The appalachian and black warrior basins: Foreland basins in the eastern united states

The Appalachian and Black Warrior basins represent only two of the more than 200 sedimentary basins that have developed across the North American landmass, but these basins have probably had more influence than many others in understanding the interplay between sedimentation, structure, and tectonics. This is especially true for the Appalachian Basin, which is a composite, retroarc/peripheral foreland basin that formed during five orogenies. In many ways, the Appalachian Basin is the “type” foreland basin and the “type area” for the Wilson cycle. Our understanding of the basin, and others like it worldwide, is largely the legacy of a single observation by James Hall in 1857, an observation that also effectively established the framework for the later plate-tectonic paradigm, not to mention major framework developments in structure, tectonics, isostasy, flexural modeling, stratigraphy, sedimentation, paleoclimate, and paleogeography. As preserved today, the basin is about 2050 km long with an area of nearly 536,000 km2, extending from southern Quebec in Canada to northern Alabama in the United States; it reflects the structural influence of earlier Grenvillian convergence and Rodinian dispersal, as well as the paleoclimatic, paleogeographic, eustatic, and tectonic history of eastern Laurentia/Laurussia from latest Precambrian to early Mesozoic time. The associated Black Warrior Basin, on the extreme southwest margin of the Appalachian Basin, is a peripheral foreland basin that developed during a single orogeny. It is a more local basin, and as preserved today, it is about 383 km long by 313 km wide and encompasses an area of nearly 63,900 km². During latest Precambrian to Early Ordovician time, the recently formed, southern to southeastern Appalachian margin of Laurentia, which now includes both basins, experienced mainly synrift and postrift, passive-margin sedimentation, largely controlled by local structure, regional climate and eustasy. However, by Cambrian time, on some of the more distal outboard parts of the Laurentian margin, the initial tectonic reorganization that would ultimately produce these basins had already begun. Major development of the Appalachian foreland basin began with the advent of the Taconian orogeny at about 470 Ma near the Early-Middle Ordovician transition and continued for nearly 200 Ma during five nearly continuous orogenies that reflect closure of the Iapetus and Rheic oceans and growth of Pangea. Closure of the Iapetus and Rheic oceans reflects the transfer of several Gondwanan terranes to the eastern margin of Laurentia and eventual collision with Gondwana during Paleozoic time. Tectonic dynamics controlled the extent and shape of the basin during various orogenies, and the resulting deformational loading seems to have largely generated the accommodation space in which Appalachian sediments accumulated. Sediment thicknesses up to 13,700 m accumulated in 13 third-order (106–107 years), unconformity-bound cycles that are clearly related to Appalachian tectophases, distinct phases of tectonism controlled by sequential convergence with continental promontories during orogeny. The Black Warrior Basin, in contrast, contains two Late Paleozoic tectophase cycles generated during the Ouachita orogeny. Appalachian tectophase cycles during the Taconian and Salinic orogenies and during the succeeding Acadian, Neoacadian, and Alleghanian orogenies form the larger, second-order (107–108 years), Caledonian and Variscan-Hercynian orogenic supercycles, which generally reflect closure of the Iapetus and Rheic oceans, respectively. In most parts of the basin, the brief transitional Siluro-Devonian, Helderberg interval separates these supercycles and is represented by a thin, widespread, shallow-water, clastic, and carbonate succession with poorly developed tectophase cycles. In contrast to the relative tectonic quiescence apparent in the foreland basin during the Helderberg interval, evidence from more outboard parts of the orogen indicates that the Helderberg interval appears to represent a transitional period of uplift, magmatism, and successor-basin formation during Taconian-Salinic orogen collapse and change to collision-related, strike-slip and transpressional regimes in succeeding orogenies. The first 11 cycles in the Appalachian foreland basin mainly reflect subduction-type orogenies and typically consist of basal, dark, marine shales succeeded in ascending order by flysch-like and molasse-like units, all of which track the progress of orogeny in time and space. The last two Alleghanian cycles, in contrast, reflect collision-type orogeny and are largely composed of clastic-dominated, terrestrial or marginal-marine sediments with a strong eustatic overprint related to Gondwanan glaciation. Although Alleghanian tectonism probably continued through Late Permian time, no foreland-basin sediments younger than Early Permian age are preserved. By Late Triassic time, thermo-tectonic thickening and uplift in the Alleghanian orogen and/or delamination of the underlying Rheic slab and resulting mantle upwelling into old crustal zones of weakness caused orogen collapse and extension, ending the Iapetan or Appalachian Wilson cycle and initiating Pangean dispersal and the current Atlantic Wilson cycle. The importance of the Appalachian Basin lies not only in its “type” status as a basis for our understanding of geomorphological, structural, stratigraphic, and sedimentary parts of the plate-tectonic paradigm, but also in the fact that it contains the relatively well-preserved 545-Ma stratigraphic and sedimentary record of one complete Wilson cycle and parts of others. The larger foreland-basin/orogen area clearly shows the orogen-collapse and extension phase of the previous Laurentian or Grenvillian Wilson cycle during dispersal of the Rodinia supercontinent, as well as late-synrift, passive-margin, active-margin, and orogen-collapse phases of the Iapetan or Appalachian cycle during accretion and dispersal of the Pangean supercontinent. The foreland-basin area itself shows evidence for all the phases except orogen collapse. Nonetheless, what is particularly apparent throughout the basin’s entire history is the fact that the zigzag shape of the old Iapetan margin and the basement structural framework remaining from the previous Laurentian cycle, combined with a series of probably global tectonic events, essentially controlled development and infill of the Appalachian foreland basin. This is apparent in the timing and distribution of the 13 sedimentary cycles that largely comprise its sedimentary infill. Even so, every cycle in the basin differs, reflecting the indelible overprint of changing climatic, geographic, and eustatic regimes. The Black Warrior Basin, in contrast, is not as well understood as the Appalachian Basin. Its roughly 30-Ma history during Mississippian-Pennsylvanian time is relatively short, but that history is very instructive in that it demonstrates how flexural forces from multiple orogenies interact to generate a unique basin response within the standard tectophase model. Moreover, it shows that flexural forces generated during nearby coeval orogenies may influence the sedimentary responses in adjacent foreland basins.