PALEOECOLOGICAL AND SEDIMENTARY DYNAMICS OF A VANISHED FACIES: DEPOSITIONAL CONTROLS ON AN EARLY SILURIAN REGIONAL ENCRINITE FROM ANTICOSTI ISLAND, QUEBEC

Regional encrinites are rock units composed of at least 50% crinoidal remains with substantial stratigraphic thickness (≥5-10 m) and areal extent (≥500 km²). These deposits were episodically abundant from the Ordovician through Jurassic during intervals of high crinoid diversity and abundance on carbonate ramps. Because echinoderm ossicles are extremely porous, they are readily entrained in low-velocity currents and formed coarse, highly mobile substrates in encrinites that promoted continued growth of crinoids through positive taphonomic feedback, while excluding many other benthic invertebrates. As a result, both biotic and abiotic factors played pivotal roles in controlling the deposition of regional encrinites. Notably, encrinites are now an extinct facies, and due to the lack of modern analogues, the complex interplay between biological, environmental, and taphonomic controls on encrinite deposition remain poorly understood.

Anticosti Island (Quebec, Canada) preserves exceptional exposures of the ~80m thick Chicotte Formation, an early Silurian (Telychian) unit that represents one of the earliest known regional encrinites. This unit serves as an ideal case-study for investigating the depositional controls on regional encrinites and their paleoecological, sedimentological, and taphonomic characteristics. Although prior work has established that the Chicotte is a crinoidal sand-shoal complex that was deposited in a mid-ramp carbonate setting above storm wave base, the paleoecological and sedimentary dynamics of this unit have not been investigated in detail. Here, we apply a suite of multivariate statistical analyses to point counts, facies attributes, and other paleoecological and taphonomic data to better characterize facies within the Chicotte Formation and interpret their depositional controls. This work serves as a foundational framework for better understanding the biotic and abiotic factors that influenced the onset, maintenance, and termination of regional encrinite deposits like the Chicotte Formation.