THE PINGUICULA GROUP (YUKON): EXTENSION OF LATEST MESOPROTEROZOIC INTRACRATONIC RIFTING INTO NW LAURENTIA?

The Pinguicula Group is one of a series of middle to late Proterozoic stratigraphic successions that outcrops in Proterozoic inliers in Yukon, Canada. In the Wernecke Mountains, where it is defined and most extensive, it unconformably overlies the ca. 1.6 Ga Wernecke Supergroup and is unconformably overlain by the earliest Tonian Hematite Creek Group. Here, the <1.5 km-thick Pinguicula Group is subdivided into the lower Mount Landreville, middle Pass Mountain, and upper Rubble Creek formations (Medig et al., 2016). However, the age and tectonic significance of the Pinguicula Group remain unclear due structural complexity and a lack of geochronological constraints, compounded by the fact that the Rubble Creek Fm. is heavily recrystallized, disturbed by extensive zebra dolomite, and difficult to study due to the rugged terrain. We show that the Pinguicula Group as a whole clearly represents a single transgressive-regressive sequence dominated by a carbonate-rich regressive systems tract. In the northernmost exposure of the Pinguicula Group in the Wernecke Mountains, the succession is thinner but well preserved, revealing a basal deepwater shale and siltstone that transitions into a storm-dominated carbonate ramp, which is then overstepped by a southward-prograding stromatolite reef and back-reef system. Stratigraphically analogous transgressive-regressive successions in the same tectonostratigraphic position occur in the Hart River, Coal Creek, and Tatonduk inliers to the west. In the Coal Creek inlier, the lowermost shales locally feature olistoliths and are organic-rich, providing a likely source for bitumen preserved in coarse vugs in overlying carbonates. We propose that the Pinguicula Group is broadly correlative with other similar shale-carbonate sequences in the Amundsen basin (lower-middle Rae Group) in northern Canada and the Bylot basins in northeastern Canada and northwestern Greenland. Re-Os dates on these successions indicate deposition ca. 1090–1050 Ma, which overlaps with the upper Keweenawan Supergroup in the Mid-Continent Rift. These correlations imply that isolated rifting occurred across a wide arc of northern Laurentia coeval with the Ottawan phase of the Grenville orogeny. These intracratonic rifts were significant depocentres for black shales and archives for early eukaryotic fossils.