EVIDENCE FOR HUGE ESTUARIES ALONG THE MARGINS OF EPICONTINENTAL SEAS, MIDCONTINENT NORTH AMERICA: PALEOZOIC AND MESOZOIC EXAMPLES

Marine transgressions episodically flooded vast areas of the North American continental interior during the Paleozoic and Mesozoic, and these transgressions were accompanied by expansion of estuarine facies along the seaway margins that extended up former river valleys. The large-scale river systems that drained the continental interior were ideal sites for the development of huge estuarine systems marginal to the transgressing epicontinental seas in the Midcontinent area. Such estuarine sedimentary units lack evidence of typical open-marine facies and faunas that characterize the expansive epicontinental seas, but instead include evidence of brackish-water faunas, tidally-influenced sedimentation, restricted oxygen-poor benthic conditions, and inter-tonguing with fluvial/paludal facies. Such estuarine facies are sometimes developed at considerable distances from the margins of the epicontinental sea as delineated at the depositional edges of open-marine shale or carbonate facies. The distances from the seaway margins are at times so great that remarkably huge estuarine systems are interpreted for specific Phanerozoic episodes in the North American Midcontinent. The Middle Ordovician (Whiterockian) Winneshiek Shale of Iowa contains brackish faunas and tidally-influenced restricted facies that lies 500-700 km from the coeval seaway margin. The basal Pennsylvanian (Morrowan) ‘Caseyville’ Formation of Iowa and northern Illinois contains tidally-modulated estuarine facies and brackish faunas, lying more than 400 km from the nearest seaway margin. The Cretaceous (upper Albian-lower Cenomanian) Dakota Formation of Iowa, eastern Nebraska, and Minnesota contains multiple horizons of marine-influenced estuarine facies marked by tidal modulation and brackish microfossils (acritarchs, dinoflagellates) within an otherwise fluvial-dominated depositional system. The basal Dakota contains estuarine facies that extend eastward to central Iowa, some 400 km from the seaway margin (Kiowa Shale). Middle Dakota estuarine facies are found at least 600 km east of coeval marine facies (Muddy/Mowry). The size of these ancient Midcontinental estuaries was truly immense, equally or exceeding in scale the largest estuaries in the modern world (e.g., Chesapeake Bay, St. Lawrence, Amazon).