Thermodynamic zonation in the black shale facies based on iron-manganese-vanadium content

Abstract

The black shale facies contains several discrete chemical groupings of Mn, Fe and V in visually similar rocks. These groups are related to four thermodynamic zones based on changing p E and pH, are identified from a collection of ∼200 Paleozoic and Mesozoic finely-laminated, low-calcic ([Ca] < 0.4%) black shales. The low-calcic nature of this data set permits the simplifying assumption that CaCO 3 is not present as a significant complication factor. The relative variations in the concentrations of Mn and Fe can be used as indicators of p EpH conditions. In Group 1 (oxic conditions: high p E), Mn and Fe occur as Mn(III,IV)- and Fe(III)-oxides, and appear in relatively high concentrations (average concentrations 1,300 and 56,000 ppm, respectively). In Group 2 (anoxic: nitrate-reducing to sulfate-reducing at intermediate pH), Mn is reduced to Mn 2+ as is evidenced by low Mn concentration (310 ppm average); in Group 2, Fe remains bound in oxides or sulfides with concentrations comparable to those in Group 1 (52,000 ppm). In Group 3, both Mn and Fe are reduced and relatively soluble, reflecting anoxic, but non-sulfate reducing, p E at intermediate to low pH. This solubility, and Mn and Fe species instability, is shown in lower average concentrations 170 and 23,000 ppm, respectively, in Group- 3 sediments. Group- 4 black shales have V concentrations > 300 ppm, averaging 1500 ppm and indicating deposition under conditions of high organic preservation. Mn and Fe concentrations are low — the Mn concentration is <260 ppm and averages 80 ppm; Fe is <36,000 and averages 19,000 ppm. The high V concentration with the relatively low Mn and Fe content suggests low pH and possible methanogenesis at low p E. This study demonstrates the wide variety of black shale depositional environments from oxic to methanic. Accordingly, black shales cannot all be assigned to anoxic, sulfidic environments without additional chemical analysis and interpretation.