Chemostratigraphy of Middle Devonian lacustrine sediments in the Orcadian Basin, north-east Scotland
During Middle Devonian times, lacustrine deposition dominated much of NE Scotland including Caithness, Orkney, Shetland and the Inner Moray Firth. Donovan classified such deposits into five distinct facies associations:- the deep water facies association A and the progressively shallower water facies associations B, C, D and sandstones. Such facies associations occur in climatically induced cycles. Facies A sediments (known as "fish beds") are organic rich, comprising triplets of carbonate, clastic and organic laminae (each triplet is c. 1.5mm thick). In the present study the fish beds have been categorised on sedimentological grounds into four subtypes:- types I, II (a and b subtypes), III and IV fish beds. The former, which were deposited under the most reducing, deep water, quiescent conditions, comprise 1.3m+ thick laterally continuous beds containing abundant and well preserved, fully articulated fossil fish. Type II(a), and II(b) and III fish beds are less than 1.3 thick and deposited under increasingly more shallow water and more oxidising conditions. Type II(a) fish beds contain both articulated and dissarticulated fish carcass material while type II(b) fish beds, of similar thickness, contain scattered fish fragments. Type III fish beds occur in close vertical and lateral proximity to fluvial sandstones. Type IV fish beds are carbonate rich and are confined to the south Moray Firth coast. Type I fish beds have the greater source rock potential. It is possible to categorise the Middle Devonian facies, including the fish bed facies, on geochemical grounds. As far as major element geochemistry is concerned, SiO2 is concentrated principally in detrital quartz, and for this reason is highest within sandstones, while K2O, Al2O3 and Fe2O3 are highest within the clay rich facies C and D. MnO is most concentrated within facies AIII and B, deposited closest to the thermocline. Trace elements were also analysed and are also useful in discriminating facies. Some elements, such as Zr and Nb are highly immobile, being concentrated in the dereital fraction of sandstones. By contrast, Rb, Ba and V are principally concentrated within clay and feldspars and, for this reason, are highly concentrated within the most clay rich deposits (facies D). The distributions of Mo, Cu, Ni, V and Cr are partly controlled by paleoredox and, consequently, may be used to discriminate relatively reducing from oxidising facies. U and Th are most highly concentrated within fish bone/scale material and it is possible to use the U/Th ratio to categorise the fish beds. This ratio is highest within the most reducing fish beds (type I) and in fish beds located close to fluvial sandstones (type III). Type II and IV fish beds have generally lower U/Th ratios. This ratio may be measured where spectral gamma ray logs have been run (e.g. Dounreay boreholes).