The Cherty Rock, Elgin: A petrographic and isotopic study of a Permo‐Triassic calcrete

The Cherty Rock is a laterally persistent horizon that extends across the Inner Moray Firth Basin, northeastern Scotland. The southern exposures of the Cherty Rock near Elgin reveal an indurated carbonate and silica‐rich horizon (0·75‐10m thick) developed in the uppermost part of the largely aeolian Lossiemouth Sandstone Formation. Laminated, pisolitic, and brecciated textures within the Cherty Rock confirm that the horizon represents an analogue to Recent calcretes of semiarid areas. Textures characteristic of silcretes are widespread in the Cherty Rock but these originate from silica replacement of calcite. The carbon and oxygen stable isotopic composition of micrite in the Cherty Rock (mean σ ¹³C = ‐6·9%°, mean σ ¹⁸O = ‐5·7%°, both relative to PDB) is comparable to that of other Permo‐Triassic calcretes and modern soil carbonate, suggesting that similar processes were responsible for their formation. The σ ¹³C and σ ¹⁸O values for calcite in the Cherty Rock may indicate that evaporative processes contributed to its development. σ ¹⁸O data from spar (mean σ ¹⁸O = ‐9·8%° PDB) and macroquartz (mean σ ¹⁸O = 19·7% SMOW) indicate that these are later (burial) cements. The Cherty Rock is a useful stratigraphical and palaeoclimatic indicator, and its presence in the Inner Moray Firth basin suggests a period of tectonic stability in the basin during late Triassic times.