Reelfoot Rift: Reactivated Precursor to the Mississippi Embayment ?' PA™CK ERVIN I Department of Geology, Northern Illinois University, DeKalb, Illinois 60115 L. D. McGINNIS ' ABSTRACT American Rift System (Midcontinent Grav- sive basinal development in Cambrian- ity High) (Ocola and Meyer, 1973), and Ordovician time (Schwalb, 1969). This Tectonic elements involved in the forma- epeirogenic uplifts (Burke and Wilson, basin, which Schwalb named the Reelfoot tion of the Mississippi embayment, as in- 1972; Faure, 1971, 1972, as reported by basin, was later transected by the gradual ferred from geophysical and geological in- Menard, 1973). Burke and Dewey (1973) uplift of the Pascola arch connecting the formation, originated in late Precambrian proposed that former plume-generated tri- Ozark uplift and the Nashville dome (Wil- time with continental rifting (the Reelfoot ple junctions lie along the present continen- son and Stearns, 1963). The center of basi- rift) and intrusion of high-density magma tal boundaries. In their model, active nal deposition concurrently shifted north- into the crust. Isostatic subsidence in early spreading occurred along two arms parallel ward from western Tennessee, reaching Paleozoic time formed the Reelfoot basin, to the continental edge, while the third arm southern Illinois by Silurian time and merg- approximately coincident with the modern often projected perpendicularly into the ing with the southward-shifting deposi- embayment, in which several kilometers of continent as a rift that failed to reach the tional center of the Eastern Interior basin. sediment were deposited. Closing of the spreading stage. On the basis of this The Pascola arch was subsequently de- proto—Atlantic Ocean and subduction in hypothesis, they suggested that the Missis- pressed and buried beneath the late the southern Appalachian Mountains in sippi embayment is the failed arm of a late Mesozoic and Cenozoic sediments that middle to late Paleozoic time were accom- Paleozoic triple junction that was located were deposited in the subsiding embay- panied by uplift and widespread erosion in near Jackson, Mississippi, and that was as- ment. the midcontinent. A period of rift reactiva- sociated with the opening of the Gulf of tion and intrusion in late Mesozoic time, in Mexico. We propose that the genesis of the REGIONAL GEOPHYSICAL SETTING association with rapid subsidence in the Mississippi embayment can be interpreted Gulf of Mexico, prompted renewed isosta- within the context of plate-tectonic theory Burke and Dewey's (1973) designation of tic subsidence within the embayment, form- but that its development is more ancient the Mississippi embayment as a failed rift ing the elongate depositional trough ob- and complex than suggested by Burke and was based on regional geology; however, served today. Continued seismicity and Dewey. this classification is also supported by positive free-air gravity anomalies indicate geophysical data. that isostatic adjustment continues to the REGIONAL GEOLOGIC SETTING present time. Key words: continental and Gravity failed rift, anomalous mantle. The Mississippi embayment is a re- entrant into the North American craton In the southeastern United States, areas INTRODUCTION from the south, delimited geographically by of positive Bouguer anomaly are found the maximum updip extent of post- along the Piedmont and within the embay- With the recent national concern about Paleozoic sedimentary rocks (Fig. 1). It is a ment (Fig. 2). The positive areas in the em- the possibility of predicting and preventing broad, spoon-shaped trough lying between bayment occur along a linear trend gener- major earthquakes, attention has focused the Ozark uplift to the west and the Nash- ally centered over the axis, which lies a few on the New Madrid earthquake zone, ville dome to the east and opening south- kilometers to the west of the Mississippi which is approximately coincident with the ward into the southerly-dipping beds of the River (Caplan, 1954) and extends north- axis of the Mississippi embayment. Studies Gulf Coastal Plain. Paleozoic strata are un- ward into the Illinois basin (Ervin and of this region are particularly important be- conformably overlain by Cretaceous and McGinnis, 1974). Superimposed on these cause an earthquake with the same intensity Tertiary unconsolidated to poorly consoli- linear anomalies are several approximately as the 1811 to 1812 sequence would cause dated sediments that thicken southward circular anomalies, 30 to 100 km in diame- extensive damage and loss of life in the from the feather edge in southern Illinois to ter, with amplitudes of 30 to 40 mgal. Phe- midcontinent. This paper presents a 1 km near Memphis, Tennessee (Stearns lan (1969) studied two of these and found hypothesis for the evolution of the embay- and Wilson, 1972). For the purposes of this them to be plutons extending through much ment that may serve as a guide for these paper, we will assume that the southern of the crust and having densities in the investigations. limit of the embayment is marked by a line range of 3.1 to 3.3 g/cm3. Plate- and block-tectonic theories have extending from the southern Appalachians provided a new basis for the interpretation to the southeastern Ouachita Mountains. Seismology of intraplate processes, such as formation Although sediment deposition indicates of the African rifts (Sowerbutts, 1969; that subsidence of the present trough began Although located within the otherwise Griffiths and others, 1971; Maasha and in Cretaceous time, subsurface information stable craton, the embayment is an area of Molnar, 1972), intracratonic basins obtained from drilling clearly shows that moderate seismicity. The majority of the (McGinnis, 1970), the Central North the embayment was also the site of exten- earthquakes, including the great New Ma- Geological Society of America Bulletin, v. 86, p. 1287-1295, 5 figs., September 1975, Doc. no. 50913. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1287/3418596/i0016-7606-86-9-1287.pdf by guest on 27 September 2021 1288 ERVIN AND McGINNIS drid sequence of 1811 to 1812, have oc- curred in a zone along its axis (Stearns and Wilson, 1972). A single reversed refraction profile fur- nishes the only deep control on the crustal structure underlying the embayment (McCamy and Meyer, 1966). The profile (Rl) is located along its western edge be- tween Little Rock, Arkansas, and Cape Girardeau, Missouri (Fig. 1). The inter- preted crustal section near Batesville, Ar- kansas, is shown in Figure 3. A reversed refraction profile between St. Joseph and Hannibal, Missouri (R2 in Fig. 1), shows the crustal structure in an area of normal craton. The interpreted structure at Hannibal (Stewart, 1968) is included in Figure 3 for comparison between normal and anomalous crustal sections. The seismic crustal structure of the em- bayment has the following three anomalous characteristics: (1) the presence of an anomalous velocity layer (7.4 km/sec) at the crust-mantle interface, (2) the great depth of the mantle (8.1-km/sec layer), and (3) the shallowness of the 6.5-km/sec crustal layer. A velocity of 7.4 km/sec is intermediate between normal continental crust and man- tle velocities, suggesting that this is a zone of transitional composition. If the anoma- lous layer is removed from the embayment profile and adjustment made for the isostat- ic depression of the mantle surface, the two velocity structure sections in Figure 3 are identical, suggesting that the overlying crust was upwarped by emplacement of the 7.4- km/sec layer. CRUSTAL MODEL FROM GEOPHYSICS AND GEOLOGY The above data were used as constraints in the construction of a crustal model along a gravity profile (G, Fig. 1) perpendicular to the embayment axis, using Woollard and Joesting's Bouguer anomaly map (1964). The profile extends from Yellville, Arkan- Figure 1. Location map. Light stippled pattern: Cretaceous and younger sediments. Rl: refraction sas, on the west to Scottsboro, Alabama, on profile by McCamy and Meyer, 1966. R2: refraction profile by Stewart, 1968. G: gravity profile in the east (Fig. 1). Because the embayment is Figure 4. AX: probable rift axis. (Adapted from Cohee and others, 1962.) an essentially linear feature, it can be ap- proximated by a two-dimensional model contour map of Stearns and Wilson (1972). is between Ste. Genevieve, Missouri, and that extends to infinity in the direction Away from the axis of the embayment, the Hercules, Missouri (Stewart, 1968), about parallel to its axis. Therefore, the gravity base of the 6.2- and 6.5-km/sec layers were 50 km north of Yellville. This profile was field to be fitted by the model is that com- constrained to approach the "standard" ignored because the data were poor and ponent of the observed field that is theoreti- crustal section of Stewart (1968). No at- would not reverse. In a study of P-wave cally invariant in the axial direction. This tempt was made to constrain the base of the spectra, Kurita (1973) also encountered regional component was estimated from 6.1-km/sec layer, because other refraction low-quality data in this area. Lateral in- both the original map and the profile by profiles in the midcontinent (Healy and homogeneities in the crust under the Ozark visual smoothing (Fig. 4). Computations Warren, 1969) suggest that this interface is uplift appear to be the cause of the data were made using the two-dimensional Tal- not laterally continuous. The structure also degradation. wani algorithm (Talwani and others, was required to conform to the seismic The second profile is between Moulton, 1959a). model (McCamy and Meyer, 1966) at the Alabama, and Tullahoma, Tennessee (War- The interpreted crustal model and its cal- intersection of the refraction profile and the ren, 1968). This profile intersects the cross culated field are shown in Figure 4. Several cross section at the western edge of the em- section about 100 km west of Scottsboro, additional conditions were imposed on the bayment.