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Upper Crustal Structure of Alabama from Regional Magnetic and Gravity Data: Using Geology to Interpret Geophysics, and Vice Versa

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Upper Crustal Structure of Alabama from Regional Magnetic and Gravity Data: Using Geology to Interpret Geophysics, and Vice Versa Upper crustal structure of Alabama from regional magnetic and gravity data: Using geology to interpret geophysics, and vice versa Mark G. Steltenpohl1, J. Wright Horton, Jr.2, Robert D. Hatcher, Jr.3, Isidore Zietz4, David L. Daniels4, and Michael W. Higgins5 1Department of Geology and Geography, Auburn University, Petrie Hall, Auburn, Alabama 36849, USA 2U.S. Geological Survey, 926A National Center, Reston, Virginia 20192, USA 3Department of Earth and Planetary Sciences and Science Alliance Center of Excellence, University of Tennessee, Knoxville, Tennessee 37996-1410, USA 4U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, USA 5The Geologic Mapping Institute, 1752 Timber Bluff Drive, Clayton, Georgia 30525-6011, USA We dedicate this paper to the memory of coauthor Isidore (Izzy) Zietz, who died at age 93 while this paper was in review. After contributing to the early theoretical foundation for analysis of airborne magnetic surveys, Izzy became a leading advocate for aeromagnetic survey acquisition and interpretation throughout the U.S., combining surveys into regional, state, and national maps, and working with regional geologists in interpret- ing geophysical anomalies. Like many geologists, we have been energized by Izzy’s contagious enthusiasm for using magnetic and gravity anomalies to delineate and characterize major tectonic features. Lessons from working with him on data from Alabama and elsewhere over the years will continue to infl uence our apprecia- tion and understanding of aeromagnetic and gravity anomalies for interpreting the Earth’s upper crust. “Magnetics is never good, and gravity is even worse!” “…all of it can just be dashed.” –Isidore Zietz ABSTRACT Gondwanan crust of the Suwannee terrane. and all other crustal blocks in the subsurface of Within the ADD, there is clear magnetic dis- Alabama are truncated along the boundary with Aeromagnetic and gravity data sets tinction between Laurentian crust and the the Suwannee terrane, a huge mass of Gond- obtained for Alabama (United States) have strongly linear, high-frequency magnetic wanan crust sutured to the Laurentian margin been digitally merged and fi ltered to enhance highs of peri-Gondwanan (Carolina-Uchee) and left orphaned here as the modern Gulf of upper-crustal anomalies. Beneath the Appa- arc terranes. The contact (Central Piedmont Mexico (Early Jurassic) and Atlantic oceans lachian Basin in northwestern Alabama, suture) corresponds to surface exposures of (Early Triassic) began to form (Applin, 1951; broad deep-crustal anomalies of the conti- the Bartletts Ferry fault. ADD magnetic and Barnett, 1975; Neathery and Thomas, 1975; nental interior include the Grenville front gravity signatures are truncated by the east- Pojeta et al., 1976; Chowns and Williams, and New York–Alabama lineament (dextral west–trending Altamaha magnetic low asso- 1983; Horton et al., 1984, 1989, 1991; Guthrie fault). Toward the east and south, high-angle ciated with the Suwannee suture. Arcuate and Raymond, 1992). The discovery in the late discordance between the northeast-trending northeast-trending magnetic linears of the 1950s of Lower Ordovician through Devonian Appalachians and the east-west–trending Suwannee terrane refl ect internal structure sedimentary rocks containing Gondwanan faunal wedge of overlapping Mesozoic and Ceno- and Mesozoic failed-rift trends. Geophysical assemblages from exploration wells penetrating zoic Gulf Coastal Plain sediments reveals data can be used to make inferences on sur- the pre-Jurassic basement (here meaning either how bedrock geophysical signatures pro- face and subsurface geology and vice versa, or both Gondwanan and crystalline Appalachian gressively change with deeper burial. High- which has applicability anywhere that bed- basement) beneath Gulf Coastal Plain sedimen- frequency magnetic anomalies in the Appa- rock is exposed or concealed beneath essen- tary rocks in south Alabama (Applin, 1951) was lachian deformed domain (ADD) correspond tially non-magnetic sedimentary cover. a critical piece of the puzzle that Wilson needed to amphibolites and mylonites outlining to help solidify his thoughts on cycles of ocean terranes, while broader, lower-amplitude INTRODUCTION basins opening and closing and continents domains include Paleozoic intrusive bodies colliding. and Grenville basement gneiss. Fundamen- Alabama (United States) contains several Thick accumulations of younger sedimentary tal ADD structures (e.g., the Alexander City, major crustal-lithospheric boundaries that rock coupled with deep, sub-tropical weathering Towaliga, and Goat Rock–Bartletts Ferry played an important role in J. Tuzo Wilson’s and saprolitization of crystalline Appalachian faults) can be traced southward beneath (1966) formulation of what later would become rocks in Alabama leave us, however, with a the Gulf Coastal Plain to the suture with known as the Wilson cycle. The Appalachians fragmented understanding of parts of this geo- Geosphere; August 2013; v. 9; no. 4; p. 1044–1064; doi:10.1130/GES00703.1; 5 fi gures; 1 table; 1 supplemental fi le. Received 17 March 2011 ♦ Revision received 28 August 2012 ♦ Accepted 5 March 2013 ♦ Published online 16 July 2013 1044 For permission to copy, contact [email protected] © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/1044/3343459/1044.pdf by guest on 27 September 2021 Magnetic and gravity structure of Alabama: Using geology to interpret geophysics, and vice versa logical history. Eighty percent of the crystalline play their interactions with one another, provid- also the Ouachita foreland basin, and perhaps rocks in the state are concealed beneath Paleo- ing new insights into the tectonic evolution of is best characterized as a foreland basin that zoic and younger sedimentary cover, with 60% southeastern North America. formed adjacent to the syntaxis of both orogenic of that cover comprising Mesozoic and younger belts. Regional dip of this part of the continen- sediments of the Gulf Coastal Plain. The high- GEOLOGIC SETTING tal platform is southwest toward the Ouachitas angle discordance between strike of the north- and subparallel to Appalachian strike. Inter- east-trending Appalachians and the generally Two complete Wilson cycles and the begin- rupting the continuity within the foreland are east-west–trending Coastal Plain onlap is one of ning of a third are recorded in the lithosphere several anticlines (Sequatchie and Birmingham; the more prominent features on the geological of southeastern North America (Hatcher, 1978, Fig. 2) cored by Ordovician and younger rocks map of North America (Reed et al., 2005), and 1987, 2004, 2010; Thomas, 2006) (Fig. 1). that constitute the western limits of thin-skinned nowhere is this difference more pronounced than The fi nal phase of assembly of the supercon- Alleghanian deformation in Alabama (Rodgers, in Alabama (Fig. 1). This southward-thickening tinent Rodinia was the closing of ocean basins 1950). The Appalachian foreland fold-thrust wedge of Coastal Plain sediments provides the and collision of eastern Laurentia with pre- belt consists of a series of major folds and large opportunity to evaluate how remotely sensed Gondwanan continents during the ca. 1 Ga imbricate thrust sheets that have a northeast- geophysical signatures from surface exposures Grenville event (Hoffman, 1991). Breakup of southwest trend and become continuous with progressively change with deeper burial. Rodinia involved separation of Laurentia from other structures to the northeast in Georgia. The objectives of this paper are to use digi- West Gondwana and opening of the Iapetus ocean These areas are underlain by platform Cambrian tal maps of aeromagnetic and gravity data to at roughly 570–535 Ma (Odom and Fullagar, to Ordovician carbonate and siliclastic rocks better understand the upper-crustal surface and 1984; Aleini koff et al., 1995). Subduction in unconformably beneath Middle Ordovician subsurface structure of Alabama with the goal Iapetus, 470–455 Ma, accreted the Taconian and Silurian carbonate and siliciclastic rocks, of incorporating its broader signifi cance for the volcanic arc system to southeast Laurentia which are themselves unconformably overlain tectonic evolution of southeastern North Amer- (Hatcher and Odom, 1980). An ocean remained by Devonian and Mississippian carbonate and ica. Aeromagnetic and gravity data that have off eastern Laurentia that closed obliquely in siliciclastic rocks, and fi nally by the Pennsyl- been obtained for parts of Alabama over several the middle to late Paleozoic with zippered col- vanian clastic sedimentary rocks of the Allegha- decades have been merged and digitally fi ltered lision of a collage of peri-Gondwanan arcs with nian clastic wedge (Thomas, 1988, 1991, 1995; to enhance anomalies. Aeromagnetic anomalies Laurentia (West, 1998; Wortman et al., 1998; Pashin, 1994, 2004). record upper-crustal structure down to the Curie Hatcher et al., 1999; Bream et al., 2000, 2004; Southeast of the Appalachian foreland are the isotherm (25–30 km) for induced magnetiza- Hibbard et al., 2002, 2007; Merschat et al., remnants of the Blue Ridge geologic province tion of the buried rocks. Temperatures beneath 2005; Hatcher and Merschat, 2006; Steltenpohl extending southwestward from Pennsylvania to these depths exceed the blocking temperatures et al., 2006, 2008). At ca. 330 Ma, thermal activ- Alabama. The Talladega fault (Fig. 2) separates of magnetic minerals such that remanent
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