Duties and Reports of the Subsurface Geologist 849 



(b) Lithology, mineralogy, and paleontology 

 Upper Cretaceous: 



Pierre (0-5,790 ft.) — Dark-gray shale and mudstone with a few thin interbeds 

 of fine-grained, gray, extremely tight quartzose sandstone containing 

 scattered nondiagnostic ilmenite, zircon, and tourmaline grains; some 

 small Foraminifera, ostracodes, mollusc fragments, and Inoceramus 

 prisms throughout. 



Niobrara (5,790-6,080 ft.) — Essentially dark-gray-black, highly calcareous, 

 foraminiferal shale; base marked by a fine-crystalline, gray limestone 

 (probably Timpas). 



Benton (6,080-6,660 ft.) — Gray-black shale containing thin bluish-white ben- 

 tonite streaks throughout; few scattered Foraminifera; mineralogy un- 

 determined. 



Dakota group (6,660-6,950 ft.) — Consists of three units, upper sandstone, 

 middle shale, and lower sandstone; sandstone fine- to medium-grained, 

 gray; shale black to dark gray, carbonaceous, and siltaceous; some mol- 

 lusc fragments noted; detrital heavy minerals essentially zircon and 

 tourmaline, and sparse garnet; a few minor oil and gas shows in upper 

 part of upper sandstone member. 

 Jurassic: 



Morrison (6,950-6,960 ft.)- — Light-gray, slightly calcareous mudstone. 



Detail Stratigraphy 

 Upper Cretaceous: 



Pierre (0-5,790 ft.) 



Lithology: Dark-gray to grayish-black shale and mudstone with many thin, 

 irregular streaks and laminae of light-gray siltstone; a few layers of fine- 

 grained, medium-gray sandstone in upper and middle part; fine carbonaceous 

 material disseminated through the sandstone. 



Stratigraphic relationship: Conformable but sharply distinguishable from the 

 underlying Niobrara. The Pierre is essentially noncalcareous, whereas the 

 Niobrara contains considerable lime. This boundary reflects a major en- 

 vironmental anomaly. 



Paleontology: Foraminifera (Cibicides, Anomalina, and Gumbelina) are 

 found sporadically throughout; a few ostracodes and moUuscan fragments 

 noted. The paleontologic aspects of the Pierre do not permit faunal zonation. 

 Mineralogy: Zircon, ilmenite, and tourmaline present in most arenaceous 

 phases. The homogeneity of this assemblage does not permit mineralogic 

 subdivision of the section. 



Electrical characteristics: As the Pierre is represented essentially by shales, 

 the electrical profile is exceptionally monotonous, although the more silty and 

 arenaceous phases are moderately well-defined, particularly in the intervals 

 l,885--2,350 and 3,285—3,445 feet. The resistivity profile shows a conspicuous 

 increase at the Pierre-Niobrara boundary. 



Correlation: The Pierre is widely distributed in eastern Colorado. The silty 

 phases in the upper part may be correlated moderately well from the elec- 

 tric log over considerable distances. The base of the Pierre is correlative 

 over wide areas from lithology and electrical characteristics. 

 Productive character and possibilities: Sandstones within the Pierre are 

 extremely fine-grained; permeability determinations from eighteen cores 

 showed average values of 35 millidarcys; porosity values are from eight 

 to twelve percent. No oil or gas shows were observed in the formation. 

 Some sandstones may prove productive, although on the whole they would 

 not be expected to contain commeicial quantities. 



Depositional environment: The Pierre shales accumulated under marine 

 conditions as indicated by the foraminiferal faunas. The sparseness of the 

 benthos microfaunas, however, suggests unfavorable bottom conditions. 



Succeeding formations showld be treated as given above. Informa- 

 tion pertaining to core descriptions, cored intervals, formation tests, 

 mechanical troubles, and hole deviation should also be included. 



