SEDIMENTOLOGY, C-S-FE RELATIONSHIPS AND STABLE ISOTOPIC COMPOSITIONS IN DEVONIAN BLACK MUDROCKS, MACKENZIE MOUNTAINS, NORTHWEST-TERRITORIES, CANADA

An integrated approach combining C-S-Fe relationships, stable isotopic compositions, and lithofacies characterization was utilized to constr ain the palaeoenvironmental and early diagenetic conditions of Middle- Upper Devonian (Eifelian-Frasnian) mudrocks from the Mackenzie Mountai ns, Northwest Territories, Canada. These rocks include the Hare Indian Formation (informally subdivided into the lower Bluefish Member and t he Hare Indian Upper Member), Carcajou Marker and Canol Formation. The Bluefish Member is dominated by black, laminated, organic-rich shales (TOC = 0.35-10.34 wt.%; av. 5.83 wt.%) with moderate degrees of pyrit ization (DOP) of 0.34-0.67 (av. 0.55). These mudrocks were deposited i n dysoxic marine bottom-waters that became progressively more oxygenat ed with time. Variations in TOC, DOP and organic matter delta(13)C(PDB ) values (-29.7 parts per thousand to -19.9 parts per thousand; av. -2 7.2 parts per thousand) are attributed to repeated elastic dilution an d increased input of terrestrial organic matter in association with sh allowing-upward ramp-clinothem cycles. Pyrite delta(34)S(CDT) values ( -32.7 parts per thousand to -18.8 parts per thousand; av. -24.9 parts per thousand) indicate an open system, bacteriogenic seawater-sulphate reduction. Conversely, the overlying Hare Indian Upper Member, charac terized by clinothem facies, is composed of grey to green mudstone wit h minor argillaceous limestones and considerably less organic matter c ontents (TOC = 0.28-2.99 wt.%; delta(13)C = -29.5 parts per thousand t o -22.5 parts per thousand). Deposition occurred in oxic to slightly d ysoxic waters (DOP = 0.20-0.54; delta(34)S = -23.0 parts per thousand to -20.9 parts per thousand), depending on the palaeotopographic locat ion along the depositional slope. A rapid rise in sea level drowned th e carbonate 'ramp' member of the Ramparts Formation and produced the t hin, organic-rich Carcajou Marker. Bottom-water stagnation that result ed from subdued ramp palaeotopography produced anoxic sea bottom. Blac k, laminated, organic-rich shales from the Canol Formation (TOC = 1.37 -6.68 wt.%) are very similar to those of the Bluefish Member, and are likewise basinal sediments. However, TOC, DOP and organic-matter delta (13)C(PDB) values (-29.1 parts per thousand to -20.8 parts per thousan d; av. -26.2 parts per thousand) do not show pronounced variations and indicate that low-energy, quiet-water conditions persisted over relat ively long, uninterrupted periods of time. High DOP values (0.72-0.93) throughout the Canol Formation suggest that deposition occurred in an oxic bottom-waters, but as basin-fill conditions continued there was a shift to a dysoxic environment (DOP = 0.55-0.65), which grades into n earshore and offshore sequences of the overlying Imperial Formation. I n contrast with the Hare Indian Formation, much heavier delta(34)S(CDT ) values of pyrite in Canol mudrocks (-11.1 parts per thousand to +5.3 parts per thousand; av. -3.1 parts per thousand) point to bacterial s ulphate reduction in a closed to semi-closed system with respect to se awater sulphate. The accumulation of large amounts of organic matter i n the Bluefish Member, Carcajou Marker and Canol Formation is attribut ed to a combination of large inputs of terrestrially-derive organic ma tter during transgressive events and a high primary productivity, whil e preservation of this organic matter was facilitated by dysoxic-anoxi c bottom-waters.