Organic facies of the Asbian (early Carboniferous) Queensferry Beds, Lower Oil Shale Group, South Queensferry, Scotland, and a brief comparison with other Carboniferous North Atlantic oil shale deposits

The 71 m foreshore exposure of the Queensferry Beds at South Queensferry is currently the best accessible section of the Oil Shale Group of the Midland Valley of Scotland. A combination of bulk organic geochemistry and kerogen microscopy (palynofacies, organic petrology) is used to define four organic facies and one variant. Organic facies 1 occurs in silty mudstones, has a mean TOC of only 2% and Type III–IV gas-prone or inert kerogen (HI<50). Organic facies 3 corresponds to true oil shales, has a mean TOC content of 12% and Type I, predominantly amorphous kerogen (mean HI>800); variant 3A is additionally characterised by an abundance of small (4 μm) spherical cells of possible cyanobacterial origin (occurring in one sample adjacent to a stromatolitic limestone). Organic facies 2 is transitional between 1 and 3. Organic facies 4 has atypically low C/S ratios and corresponds to the Pumpherston Marine Band. These organic facies can also be distinguished using biomarker parameters (Pr/Ph, OEP, C25:C15, predominant carbon number, etc.). The HI is very strongly correlated with the relative fluorescence of the organic matter, and strongly correlated with the AOM:phytoclast ratio, and the Botryococcus and total alginite contents; it is inversely correlated with the diameter of black equant phytoclasts. The organic-rich shales thus correspond to distal lower energy anoxic conditions during periods of meromixis (high preservation, low dilution). Similar Carboniferous lacustrine source rocks occur in eastern Canada and East Greenland; all are characterised by large lateral extents (500–2,000 km2), a geographic and temporal association with strong syndepositional subsidence, generally low Botryococcus contents, and a dominance of amorphous Type II kerogen.