DUROCHER, SCOTT, and IHSAN S. AL-AASM, University of Windsor, Windsor, Ontario, Canada

ABSTRACT: C-S-Fe Relationships and Stable Isotopes of Devonian Black Shales, MacKenzie Mountains, N.W.T. Canada

Devonian black shales from the Hare Indian Formation (Eifelian-Givetian) and Canol Formation (Givetian-Frasnian) are the most probable source rocks for the Norman Wells oil reserves in the Mackenzie Mountains area, Northwest Territories. Black, organic-rich, laminated shales dominate the Hare Indian Bluefish Member and Canol Formation. The Hare Indian Upper Member is characterized by grey to green calcareous shale and minor argillaceous limestone beds.

Total organic carbon (TOC) content for the Bluefish Member averages 5.60 weight percent (wt. %) and displays cyclic variations due to repeated clastic dilution. This dilution is also confirmed by varying (isotope){13}C values of organic matter ((isotope){13}C = -20.0 to -29.7; mean = -27.1 o/oo) which show higher contributions from a terrestrial source. In general, the Degree of Pyritization (DOP) decreases with time 0.66 to 0.34, suggesting that deposition occurred in a restricted environment which became increasingly more oxygenated. Values of (isotope){34}S for pyrite sulfur ((isotope){34}S = -18.8 to -32.7; mean = -24.9 o/oo) are indicative of an open system for sulfate reduction.

Initial low DOP values (mean = 0.36) for the Hare Indian Upper Member indicate that deposition continued in oxygen-rich, aerobic waters where organic carbon was easily oxidized, as evidenced by a much lower TOC content (mean = 1.75 wt. %). DOP values then increase, implying a loss of oxygen and a relative rise in sea level that terminated Hare Indian deposition and produced a thin, organic-rich lithosome known as the Carcajou Marker.

The Canol Formation contains an average of 5.20 wt. % organic carbon and initial high DOP values (0.72 to 0.93) which represent deposition in euxinic waters. A decrease in DOP at the end of Canol deposition signifies a change to a sub-oxic depositional environment. Variations in (isotope){13}C values in organic matter ((isotope){13}C = -20.7 to -29.0; mean = -26.8 o/oo) represent differences in the source of organic matter. In contrast with the Hare Indian Upper Member, much heavier (isotope){34}S values of pyrite sulfur ((isotope){34}S = +5.3 to -11.1; mean = -3.0 o/oo) point to a closed or semi-closed system for sulfate reduction.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.