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The basal contact of the Niobrara Formation with the underlying Carlile Shale is a regional unconformity or sequence boundary which formed about 88.5 Ma. Because the Carlile Shale and Niobrara Formation both represent offshore to basinal deposits, most previous workers have suggested that erosion and truncation at the sequence boundary occurred in a submarine environment.

In the present study, 790 well logs penetrating the Niobrara-Carlile contact were correlated in the central Powder River basin, representing an area of 2800 mi2. Field studies established the character of the Niobrara-Carlile contact in outcrop. Two sequence boundaries were recognized during log correlation, the older “blue” and the younger “green.” These two sequence boundaries define a sequence between the Niobrara Formation and Carlile Shale, referred to as the intermediate unit. The lithology of the intermediate unit is inferred to be shales and calcareous shales based on mud-logging reports and interpretation of geophysical logs. The intermediate unit was observed in outcrop in one or two locations in the Powder River basin area and consists of concretion-bearing microconglomerate.

Isopach maps from the sequence boundaries to a chronostratigraphic surface inferred to represent a paleohorizontal datum (a prominent bentonite layer in the Niobrara Formation) were used to reconstruct the topography of the unconformity. The erosional surfaces are characterized by mappable highs and lows (“thins” and “thicks,” respectively, on an isopach map): lows are associated with areas where, based on correlation of cross sections, the underlying Carlile Formation is incised. These topographic lows characterized by incision of underlying units are interpreted as paleovalleys that trend approximately NNW, parallel to, and possibly associated with the Gillette/Keeline structural lineament. The valleys together form what is interpreted to be a subparallel drainage pattern, with paleoflow in the valleys to the NNW. Individual valleys are on the order of 1-5 mi in width and are contained within a larger valley system encompassing the entire study area, at least 70 mi in width. The valley system has a strongly asymmetric geometry, with the eastern valley margin more steeply dipping and the western valley margin more gently dipping. The valley system apparently followed a structural weakness caused by a half-graben which was actively subsiding during deposition.

Field studies of the Carlile Shale to Niobrara Formation transition indicate the local presence of a microconglomeratic unit rich in rip-up clasts from the underlying Carlile Shale. Calcareous concretions in this transition unit have isotopic signatures indicative of meteoric diagenesis, a marked contrast to the marine shales above and below the unit.

Based on the inferred subparallel drainage patterns, and presence of microconglomerate and calcareous concretions of meteoric derivation at the sequence boundary, formation of the sequence boundaries occurred by fluvial erosion.

It is therefore concluded that: (1) two discrete sequence boundaries occur at the Carlile Shale-Niobrara Formation contact, (2) erosion and formation of both sequence boundaries occurred in a fluvial environment, and (3) the loci of valley incision and probably drainage were directly linked to active tec- tonism during sedimentation.

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