Rift-Related Deepwater Carbonate Mounds in the Mesoproterozoic Borden Basin, Nunavut

The Mesoproterozoic Borden Basin (<1270 Ma) developed in northern Laurentia during extension related to the Mackenzie igneous event. Marine mudstone of the Arctic Bay Formation overlies the earliest stratigraphic units of the Borden Basin and records syndepositional extension. Isolated, Deepwater carbonate mounds of the Ikpiarjuk Formation accumulated at the same time as the upper Arctic Bay Formation shale. These carbonate mounds are kms in diameter, hundreds of ms thick, and lack “normal” facies characteristic of Proterozoic microbial reefs. Two well-exposed sections through the Ikpiarjuk Formation were documented using standard lithostratigraphic techniques to determine the mounds’ depositional setting.

The mounds are elongate parallel to syndepositional faults that belong to a suite of basin-scale rift structures. Mounds are characterized predominantly by massive, seemingly featureless dolostone, although mottled to clotted fabrics are rarely present. Mound-related intraclast debrites are present along the flanks of some mounds, and purple-cemented conglomeratic pipes are present near their tops. In two locations, smaller “moundlets” (<100 m wide and only tens of m thick) are present lower in Arctic Bay stratigraphy than the larger mounds. These moundlets are dominated by clotted fabric and isopachous, formerly calcitic cement in voids among the clots. In one location a moundlet is overturned within flat-lying Arctic Bay Formation shale, recording fault activity during mound growth. Each mound had a different terminal growth history. Some were subaerially exposed and karsted, some were drowned, and some developed mound-capping ooid shoals and tidal-flat tufa crusts. The mounds were eventually overlain by Deepwater laminated dolostone of the Nanisivik Formation.

The Ikpiarjuk mounds are interpreted to represent fossilized cold seeps of as-yet unknown fluid composition. Fluids rose from sea-floor fissures associated with intra-graben fault zones and precipitated local carbonate accumulations in nearby deep water settings. The local presence of clotted texture suggests that carbonate precipitation was influenced by benthic microbial communities.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California