Uppermost Precambrian(?)–Lower Cambrian of mainland Nova Scotia: faunas, depositional environments, and stratigraphic revision

Abstract

The uppermost Precambrian(?)–Lower Cambrian of the Avalon Zone in the northern Antigonish Highlands is composed of two dissimilar sequences in thrust contact. These include the sandstones and slates of the Doctor's Brook allochthon and the volcanoclastic-rich Malignant Cove authochthon.Lithostratigraphy of the “Black John Formation” (designation abandoned) in the Doctor's Brook allochthon is comparable to the uppermost Precambrian–Lower Cambrian in eastern Placentia Bay, southeastern Newfoundland, and Cape Breton Island. A unified stratigraphic nomenclature is appropriate in these Avalonian areas. The lower part of the “Black John” is an unconformity-bounded depositional sequence with subaerial rift facies (Rencontre Formation, 178+ m), overlying marine siliciclastic mudstones and fine sandstones (Chapel Island Formation, 59 m), and a quartzite cap (Random Formation, 2.05 m). The Chapel Island Formation has the oldest faunas from mainland Nova Scotia (Watsonella crosbyiZone, lower Placentian Series). A post-Random unconformity known in Newfoundland and Cape Breton Island lies at the Random-Bonavista Group (Cuslett Formation) contact in the upper “Black John Formation.”Fossiliferous Lower Cambrian limestones and siliciclastic mudstones previously reported from the Malignant Cove autochthon are actually clasts in basalt pebble-dominated slope deposits of the Arbuckle Brook Formation. These clasts were eroded from shallow-marine facies comparable to those in the Doctor's Brook allochthon during local uplift associated with Middle Cambrian(?) extension and mafic volcanism.Eight species are illustrated from the Placentian and Branchian Series.AnabaritellusMissarzhevsky, 1974, emend. (=SelindeochreaValkov, 1982) is a Lower Cambrian calcareous tube-dwelling metazoan(?) known from tri- through multisulcate conchs that are morphologically intermediate betweenAnabaritesandColeoloides.