Stratigraphy and Petrography of the Codell Sandstone and Adjacent Units in the Subsurface of the West-Central Denver Basin, Adams, Larimer, and Weld Counties
Authors:   , Sablina Yelena 
Published: February 2023

The Codell Sandstone (middle to late Turonian) Member of the Carlile Shale was deposited in a shallow marine environment in the Western Interior Cretaceous Basin during the third order (9-10 Ma) Greenhorn regressive phase. The Codell unit is widespread at the surface and in the subsurface of the west-central Denver Basin, Colorado, and is absent within a northeast-trending area of the central part of the basin between Colorado Springs and Fort Collins. Its absence is due to the presence of an uplifted high, the so-called Transcontinental Arch, which developed in Albian time and caused subsequent erosion of the Codell. This study of the Codell Sandstone is based on a detailed description of eight cores (Brooks Exploralion. #2-30 Harrington; Byron Oil Industry, # I Degenhart; Tudex Petroleum; #12-9 Barker; Petro-American Energy, #2 Young; Calvin Petroleum, Runyan; Petro Quest, #41-36 State-B; Rock Oil, #1 Ronald; Andrau Enterprises, # 13 Owl Creek) stored in the Denver Core Library, which were obtained from wells drilled in northern Colorado along the Front Range in Adams, Larimer, and Weld counties. The underlying Blue Hill Shale Member of the Carlile Shale reaches its limit south of the study area. It was completely eroded in the study area during sub-aerial exposure and beds of the Codell sandstone disconformably overlie the underlying Fairport Shale. The Codell Sandstone, in the study area, consists of 12 to 26 ft (3.6 to 7.8 m) of interlaminated muddy and silty sublitharenite and quartz-arenite beds, and olive-black to black heavily bioturbated shaly sandstone intercalated with olive-gray hummocky- stratified, discontinuously laminated and bioturbated silty sandstone. The CodeII beds in this area can be subdivided into three units. They were deposited on the inner and outer shelf, and possibly shoreface environments. The lower stratified and bioturbated fining-upward succession represents storm-deposit facies, which grade into shaly and biogenically reworked sandstone and mudstone intercalations. These show characteristics of the more distal and quieter shelf environment. The upper unit consists of an intensively bioturbated coarsening-upward succession with little or no shale laminae, which most likely reflects deposition between storm and fair-weather wave bases. The third unit, delineated only in the southwestern part of the area (Adams county), was in proximity to the Transcontinental Arch. This unit comprises the lowermost part of the Codell and consists of massive to bioturbated, fine-to medium- grained, calcite-cemented, mature sandstone with little mud and abundant biogenically reworked, silty material. The totally different nature of this unit may indicate different Codell source areas. This unit may represent a sand body deposited on a progradational shoreface. The Codell Sandstone consists of nearly mature sediments with quartz, chert, biotite, and muscovite being the main framework-building minerals. The rock is cemented by chert, calcite, and quartz overgrowths combined with abundant clay and sericite matrix. Clay minerals are represented mainly by mixed layer illite/smectite, followed by illite, chlorite, and kaolinite, which are found only in trace amounts. Porosity is present; however, its percentage is low. It developed after chemical leaching of quartz, feldspar and calcite cement or/and micro-fracturing in organic layers. The Juana Lopez, the uppermost member of the Carlile Shale, pinches out north of Morrison, Colorado, and is completely absent in the study area. The Codell Sandstone is disconformably overlain by a thin calcareous shale, which belongs to the lowermost Fort Hays Limestone-shale couplets.