Paper No. 1
Presentation Time: 1:20 PM
A COMPLEX BUT REPETITIVE RECORD OF GLACIO-EUSTATIC CYCLICITY IN LOWER SILURIAN TROPICAL CARBONATE RAMP DEPOSITS, ANTICOSTI ISLAND, QUEBEC, CANADA
The Lower Silurian (upper Llandovery) Chicotte Formation is a regional 80 m thick encrinite (crinoid-rich) deposit that accumulated on a slowly subsiding cratonic tropical carbonate ramp now exposed in superb coastal and river exposures on Anticosti Island. Chicotte encrinites represent extensive crinoid blankets formed near fairweather wave base. These were frequently reworked by storms resulting in the selective landward and seaward transport and rapid burial of various crinoidal skeletal elements (i.e., stem versus calyx and holdfast). Small coral-stromatoporoid and sponge-bryozoan buildups occurred locally. Vertical and lateral facies successions in the Chicotte encrinites record three distinct hierarchical cycles at sub-meter, meter, and formation scales. The predominant depositional unit is a meter-scale subtidal cycle (3-8 m thick) typically characterized by upward coarsening, locally cross-bedded encrinitic debris capped sharply by an erosion surface. These surfaces display low scalloped relief but up section become more complex with a highly irregular relief similar to modern rocky shorelines. Several closely-spaced encrinitic sub-units (< 1 m thick) capped by erosion surfaces with low scalloped relief are also present in the upper part of individual meter-scale encrinitic cycles. At the formation scale, meter-scale encrinite cycles display an aggrading to prograding stacking pattern truncated locally by major incised paleovalleys (at least 1 km wide and 50 m deep) that developed during a long term sea level fall and subsequently were filled with encrinitic debris during the ensuing long term trangression. This complex but repetitive record of sea level changes preserved in the Lower Silurian Chicotte encrinites was likely forced by multi-order glacio-eustatic fluctuations associated with the South American Gondwanan ice sheet growth and melting.