Origin and Depositional Environment of the Codell Sandstone Member of the Carlile Shale (Upper Cretaceous) Southeastern Colorado

ABSTRACT: The Codell Sandstone Member of the Carlile Shale (Upper Cretaceous, southeastern Colorado) is a widespread unit of marine derivation. Detailed correlations of 38 complete measured sections disclose that the unit can be segregated into two distinct facies: (1) a lower "basal sequence" and (2) an "upper sequence". The thicker (0'-33') "basal sequence" is bounded below and above by provincial disconformities, and consists of very pale orange, calcareous sandstones interbedded with very thin olive-gray shales. Bioturbation, spill-over cross-beds, penecontemporaneously deformed sandstones, irregularly occurring oscillation and current ripple marks, and scattered molluscan remains point to a shallow marine origin (water depths probably less than 10') for this facies. Several minor diastems break the continuity of this sequence; these have been interpreted as large-scale current lineations. All these sedimentary structures suggest an environment of deposition similar to that occurring on stable bottoms of the shelf-lagoon area of the present Bahaman chain. Rocks of the "upper sequence" are relatively thinner (nowhere more than 6'), and are separated from the lower "basal sequence" by a disconformable surface marked by extensive borings which penetrate underlying beds. The top surface of this sequence is usually a bedding plane break, apparently representing a surface of continuous deposition. It reflects passage into shales or micrites of overlying Niobrara rocks. Therefore the major disconformable surface separating Benton and Niobrara Groups lies slightly below the top of the Codell Sandstone Member of the Carlile Shale, not at the actual top of the Codell, Abundant fragmental molluscan remains, collophanized shark and ray teeth, low angle cross-stratification, and porosity voids lined with blocky sparry calcite suggest a general environment of deposition similar to the "winnowed-shelf carbonate sand environment" forming at present on the outer part of the modern Florida reef tract. Five facies and one sub-facies have been recognized Within "upper sequence" rocks: (1) a calcarenite or limey sandstone unit, (2) a sandstone containing limonitized borings, (3) a shaly to massive sandstone sequence, (4) a sandy limestone, and (5) a sandy shale of restricted areal extent. A sub-facies of the calcarenite (1) is also present. All these appear to be either submarine or supratidal in origin. Isopachous and lithofacies studies suggest that the basal surface of the Codell Sandstone Member rests disconformably on upper Carlile Shale. Two interpretations of these data are: (1) the surface of the upper Carlile Shale is very gently flexed into northeasterly aligned "protosynclines" and"protoanticlines", (2) structural influence had little or nothing to do with the patterns evidenced on the facies maps. Acceptance of interpretation (1) implies a pre-Laramide stress field oriented in a northwesterly direction. Such a stress field is almost at right angles to the northeasterly aligned stresses implied by most Laramide structural features of this area. I£ interpretation (2) is accepted. several sedimentary models could generate the patterns mapped. A suggested model involves nearshore currents flowing across the study area from southwest to northeast.