High Resolution Chemostratigraphy and Cyclostratigraphy of Lower Silurian Neritic Carbonates from Anticosti Island, Quebec, Canada.

Abstract

The storm-dominated paleotropical carbonate succession exposed on Anticosti Island in Eastern Canada represents one of the most complete, thickest, and well-preserved successions in the world spanning the O/S Boundary. This study develops a new high resolution integrated lithostratigraphic, cyclostratigraphic, and chemostratigraphic framework for the upper Hirnantian to lower Telychian (Upper Ordovician to lower Silurian) succession on Anticosti, by examining ~450 m of strata from a recent stratigraphic drill core (Martin La Mer), supplemented by ~120 m of outcrop, all from the south-central part of the island. Four facies assemblages and three time-specific facies were identified in this succession and can be organized into three orders of superimposed transgressive-regressive cycles. New high resolution isotopic curves were produced by sampling well-preserved bulk micrite at a resolution of 0.5-1.0 m per sample; in total 443 samples were taken from core and 168 from outcrop, corresponding to the Ellis Bay, Becscie, Merrimack, Gun River, Menier, Jupiter and Chicotte formations. Four distinct positive carbon isotope excursions are recognized in the study interval; the upper Hirnantian (+5‰), Lower Aeronian (+2‰), Upper Aeronian (+6‰), and Valgu (+3.5‰) excursions. These δ13C excursions accompany lithology, and sea level changes and are likely driven by transitions between humid and arid climate states coupled with ocean changes. Multi-ordered δ18O trends are recognized to occur in association with δ13C trends; long-term and intermediate scale δ18O fluctuations are likely controlled by glacio-eustasy and Silurian climate fluctuations, while high-frequency fluctuations may record an astronomical forcing signal.