CHEMICAL DEPOSITIONAL-ENVIRONMENTS OF CALCIC MARINE BLACK SHALES

The economically important black shale facies contains several discret e chemical groupings in visually similar rocks. We examined the elemen tal 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 law calcic shales were found to contain at least four discrete groupings (1-4) whose depositional and early d iagenetic redox conditions are suggested by their Fe, Mn, and V concen trations. The calcic shales (72 Paleozoic, 50 Mesozoic, and 42 Cenozoi c; >50,000 ppm Al, >4,000 ppm Ca) were examined in terms of the four t hermodynamic 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 t he stability of Mn minerals in overlying and interstitial waters under varying redox conditions. One cluster (Ca/Mn I) from a period of glac iation (Ordovician-Silurian boundary) has high Mn concentrations and f alls 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 set tings 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 depo sition, Ca/Mn II samples became anoxic, releasing Mn to interstitial w aters. The Ca/Mn III and IV group samples were deposited under anoxic waters. In order to establish the thermodynamic environment of the sha le component of the calcic shales, it is necessary to extrapolate to t he no calcium intercept to determine the Mn content, then apply the ru les determined for the low calcic shales. This study demonstrates the wide variety of black shale depositional environments from oxic to met hanogenic. Accordingly, black shales can not all be assigned to anode, sulfidic environments without additional chemical analysis and interp retation. The proper environmental assignment should be useful in asse ssing the economic potential of particular black shale deposits.