PRELIMINARY ANALYSIS OF THE UPPER JURASSIC MORRISON FORMATION IN THE REBECCA BOUNDS CORE, WESTERN KANSAS

The Upper Jurassic Morrison Formation in the Amoco #1 Rebecca Bounds core from Greeley County, Kansas consists of: (1) fine sandstones, siltstones, and heterolithic sandstone-siltstone, containing a basal bed (1617.0-1627.3 ft) of well-sorted fine to medium sandstone containing large-scale cross-strata that may be eolian in origin (1576.4-1627.3 ft); (2) heterolithic fine sandstone-siltstone strata containing gypsum/anhydrite (1550.0-1576.4 ft); (3) gray laminated mudstone and heterolithic fine sandstone-mudstone containing gypsum/anhydrite and evaporate-replacing chert (1533.3-1550.0 ft); (4) gray to white calcareous mudstones and muddy limestone, with laminated and speckled to birdseye-like fabrics (1526.9-1533.3 ft); (5) gray, greenish gray, and weak to pale red, noncalcareous to strongly calcareous, massive mudstone exhibiting blocky fracture and small slickensides interpreted as weak pedogenic overprinting (1512.0-1526.9 ft); and (6) light gray and white muddy limestone (“marl”) and calcareous mudstone with laminated to crinkly and birdseye-like fabrics (1496.0-1512.0 ft).

Mudstones in the Morrison Formation in the Bounds core are dominantly illitic; kaolinite is a minor component in some, but by no means all, samples. This overall pattern of clay mineralogy could be largely detrital, having been derived from distant sedimentary source areas. Similar clay mineralogies have been found in a few samples of Morrison Formation cuttings from petroleum wells located a few hundred kilometers northeast in Nebraska. Speculatively, however, discrete smectite dominating the clay fraction in one calcareous mudstone sample from the upper Morrison (informal unit 6 above) in the Bounds core may result from the diagenetic alteration of volcanic ash. Reddish brown, thinly-laminated, light-greenish-gray-mottled shales (Sundance Formation?) with gypsum seams in the interval below 1627.3 ft in the Bounds core are distinct from the overlying Morrison Formation sediments. Moreover, the clay fraction of these reddish brown shales is dominated by corrensite (smectite/chlorite), although illite and kaolinite are minor components.