Abstract

The Cretaceous (Gulfian) sediments in the upper Mississippi embayment are the result of deltaic deposition. The deltaic system (the "McNairy Delta") centered in the northeastern end of the embayment and dispersed the sediments toward the SW. Three major environments of deposition are represented by the embayment Gulfian sediments: 1) the fluvial and upper deltaic environment, represented by the nonmarine, arenaceous Coffee and McNairy formations; 2) the pro-delta shelf environment, represented by the marine, calcareous, and argillaceous Selma and Owl Creek formations; and 3) the transitional delta platform environment, represented by the marine, argillaceous Coon Creek formation and the marine parts of the arenaceous McNairy and Coffee formations. Several regional trends are apparent from subsurface studies: the sedimentary body thickens from less than 100 ft. in the N. to more than 1,100 ft. in the SW.; the clastic ratio decreases from greater than 2:1 in the N. to less than 1:2 in the S.; the sediments range from totally nonmarine in the N. to totally marine in the SW. McNairy formation sands are first-cycle protoquartzites. The heavy-mineral assemblages of the McNairy formation and the adjacent Coffee, Selma Coon Creek, and Owl Creek formations are identical and are composed chiefly of kyanite, staurolite, tourmaline, zircon, and sillimanite, with minor amounts of other minerals. Major sand bodies in the McNairy formation trend SW., parallel with the paleoslope and the regional transport direction. Local trends in the McNairy formation can be predicted from studies of cross-stratification and fabric. The Gulfian sediments in the upper Mississippi embayment were derived chiefly from the high-rank metamorphic rocks of the Blue Ridge and Piedmont plateau of the southern Appalachians and were transported to the embayment by a single, large stream system. Humid, temperate to subtropical climatic conditions were prevalent in both the source and depositional areas. The depositional history involves 2 transgressive-regressive cycles of late Cretaceous seas, separated by an erosional interval. The initial transgressive-regressive cycle resulted in the deposition of the Coffee-Selma-Coon Creek-McNairy sequence, and the later cycle, apparently more rapid and widespread, resulted in the deposition of the Owl Creek-Prairie Bluff sequence. A depositional model, postulated for the Cretaceous (Gulfian) upper Mississippi embayment, includes as primary elements 1) basin geometry: a shallow, open-end, intracratonic basin, plunging SW.; 2) depositional pattern: deltaic with longitudinal filling; 3) paleoslope: parallel with basin axis, toward the SW.; 4) depositional strike: normal to paleoslope, NW.-SE.; 5) transport direction: parallel with paleoslope and normal to depositional strike, toward the SW.; 6) sand trends: parallel with transport direction and paleoslope and normal to depositional strike, toward the SW.; 7) sedimentary association: protoquartzite grading down paleoslope into impure carbonates; and 8) tectonic activity: mild subsidence and uplift.

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