The stratigraphic and geochemical imprints of Late Ordovician glaciation on far-field neritic carbonates, Anticosti Island, eastern Canada

The storm-dominated, open marine neritic carbonate facies, continuously exposed in coastal cliffs and wide rocky tidal flat exposures at the western end of Anticosti Island, form a 315-m-thick, expanded section spanning the Ordovician-Silurian boundary. These carbonate facies display deepening-shallowing trends of different orders, resulting in a hierarchical stacking of sedimentary cycles. The carbonate facies within these multi-order cycles represent sediments that were deposited from shoreline to deeper outer carbonate ramp (>80 m in water depth) and exhibit sea-level changes of several tens of metres in magnitude. Field, petrographic and geochemical evidence support a well-preserved primary δ18O signal in close equilibrium with the original paleoenvironmental conditions. The δ18O intracycle variations, up to 1.0‰ in magnitude, reveal repeated decreasing and increasing values during the transgressive and regressive portions of decametre TR cycles respectively. Although temperature, salinity, and ice volume influenced the recorded primary δ18O signal, orbital scale glacioeustasy ultimately controlled facies and cycle development and δ18O signal during the Katian immediately before the latest Ordovician (Hirnantian) glaciation. The ~3.5 myr-long δ18O signal recorded by our entire succession supports a protracted period of cooling leading into the main phase of latest Ordovician (Hirnantian) glaciation. Spectral analysis of the δ18O record also reveals decametre variations, which are associated with decametre scale transgressive-regressive (TR) cycles; possibly reflecting eccentricity orbital signals. Our study shows the importance of a thick expanded stratigraphic succession sampled at the highest resolution possible to decipher potential linkages between facies changes, multi-order stratigraphic cycles, δ18O signal and the Earth's orbital parameters.