Ecological zonation during the carbonate transgression of a late Ordovician rocky shore (Northeastern Manitoba, Hudson Bay, Canada)

Abstract

Late Ordovician (Ashgillian) carbonates belonging to the Port Nelson Formation transgress a resistant rocky shore near Churchill, Manitoba on the southwest flank of the Hudson Bay Basin in Canada. Examples of ancient rocky shores are very rare and this locality offers the first opportunity to document extensive offshore biological zonation with spatial reference to a well defined shoreline. Five biofacies zones are parallel to a 350 m-long shoreface carved into the Precambrian Churchill Quartzite. Eroded boulders of the dark quartzite blanket the shoreface and clast size is observed to decrease dramatically up section through 4.5 m of tan-colored carbonates. From an onshore to offshore position, the ecological zones include (1) boulder-encrusting favositid corals and a related high-energy fauna, (2) a shell pavement dominated by the inarticulate brahciopod Monomerella, (3) large, free-standing tabulate corals including Palaeophyllum, Calapoecia, and Catenipora, (4) tunneled trace fossils including chondritid, thalassinoid, and less common vertical “volcano” burrows, and (5) surface Cruziana burrows associated with rare, large isotelid (asaphacean) trilobites. Siphuncular debris from aminocerid cephalopods abundantly scattered through zones 2–3, provides data on current orientation consistent with long-shore drift. A wide field of ripple marks preserved near the top of the stratigraphic sequence confirms that wave trains advanced shoreward at an indirect angle of about 45°. Although currents deflected by natural groin fields may have swept coarse clastics more seaward at some spots than others, the biological zones are remarkably linear and persistent. The Churchill site is a comparatively simple case of landward migration by shore-parallel biofacies primarily controlled by factors related to water depth and distance from shore. This interpretation does not preclude the possibility that more random facies patterns may have existed in protected embayments or along more irregularly cliffed sectors of the ancient Churchill coast.