setting of the Talladega slate belt, southern Appalachians. JAMES F. TULL Department of Geology, Florida State University, Tallahassee, Florida 32306.
New paleontologic evidence constraining the age and paleotectonic setting of the Talladega slate belt, southern Appalachians JAMES F. TULL
Department of Geology, Florida State University, Tallahassee, Florida 32306
^ « I f t P o c t c ^ i 1 U.S. Geological Survey, 970 National Center, Reston, Virginia 22092 JOHN t . Kfcrh 1 aKl ) FRANK K. McKINNEY Department of Geology, Appalachian State University, Boone, North Carolina 28608 CONNIE B. GARRETT* Department of Geology, Florida State University, Tallahassee, Florida 32306 DENNY N. BEARCE Department of Geology, University of Alabama at Birmingham, Birmingham, Alabama 35294
ABSTRACT
INTRODUCTION
The Talladega slate belt in the southern Appalachian orogen of Alabama and Georgia is a thick sequence of lower greenschist-facies metaclastic, metacarbonate, and metavolcanic rocks thrust above miogeoclinal rocks of the foreland and overthrust by higher grade metamorphic rocks of the eastern Blue Ridge terrane(s). The age assignments and tectonic affinities of this sequence have been highly controversial. Recent fossil discoveries in key stratigraphic units, however, combined with confirmed fossil occurrences in the Jemison Chert, have established a firm correlation with the Appalachian foreland, thus stratigraphically linking the Talladega belt with Laurentia. The lower predominantly clastic sequence, tectonically bounded at its base by the frontal Blue Ridge thrust system, grades upward into a 3.5-km-thick marble sequence. The basal carbonate unit (Jumbo Dolomite) contains Early Cambrian archaeocyathids, and these fossils, in addition to the stratigraphic position and carbonate iithofacies, establish correlation of this unit with the Lower Cambrian Shady Dolomite. The uppermost unit in the marble sequence (Gantts Quarry Formation) contains Early Ordovician (middle to late Canadian; = early to middle Arenigian) conodonts that confirm correlation of this unit with the Newala Limestone and Kingsport and Mascot Formations of the Appalachian foreland. The carbonate platform sequence is unconformably overlain by a thick clastic sequence that accumulated in a deep successor basin; the Lay Dam Formation is a marine fanlike deposit at the base of this sequence. Conodont molds from the top of the Lay Dam and fossils from the stratigraphically higher Jemison Chert indicate a Silurian to Early Devonian age for the Lay Dam Formation. Paleontologic data and the stratigraphic and structural setting indicate that the Talladega slate belt is the most distally preserved and relatively complete fragment of the Appalachian miogeocline; thus, the tectonic evolution of the Talladega belt is crucial to understanding the western margin of Iapetus. Linkage of the Talladega slate belt rocks with those of the western Blue Ridge to the northeast suggests that the latter once contained a thick Cambrian to Devonian cover sequence which subsequently has been mostly removed.
The Talladega slate belt, the westernmost crystalline thrust sheet of the Blue Ridge province at the southern terminus of the exposed Appalachian orogen, is a key link between relatively unmetamorphosed Paleozoic miogeoclinal rocks of the foreland thrust belt on the northwest and the ancient Laurentian continental margin on the southeast. It contains a stratigraphic thickness of more than 10 km of lower greenschist-facies clastic, carbonate, and volcanic rocks bounded on the northwest and southeast by major late Paleozoic fault systems. Crystalline thrust sheets to the southeast form the eastern Blue Ridge and constitute suspect terranes that cannot be linked stratigraphically to the Laurentian margin. The Talladega sequence structurally thins northeastward as its bounding faults converge near Cartersville, Georgia, and is overlapped on the southwest by Upper Cretaceous deposits of the Gulf Coastal Plain (Fig. 1). The nature of many of the contacts, within and bounding the Talladega belt, and the age assignment of most of the units have been, or still are, controversial. The combination of low-grade regional metamorphism and low penetrative strain in part of the belt, however, has resulted in local, rare preservation of fossils. Discoveries of conodonts and archaeocyathids from three previously undated stratigraphic units of the Talladega belt in Alabama provide a definitive link between this belt and the southern Appalachian western margin of Iapetus and provide new chronologic constraints on the evolution of this, the most distally preserved, relatively complete fragment of that margin.
*Present address: Florida Department of Environmental Regulation, Tallahassee, Florida 32301.
TALLADEGA BELT STRATIGRAPHY The stratigraphy of the Talladega belt (summarized by Bearce, 1973; Carrington, 1973; Guthrie, 1985; and Tull, 1979,1982,1985) consists of four distinct lithologic sequences: a lower clastic sequence (Kahatchee Mountain Group), conformably overlain by a middle carbonate sequence (Sylacauga Marble Group), unconformably overlain by an upper clastic sequence (Talladega Group), which is conformably overlain by a volcanic sequence (Hillabee Greenstone). The Kahatchee Mountain Group of Tull (1982) is the oldest and structurally lowest sequence preserved in the belt. It crops out along the northwest margin of the belt and is bounded below and on the northwest by the Talladega-Cartersville fault system, which locally includes the Kahatchee Mountain Group in a frontal imbricate fan. This sequence consists
Geological Society of America Bulletin, v. 100, p. 1291-1299,7 figs., August 1988. 1291
Figure 1. Location maps of the Talladega slate belt in the southern Appalachians and generalized geologic map of Talladega slate belt showing locations of Figures 2,4, and 6. Superscripts indicate source of stratigraphie nomenclature: 1usage of Tull (1982), 2usage of Tull (1985), 3 usage of Griffin (1951), *usage of Tull and Stow (1980b), 'usage of Tull (1978).
NEW PALEONTOLOGIC EVIDENCE, SOUTHERN APPALACHIANS
of more than 1.5 km of clastic and minor carbonate rocks and has generally been considered to be stratigraphically equivalent to the lowermost Cambrian Chilhowee Group, as well as perhaps the highest part of the uppermost Precambrian Ocoee Supergroup (Butts, 1926, 1940; Guthrie, 1985; Prouty, 1922; Rodgers and Shaw, 1962; Shaw, 1970; Tull, 1982; Tull and Guthrie, 1985). Carbonaceous slates in the uppermost Kahatchee Mountain Group grade into dolomitic marbles of the overlying Sylacauga Marble Group, a sequence of calcite and dolomite marbles, pelitic units, and cherty dolomites as redefined by Tull (1982, 1985). The Sylacauga Marble Group is cut out along a regional unconformity and thins from as much as 3.5 km on the northeast, to a feather edge on the southwest (Fig. 1). Rock type and stratigraphic succession have suggested that these rocks correlate with the Cambrian-Ordovician carbonate sequence of the foreland thrust belt to the northwest (McCalley, 1897; Prouty, 1916; Shaw, 1970; Gilbert, 1973; Tull, 1982; Bocz, 1985). Until recently, however, no fossils have been recovered from these marbles to confirm this correlation. This study reports discoveries of Early Cambrian archaeocyathids from the lowermost unit, the Jumbo Dolomite as redefined by Tull (1985), and Early Ordovician conodonts from the uppermost unit, the Gantts Quarry Formation of Tull (1985), of the Sylacauga Marble Group, establishing its correlation with the Cambrian and Ordovician carbonate platform rocks to the northwest. The Sylacauga Marble Group is unconformably overlain by the Lay Dam Formation (Carrington, 1973), the lowermost and most extensive unit of the Talladega Group as redefined by Tull (1982). The unconformity increases in magnitude westward; the Sylacauga Marble Group is entirely cut out near the Coastal Plain overlap where the Lay Dam lies on the upper Kahatchee Mountain Group (Shaw, 1970) (Fig. 1). The unconformity is characterized by a low-angle regional truncation of units beneath it, local relief of several meters above a paleokarst surface (Cook, 1982, Fig. 5), a sharp boundary marking a distinctive change in lithology, and overlying units containing clasts derived from basement and cover units below the unconformity (Tull and Telle, 1988). The Lay Dam Formation consists of as much as 2.5 km of homogeneous and rhythmically layered and laminated phyllite, metasandstone, and a distinctive olistostromal facies consisting of unsorted, unbedded, polymictic, bouldery and pebbly metamudstone. These rocks are interpreted to have formed in a submarine fanlike environment in a foreland successor basin that developed on the collapsing carbonate platform (Telle, 1983; Tull and Telle, 1988). Clast compositions indicate that the source included the Kahatchee Mountain and Sylacauga Marble Groups as well as the underlying Grenvillian (1.1 b.y.) crystalline basement. Until now, the Lay Dam Formation has been paleontologically undated. We have recovered post-Ordovician conodonts from the upper part of the formation. Stratigraphically overlying the Lay Dam Formation, there are coarse, thick (commonly several hundred meters), shallow-water metasandstone units, the Cheaha Quartzite as used by Griffin (1951) and the Butting Ram Quartzite as used by Tull (1982) and Tull and Telle (1988), and an associated black slate facies, the Erin Slate as used by Prouty (1923) and Griffin (1951). Overlying the Butting Ram and intercalated with the Erin Slate, is the Jemison Chert as used by Carrington (1973), Sutley (1977), and Tull (1979,1982)—a siliceous argillite, phyllite, and locally massive thick-bedded chert unit normally between 200 and 300 m thick. The Jemison Chert grades upward into the Hillabee Greenstone, as redefined by Tull and Stow (1980b), through an interval of interlayered epiclastic, volcaniclastic, and pyroclastic rocks. The nature of the contact between the Jemison and Hillabee is discussed in detail by Tull (1979) and Tull and Stow (1980a). These authors, as well as Thomas and Neathery
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(1980), among others, interpret this contact as stratigraphic. Other investigators (Prouty, 1923; Griffin, 1951; Carrington and Wigley, 1967) have interpreted the Jemison Chert-Hillabee Greenstone contact as intrusive. The Hillabee is dominated by metabasalts but also contains significant units of felsic volcanic rocks (Tull and Stow, 1980a, 1980b). The bimodal volcanic complex, locally as much as 2.5 km thick (Tull, 1979), is overlain tectonically by medium- to high-grade rocks of the eastern Blue Ridge, the Ashland Supergroup, as redefined by Tull (1978), along the Hollins Line fault. Zircons from a felsic unit have yielded a 2 0 7 Pb/ 2 0 8 Pb age of approximately 450 m.y. and have introduced controversy about the age assignment of the Hillabee, suggesting a tectonic boundary at its base (Russell, 1978; Russell and others, 1984). The implications of this radiometric age determination are discussed in Tull (1979) and Tull and Stow (1980a). Map relationships and exposures of the Jemison-Hillabee contact, however, suggest that the Hillabee stratigraphically overlies the Jemison in a chert-volcanic couplet similar to the Huntersville Chert-Tioga Bentonite sequence of Early to Middle Devonian age in the central Appalachian foreland (Sherrard and Heald, 1984). PREVIOUSLY REPORTED FOSSILS FROM THE TALLADEGA BELT There have been several previously reported occurrences of fossils from the Talladega belt, but few have been confirmed. The reports have, however, created considerable controversy for decades, and many of the unconfirmed reports continue to fuel debate. Carrington (1973) reported well-preserved, genetically identifiable Carboniferous palynomorphs from the Kahatchee Mountain Group. This palynomorph occurrence was never duplicated, and it seems unlikely that such specimens originated from rocks as thermally mature as phyllites and thus likely represent contamination. This conclusion is supported by Early Devonian or slightly older K-Ar whole-rock phyllite ages (380 ± 7 m.y., 396 ± 8 m.y., 417 ± 8 m.y.) from this group (Kish and Harper, 1973; S. A. Kish, 1982, written commun.) which most likely represent the approximate time of regional metamorphism in the Talladega belt. Carrington (1973) also reported a variety of fossils from the upper Butting Ram Quartzite, similar to those from the overlying Jemison Chert, but none were diagnostic enough for a definitive age assignment. The Cheaha Quartzite lies in a stratigraphic position similar to, but northeast of, the area containing the Butting Ram. Griffin (1951) reported a crinoid stem from the Cheaha which was not diagnostic of age. The Jemison Chert stratigraphically overlies the Butting Ram Quartzite and contains a diverse assemblage of Early Devonian fossils, which we and others have recovered from several localities. The marine invertebrate fauna is dominated by brachiopods [Acrospirifer murchisoni (Castlenau), Delthyris sp., Leptaena rhomboidalis Wilckens, Meristella sp., Stropheodonta sp., chonetids and spiriferids] but also includes tentaculitids (Tentaculites cf. T. elongatus Hall), bryozoans (Cystodictya sp.), corals (Favosites sp.), trilobites, and sponge spicules, indicating Early Devonian shelf deposition (Butts, 1926; Carrington, 1973; Sutley, 1977; and J. T. Dutro, Jr., and E. L. Yochelson, written commun. to M. Higgins, 1982,1983). The faunal elements, particularly A. murchisoni, and rock type are typical of the Oriskany Group and its equivalents in the Appalachian foreland. Perhaps the most controversial unit in the Talladega belt is the Erin Slate, which interfingers with the upper parts of the Cheaha and Butting Ram Quartzites near the top of the Talladega Group. Recent geologic mapping (J. F. Tull, 1978-1987, and W. B. Moore, 1982-1987, unpub. data) indicates that the Erin is a facies of the Jemison Chert. Although Smith (1903) reported Carboniferous lycopods and other plant materials
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Figure 2. Geologic map of part of the Wilsonville IVi-xnm. quadrangle, showing archaeocyathid locality near Fayetteville, Alabama (geology mapped by J. F. Tull). See Figure 1 for location. Superscripts indicate source of stratigraphic nomenclature: 'usage of Tull (1982), 2 usage of Tull (1985).
from Erin, in Clay County, in the central part of the Talladega belt (Fig. 1), the best material was collected by a local resident near a railroad cut. Subsequent search by numerous geologists for identifiable material in situ has failed (Gastaldo and Cook, 1982). We believe that it is likely that the original collection of well-preserved floral elements is exotic to the Erin. We have collected an exotic well-preserved Calamites sp. fragment from atop the metavolcanic Hillabee Greenstone at Pyriton, 6 km to the northeast of Erin, near the remains of the old sulfide ore smelter where the Erin resident who collected the original specimens served as mine foreman. The reported Carboniferous plant materials led some workers to exclude the Erin from the Talladega belt sequence (which at the time was believed to be correlative with the Precambrian Ocoee Supergroup) and to consider it to be exposed within a window of foreland rocks (Jonas, 1932; Park, 1935). More recently, however, geologists have recognized that the Erin has the same low-grade regional metamorphic overprint as the surrounding Talladega belt rocks and that it constitutes a contiguous part of the stratigraphic sequence (Griffin, 1951; Tull, 1979,1982).
Harris and others (1984) presented a preliminary report on the discovery of Early Ordovician conodonts in the upper part of the Sylacauga Marble Group and Silurian to Early Devonian conodonts in the upper part of the Lay Dam Formation. These discoveries are presented in detail below. NEW FOSSIL DISCOVERIES Archaeocyathids from the Sylacauga Marble Group The basal formation of the Sylacauga Marble Group is the Jumbo Dolomite. This unit thins southwestward below the pre-Lay Dam Formation unconformity; west of Sylacauga, it is 300 to 1,000 m thick and is gradationally overlain by a maroon phyllite and fine-grained sandstone sequence, the Fayetteville Phyllite of Tull (1985) (Figs. 1 and 2). The lowermost Jumbo is a thin-bedded, ribbon dolomite, grading upward into a more massive, thickly bedded dolomite. Metachert-rich intervals occur
NEW PALEONTOLOGIC EVIDENCE, SOUTHERN APPALACHIANS 5
Figure 3. Archaeocyathids from near the base of the Jumbo Dolomite, on Cedar Creek, 1.2 km north of Fayetteville, Alabama (SEV4 sec. 35, T. 21 S., R. 2 E., lat. 33°09' N., long. 86°24.5' W., Wilsonville 7.5-min. quadrangle). (A) Cross section of irregular archaeocyathid (USNM 422375); (B) the same form, but different specimen, near base of cone where septa are planar (USNM 424363).
in the middle of the unit, and calcite marble dominates the upper part. Archaeocyathids were found a few hundred meters above the stratigraphic base of the Jumbo, on Cedar Creek, 1 km north of Fayetteville (Figs. 1 and 2). In addition to the archaeocyathids, small (
