The St George Group (Lower Ordovician) of western Newfoundland: tidal flat island model for carbonate sedimentation in shallow epeiric seas

Abstract

The St George Group consists of peritidal carbonate rocks deposited on the continental shelf of North America bordering the ancient Iapetus Ocean. These Lower Ordovician rocks are similar to other lower Palaeozoic limestones and dolostones that accumulated in epeiric seas and veneer cratonic areas worldwide. A wide variety of facies in the St George is grouped into seven lithotopes, interpreted to represent supratidal, intertidal and shallow, high‐ and low‐energy subtidal environments. Rapid lateral facies changes can be observed in some field exposures, and demonstrated by correlation of closely spaced sections. The stratigraphic array of these lithotopes, although too irregular to be simplified into shallowing‐upward cycles, suggests that they were deposited as small tidal flat islands and banks. Shallow subtidal areas around islands generated sediment and permitted tidal exchange. Tidal flat islands were somewhat variable in character at any one time, and evolved with changing regional hydrographic conditions. The St George rocks suggest an alternative theory of carbonate sedimentation in large, shallow epeiric seas, namely as small islands and banks built by processes that operated in a tidal regime. Furthermore, this island model provides a framework for a mechanism of cyclic carbonate sedimentation, by which small‐scale, peritidal cycles represent tidal flat islands that accreted vertically and migrated laterally as local sediment supply from neighbouring subtidal areas waxed and waned during relatively constant subsidence.

A comparison of recent and fossil large, high-spired gastropods and their environments: the Nopparat Thara tidal flat in Krabi, South Thailand, versus the Swiss Kimmeridgian carbonate platform

“…The evolution from bioturbation to lamination to dinosaur track preservation indicates a succession from marginal-marine to tidal-Xat to supratidal-Xat environment during relative sea-level fall. Bed 12 and the overlying succession of marly laminites (bed 11) are interpreted as intertidal to supratidal regime (Pratt and James 1986;Marty 2008) and correspond to depositional condition No. 5 in Fig.…”

Tethyan-to-boreal correlation in the Kimmeridgian using high-resolution sequence stratigraphy (Vocontian Basin, Swiss Jura, Boulonnais, Dorset)

“…The methodology used in this study follows the one proposed by Strasser et al (1999) for shallow-water carbonates. The depositional sequences are first defined independently in each section and then correlated to filter out beds that result from autocyclic processes such as progradation or lateral migration of sedimentary bodies (Ginsburg 1971;Pratt and James 1986;Strasser 1991) or to identify local and/ or regional gaps in the sedimentary record. Based on the high-resolution sequence stratigraphic interpretation and lithological markers, the Cras d'Hermont and Roche de Mars sections are then correlated with the Gorges du Pichoux, Pé ry-Reuchenette, and Noirvaux sections that are located in the central and southern Swiss Jura and that are known from earlier work (Colombié 2002;Colombié and Strasser 2005;Rameil 2005) (Figs.…”

Recent studies on the Silurian of the western part of Ukraine

“…The biogenic material derived from the shoals, or even from shallow open shelf areas, was swept into the lagoon and laid down in various peritidal settings. In Kubachivka Quarry, it was situated in the intertidal zone located on the lee sides of stromatoporoid shoals (according to a model of tidal flats proposed by Pratt and James (1986)). In Skala Podil'ska Quarry the transported material reached the onshore supratidal zone on the other side of the lagoon.…”