Biostratigraphic zones based on fossil fish are identified within the Middle Devonian Rousay sequence in the upper part of the main lacustrine sequence of Orkney. The Rousay sequence forms the majority of the bedrock of Orkney and the original Geological Survey had estimated its thickness to be over 1500 m. Some recent geological investigations have redefined the Rousay sequence and reduced its thickness to such an extent (around 200 m) that the latest (2000) BGS geological map (1:100 000) includes a Rousay Flagstone Member as only an upper member of the Upper Stromness Flagstone Formation. Our new investigations have confirmed that the fossil fish clearly separate the Rousay sequence from the underlying Stromness Flags and show that its thickness is over 800 m. The osteolepid fish Osteolepis panderi occurs in a limited zone at the base of the Rousay sequence in a set of two or three fish beds with prolific occurrences of O. panderi and occasional specimens of the osteolepid fish Thursius pholidotus. Above these fish beds, O. panderi becomes extinct within the basin (but may have persisted outside the basin in the marine realm). Several fish beds containing numerous specimens of the coccosteid fish Millerosteus minor, again with occasional specimens of T. pholidotus, occur from about 205 m above the base of the Rousay sequence in a middle unit of the sequence. This unit is about 195 m thick and contains the first significant coarse to pebbly sandstones above the basal transgression of the main lacustrine sequence. Above this sandy member, these fossil fish become extinct within the basin (but may persist outside the basin) and then there are over 400 m of dominantly finer-grained lacustrine strata up to the overlying sandstones of the Lower Eday Sandstone Formation, the lowest formation of the Eday Group. Fossil fish are present in this uppermost part of the Rousay sequence, but do not include Osteolepis panderi or Millerosteus milleri and instead are dominantly of species with long ranges in the flagstone sequence. A poorly defined biozone containing rare individuals of the antiarch, Asterolepis orcadensis, occurs in a limited interval near the top of the lacustrine sequence. Based on the identification of these biozones and their relationship to distinct sedimentary units, we have provisionally identified within the Rousay sequence Lower, Middle and Upper Rousay units, but these are not formally defined as formations.

These biostratigraphic zones can be correlated in the equivalent sequence in Caithness, except for the uppermost Asterolepis orcadensis zone probably because the transition between the lacustrine sequence and the overlying sandstones is not exposed. Equivalent biostratigraphic units, provisionally identified as the Thurso Flagstones, Mey and Huna units, can be directly related in upward sequence to the units recognized on Orkney. By comparison with the position of the first appearance of the spore Geminospora lemurata, which marks the base of the Givetian, the Eifelian/Givetian boundary is interpreted to lie above the Millerosteus minor Biozone.

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