Rifting of Columbia to form a deep-water siliciclastic to carbonate succession: The Mesoproterozoic Pinguicula Group of northern Yukon, Canada
Graphical abstract
Introduction
The evolution of western ancestral North America (Laurentia) from late Paleoproterozoic to Paleozoic time was dominated by the formation of sedimentary basins (Gabrielse, 1967, Fritz et al., 1991, Poole et al., 1992, Link et al., 1993, Norris and Dyke, 1997). Together, these basins record a pattern of repeated lithospheric extension and sedimentary fill separated by long hiatuses. The basinal successions are typified by substantial thicknesses of both siliciclastic and carbonate rocks. Igneous activity, although locally important, was secondary to sedimentation. Activity during the sedimentary hiatuses varied from quiescence to basin inversion, terrane accretion, magmatism, and hydrothermal surges (Eisbacher, 1978, Aitken and McMechan, 1992, Colpron et al., 2002, Thorkelson et al., 2005, Milidragovic et al., 2011, Doe et al., 2012, Furlanetto et al., 2013, Thorkelson and Laughton, in press).
In northwestern Laurentia, the Precambrian sedimentary record consists of at least five intervals of basin formation that span over a billion years (Young et al., 1979, Thorkelson et al., 2005, Macdonald et al., 2012, Medig et al., 2014). The record was divided into three unconformity-bounded successions named sequences A, B and C by Young et al. (1979). Since then, the understanding of sedimentation and other processes in the region has expanded appreciably, and the history of basin formation is now known to be far more complex than previously envisaged. Despite advancements in understanding, the significance of each sedimentary succession in terms of cause, paleogeography, and nature of bounding surfaces remains largely unanswered.
The Mesoproterozoic Pinguicula Group is the third in a series of five sedimentary successions that crop out in extensive Proterozoic inliers of Yukon, Canada. The succession is well exposed in mountainous terrain in northern Yukon over an area of at least 100 × 190 km and as much as 100 × 330 km, depending on the validity of stratigraphic correlations. Its deposition was preceded by deposition and ensuing contractional deformation of the Paleoproterozoic Wernecke Supergroup (Delaney, 1981, Furlanetto et al., 2013), and deposition and uplift of Mesoproterozoic unit PR1 (Medig et al., 2014). The Pinguicula Group was succeeded by the Neoproterozoic Mackenzie Mountains Supergroup and its correlatives, the Fifteenmile and Hematite Creek groups (Thompson et al., 1992, Thorkelson, 2000, Turner, 2011a, Turner, 2011b, Macdonald et al., 2012), and the late Neoproterozoic Windermere Supergroup (Eisbacher, 1985).
Prior to this study, the Pinguicula Group had been mapped and studied at only the reconnaissance level by Green, 1972, Blusson, 1974, Eisbacher, 1981, Abbott, 1997a, Thorkelson, 2000. Without a focused sedimentological study, many questions remained regarding sedimentological and stratigraphic detail, depositional environment, paleocurrents, basin architecture, composition, and age. Accordingly, this study involved detailed mapping, section measurement, detailed petrography, detrital mineral geochronology, and geochemistry. In this paper, the stratigraphic, lithologic, and petrographic data are used to define the stratigraphy of the Pinguicula Group formally and to describe its sedimentological history in terms of environments of deposition, direction of sediment transport, and basin evolution. Particular attention is given to the carbonate rocks in the Pinguicula Group, and comparisons are drawn with Mesoproterozoic carbonate rocks globally.
Section snippets
Geologic setting
The Mesoproterozoic Pinguicula Group, as originally defined, is exposed in the Wernecke inlier (Eisbacher, 1978). Possible correlatives were identified in the Hart River inlier (referred to as the Pinguicula Group by Abbott, 1997b) and Coal Creek inlier (Fig. 1, Fig. 2; as part of the Fifteenmile group; Thompson et al., 1992). The underlying Wernecke Supergroup is estimated to be >14 km thick and includes, from oldest to youngest, the Fairchild Lake Group, the Quartet Group, and the Gillespie
Stratigraphic relationships and descriptions
In the Wernecke inlier, the Pinguicula Group overlies the Gillespie Lake and Quartet groups of the Wernecke Supergroup with angular unconformity (Fig. 2; Thorkelson, 2000). The Mount Landreville Formation also unconformably overlies zones of Wernecke breccia and Hart River sills (Thorkelson, 2000, Medig et al., 2010). Previously, the Hart River sills in the Wernecke Mountains were thought to cut the Pinguicula Group (Thorkelson, 2000, Thorkelson et al., 2005), but an unconformable rather than
Mapping and section measurement methods
The Pinguicula Group was mapped in the Wernecke inlier on 1:50,000 map sheets NTS 106C/11, and NTS 106C/12, south of areas NTS 106C/13 and NTS 106C/14, previously mapped by Thorkelson (2000). Type sections for the Mount Landreville, Pass Mountain, and Rubble Creek formations were measured and described in detail at three different locations (Fig. 3). A detailed section through the Mount Landreville Formation was measured in a creek bed at the PIKA mineral occurrence (Fig. 4, Fig. 5). At this
The Mount Landreville Formation
At the measured section, the Mount Landreville Formation (formerly unit A) is 306 m thick and is predominantly siltstone with minor conglomerate and sandstone (Table 1 and Supplementary Table 1; Figs. 4 and 9a). A 50 cm-thick layer of polymictic pebble conglomerate is the lowest unit in the section (unit 1) and unconformably overlies a zone of Wernecke Breccia (Fig. 9b). The conglomerate is matrix-supported with well-rounded pebble clasts approximately 4–15 mm long. The clasts are composed of
Pinguicula Group Lithostratigraphy, Hart River inlier
In the Hart River inlier, the Pinguicula Group displays similar lithofacies to those in the Wernecke inlier (Table 1 and Supplementary Table 1; see Supplemental Data). One exception is at the measured section, where the Rubble Creek Formation is composed almost entirely of stromatolites.
Reconnaissance mapping of the Pinguicula Group in the Hart River inlier was completed by Abbott (1997b) who described the conformable Mount Landreville, Pass Mountain, and Rubble Creek formations as an
Mount Landreville Formation: deep-water siliciclastic sediment
The Mount Landreville Formation records the onset of basin formation. Polymictic pebble conglomerate at the base of the unit is interpreted as channels cutting into the otherwise sandy substrate in the basin as transgression and submersion of the underlying Wernecke Supergroup, Wernecke Breccia, and Hart River sills progressed. Most of the succession consists of siltstone and calcareous siltstone with planar, parallel lamination. These facies are interpreted to have formed in a deep-water
Tectonic setting of the Pinguicula Group
The Pinguicula Group is the middle of five unconformity-bounded stratigraphic successions deposited on the northwestern margin of Laurentia between the late Paleoproterozoic and the Cambrian (Eisbacher, 1978, Delaney, 1981, Thorkelson et al., 2005, Medig et al., 2014). Together, these successions record repeated episodes of subsidence, marine transgression and clastic-carbonate sedimentation, commonly followed by uplift and erosion. Significant magmatism synchronous with sedimentation did not
Conclusions
- 1.
The Pinguicula Group is a siliciclastic and carbonate succession that consists of the newly formalised Mount Landreville, Pass Mountain, and Rubble Creek formations (formerly known as units A, B and C). The succession was deposited on a low-energy, gentle slope that recorded episodic, high-energy, events in an extensional epicratonic basin that deepened to the south (present day coordinates). The majority of the sediment was deposited below storm wave-base, but rare lithofacies deposited above
Acknowledgements
Funding for the project was provided by the Yukon Geological Survey, The Geological Survey of Canada, Northern Scientific Training Program, and an Natural Sciences and Engineering Research Council of Canada grant to Derek Thorkelson. Katherine Hahn, Geoff Baldwin, Tim Peters, Francis Macdonald, and Robbie Dunlop are thanked for their assistance in the field. Dr. Linda Kah and one anonymous reviewer are thanked for their thorough reviews that led to improvements in the quality of this manuscript.
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