Abstract

The Lakota Formation is the lower unit within the Early Cretaceous Inyan Kara Group of the Black Hills region. The thickest portion of the Lakota Formation is exposed in the southern Black Hills, between Hot Springs and Edgemont, SD, where it is comprised of three members: the basal Chilson Member, a sequence of superimposed point bar and flood-plain sediments; the middle Minnewaste Limestone Member, representing deposition within alkaline lakes; and the upper Fuson Member, which includes lacustrine strata overlain by tidal-delta and tidal-flat deposits.

Fluvial Lakota strata vary from vertically repeated upward-fining meandering stream sequences to clayey abandoned-channel fills, richly fossiliferous flood-plain deposits containing fish, ostracods, and coal beds.

The thickest coal lies near the ghost town of Cambria 6 mi north of Newcastle. Pronounced vertical and lateral variability in the seam, abundant dispersed mineral grains, dominance of detrital macerals, total lack of megascopic plant remains, and absence of a rooted underclay suggest derivation from a transported lacustrine organic mud.

Palynomorphs in the coal are dominated by trilete spores (e.g. Stereisporites antiquas-porites, Cyathidites minor, Deltoidospora hallii, Osmundacites wellmanii). Conifer pollen include Inaperturopollenites dubius, Alisporites grandus, A. bilateralis and Cedripites cretaceous. Assemblages show that a stable community existed through much of peat deposition, though incursion of clastic detritus late in peat bed formation caused a decrease in conifer pollen, among other things. Biostratigraphic ranges of six time-restricted palynomorphs and the presence of probable angiosperm pollen indicate an early Cretaceous (Albian) age for the Cambria coal.