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<p>The Mesoproterozoic Ikpiarjuk Formation (Borden Basin, Nunavut) consists of a series of very large (&gt;200 m thick; kms diameter) dolostone mounds that accumulated on the floor of a restricted basin. The mounds are isolated from contemporaneous shallow-water carbonate rocks, accumulated at the same time as black shale in deep-water regions of the basin, and are geographically limited to the vicinity of basin-scale fault zones. The mounds contain extensive deposits of benthic thrombolites, a microbial texture that is not common in Mesoproterozoic carbonate rocks.<br />
Field relationships of individual mounds indicate that mound growth was initiated during regional deposition of organic-rich black shale, mounds developed significant relief above the basin floor, and some mound tops were eventually exposed and karsted during differential uplift. The mounds accumulated in a deep-water, sub-photic-zone setting as a result of fluid venting along basin-scale faults. Interpreted basin depths indicate that the thrombolitic mound texture is probably not related to cyanobacteria. Chemosynthetic microbes exploited the chemical gradient in the vicinity of the vents and built a mound framework of thrombolites. Framework voids contain both carbonate mud and isopachous linings of fibrous cement, indicating that the mixing of vent fluid with basin water favoured production of abiogenic carbonate minerals in addition to providing a chemical gradient for microbes to exploit.<br />
Proterozoic deep-water carbonates and thrombolites are rare, and the Ikpiarjuk Formation mounds represent microbial buildups that are fundamentally different from other examples in the Precambrian rock record. (C) 2017 Elsevier B.V. All rights reserved.</p>

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