The Goldenville–Halifax transition, Mahone Bay, Nova Scotia: relative sea-level rise in the Meguma source terrane

Abstract

In the Meguma Group of Nova Scotia, the transition from sand-rich turbidites of the Goldenville Formation to overlying mud-rich turbidites of the Halifax Formation shows unusually rapid vertical facies variations. The Tancook member of the Goldenville Formation is divided into four laterally continuous units. Unit 1 consists of generally thinly bedded classical turbidites interpreted as lobe facies. Unit 2 comprises mottled (bioturbated) and laminated muds. Units 3 and 4 contain packets of thick amalgamated sandstone beds, alternating with more thinly bedded muddy sections. A unique bioclastic bed occurs near the top of unit 3. In unit 4, burrowed sandstone packets traced along strike (across paleocurrent) are not significantly incised into underlying muds. Laminated slates and siltstones of the overlying Mosher's Island member (Halifax Formation) are enriched in manganese and trace metals and contain diagenetic concretions of manganoan carbonate rimmed by metamorphic garnet. The overlying Cunard member comprises dark grey to black, strongly cleaved, carbon-rich slates, interbedded with thin, pyrite-rich, graded siltstones.Changes in the transition zone can be explained by a relative sea-level rise in the Gondwanaland source area of the Meguma Group. During deposition of the Tancook member, turbidite sand supply was progressively and intermittently interrupted as sand became trapped on the shelf. In unit 1, deposition occurred in a submarine-fan system; in higher units, deposition of sandstone packets was probably controlled by transgressions and regressions in the source area. Expanded shelves, briefly colonized by a shelly fauna, led to high organic productivity and eventual anoxia. During deposition of the Mosher's Island member, manganese was concentrated in reducing waters, delivered to the sea floor close to the oxic–anoxic boundary, and trapped as carbonate during early diagenesis. Expansion of fully anoxic conditions led to deposition of black, sulphide-rich shales of the Cunard member.