All 'black' shales are not chemically similar. Quinby-Hunt and Wilde, (1991, 1994, 1996) placed black shales into four zones on an elemental and thermodynamic basis using a cluster approach on the 287 syngenetic sample data base of the Marine Sciences Group (Quinby-Hunt et al., 1989). These zones are redox based going from oxic to highly anoxic even though lithologically these rocks are considered 'black' shales. The 'black' color seems to be related to the Mn-Fe content and solubility, with oxic black shales having stable Mn oxides that color the rock. In anoxic black shales with lower Mn concentrations, Fe minerals color the rock. We have compared our zones with (1) other published analyses of aperiodic black shale events from the Archean, Early Proterozoic, Late Proterzoic, Ordovician, Silurian and Jurassic using Drever's (1988) average shale values as a baseline. Certain trace metal concentrations are higher in the more anoxic zones. These signatures might be used to identify potential cryptic petrochemical source beds even when the initial high carbon values may be lost via metamorphism or migration. This assumes that trace metal signatures in the potential petroleum source rocks are from marine organic matter (Martin and Knauer, 1973, Ho et al., 2003), which can be distinguished from that of detrital sediments from non-organic rich rocks. Carbon (Wilde, Lyons, and Quinby-Hunt, 2004) and Sulfur (Wilde, Quinby-Hunt and Lyons, 2005) initial values might be recovered using trace metal proxy equations for such depleted rocks.
ReferencesDrever, J. I., Li, Y-H, and Maynard, J. B., 1988, Geochemical Cycles: The Continental Crust, in Geochemical Cycles in the Evolution of the Earth, C. B. Gregor, Garrels, R. M. , Mackenzie, F. T. and Maynard, J. B. (eds.) J. Wiley, p. 17-53
Ho T-Y., Quigg A., Finkel Z.V., Milligan A.J., Wyman K., Falkowski P.G., Morel F.M.M. 2003, The Elemental Composition Of Some Marine Phytoplankton. Journal of Phycology, v. 39, no. 6, 2003 , p. 1145-1159. pdf format
Martin, J. H. and G. A. Knauer, 1983, The elemental composition of plankton. Geochimica et Cosmochimica Acta, v.3 7, p. 1639-1653. pdf format
Quinby-Hunt, M. S., P. Wilde, and W. B. N. Berry, 1991, The Provenance of Low-Calcic Black Shales. Mineralium Deposita, v. 26, p. 113-121
Quinby-Hunt, M. S., P. Wilde, C. J. Orth, and W.B.N. Berry, 1989, Elemental Geochemistry of Black Shales-Statistical Comparison of Low-Calcic Shales with Other Shales. In Metalliferous Black Shales and related Ore Deposits, R. I. Grauch and J. S. Leventhal (eds.), U. S. Geological Survey Circular 1037, p. 8-15.
Quinby-Hunt, M. S. and P. Wilde, 1994, Thermodynamic zonation in the black shale facies based on iron-manganese-vanadium content: Chemical Geology, v. 133, p. 297-317.
Quinby-Hunt, M. S. and P. Wilde. 1996, Chemical Depositional Environments of Calcic Black Shales: Economic Geology, v. 91, p. 4-13.
Wilde, P., T. W. Lyons and M. S. Quinby-Hunt, 2004, Organic proxies in Black Shales: Molybdenum: Chemical Geology, v. 206, p. 167-176.
Wilde, P., M. S. Quinby-Hunt and T. W. Lyons, 2005, Sulfur Proxies in type III Black Shales: Fe, Mn, Co, Cu, Ni, Zn, Sc, Expanded Abstract Siever Symposium: Salt Lake Geological Society of America Meeting, October, 2005.Power Point Presentation