, 1982; Kisvarsanyi, 1984; Simpson et al., 1986; Sims et al., 1987; Haley et al., 1993), and hypocenter alignments (J. Pujol, 2006, personal commun.; Csontos, 2007). Hypocenters were viewed within a Landmark GeoprobeTM three-dimensional (3-D) model to identify fault planes. Hypocenter clusters appearing to follow planar surfaces were grouped into subsets and planar trend surfaces were created using ArcGISTM and Landmark ZmapTM. The Eastern Rift margin, Western Rift margin, and Reelfoot faults were assigned dip angle, dip azimuth, heave, and throw values where available with ZmapTM. The southeast-striking Ozark Plateau faults and Axial fault were assigned vertical dips with no throw. Surface interpolation then utilized faults with dip, throw, and heave data to produce fault plane polygons, while faults without these data were treated as vertical break lines. These fault polygons and break lines were used to interrupt the contouring algorithm at the fault planes and continue interpolation on the opposite side of the fault using a new trend surface created for the fault-bound data subset. These faults were then incorporated into the ZmapTM interpolation algorithm to produce a Precambrian surface structure contour map displaced by faults.
The Pliocene–Pleistocene unconformity surface was mapped using ArcGISTM. Universal kriging was used to interpolate the surface, and then the surface was clipped along the periphery data points.
Owing to the regional extent of these surfaces and their three-dimensionality, visual comparison of these surfaces to control data (drill hole, seismic reflection data) was made easier through viewing them stereoscopically in GeoprobeTM. Both ArcGISTM and LandmarkTM provide 3-D stereoscopy through the extensional packages ArcSceneTM and GeoprobeTM, respectively, and using Sharper Technology Incorporated Crystal-eyes stereo goggles, an emitter, and a DepthQ Stereo3D projector.
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE REELFOOT...FAULTING WITHIN THE REELFOOT...METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE REELFOOT...FAULTING WITHIN THE REELFOOT...METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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The Reelfoot rift is subdivided into eight fault-bound blocks (Fig. 6). At the largest scale, the rift consists of two basins divided by a structural high. This intrarift high is bound on the north by the Osceola fault zone and on the south by the Bolivar-Mansfield tectonic zone. Major changes in strike of the Eastern Rift Margin and Western Rift Margin faults occur near their intersection with the Bolivar-Mansfield tectonic zone and the Osceola fault zone, also indicating that these southeast-trending faults influenced the geometry of the Reelfoot rift.
Although we do not have well or seismic data between the Reelfoot rift and the Rough Creek graben, regional depth to magnetic basement maps (Hildenbrand, 1985; Langenheim and Hildenbrand, 1997) suggest that the basement rises between these two down-dropped structures (Fig. 4). Thus, we have chosen to close the Reelfoot rift at its northern end with a down-to-the-southwest normal fault (Fig. 6). This fault coincides in location with the Reelfoot thrust and reverse fault. However, the only displacement information we have on the Reelfoot fault shows that there is 70 m of reverse displacement on the top of the Paleozoic section that diminishes up section (Van Arsdale, 2000). We thus propose that the Reelfoot fault was a normal fault during Reelfoot rifting, but was subsequently inverted and is a reverse fault in the post-Paleozoic section. The southern end of the Reelfoot rift appears to be closed by the up-to-the-southwest White River fault zone. Our data are also limited in this area and it is possible that the Reelfoot rift continues south beneath the Appalachian-Ouachita thrust belt.
Extending down the center of the Reelfoot rift is the Axial fault. Although we have modeled the Axial fault as terminating against the Reel-foot fault, this is speculative. The Axial fault appears to have undergone both dip-slip and strike-slip movement. The northwestern side of the fault is generally downthrown relative to the southeastern side (Howe and Thompson, 1984). Left-lateral strike-slip offset is also suggested by a series of offset ridges and depressions on the Precambrian surface (Fig. 6).
Reelfoot Rift Faults Projected to the Pliocene–Pleistocene Unconformity Surface
In Figure 9 we project the Reelfoot Rift basement faults to the Pliocene–Pleistocene unconformity surface. In addition, we have included the base of the Pliocene Upland Gravel on Crowley's Ridge in our Pliocene–Pleistocene unconformity surface. Thus, the Eastern and Western Lowlands and Crowley's Ridge were contoured together in Figure 9. The Pliocene–Pleistocene unconformity surface is an erosional surface that has been tectonically modified. In the Eastern Lowlands, Quaternary uplift has occurred along the Reelfoot fault (Grand River tectonic zone) to form the Lake County uplift, along the Axial fault to affect regional drainage, and along Joiner ridge. Tectonic subsidence in the Eastern Lowlands has occurred beneath Reelfoot Lake, northwest of the Axial fault, and perhaps immediately east of Joiner ridge. In the Western Lowlands there are one and perhaps two major structural basins that are separated by an apparently uplifted block bound by the Axial fault and a possible extension of the Eastern Margin faults southwest of the White River fault zone. Perhaps most dramatic in Figure 9 is the elevation of Crowley's Ridge. Upon viewing Figure 9 stereoscopically (see Animation 2), it becomes apparent that most of the basement faults produce saddles, or appear to divide Crowley's Ridge into sections, as one would expect if the faults were active during the Quaternary.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE REELFOOT...FAULTING WITHIN THE REELFOOT...METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES CITED |
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The Pliocene–Pleistocene unconformity beneath the Mississippi River alluvium is an erosional unconformity that has been tectonically altered at different scales. On a regional scale, local field studies and inspection of Figure 9 suggest that the southeastern half of the Reel-foot rift has undergone Quaternary uplift while the northwestern half has undergone subsidence. On a more local scale, tectonic uplift has occurred on the Lake County uplift, along the Axial fault, along the southern half of Crowley's Ridge, and on Joiner ridge. Tectonic subsidence has occurred east of the Lake County uplift at Reelfoot Lake and perhaps on the eastern sides of Joiner ridge and the southern half of Crowley's Ridge. Any tectonic low east of Crowley's Ridge, however, is masked by the Pleistocene river channel.
Right-lateral strike slip on the Axial and Western Rift Margin faults near New Madrid, Missouri, has been explained as causing the Reelfoot thrust stepover between these faults. We wish to extend this explanation to propose that the southern half of the Lake County uplift, the southern half of Crowley's Ridge, and Joiner ridge are all right-lateral compressional stepovers (Fig. 10). These stepovers formed as a consequence of the Quaternary N60°E maximum horizontal compressive stress field of eastern North America (Zoback and Zoback, 1989). The regional stress field has imposed, and is imposing, right-lateral shear across the N45°E striking East Rift Margin faults, Axial fault, and West Rift Margin fault. Essentially, the entire Reelfoot rift has undergone Quaternary right-lateral transpression resulting in relative uplift and subsidence of the eastern and western halves of the Reelfoot rift, respectively, and formation of compressional stepovers. The stepover that is currently seismically active is at the northern end of the Reelfoot rift. It is reasonable to assume, however, that the different stepovers activate at different times, perhaps as shear displacement propagates along the rift faults.
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ACKNOWLEDGMENTS |
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MANUSCRIPT RECEIVED BY THE SOCIETY 13 March 2007
REVISED MANUSCRIPT RECEIVED 27 August 2007
MANUSCRIPT ACCEPTED 2 September 2007
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1.55 Ga; open circles are drill-hole and outcrop samples for which TDM ≤ 1.55 Ga. Black cross in northwestern Ohio is drill-hole OHSN-001; solid square in southeastern Michigan is drill-hole MIHI-001.
