ISOTOPIC COMPOSITION OF MIDDLE OLD RED SANDSTONE CARBONATES - IMPLICATIONS FOR THE PALEOENVIRONMENT

The carbonates occurring within the sandstones of the Middle Old Red S andstone of the Orcadian basin (Middle Devonian) in Caithness, N.E. Sc otland are of minor abundance, predominantly calcite and may be petrog raphically ascribed to early diagenetic origin.. Isotopic analyses (C, O, Sr) have been undertaken to elucidate the depositional mileu of th ese carbonates at two localities, Wick and Reay. Calcite and dolomite coexist in carbonate facies rocks and their similarity in delta(13)C(P DB) (-4.57 +/- 0.64 and -4.56 +/- 0.53 parts per thousand respectively at Wick and -2.39 +/- 0.30 and -2.00 +/- 0.48 respectively at Reay) s uggest a cogenetic origin at each locality. The delta(13)C values also suggest deposition from lighter, freshwater. Mean calcite delta(18)O( PDB) values of -7.76 +/- 1.92 parts per thousand and -8.76 +/- 2.23 pa rts per thousand for Wick and Reay samples respectively also support t his contention. Dolomite delta(18)O(PDB) values are consistently heavi er which is not inconsistent with a generally cogenetic origin. The ox ygen isotope data reflects a lighter pore fluid composition either bec ause of diagenetic modification by carbonate recrystallization or a gr eater contribution of freshwater via river supply during sandstone dep osition in an ephemerally evaporative lacustrine environment. Mean Sr- 87/86 value of 0.711167 for the carbonates of Wick confirm a nonmarine origin.