Tectonic Influence on Deposition and Stratigraphic Relationships of the Ordovician-Silurian Carbonates in the Mackenzie Mountains, NWT, Canada

The stratigraphic relationships between the shallow-marine Late Ordovician – Early Silurian Mount Kindle Formation and basinal Late Ordovician – Middle Silurian Cloudy Formation are not well-constrained in the Mackenzie Mountains, Canada. These units were studied to better understand the geologic history of northwestern Laurentia in the Late Ordovician – Silurian. The Misty Creek Embayment (MCE), a deep-water intrashelf rift basin along northwestern Laurentia’s extensive passive margin, records active rifting and sediment infill in the Early to Middle Cambrian, and Early to Middle Ordovician. Evidence from measured sections, thin sections, stable isotope geochemistry (C, O) and conodont data from the Cloudy Formation suggests the southern portion of the MCE was still experiencing active rifting through the Middle Silurian, whereas the northern MCE remained tectonically quiescent following Middle Ordovician rifting. A measured section of the Cloudy Formation (~720 m thick) in the southern MCE records anomalous thickness changes and distinct facies transitions relative to the northern MCE. This section was sub-divided into three distinct members: Lower, Middle, and Upper. The Lower Member of the Cloudy Formation records deep subtidal facies of laminated dolosiltstone to dolomudstone with chert interbeds. Similar to the Lower Member, the Middle Member of the Cloudy Formation records deep subtidal facies, but interbedded with frequent coarser-grained debrites of skeletal dolowackestone to dolograinstone. The Middle Member of the Cloudy Formation transitions into large (up to 300 m thick) microbial, coral, stromatoporoid and bryozoan mounds within the Upper Member of the Cloudy Formation, indicating a shallow, open marine setting. The Upper Member grades vertically into inner ramp facies and eventually, tidal flat facies of the Late Silurian – Early Devonian Tsetso Formation. The Middle and Upper Member facies transitions are not recorded in the northern MCE, suggesting accommodation was created for these members to infill in the southern MCE. In the northern MCE, the Cloudy Formation is considered coeval to the Mount Kindle Formation, deposited during the Late Ordovician – Early Silurian in a post-rift subsidence phase. Carbon isotope stratigraphy and conodont biostratigraphy indicates the Cloudy Formation in the southern MCE was deposited between the Early to Middle Silurian; therefore, the Cloudy Formation is younger than a majority of the Mount Kindle Formation in the southern MCE. Anomalous thickness changes, facies associations, and relative age dates suggest rifting within the southern MCE began in the Early Silurian during coarser sediment deposition of skeletal debrites in the Middle Member of the Cloudy Formation, and continued until tidal flat deposition of the Late Silurian – Early Devonian Tsetso Formation