TECTONIC EVOLUTION OF THE SOUTHERN MARGIN OF LAURENTIA

The Ouachita orogen adapted to the existing Iapetan rifted margin of Laurentia during the late Paleozoic assembly of Pangaea. During diachronous continental rifting and breakup of Rodinia (~765 to 530 Ma), transform offsets of the rift framed the Alabama promontory, Ouachita embayment, Texas promontory, and Marathon embayment of southern Laurentia. Post-rift thermal subsidence and transgression of passive-margin carbonate-shelf deposits began diachronously during the Cambrian; a passive margin persisted into the Mississippian west of the Alabama promontory. Deposition of synorogenic clastic sediment, tectonic-load-driven foreland subsidence, and cratonward propagation of foreland thrusting began diachronously along the Ouachita orogen in the Mississippian and continued into Middle Pennsylvanian in the Ouachita embayment and into Early Permian in the Marathon embayment. Synorogenic deposition began with a thick succession of Mississippian muddy turbidites, deposited over off-shelf deep-water passive-margin facies, while a relatively thin heterolithic succession was deposited on the adjacent shelf. Ouachita allochthons of Cambrian–Mississippian off-shelf passive-margin deposits and Mississippian–Pennsylvanian synorogenic turbidites were thrust over the Cambrian–Mississippian passive-margin shelf succession. As the tectonic load propagated toward the craton, foreland subsidence, a component of which included large-scale down-to-basin basement normal faults, accommodated thick successions of Pennsylvanian turbidites in the foreland basins. Intracratonic basement uplifts and fault systems (Mississippi Valley, Southern Oklahoma), trending orthogonal to the continental margin, partitioned the foreland; a string of foreland basins (Black Warrior, Arkoma, Fort Worth, Kerr, Val Verde) rimmed the margin. Southward subduction of Laurentian crust and accretion of the Sabine microcontinental terrane in the Ouachita embayment generated a continental-margin arc, an accretionary prism and forearc basin, and incomplete closure of a remnant ocean basin. Mesozoic extension during opening of the Gulf of Mexico and Atlantic Ocean affected the interior of the Ouachita orogen, and transgressive passive-margin deposits of the Gulf Coastal Plain covered much of the orogen.