AN INTEGRATED HIGH-RESOLUTION STRATIGRAPHIC FRAMEWORK OF THE HIRNANTIAN TO TELYCHIAN SUCCESSION OF ANTICOSTI ISLAND, QUEBEC, EASTERN CANADA

The early Paleozoic Icehouse, which spans the Ordovician-Silurian (O/S) boundary, comprises a series of climatic shifts and sea level fluctuations driven by the advance and retreat of the Gondwana ice sheets. The storm-dominated palaeotropical carbonate succession superbly 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 integrated high resolution litho-, cyclo-, 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 supplemented by ~120 m from outcrop. Through a lithofacies analysis, four facies assemblages and three time-specific facies were identified in this succession; all facies are interpreted to have been deposited below fairweather wave base on a storm dominated carbonate ramp. Temporal facies shifts appear to be controlled by both long-term and short-term sea level fluctuations and can be organized into three orders of superimposed transgressive-regressive cycles. Isotopic curves for ¹³C and ¹⁸O were produced by sampling well-preserved bulk micrite at a resolution of 0.5-1 m per sample; in total 611 samples from core and outcrop were taken, corresponding to the Ellis Bay, Becscie, Merrimack, Gun River, Menier, Jupiter, and Chicotte formations. Four distinct positive carbon isotope excursions are recognized in the succession; the upper Hirnantian (+5‰), Lower Aeronian (+2‰), Upper Aeronian (+6‰) and Valgu (+3.5‰) excursions. Multi-ordered δ¹⁸O trends are recognized to occur in association with δ¹³C trends and are likely controlled by glacio-eustatic fluctuations. Determined through a combination of trace element geochemistry and a lack of covariance between ¹³C and ¹⁸O, the Anticosti succession appears to record near pristine isotopic signals for both carbon and oxygen; although unusual in deep geological time, preservation is possible when lithification occurred shortly after deposition in a closed diagenetic system. Ultimately the integrated stratigraphic framework produced in this study is the most complete, well preserved, and highest resolution dataset ever produced for strata of this age.