Chemical depositional environments of calcic marine black shales

Abstract

The economically important black shale facies contains several discrete chemical groupings in visually similar rocks. We examined the elemental compositions of more than 300 black shales and anoxic sediments in order to understand the relation of their chemical variability to the depositional environment. The low calcic shales were found to contain at least four discrete groupings (1-4) whose depositional and early diagenetic redox conditions are suggested by their Fe, Mn, and V concentrations. The calcic shales (72 Paleozoic, 50 Mesozoic, and 42 Cenozoic; >50,000 ppm AL, >4,000 ppm Ca) were examined in terms of the four thermodynamic groups previously defined for the low calcic shales. The Ca/Mn ratios of the calcic black shales fall into four (I-IV) discrete clusters (as opposed to a continuum). This differentiation reflects the stability of Mn minerals in overlying and interstitial waters under varying redox conditions. One cluster (Ca/ Mn I) from a period of glaciation (Ordovician-Silurian boundary) has high Mn concentrations and falls on the line: [Mn] = 0.065[Ca] + 1,280 (ppm) (r = 0.95, n = 7). A second group (Ca/Mn II) from many different localities and ages falls on a line with a lower slope: [Mn] = 0.038 [Ca] + 187 (ppm) (r = 0.92, n = 21). The third group (Ca/Mn III) falls on the line: [Mn] = 0.011 [Ca] + 152 (ppm) (r = 0.95, n = 19). The greatest number of samples (Ca/Mn IV) fall on the line: [Mn] = 0.0027 [Ca] + 144 (ppm) (r = 0.85, n = 69). In order to establish the depositional environment associated with each of the groups, more modern sediments whose depositional settings are better defined were explored. As a result, the samples from both Ca/Mn I and II can be associated with deposition under oxic water, those from Ca/Mn I remained oxic during early diagenesis. After deposition, Ca/Mn II samples became anoxic, releasing Mn to interstitial waters. The Ca/Mn III and IV group samples were deposited under anoxic waters. In order to establish the thermodynamic environment of the shale component of the calcic shales, it is necessary to extrapolate to the no calcium intercept to determine the Mn content, then apply the rules determined for the low calcic shales. This study demonstrates the wide variety of black shale depositional environments from oxic to methanogenic. Accordingly, black shales can not all be assigned to anoxic, sulfidic environments without additional chemical analysis and interpretation. The proper environmental assignment should be useful in assessing the economic potential of particular black shale deposits.