Chapter 4 The Appalachian Foreland Basin in Eastern United States

Abstract

The Appalachian Basin is a composite, retroarc foreland basin that in many ways 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 2,050 km long with an area of nearly 536,000 km2, extending from southern Quebec in Canada to northern Alabama in the U.S., and 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. During latest Precambrian to Early Ordovician time, the recently formed, southern to southeastern, Appalachian margin of Laurentia 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 the Appalachian foreland basin had already begun. Major development of the Appalachian foreland basin began with the advent of the Taconian orogeny at about 472 Ma near the Early–Middle Ordovician transition and continued for nearly 200 Ma during four nearly continuous orogenies that reflect closure of the Iapetus and Rheic oceans and growth of Pangea. Tectonic dynamics controlled the extent and shape of the basin during various orogenies, and the resulting deformational loading is largely thought to have 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. Tectophase cycles during the Taconian and Salinic orogenies and during the succeeding Acadian 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. These supercycles are separated by the brief, Siluro-Devonian, Helderberg interval, which in the foreland basin 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, 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 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 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 at the end of 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.