SEDIMENT DYNAMICS AND STRATIGRAPHIC ARCHITECTURE OF A MIXED CARBONATE-SILICICLASTIC RAMP: THE UPPER ORDOVICIAN (HIRNANTIAN) ELLIS BAY FORMATION, ANTICOSTI ISLAND, QUÉBEC, CANADA

The Upper Ordovician (Hirnantian) Ellis Bay Formation on Anticosti Island, Québec, Canada, provides a unique opportunity to study the stratigraphic architecture of mixed carbonate-siliciclastic deposits that formed during a period of major glaciation on a storm-influenced, slowly subsiding equatorial ramp. The west-east-trending Ellis Bay outcrop belt is ~180 km long, slightly oblique to the paleoshoreline, and offers superb coastal exposure at both ends. The western sections (90 m thick) consist of stacked cycles of mid- to outer-ramp, storm-dominated carbonates with argillaceous mudstones. Inner ramp oncolitic and reefal carbonates are also present in the uppermost part of the formation. The thinner eastern sections (45 m thick) are composed of basal sandstone and argillaceous mudstone units forming a continuous succession from 1) tide-dominated estuary to 2) storm-influenced delta to 3) storm-dominated strandplain (updrift or downdrift) deposits. The latter succession likely resulted from 1) infilling during transgression of an estuary that was first developed as an incised valley during the previous regressive event, 2) regression and progradation of a delta fed by the same river that formed the incised valley and the estuary, and 3) avulsion of the delta river coupled with an initial transgression rapidly followed by regression. Overlying these basal sandstones are mid- to outer-ramp, texturally-mixed carbonate-siliciclastic tempestites capped by pure carbonate tempestites, with oncolitic and reefal units that correlate westward. This decrease in siliciclastic content is thought to represent the side-stepping (avulsion) of the siliciclastic-supplying delta combined with an initial major transgression, thus leading to carbonate-dominated storm-reworked deposits. Alternatively, the upward decrease in siliciclastic content in the eastern sections could result from river avulsion coupled with a gradual shift from relatively humid to more arid climatic conditions. Despite these important lateral facies changes, the recognition of four major transgressive-regressive cycles based on sequence stratigraphy allowed a precise correlation between the western and eastern sections. These cycles are likely driven by glacio-eustasy in association with the Hirnantian Gondwana glaciation.