Full article available in PDF version

The Macon Complex, which extends from eastern Alabama to northern North Carolina, is a late Precambrian–Middle Cambrian accretionary complex comparable in size to the Franciscan Complex of California and Oregon. Much of the complex is tectonic, sedimentary, and metamorphic chaos, properly termed mélange, where well-rounded to angular fragments, blocks, and slabs of contrasting metamorphic grades, different igneous parentages, drastically different sedimentary facies, and different degrees of deformation “float” in highly imbricated and tectonized matrices, the whole having been intruded by Devonian mafic plutons and associated syenites, and by Carboniferous granitic plutons. We have divided the complex into three mélanges that probably reflect different structural regimes within the accretionary wedge: (1) the Juliette mélange, with two tectonostratigraphic lithofacies, the clastic-rich, partly olistostromal Potato Creek facies and the Gladesville facies, rich in mafic and ultramaflc fragments, blocks, and slabs; (2) the Po Biddy mélange, characterized by metamorphosed manganiferous sediments, metavolcaniclastic rocks, graphitic schists, and locally by metamorphosed thinly bedded pyritiferous limestones, and a wide variety of mineral deposits; and (3) the Falls Lake mélange, which is quite similar to the Juliette mélange and probably represents the same tectonostratigraphic horizon in the accretionary prism. The matrices of the mélanges contain a wide variety of metaigneous and metasedimentary exotic clasts, including mafic and ultramaflc rocks. The Macon Complex is structurally overlain by the late Precambrian–Middle Cambrian Little River Complex, made up of thick piles of mostly felsic calc-alkaline metavolcanic rocks, and lesser amounts of metaplutonic rocks, that originated in a continental-margin volcanic arc (Little River arc). Trilobites from near the top of one of the youngest sections are restricted to the upper two-thirds of the Middle Cambrian and are characteristic of the Atlantic faunal province. The Little River Complex is overlain, beneath the Atlantic Coastal Plain, by the African cratonic Northern Florida platform sequence; the Macon and Little River complexes and the Northern Florida platform sequence make up the Little River thrust stack. The magmas of the Devonian plutons that have intruded the Macon Complex probably formed when the Little River stack was thrust upon the underlying Georgiabama thrust stack, which was itself still being thrust toward the North American craton. The Macon Complex is interpreted to have formed between a trench and the Little River island arc at the oceanward edge of what was either a microcontinent off the African continent or the core of the present African continent. Many mafic and all ultramaflc bodies in the mélange are probably pieces of Iapetus Ocean crust and mantle offscraped from the downgoing slab and imbricated into the accretionary wedge. Rocks of the Macon Complex have previously been assigned to the “Charlotte,” “Kiokee,” “Kings Mountain,” and “Lowndesville” belts and to parts of the “Uchee,” “Raleigh,” “Pine Mountain,” and “Inner Piedmont” “belts.”

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.