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Sequence stratigraphy and sequence boundary characteristics for upper Tournaisian (Mississippian) strata in the greater Williston basin area: an analysis of a third-order cratonic carbonate-evaporite depositional cycle
Sequence stratigraphy and sequence boundary interpretations for Mississippian Madison stratigraphic units in the greater Williston basin area illustrate some of the challenges for sequence stratigraphy in cratonic carbonate-evaporite depositional systems. Some of the most significant interpretation differences occur in middle Madison strata that correlate with the upper Tournaisian Frobisher-Alida interval in North Dakota which, in the present paper, is considered to represent a third-order (2–3 million-year) sequence. Widely published surface studies in Wyoming place a third-order sequence boundary near the base of a regional solution breccia bed that corresponds with the Frobisher-Alida evaporite. In the subsurface, it can be demonstrated that most of the evaporite formed during regressive deposition and a stratal surface near the base of the evaporite is not a viable choice for a third-order sequence boundary.
The Frobisher-Alida third-order maximum regressive surface separates underlying regressive deposits from overlying transgressive deposits and it broadly corresponds with conventional sequence boundary placement. This is a subtle surface that may lie either within or on top of the Frobisher-Alida evaporite or solution breccia equivalent. The third-order maximum flooding surface is placed on a widely mappable submarine unconformity that occurs as a sharp contact on an eroded Thalassinoides-burrowed firmground or a breccia-conglomerate lag deposit. This surface separates transgressive deposits from overlying open-marine strata. Fourth-order Frobisher-Alida sequences are defined in landward areas by the cyclic alternation of anhydrite-dominated strata, with dolomitic and siliciclastic marker beds. Both low-magnitude sea-level movements and the paleoclimatic change from a hot, arid climate (anhydritic strata) to a hot, subhumid climate (dolomitic and siliciclastic marker beds) are responsible for the cyclicity. Abrupt sea-level falls sometimes caused exposure that generated local but prominent fourth and fifth-order subaerial erosion surfaces. In this setting, caution should be employed if subaerial erosion is used to indicate the location of a third-order sequence boundary.
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