Sedimentology, sea-level history, and tectonic context of a mesoproterozoic carbonate ramp, Baffin Island, Nunavut

The ∼1.2 Ga Victor Bay Formation is a ∼300–600-m-thick succession of predominantly carbonate rocks within the Mesoproterozoic Borden Basin of northern Baffin Island in Arctic Canada. These sediments are dominated by lime mudstone facies and, given their Precambrian age, contain an anomalously low proportion of shallow-water stromatolitic rocks. They form one of the first examples of a Phanerozoic-style carbonate ramp, and reflect the interplay between sea level and tectonics during a critical period in the evolution of the Borden Basin aulacogen.; Of the wide spectrum of carbonate depositional systems that had evolved by the late Mesoproterozoic, the Victor Bay ramp succession represents a muddy endmember wherein the carbonate factory produced lime mud in far greater amounts than ooids, microbialites, or benthic precipitates combined. Lime mud precipitated from the water column and grains formed by early lithification of the muddy sediments. Episodic storms generated a variety of intraclast particles and redistributed mud and grains on this otherwise low-energy microtidal ramp. Stromatolites were deposited only when accommodation space increased rapidly, but during such times flourished and accumulated as large reefs.; The ramp carbonates are demonstrably cyclic at several scales. Most are packaged in decametre-scale cycles of 20–50 m, thicknesses that exceed those of classic shallowing-upward cycles by a factor of ten. Three types of cycles are recognized: (1) deep-subtidal cycles on the outer-ramp, with black dolomitic shale at the base overlain by ribbon, nodular, and carbonaceous carbonate facies, all of which exhibit signs of synsedimentary disruption; (2) shallow-subtidal cycles with basal deep-water facies and an upper layer of subtidal molar-tooth limestone tempestite interbedded with microspar calcarenite facies; and (3) peritidal cycles similar to shallow-subtidal cycles and capped by dolomitic tidal-flat microbial laminite, red shale sabkha facies and sandy polymictic conglomerate. Maximum progradation of inner-ramp tidal flats over outer- and mid-ramp facies during shoaling coincides with a zone of slope failure that may have created accommodation space and therefore promoted the development of the stromatolitic reefs. Consistently thick and complete decametre-scale cycles are compatible with the rapid creation of accommodation space during high-amplitude, high-frequency eustasy. A glacio-eustatic interpretation of these cycles supports the global climatic supercycle theory that predicts icehouse conditions in the late Mesoproterozoic. At the hectometre scale, the eustatic signal was overprinted by pulses of tectonism that defined major sequence-bounding unconformities and flooding surfaces.; Tectonic effects fundamentally altered patterns of sedimentation in the Borden Basin during Victor Bay time. Regional correlations indicate that during deposition of the carbonate ramp, the overall deepening trend within the basin was reversed, leading to karsting in the west and drowning in the east. This sequence of events and the disruption of the paleogeography across the structural grain of the aulacogen are best reconciled with reactivation of basement faults in a foreland basin setting. This new interpretation is consistent with observations of late Mesoproterozoic deformation in the northwestern Arctic.