Paleoclimatic control on porosity occurrence in the Tilston interval, Madison Group, Williston basin area

Abstract

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 m) and controls porosity-permeability relationships.