Evidence for evaporites in the ORS of northern Scotland: replaced gypsum horizons in Easter Ross

Abstract

The thick lacustrine sequence of the Old Red Sandstone (ORS) Orcadian Basin might be expected to yield evidence of evaporites. Lithological evidence suggests sabkha/playa-type environments in the Lower and Middle ORS, with widespread deposition of algal carbonates ([Donovan 1978][1]). However records of pseudomorphs after evaporites in the basin ([Donovan 1978][1]; [Westoll 1979][2]) are scant and uncertain, and the extensive lacustrine deposits of the Middle ORS are generally considered to have been laid down in a non-evaporative environment (e.g. [Leeder 1982][3]). Pseudomorphs after gypsum (usually moulds due to differential weathering) are in fact widespread and occur in each of the Upper, Middle and Lower divisions of the ORS. Scarce pseudomorphs occur in Upper ORS green siltstones at Dunnet Head and Hoy (A. McAlpine pers. com. 1984). Within the Middle ORS, dolomicrites, siltstones and sandstones at most stratigraphic levels yield sparite or dolosparite pseudomorphs after gypsum. Examples in Orkney occur in the Eday Marls at Hoxa [ND 424 931] and Eday [HY 551 365], Rousay Group siltstones at Gorseness [HY 421 187], and in Stromness Group stromatolitic dolomites at Hesta Geo [HY 252 287], Stellar quartz pseudomorphs, possibly after a sodium carbonate mineral, occur in Middle ORS nodular cherts in Orkney and Caithness, interpreted as Magadi-type cherts ([Parnell 1983][4]), and in thin chert bands within the Sandwick Fish Bed west of Stromness [HY 242 081]. Pseudomorphs after gypsum are particularly prevalent in the Lower ORS of Easter Ross and are the main subject of this communication. The Lower ORS of Easter Ross . . . [1]: #ref-1 [2]: #ref-9 [3]: #ref-4 [4]: #ref-8