Laminated Black Shale-Chert Cyclicity in Woodford Formation (Upper Devonian of Southern Mid-Continent)

Charles T. Roberts, Richard M. Mitterer

The Woodford formation, a known hydrocarbon source rock, is contemporaneous with other mid-Paleozoic (Frasnian to Tournasian) black shales occurring throughout North America and Europe. Much of the Woodford formation in outcrop along the southern flank of the Arbuckle uplift (Carter County, Oklahoma) consists of rhythmic alternating beds of laminated black shale and chert. A time series analysis of the chert-black shale couplets displays an approximately 20-ka periodicity, suggesting that deposition was controlled by external orbital forcing of the earth (Milankovitch cyclicity).

Total organic carbon ranges from 3 to 9% in the chert and from 10 to 28% in the black shale; carbonate carbon content is essentially zero. Carbon isotope (^dgr¹³C = -29 per mil), pyrolysis-gas chromatography, and Rock-Eval analyses indicate that the kerogen is of marine origin and is oil-prone type II. Except for the difference in organic carbon contents, the kerogens of the chert-black shale couplets are analytically similar.

The high organic carbon concentrations and the presence of laminated black shales signify deposition under anoxic conditions. However, C/S values for both the chert and black shale are about 10, which is significantly higher than C/S values in sediments deposited in euxinic and normal marine environments. Iron availability, rather than sulfate, apparently limited pyrite formation in this anoxic depositional setting.

Presence of phosphorite-rich zones indicates that upwelling conditions were prevalent during parts of Woodford deposition. The chert-black shale cyclicity is interpreted to represent deposition during periods of upwelling and high productivity (siliceous ooze) alternating with deposition during times of lower productivity (laminated black shale) in a restricted basin (Southern Oklahoma aulacogen).

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.