Abstract The Martinsburg-Reedsville, Juniata-Sequatchie, and Tuscarora-Clinch-Rockwood-Red Mountain Formations, (Upper Ordovician to Lower Silurian) comprise the major part of the Taconic clastic wedge in southwestern Virginia, eastern Tennessee, northeastern Alabama, and northwestern Georgia. This clastic wedge constitutes the fill of a peripheral foreland basin which formed in response to thrust loading. The well-known disconformity at the base of the Tuscarora Fm. in the eastern Valley and Ridge of Virginia was recently traced as a paraconformity to the northwest (i.e., basinward). A white, thick-bedded "upper" Tuscarora abruptly overlies the Martinsburg or Juniata Formations. with a distinctive basal conglomerate in the southeastern Valley and Ridge; a yellowish, thin-bedded, more mudstone-rich "lower" Tuscarora interpreted as facies-equivalent for parts of the Juniata Formation, occurs beneath the "upper" Tuscarora in the northwestern Valley and Ridge. This regional unconformity, with maximum lacuna towards the southeast, divides the clastic wedge into two parts. The nature of the unconformity along and across structural strike, its regional extent and disappearance towards Tennessee, and the overall tectonic setting all favor a tectonic origin for this unconformity. Basin rebound following the cessation of thrusting, concomitant with erosional/tectonic unloading of the Taconic orogen farther to the southeast, is suggested as the causal mechanism for the erosional unconformity. This interpretation contrasts markedly with stratigraphic relationships observed in eastern Tennessee, northeastern Alabama and northwestern Georgia, where the equivalent stratigraphic interval exhibits a small lacuna between the Juniata-Sequatchie and Clinch-Rockwood-Red Mountain Fms. Basin development was not related to collisional tectonism, but represents a "left-over moat" of the Blount-phase. Paleosols occur within the upper Juniata Fm. in the eastern Valley and Ridge, sharply overlain by a 0.6- to 3-m thick, very fine- to medium-grained and phosphatic, highly bioturbated, transgressive sandstone of marine shoreface origin that occurs in the basal Clinch, Rockwood and Red Mountain Fms. This regionally correlative sandstone, informally known as the "basal transgressive sandstone" (bts), was deposited along ravinement surfaces during erosional shoreface retreat; it therefore documents a lowstand followed by a eustatic rise. To the west, the bts overlies open-marine shale and limestone deposits containing a typical Ashgillian brachiopod fauna; shelf strata above the bts contain a Lower Llandovery (A1-2) brachiopod fauna. Hence, the magnitude of the lacuna appears decrease in a northwesterly direction, where the Ordovician to Silurian boundary is nearly conformable.

A Mississippian carbonate facies in north-central Alabama is bordered on the southwest and on the northeast by separate prograding clastic wedges. The southwestern clastic wedge comprises a major delta complex which prograded northeastward onto the carbonate shelf and associated barrier sediments. The clastic sequence grades upward from shale (Floyd) through sandstone and shale (Parkwood) to a quartz-pebble conglomerate and coal-bearing interval (Pottsville), and the Floyd-Parkwood grades laterally northeastward into the carbonate facies across a transition zone marked by shelf-edge barrier sands and oölites. Stratigraphic strike of the Floyd-Parkwood trends southeastward across northwestern Alabama, but in the Appalachian structural system it is deflected northeastward along structural strike of contemporaneous synclines. The Floyd-Parkwood-Pottsville thickens southwestward and is probably continuous with the Ouachita clastic wedge. The northeastern clastic wedge includes a shale and sandstone unit (Pennington) overlain by quartz-pebble conglomerate and associated coalbearing strata. The Pennington grades southwestward to the upper part of the carbonate facies in northeastern Alabama and apparently is only the fringe of a clastic wedge centered farther northeast. The prograding clastic wedges, which converge in Alabama from the southwest and northeast, suggest major clastic provenances in the Ouachita and Tennessee Appalachian structural salients. The carbonate shelf in the Alabama Appalachian structural recess was progressively overlapped from the southwest and northeast by laterally derived clastic sediments which were partly channeled along contemporaneous synclines in the recess.

Abstract The allochthonous Talladega belt of eastern-northeastern Alabama and northwestern Georgia is a northeast striking, fault bounded block of lower greenschist facies metasedimentary and metaigneous rocks that formed along the margin of Laurentia at or outboard of the seaward edge of the Alabama promontory. Bounded by metamorphic rocks of the higher grade Neoproterozoic(?) to Carboniferous eastern Blue Ridge on the southeast and unmetamorphosed to anchimetamorphic Paleozoic rocks of the Appalachian foreland on the northwest, the Talladega belt includes shelf facies rocks of the latest Neoproterozoic/earliest Cambrian Kahatchee Mountain Group, Cambrian-Ordovician Sylacauga Marble Group, and the latest Silurian(?) to uppermost Devonian/earliest Mississippian Talladega Group. Along the southeastern flank of these metasedimentary sequences, a Middle Ordovician back-arc terrane (Hillabee Greenstone) was tectonically emplaced along a cryptic pre-metamorphic thrust fault (Hillabee thrust) and subsequently dismembered with units of the upper Talladega Group along the post-metamorphic Hollins Line fault system. Importantly, strata within the Talladega belt are critical for understanding the tectonic evolution of the southern Appalachian orogen when coupled with the geologic history of adjacent terranes. Rocks of the lower Talladega Group, the Lay Dam Formation, suggest latest Silurian–earliest Devonian tectonism that is only now being recognized in other areas of the southern Appalachians. Additionally, correlation between the Middle Ordovician Hillabee Greenstone and similar bimodal metavolcanic suites in the Alabama eastern Blue Ridge and equivalent Dahlonega Gold belt of Georgia and North Carolina suggests the presence of an extensive back-arc volcanic system on the Laurentian plate just outboard of the continental margin during the Ordovician and has significant implications for models of southern Appalachian Taconic orogenesis.

Abstract The eastern Gulf Coast, comprising the states of Mississippi and Florida and those portions of Alabama and Georgia not included in the Appalachian Mountains and the Piedmont Plateau, covers about 166,000 square miles. About 95,000 cubic miles of Paleozoic sediments are included in this area, occupying two widely separated basins. The better known of these is the Black Warrior basin, of northeastern Mississippi and northwestern Alabama, in which the first hydrocarbon production of both these states was found. The other, herein named the "Suwannee River basin," lies in southern Georgia, southeastern Alabama, and northern Florida. It is nonproductive. The most important basin within the eastern Gulf Coast from the standpoint of oil production is the Mississippi salt basin, which is continuous with a similar basin in Louisiana on the west, and extends into Alabama on the east. Minor production has been obtained from four of the 49 shallow salt domes proven by drilling in this basin. The major production from Mesozoic reservoirs in the province is derived from salt-uplifted structures which flank the salt basin on the north and south. Important Tertiary (Eocene Wilcox) production is largely confined to four counties in southwestern Mississippi. It is obtained principally from structurally modified stratigraphic traps. Southern Florida, since early Cretaceous time at least, has been a shallow platform area characterized by carbonate and evaporite deposition with no important noncarbonate clastics. Two fields have been discovered to date, producing from a highly organic limestone of Lower Cretaceous, Glen Rose age. The present northeastern boundary of rich Miocene production in South Louisiana and the Louisiana tidelands is approximately coincident with the 10,000-foot isopach of Miocene sediments. Present information, which indicates rapid thinning of the Miocene and its gradation to a carbonate facies east and northeast of this line, suggests objectives for exploration in the Mississippi-Alabama-Florida coastal and offshore areas will be in sediments other than Miocene, on structures other than salt domes.

Salient features of the new aeromagnetic map of the Southeastern United States (Zietz and Gilbert, 1980) are: (1) the Charleston magnetic terrane that is generally high magnetically and has numerous distinct, very high magnetic anomalies within it; (2) the northern Florida magnetic terrane that is virtually identical magnetically to the Charleston magnetic terrane; (3) a linear series of magnetic low anomalies within a continuous magnetic low anomaly, collectively called the Altamaha magnetic anomaly, that is more than 1,150 km long and that arcs across the Continental Shelf, from about 33°30’N., 76°30’W., to the Georgia coastline at the mouth of the Altamaha River and trends inland across southern Georgia and Alabama; (4) the southern end of the East Coast anomaly and its shoreward branch, the Brunswick anomaly, which is part of the northern Florida terrane; (5) the characteristic magnetic terranes of the Piedmont and Valley and Ridge provinces; (6) the southern end of the New York-Alabama lineament and the magnetically high terrane west of it; and (7) the lineament formed by the nearly straight northwestern margin of the Charleston magnetic terrane in South Carolina and Georgia, the northwestern margin of the Altamaha anomaly, and the southern margin of the magnetically high terrane northwest of the New York-Alabama lineament in Alabama. A newly compiled gravity map of part of the Southeastern United States shows that the Charleston and northern Florida magnetic terranes have mixed gravity expression, with gravity high anomalies coinciding with the distinct, very high anomalies on the aeromagnetic map. In general, the trace of the Altamaha magnetic anomaly either coincides with or lies just south of gravity low anomalies that are linear and form a linear series along or closely parallel to the magnetic anomaly. Near its northeastern end, the Altamaha magnetic anomaly lies along a relatively steep, northwestward-sloping gravity gradient. The East Coast anomaly coincides with linear gravity high anomalies. The Charleston and northern Florida magnetic terranes are interpreted as being virtually the same geologic terrane, a predominantly mafic terrane, intruded by mafic plutons of batholithic proportions, that, until late Paleozoic time, was part of the African or African/South American plate. The northwestern margin of the Charleston terrane is interpreted as the Alleghanian suture between Africa and North America, and also as a major strike-slip fault, the Carolina-Mississippi fault, that extends to the southwest along the northwestern border of the Altamaha anomaly and the southern border of the magnetically high terrane northwest of the New York-Alabama lineament. The Altahama anomaly is interpreted as being caused by a fault-bounded, sediment-filled trough, locally as deep as 6 km, that divides the Charleston-northern Florida terrane. The Carolina-Mississippi fault truncates the Charleston terrane, the Piedmont and Valley and Ridge provinces, the New York-Alabama lineament, and the magnetically high terrane northwest of the New York-Alabama lineament, and the absence on the southeastern side of the fault of magnetic terranes found on the northwestern side suggests right-lateral displacement.