Cratonic unconformities represent (1) coincidence of surfaces of sedimentary accumulation with depositional base level, or elevation of depositional surfaces above erosional base level, and (2) renewed deposition covering surfaces of nondeposition or erosion. The chronostratigraphic record of unconformities is best displayed on Wheeler diagrams on which geographic distances are plotted against chronostratigraphic intervals or absolute time. Assessing the lithostratigraphic significance of unconformities requires reconstruction of the pre-unconformity stratigraphy and estimation of the thickness (or volume) of the strata eroded and of their lithologic character.
Interregional cratonic unconformities fall into two major types: (1) those marked by subequal values of nondeposition and erosion, commonly involving 5 to 30 m.y. and the stripping of as much as 1 km (.6 mi) over very broad areas (for example, the sub-Kaskaskia unconformity-Early Devonian to Early Carboniferous); and, (2) unconformities characterized by short-term nondeposition (< 5 m.y.) and extremes of erosional vacuity (for example, the sub-Absaroka surface-Late Carboniferous).
Conventional wisdom suggests that episodes of cratonic nondeposition and erosion should equate with accelerated detrital deposition at continental margins and with perturbations of marine chemistry. Evidence is accumulating to indicate a degree of concomitance between cratonic events and oceanic geochemistry but no complementary pattern is clear in terms of slope/rise depositional rates. Indeed, certain major unconformities identified on continental slopes appear to have equivalents on cratons. These and related questions demand increased communication between land-based and seagoing stratigraphic and tectonic specialists.
Figures & Tables
The result of a session at the 1981 AAPG Annual Meeting, this volume attempts to document global age and magnitude of sea-level shifts, and ultimately, the cause of the short-term shifts. Twelve individual papers were published on topics such as: comparative anatomy of cratonic unconformities; relation of unconformities, tectonics, and sea-level change; outcrop features and origin of basin margin unconformities; significant unconformities and the hiatuses represented by them; regional unconformities and depositional cycles; relative sea-level changes during the Middle and Late Cretaceous; Late Oligocene-Pliocene transgressive-regressive cycles of sedimentation; oxygen-isotope record of ice-volume history; oceanic ridge volumes and sea-level change; Jurassic unconformities, chronostratigraphy, and sea-level changes; Cenozoic regional arosion of the Abyssal sea floor; and depositional sequences and stratigraphic gaps on submerged United States Atlantic margin.