Allochthonous slope sequence at Lobster Cove Head: evidence for a complex Middle Ordovician platform margin in western Newfoundland

Abstract

The upper part of an intact sequence of Lower to Middle Ordovician deep-water sediments, which now form a large, disrupted raft within the Rocky Harbour Mélange at Lobster Cove Head, is interpreted as having been deposited downslope from a drowned carbonate platform margin. The entire 50 m thick section is Arenig (late Canadian or Ibexian to early Whiterock) in age; graptolite biostratigraphy demonstrates a correlation with upper parts of the Cow Head Group to the north. The basal part of the section is a proximal facies of the Cow Head Group (Shallow Bay Formation, Factory Cove Member, Beds 9, 10, and part of Bed 11). The upper part of the section consists of interbedded dolostone and shale and is unlike any other sequence in the Cow Head Group. This upper sedimentary sequence is defined as the Lobster Cove Head Member of the Shallow Bay Formation, Cow Head Group. Contact between the two sedimentary packages is also marked by a faunal break and coincides with emplacement of megaconglomerate Bed 12 at Cow Head.This break marks the change from a uniform to complex carbonate platform margin configuration and is here interpreted as the result of synsedimentary faulting. The margin upslope from Cow Head remained in shallow water during the final stages of Cow Head Group deposition, whereas that upslope from Lobster Cove Head was drowned and shed little sediment into deep water. The synsedimentary faulting, which led to rapid subsidence and platform-margin drowning upslope from Lobster Cove Head and possibly the deposition of megaconglomerate Bed 12 at Cow Head, coincides with the onset of the Taconic Orogeny in western Newfoundland.