Palaeozoic microfossils from Orphan Knoll, NW Atlantic Ocean

Orphan Knoll is an isolated, drowned continental fragment 550 km northeast of Newfoundland. The top of Orphan Knoll stands at 1800 to 2000 m and is marked by a series of protruding mounds. Dredging at the base of one of the mounds in 1971 obtained a suite of fossiliferous limestone pebbles that are interpreted to reflect nearby bedrock. The shallow marine limestone facies include mudstone, wackestone, packstone, and grainstone. The pebbles yielded Late Ordovician, Silurian and Devonian conodonts, as well as Ordovician/Silurian scolecodonts, chitinozoans and graptolites. Similarly, sponge spicules suggest the presence of Late Ordovician as well as Middle Devonian material including a Silicunculus-type of hexactinellid anchoring basalia and an octactine heteractinid spicule of Ensiferites, respectively. A Middle to Late Ordovician silicified ostracod fauna appears to be endemic at the species level including forms with both North American and North European affinities with a genus and two species that are new to science. We conclude that the Upper Ordovician to Silurian and Devonian material is not ice-transported but reflects the bedrock of nearby mounds. Thus, the Ordovician marine intracratonic platform sediments were much more extensive than previously known and a marine re-entrant penetrated the Devonian Old Red Sandstone province in an area where marine limestones were previously unknown.