ECOLOGICAL AND SEDIMENTOLOGICAL CHARACTERIZATION OF AN EXTINCT FACIES: THE EARLY SILURIAN REGIONAL ENCRINITE, CHICOTTE FORMATION (ANTICOSTI ISLAND, QUEBEC, CANADA)

Regional encrinites are rock units that are composed of at least 50% crinoidal remains and have substantial stratigraphic thickness (≥5-10 m) and areal extent (≥500 km²). These deposits were episodically abundant from the Ordovician through Jurassic at times when carbonate ramps became dominated almost exclusively by crinoids. The lightweight nature of echinoderm ossicles proportional to their size allows them to be entrained in low-velocity currents, resulting in coarse, highly mobile substrates that promoted continued growth of crinoids through positive taphonomic feedback, while excluding many other benthic invertebrates. As a result, regional encrinites are unusual sedimentary units because both biological and environmental factors are equally important for their deposition. However, encrinites are now an extinct facies with no modern counterparts, and the interplay between biotic and abiotic controls that drove the onset, maintenance, and termination of encrinites remains poorly understood.

The Lower Silurian (Telychian) Chicotte Formation of Anticosti Island, Quebec is an ~80 m thick regional encrinite containing several reefal horizons that serves as an ideal setting for a detailed investigation of the deposition and characteristics of regional encrinites. The Chicotte is interpreted as a crinoidal sand-shoal complex that was deposited in a mid-ramp carbonate setting above storm wave base. Although the stratigraphic architecture and general depositional setting of the Chicotte Formation have been described, the ecological and sedimentary dynamics of this unit have not been investigated in detail. Here, we apply multivariate statistical analyses to point counts, facies attributes, and other paleoecological data to better characterize facies within the Chicotte Formation. This work serves as a framework for better understanding the biotic and abiotic factors that influenced the deposition of regional encrinites, such as the Chicotte Formation.