FILLING A FORELAND BASIN: MODELS FROM THE APPALACHIAN BASIN

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. The basin exhibits 13 third- to fourth-order unconformity-bound cycles that represent flexural, foreland-basin manifestations of distinct episodes of tectonism, called tectophases, during five orogenies.

The orogenies were polyphase events that included two or more tectophases, or deformational episodes, commonly focused at different places and times along the orogen. The sedimentary record in the basin suggests that during five Paleozoic orogenies along the Laurentian/Laurussian margin, tectophase timing and occurrence were mediated by diachronous convergence at successive promontories. Projecting promontories served to localize tectonism because of greater shortening and more intense deformation, while intervening reentrants served as sediment sinks. The greater intensity of deformation and resulting deformational loading at promontories, moreover, insured greater flexural subsidence in adjacent parts of the foreland basin so that major basin depocenters or sub-basins are located just behind or adjacent to respective promontories. Hence, diachronous, oblique convergence with successive promontories generated tectophases during each orogeny that are reflected by distinct, unconformity-bound, flexurally related sedimentary cycles in the foreland basin.

The cycles show a consistent sequence of lithologies, called tectophase cycles. Overlying a bulge-related unconformity, marine black shales initiate most cycles and mark the time of maximum deformational loading and flexural subsidence; mapping the distribution of these shales in time and space can show the along-strike progress of orogeny relative to promontories. The shales in each cycle are overlain by a series of relaxational clastics, including deeper-water, flysch-like clastics followed by more shallow, marginal-marine, molasse-like clastics.

As for resources, foreland-basin cycles like these suggest that the black shales, as hydrocarbon source and reservoir rocks, and the overlying clastics, as major reservoir rocks, are products of distinctive tectonic frameworks and histories and may provide additional controls on the timing and location of tectonic events.