Isotopic Composition of Middle Old Red Sandstone Carbonates: Implications for the Palaeoenvironment

The carbonates occurring within the sandstones of the Middle Old Red Sandstone of the Orcadian basin (Middle Devonian) in Caithness, N.E. Scotland are of minor abundance, predominantly calcite and may be petrographically ascribed to early diagenetic origin.

Isotopic analyses (C, O, Sr) have been undertaken to elucidate the depositional mileu of these carbonates at two localities, Wick and Reay. Calcite and dolomite coexist in carbonate facies rocks and their similarity in δ¹³C_PDB (-4.57 ± 0.64 and -4.56 ± 0.53‰ respectively at Wick and -2.39 ± 0.30 and -2.00 ± 0.48 respectively at Reay) suggest a cogenetic origin at each locality. The δ¹³C values also suggest deposition from lighter, freshwater.

Mean calcite δ¹⁸O_PDB values of -7.76 ± 1.92‰ and -8.76 ± 2.23‰ for Wick and Reay samples respectively also support this contention. Dolomite δ¹⁸O_PDB values are consistently heavier which is not inconsistent with a generally cogenetic origin. The oxygen isotope data reflects a lighter pore fluid composition either because of diagenetic modification by carbonate recrystallization or a greater contribution of freshwater via river supply during sandstone deposition in an ephemerally evaporative lacustrine environment. Mean Sr^87/86 value of 0.711167 for the carbonates of Wick confinn a nonmarine origin.