PALEOZOIC CRATONIC STRATIGRAPHY IN THE NORTH AMERICAN CONTINENTAL HEARTLAND, A COMPLEX "LAYER CAKE"

Stratigraphic successions of Paleozoic carbonate, carbonate-evaporite, and mixed carbonate-siliciclastic facies stretch across vast areas of the cratonic interior in the U.S. and Canada. Depositional phases in the Paleozoic tropical epeiric seas lasted millions of years, each major phase punctuated by prolonged episodes of subaerial exposure and erosion that bound Sloss’ (1963) cratonic (mega)sequences. Shorter duration cyclic changes in sea-level (eustatic) of varying scale resulted in deposition of widespread transgressive-regressive stratigraphic sequences on the craton (3rd- and 4th-order). In shoreward regions, regression led to subaerial exposure, but more offshore regions remained entirely subtidal even during sea-level lowstands in many sequences. Cratonic sediment accumulation generally was slow, compared to coeval continental margins and foreland basins. Condensed sedimentation or starved submarine surfaces commonly characterize transgressive episodes in the central epeiric seas (“middle shelf”), and each cratonic sequence typically is dominated by shallowing-upward regressive deposition. In some sequences, individual beds and parasequence-scale packages (decimeter- to meter-scale in thickness) are remarkably continuous and can be traced over areas hundreds of km in extent, forming widespread “layer cake” stratigraphy. However, examples from the central Midcontinent show complex stratal geometries above regional surfaces that span many hundreds of kilometers. Ordovician and Mississippian examples from Iowa show submarine surfaces, some with indicators of bottom dysoxia to anoxia, that are proximally downlapped and distally onlapped producing heretofore unrecognized stratal geometries. In extreme cases, entire 3rd-order sequences are locally absent, marked only by submarine surfaces (some with omission sequences). Our studies suggest that stratal geometries on the craton grossly resemble parasequence-scale clinoforms seen in thicker continental-margin and basinal successions, but cratonic clinoforms are significantly compressed vertically and “stretched-out” laterally over hundreds of kilometers.