Depositional Environment, Stratigraphy, Paleogeography, and Organic Maturation of Desmoinesian Cyclothemic Excello Black Shale in Oklahoma, Kansas, and Missouri

Omer Isik Ece

The thick, laterally continuous Excello black shale was studied along the 400-mi-long outcrop belt from Oklahoma to Iowa. The Excello is perhaps the best example of a Pennsylvanian cyclothemic black shale in the Mid-Continent area. The distribution of total organic carbon (TOC) and kerogen types indicate that the Excello Shale was deposited in an epeiric, highly productive sea with bottom anoxia. Thin laminations and grainsize analysis indicate stagnant water conditions, whereas gentle undulations suggest a flat bottom topography. The TOC content averages about 10 wt. %, but can be as high as 17 wt. %. The changes in shape and size of phosphate nodules along the outcrop belt (spherical, bladed, elongated, and laminated) appear to be related to changes in the seawater chem stry, fluctuations in productivity, terrestrial input, and local diagenetic effects.

Vitrinite reflectance (R_o = 0.51-0.63%) and elemental kerogen analysis indicate that the outcrop samples are immature, but cores from west-central and northwestern Oklahoma are mature (R_o = 0.61-1.44%) with respect to hydrocarbon generation. This study suggests that a humid, warm paleoclimate with high rainfall and abundant terrestrial vegetation produced structured kerogen, which was transported into the Excello sea.

The Excello was probably deposited in less than 60 m (197 ft) of water. Surface temperature of the seawater was above 28°C (82°F) at the time of deposition in the ice-free polar regions. The anoxic episode lasted approximately 10 k.y. with the cyclicity every 500 k.y./cycle. The cyclic changes in atmospheric conditions corresponding with cyclic shifts in the earth's axis may have caused the anoxic episode.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.