Abstract

The stratigraphic relationships between the Upper Ordovician-Lower Silurian Mount Kindle Formation and Upper Ordovician-Middle Silurian Cloudy Formation were studied to better understand the geologic history of northwestern Laurentia in the Late Ordovician to Silurian. The Misty Creek Embayment (MCE), a deepwater 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 (Fig. 1) 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. This section was subdivided into three distinct members: Lower, Middle, and Upper (Fig. 2). 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 dolograin-stone. 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 Upper Silurian-Lower Devonian Tsetso Formation (Fig. 2). The stratigraphy suggests the Cloudy Formation records a single rise and subsequent fall of sea level during deposition.

In the northern MCE, the Cloudy Formation is considered coeval to the platformal Mount Kindle Formation, deposited during the Late Ordovician - Early Silurian in a post-rift subsidence phase. Stable isotope chemostratigraphy (C, O) 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 (Fig. 2). Anomalous thickness changes, facies associations, and relative age dates suggest rifting within the southern MCE began during coarser sediment deposition in the Middle Member of the Cloudy Formation, and continued until tidal flat deposition of the Upper Silurian-Lower Devonian Tsetso Formation.