Glacial trinity: Neoproterozoic Earth history within the British-Irish Caledonides Geology

Two distinct Neoproterozoic glacial episodes are known for the Dalradian Supergroup in the British-Irish Caledonides, the Port Askaig Formation and the Inishowen-Loch na Cille ice-rafted debris (IRD) beds. Here we describe a third, intermediate between those two, the Stralinchy-Reelan formations, composed of diamictite and IRD. Developed directly above these rocks is the Cranford Limestone, which consists of a basal, 1-6-m-thick, tan-gray dolostone overlain by a variably developed, but as much as 340-m-thick, sequence of thin-bedded limestone and dolostone. This unit exhibits a C isotopic trend that begins negative in the basal dolostone, reaches a nadir of -7 parts per thousand, and then rises to 0 parts per thousand.-2 parts per thousand. These characteristics match strikingly those of Marinoan-style cap carbonates. Consequently, we interpret the Port Askaig Formation, the Stralinchy-Reelan units, and the Inishowan-Loch na Cille beds as equivalents of the ca. 700 Ma Sturtian, the 635 Ma Marinoan, and the ca. 580 Ma Gaskiers glacials, respectively. Two additional observations are noteworthy. Carbonate rocks below the Port Askaig Formation record a delta C-13 decline to -6 parts per thousand that implies that such downturns may occur in both pre-Sturtian and pre-Marinoan strata. In addition, the Bonahaven Dolomite is not a cap carbonate to the Port Askaig Formation, but exhibits a delta C-13 rise to 12 parts per thousand, which we correlate with the inferred global Keele peak. These data further document the utility of Neoproterozoic glacial-cap carbonate sequences in global correlations and denote the base of the Cranford Linestone as the Cryogeman-Ediacaran boundary.