Abstract:
The Verulam Formation (Middle Ordovician) at the Lakefield Quarry
and Gamebridge Quarry, southern Ontario, is comprised of five main
lithofacies. These include shoal deposits consisting of Lithofacies 1,
winnowed crinoidal grainstones and, shelf deposits consisting of:
Lithofacies 2, wackestones, packstones, grainstones, and rudstones;
Lithofacies 3, laminated calcisiltites; Lithofacies 4, nodular wackestones
and mudstones; and, Lithofacies 5, laminated mudstones and shales. The
distribution of the lithofacies was influenced by variations in storm
frequency and intensity during a relative sea level fall.
Predominant convex-up attitudes of concavo-convex shells within
shell beds suggest syndepositional reworking during storm events. The
bimodal orientations of shell axes on the upper surfaces of the shell beds
indicates deposition under wave-generated currents. The sedimentary
features and shell orientations indicate that the shell beds were deposited
during storm events and not by the gradual accumulation of shelly material.
Cluster and principal component analysis of relative abundance data
of the taxa in the shell beds, interbedded nodular wackestones and
mudstones, and laminated mudstones and shales, indicates one biofacies
comprised of three main assemblages: a strophomenid (Sowerbyelladominated)
assemblage, a transitional mixed strophomenid-atrypid
assemblage and an atrypid (Zygospira-dominatQd) assemblage. The
occurrence of the strophomenid, the strophomenid-atrypid and atrypid
assemblages were controlled by storm-driven allogenic taphonomic
feedback.
