Cambrian-Ordovician acritarchs in the Meguma terrane, Nova Scotia, Canada: Resolution of early Paleozoic stratigraphy and implications for paleogeography

Abstract

Organic-walled microfossils (acritarchs) provide age constraints for the previously poorly dated Cambrian and Lower Ordovician turbiditic Goldenville and Halifax Groups in the northern Appalachian Meguma terrane of Nova Scotia and address controversies about stratigraphy and provenance. The oldest exposed formation of the ∼8-km-thick Goldenville Group contains the trace fossil Oldhamia , which suggests an age of late Early Cambrian, whereas the coticule-bearing uppermost formation yielded an acritarch species consistent with Middle Cambrian (Epoch 3) age. The conformably overlying ∼5-km-thick Halifax Group includes basal pyritiferous units that yielded a Late Cambrian (Furongian) assemblage of acritarch species, providing further confirmation that the underlying manganese-bearing formations are of Cambrian Epoch 3 age, and not Ordovician as recently claimed. Overlying nonpyritiferous formations contain the Early Ordovician graptolite Rhabdinopora flabelliformis flabelliformis and acritarchs of similar age. Samples collected up section from the graptolite occurrence yielded acritarch species that are indicative of the later Tremadocian and Floian. The new fossil data confirm a gap in age of ∼30 m.y. between the Halifax Group and the overlying Silurian to Lower Devonian Rockville Notch Group. The new ages are consistent with stratigraphic units defined in regional mapping and support recent interpretation of fundamental differences in depositional and tectonic environments between the northwestern and southeastern parts of the Meguma terrane. They also provide new constraints on paleogeography of the terrane by confirming age similarities to stratigraphy in north Wales, with which correlation has been proposed previously based mainly on lithological similarities.