Rift Dolostones of the Mesoproterozoic Borden Basin, Nunavut (Arctic Canada)
The Borden Basin is a Mesoproterozoic (~1.2-1.1 Ga) aulacogen in Arctic Canada that contains thick carbonate formations. The main part of the basin, the Milne Inlet Graben, is the focus of a multi-year field mapping and stratigraphy program. The basin went through an early sag phase (volcanic and shallow-marine terrigenous clastic rocks) that was followed by accumulation of Deepwater black shale and a laterally equivalent mixed carbonate-clastic open-marine ramp (Iqqittuq Fm.) as the basin evolved into a true rift. Evidence of extensional tectonism continued through deposition of two thick (hundreds of m to >1 km), laterally equivalent, but markedly dissimilar carbonate units, the Angmaat (platformal) and Nanisivik (basinal) formations, that were separated from each other by a transverse hinge zone and a narrow region populated by large fault-related Deepwater carbonate mounds (Ikpiarjuk Fm.). The Angmaat Formation consists of hectometre-scale platformal subtidal (microbial) -peritidal (sea-floor precipitates) cycles that were deposited under restricted basin conditions, and a narrow, structurally controlled hinge zone populated by peritidal tepee-ooid cycles. The Nanisivik Formation consists of millimetrically laminated dolostone with pervasive early brecciation that cross-cuts layering and which is presumed to record gas- escape from early lithified carbonate; shallow-water mechanical and biological features are completely absent. Both formations show sedimentological evidence of synsedimentary fault activity in the form of graben-margin clastic aprons, local subaqueous fault scarps that shed mixed detritus, and local, fault-related slopes associated with debrites, turbidites, soft-sediment deformation, and brecciation. The two formations are truncated by a pronounced unconformity along which up to hundreds of metres of dolostone were removed, and a hill-and-valley topography erosionally developed, prior to deposition of marine mudstone of the overlying Victor Bay Formation, and ensuing terrigenous clastic deposits. Topographic highs in this unconformity surface later acted as traps for methane, which acted as a reductant in the formation of geometrically distinctive MVT-style mineralisation. The unconformity marks a change in tectonic regime from extensional (rift) to contractional; the broader implications of this drastic change remain to be explored.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California