The Tilston interval of the Madison Group in the Williston basin area is sparsely well constrained by biostratigraphic data; it is confined to the lower half of Mamet foraminifer Zone 8 in the upper Tournaisian. The distribution of lithologies relative to biozones and marker beds is used to define two equatorial, shallow-water paleoclimatic facies associations. These are (1) a grain-rich carbonate association that formed during a temperate, humid climate in the lower Tilston and (2) a mud-rich carbonate and evaporite association that formed during a hot, arid climate in the upper Tilston.
Skeletal-oolitic grainstone and crinoid-dominated grainstone and packstone of the lower Tilston formed a near-continuous sheet that extended more than 1500 km (930 mi) parallel to the platform margin and more than 500 km (310 mi) perpendicular to the platform margin. Compaction-reduced, primary interparticle porosity (5–15%), with good permeability (10 to >100 md), occurs widely in the Williston basin. However, compaction and burial cements have occluded interparticle pore space in outcrop areas to the west.
Dolomite intercrystal and fossil-moldic porosity are the main porosity types in the strata that formed during the hot, arid climate of the upper Tilston. Dolomite developed as a replacement of the matrix in mudstone and wackestone. Porosity and permeability are as high as 38% and more than 100 md, but average dolomite crystal size varies widely (5–90 μm) and controls porosity-permeability relationships.