TECTONIC EVOLUTION OF THE SOUTHERN MARGIN OF LAURENTIA (Invited Presentation)

Bends of the late Paleozoic Pangaean suture belt mimic the shapes of promontories and embayments of the Iapetan rifted margin in southern North America. Pangaean assembly began with the Ordovician–Silurian Taconic and Devonian–Mississippian Acadian orogenies northeast of the Alabama promontory. To the west, a passive margin persisted into Mississippian. The Mississippian–Permian Alleghany–Ouachita–Marathon orogeny completed assembly of Pangaea. Mesozoic rifting fragmented parts of the Pangaean suture belt.

Off the shelf margin west of the Alabama promontory, thick muddy turbidites overlie Devonian–Mississippian novaculite, indicating an abrupt increase in sedimentation rate and initiation of orogenic sediment supply, first in the Marathon embayment (Early Mississippian) and later in the Ouachita embayment (Middle Mississippian). Mississippian to Middle Pennsylvanian muddy to sandy turbidites began to fill arc-related basins; starved basins persisted on the adjacent Laurentian shelf. From Middle Pennsylvanian through early Permian (Wolfcampian), synorogenic siliciclastic sediment filled arc-related basins and prograded into peripheral foreland basins on the shelf. Thin-skinned thrust faults propagated into the foreland clastic wedges from accretionary prisms thrust onto the shelf. Angular unconformities indicate latest thrusting in Marathon during Wolfcampian and in Ouachita by Desmoinesian (Middle Pennsylvanian).

In the Marathon embayment, the accreted Coahuila terrane includes a late Paleozoic arc; early to late Paleozoic detrital zircons; and Pan-African (Neoproterozoic), Sunsás (Mesoproterozoic), and older basement rocks. Tuff beds in the Marathon turbidite succession are coeval with arc rocks in Coahuila. Other accreted terranes in the Marathon embayment include Sierra Madre (Mississippian arc rocks and Sunsás basement) and Maya (middle Paleozoic and Pan-African–age rocks). Two age groups (Pan-African and Trans-Amazonian) dominate the Suwannee terrane at the corner of the Alabama promontory. No samples are available directly from the Sabine terrane in the Ouachita embayment. Late Paleozoic tuffs within Ouachita muddy turbidites indicate a Sabine continental-margin arc; xenocrystic zircons span middle Paleozoic to Archean.