Late Mississippian Cyclothems of Heath Formation, Western North Dakota: ABSTRACT

Abstract

On the south flank of the Williston basin in western North Dakota, the Heath Formation (Chester) produces from several fields including Rocky Ridge field. In 1969 Shell Oil Company discovered additional Heath production several miles southeast of Rocky Ridge. Cores cut during this drilling program provided the basic data for this study. Lithologies represented in the cores run the complete spectrum of sandstone, siltstone, shale, limestone, dolomite, coal, and even subaerial, lateritic soils and subaquaeous, underclay soils. Mechanical log correlations create the impression of erratic depositional patterns typical of alluvial deposits. Faunal data, however, indicate oscillating brackish to shallow-marine depositional environments, the several coals and underclays indicate periods of marsh conditions, the crossbedded, conglomeratic sands are interpreted as fluvial to estuarine, and the lateritic soils are indicative of subaerial exposure. Such sequences or rock types are typical of the cyclothems of Illinois and Kansas. At least 3 cyclothems are represented in the Heath. The initial basal transgressive sands were deposited above or at sea level in the topographic lows eroded during the preceding regression. As sea level rose, depositional environments covered a progressively greater area, depending on the amount of topographic relief. Consequently, an underclay lying on a previously formed lateritic soil may represent the basal unit of the cycle. In areas of greatest relief only the deposits of maximum transgression are represented. The complete succession of members of the classical cyclothem is present in very few places. End_of_Article - Last_Page 961------------