Stratigraphic Framework of Lagoonal and Evaporitic Facies Within the Lower Mississippian Madison Limestone: Northern Bighorn Basin, Wyoming and Montana

Mark D. Sonnenfeld

Unquestioning application of the asymmetric, shoaling-upward paradigm for stratigraphic cycles tends to lead to misinterpretation of thick successions of shallow water facies as cycle caps. Many of the same stratigraphic generalizations increasingly accepted for siliciclastic fluvial and aeolian strata are also applicable to lagoonal and certain evaporitic facies; namely, they tend to occur preferentially as base-level rise strata overlying sequence boundaries. Carbonate and evaporite facies within the Madison provide an example of dynamic platformal facies associations that vary over base-level transit cycles of several temporal scales (approximately second- to fourth-order).

Accommodation "turnarounds" that represent culminations of thinning-upward high-frequency cycles define the five 3rd-order sequence boundaries identified within the Madison. Overlying these boundaries, four of which are karsted in the study area, are vertically stacked to landward-stepping cycle sets deposited during progressively increasing accommodation/sediment supply ratios. Boundaries capping sequences II and III occur at or near the base of prominent evaporite solution collapse breccias that occur within thickening-upward cycle sets. Major solution collapse intervals are inferred to represent former evaporites (possibly subaqueous, in part) interbedded with restricted lagoonal rocks such as argillaceous dolomudstone, fossil-poor dolomudstone/wackestone, and ostracodal m cropeloidal grainstone. Each of these intervals accumulated as a lowstand to transgressive phase when sedimentation rates kept up with increasing accommodation, maintaining shallow water restriction and hypersalinity prior to onset of open marine conditions. Argillaceous lagoonal rocks also accumulated preferentially within base-level rise deposits of the remaining Madison sequences; however, within the study area evaporites are subordinate in these intervals.

The aggradation of "detached" grainstone barrier complexes during base-level rise is critical to the development of lagoons, particularly those with potential for hypersalinity and thick evaporite accumulation. By contrast, during mid- to late highstand times of decreasing accommodation/sediment supply ratios (base-level fall) lagoons fill up and shrink with a corresponding increase in the tendency for progradational, "shore-attached" grainstone complexes that are immediately overlain by thin peritidal to sabkha deposits without intervening lagoonal rocks.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995