Abstract

The "D" zone cyclothem of the Lansing-Kansas City Groups of southwestern Nebraska was deposited during one major oscillation of the epeiric sea in the Late Pennsylvanian (Missourian). This cyclothem records a transgression of sea level followed by a major regression. Perturbations in the cycle record changes in the influx of terrigenous clastics from outside the study area.

The "D" zone cyclothem consists of the four basic lithofacies common to most cyclic deposits of this age in northwestern Kansas and southwestern Nebraska: (1) a thin, Lower Carbonate unit deposited in a shallow-marine environment; (2) a laterally extensive, Lower Shale unit of marine origin resulting from a terrigenous influx from the north; (3) a complex, Upper Carbonate unit deposited in shoaling water during waning terrigenous influx; and (4) an Upper Shale unit deposited in nearshore and/or tidal flat to nonmarine environments.

Core data and an isopachous map of the Upper Shale unit suggest that several shoal-water sand bars existed in the area of Hitchcock County during part of Missourian time. Pellet, ooid grainstone deposition in the shoal areas were localized on bathymetric highs. The bathymetric highs were apparently formed by (1) differential compaction of the Upper Shale unit of the underlying "E" zone over erosional topography and/or (2) movement on the ancestral Las Animas arch.

The presence of equant-calcite fringing cements in pores of the grain-supported rock in the Upper Carbonate indicates early diagenesis in a freshwater-phreatic lens formed during initial subaerial exposure. Limpid dolomite rhombs intergrown with the early calcite cements and replacing the edges of some framework grains suggest cementation in a mixing zone, which was part of a regional freshwater phreatic zone that migrated basinward in response to falling sea level.

The highest stratigraphic occurrence of dolomite when plotted on a cross section forms a line which transects facies boundaries and may represent the position of the mixing zone below the early paleowater table. Some of the leached porosity in the grain-supported rock in the Upper Carbonate formed in the vadose zone above this line. Dolomitization of underlying carbonate facies probably took place contemporaneously as the mixing zone migrated through the still-porous, mud-supported sediments.

Further leaching of the Upper Carbonate may have occurred in a vadose zone above a later paleowater table. The position of this paleowater table is indicated by the distribution of skeletal fragments replaced by red silica. Associated diagenetic features are dissolution breccias infiltered with nonmarine clay, and authigenic gypsum. These features formed in a fluctuating continental ground-water system that existed during a later stage of prolonged subaerial exposure. Soil formation and calichification of the Upper Shale occurred during this time in a semi-arid to arid climate.

The presence of early freshwater cements and leached porosity in the Lower Carbonate indicates updip edges of this unit were exposed during this phase of diagenesis allowing for influx of freshwater. Authigenic gypsum cement in the leached pores of the Lower Carbonate indicate that the freshwater in the aquifer became hypersaline.

Near-surface diagenesis ended with transgression of the Late Pennsylvanian sea into the study initiating deposition in the overlying "C" zone cyclothem. Styolitization of terrigenous mudrich carbonate units in the Upper Carbonate record the latest phase of diagenesis which occurred during (deep?) burial.