Distribution and origin of elongate sandstone concretions, Bullion Creek and slope formations (Paleocene), Adams County, North Dakota

Rectilinear patterns in sandstone are visible on aerial photo graphs of Adams County, North Dakota. These patterns result from the differential erosion of elongate, calcareous, sandstone concretions that have formed in fluvial channel sand units. The distribution and orientation of the elongate concretions were mapped from aerial photographs. The concretions have a strongly east-west orientation. Averages of paleocurrent measurements of the associated sand and sandstone are also easterly. The elongate concretions occur slightly above and below the Rhame bed (a white marker zone at the top of the Slope Formation), in sand units at the base of the Bullion Creek Formation and at the top of the Slope Formation. The concretions are commonly one to two metres thick, several metres wide, and at least ten metres long. Their longest dimension is parallel to bedding in the sand. Bedding commonly continues from the sand into the sandstone concretion. The concretions occur along distinct horizons that terminate laterally and recur vertically, ruling out their use as stratigraphic markers. In some areas, individual elongate concretions are laterally connected, forming a tabular bed with aligned pinchings and swellings. This indicates an advanced stage of concretion growth. The concretions do not contain a recognizable, macroscopic, organic nucleus ... No compositional differences were found between the concretions and the enclosing sand, but the sand has a significantly lower porosity. This suggests the sand has undergone more compaction, and that the concretions formed before the sediment reached its presently compacted state. In light of this evidence, a late diagenetic age seems most likely. At some time in the past, calcium- and carbonate-enriched ground water probably flowed eastward through Adams County, following the positions of buried paleochannels. Calcite precipitation occurred in discrete centers when the pH rose above 7.8. The elongate form of the concretions is a result of preferred growth parallel to the direction of groundwater flow. Therefore, the concretions can be used as directional indicators of paleogroundwater flow