Abstract:

The Late Llandovery Chicotte Formation consists predominantly of crinoidal grainstones interspersed with reefs, deposited in a tropical foreland basin, open marine setting. A diverse and well-preserved tabulate and rugose coral fauna grew in reefal and peri-reefal settings. Growth periodicity in 12 species was measured from internal and external skeletal features preserved as pristine primary calcite. Annual cycles were exhibited by internal skeletal features as couplets of high- and low-density bands. High-density bands, considered as storm or monsoonal season growth, generally consisted of closely spaced tabulae or tabellae, greater abundance and/or thickening of septa, development of septal crests on dissepiments or tabulae, and thickening of corallite walls. In contrast, low-density bands have relatively widely spaced tabulae and sparse to rare septa, or featured zoning of cysts in coenenchyme or dissepimentaria. Periodic external skeletal structures in colonial rugosans included fine growth ridges and higher-order rugae. In Petrozium pelagicum, Entelophyllum sp., Strombodes sp., and Thecia sp., the distances between tabulae were plotted and moving averages used to yield smooth growth cycles. Average annual growth rates ranged from 0.7-16,9 mm/a (Thecia sp., Favosites gothlandicus) in tabulates and from 0.7-16.3 mm/a (Arachnophyllum pentagonum) in colonial rugosans. Growth from external features ranged from 0.6-12.6 mm/a (P. pelagicum, Strombodes sp.), and fine growht ridges averaged 0.09 mm in thickness (P. pelagicum) and numbered 23-25/ruga (Entelophyllum sp.). Annual growth rates related to skeletal complexity and were generally lower in Chicotte species than those from the Michigan Basin, which was, in contrast, developed as a semi-enclosed, epeiric sea. Cycles exhibited by external and internal skeletal features were similar in magnitude, suggesting they may both be used for growth rate studies. Compared to Holocene reef-building corals, Early Silurian species had comparable overall skeletal sizes and annual growth rates, supporting the concept that most Paleozoic corals probably possessed algal or microbial symbionts.