The development of carbonate sequences that grade toward a source region into terrigenous clastic sediment is poorly understood. In both carbonate and terrigenous clastic regimes, environments of rapid sedimentation have comparable rates of deposition (102 to 102.5 cm/1,000 yr); slowest continuous rates are also comparable (100.5 to 101 cm/1,000 yr). Because of the inhibiting effect of terrigenous deposition on carbonate (skeletal) production, the slowest rates in a transect tend to occur where mixed lithologies are deposited—midway between coarse clastic and pure carbonate sediments. This leads to a bimodal distribution of rapid depositional rates and invariably to the development of a basin in the intervening area.
To develop a model, we combine concepts of depositional rate with knowledge of the Middle Ordovician carbonate to terrigenous clastic sequence of the southern Appalachians. Deposition began on a shallow-water ramp sloping gently eastward, but everywhere within the photic zone. An episode of tectonic subsidence differentiated this ramp into a shallow shelf on the northwest and a deep, starved basin on the southeast, separated by a linear shelf margin with organic build-ups. Subsequent development of the sequence consisted of the filling of the deep basin (first with pelagites, then with turbidites), during which the carbonate shelf was isolated from terrigenous deposition. Once the basin filled, terrigenous deposition began to affect the shelf. Inhibition of carbonate deposition near the shelf edge led to formation of successively smaller basins within the old shelf area. After the initial pulse of tectonic subsidence, deposition of the sequence can be explained by sediment-loading subsidence.
Thus, the Ordovician sequence, coupled with the concept of bimodal distribution of sedimentation rates in carbonate to terrigenous clastic facies patterns, allows development of a model for basins adjacent to active tectonic lands. Five model stages are derived by application of calculated deposition rates and sediment loading along a transect away from the terrigenous source. These stages are (1) an initial carbonate ramp stage; followed by (2) a carbonate shelf-starved deep-basin stage; (3) a carbonate shelf-turbidite basin-terrigenous shelf stage; (4) a carbonate shelf-narrowed and shallowed basin-broad terrigenous shelf stage; and finally (5) a stage in which the initial basin is filled, a subsequent starved basin forms on the old carbonate shelf, and a new carbonate shelf margin forms a few to tens of kilometres farther from the terrigenous source. The model should be useful in predicting depositional facies in analogous basins.