Abstract

The Madison Limestone forms a 2nd-order sequence of approximately 12 m.y. duration that is capped by a karsted unconformity representing a hiatus ranging geographically from 5 to 34 m.y. A complex five-fold physical stratigraphic hierarchy of sequences exists within the Madison. “Fundamental” cycles (2-20 ft thick) and six 3rd-order sequences (30-200 ft thick) are the scales most easily recognized within the Madison of the Bighorn Basin area. Generally the Madison is viewed as a single transgressive-regressive cycle. Results from this study show that the six 3rd-order sequences stack into two important long-term composite sequences that are intermediate in scale between the entire 2nd-order Madison and its component 3rd-order sequences. Recognition of this long-term pattern is critical to regional physical stratigraphic correlation, to appreciating an evolution from ramp to shelf systems, and to resolving the long-term accommodation controls of facies arrangements and sequence styles within the Madison carbonate platform. The sequence stratigraphic framework discussed in this paper provides a predictive tool for reservoir characterization that enables the reservoir contribution of early stratigraphically controlled, fabric-selective dolomites to be differentiated from later diagenetic and structural overprints.

The five 3rd-order sequence boundaries identified within the Madison are placed at accommodation “turnarounds” that generally represent culminations of progradational cycle sets. Four out the five internal sequence boundaries also show evidence of subaerial exposure and microkarsting across most of the Madison platform and probably formed in response to relative falls of sea level. Boundaries capping sequences III, IV, and locally V occur at the base of prominent solution collapse breccias that coincide with the onset of aggradational to retrogradational cycle sets. Major collapse breccias reflect evaporite dissolution and are inferred to represent former evaporites interbedded with restricted argillaceous dolomudstones and micropeloidal grainstones. During early phases of 3rd-order transgressive systems tracts, sedimentation rates nearly kept up with increasing accommodation. This allowed shallow water, hypersaline conditions to be maintained prior to onset of open-marine conditions. This stratigraphic context for evaporite occurrences contrasts with the conventional paradigm of asymmetric, evaporite-capped cycles.

The two oldest 3rd-order sequences (I, II) are ramp systems correlative with the Lodgepole Formation to the northwest and exhibit an overall progradational stacking geometry, implying a long-term decrease in accommodation/sediment supply ratio. Associated with this long-term trend was a tendency for progradationally stacked, “shore-attached” grainstone complexes to be directly overlain by thin peritidal to sabkha deposits without intervening lagoonal rocks. Limited lagoon development during the TST of Sequence II reflects episodic aggradational phases of intermediate-scale (4th-order) cycles. Sequence III, confined to the Big Goose Member of the Mission Canyon Formation is a highly aggradational succession that shallows upward very gradually. The onset of “detached” grainstone barrier complexes at the outset of Sequence III was critical to the aggradation of thick lagoonal deposits throughout northwest Wyoming and corresponds to the advent of a much flatter, shelf-like morphology. Sequence III represents near balance in the long-term accommodation/sediment ratio and forms the lower portion of the upper Madison composite sequence. The relatively high accommodation during deposition of Sequence III enabled it to culminate with a thicker accumulation of shallow water facies than did all five other 3rd-order Madison sequences. The sequence boundary capping sequence III shows the least evidence of all intra-Madison sequence boundaries for a relative fall of sea level, with possible karsting limited to the inner platform. The peak transgression of the upper Madison composite sequence is represented by the widespread skeletal lime packstone and grainstone of the Little Tongue Member (Sequence IV). Upper Osagean to lower Meramecian sequences IV-VI thin upward, involve karsted sequence boundaries, and are associated with increasing argillaceous influxes above sequence boundaries. This is indicative of the long-term decrease in accommodation, coupled with increasingly humid conditions that culminate in the karsted master unconformity capping the Madison.