The Middle Cambrian Mount Roosevelt Formation (new) of northeastern British Columbia: evidence for rifting and development of the Kechika Graben System

Abstract

The eastern margin of the Kechika Graben in the vicinity of Muncho and Moose lakes, northeastern British Columbia, contains Middle Cambrian rift-related strata herein named the Mount Roosevelt Formation. The formation conformably overlies a quartzite package equivalent to the uppermost clastic unit of the informal late Early Cambrian Gataga group (Bonnia-Olenellus zone) and is conformably overlain by a thick, unnamed carbonate sequence of Middle Cambrian age (Plagiura-Poliella zone). The Mount Roosevelt Formation is subdivided into three members. The lowermost member is characterized by oöid-bearing siltstone and sandstone, interbedded with dolostone, limestone, and hematitic conglomerate. Conformably above this, the middle member is a thick sequence of polymict cobbly pebble conglomerate. The upper member includes karstified dolostone, calcareous-cemented conglomerate and sandstone, and limestone. Collectively the Mount Roosevelt Formation reflects alluvial fan delta progradation into a transgressive marine environment. Deposition occurred in an active fault-controlled basin, located on the eastern margin of the Kechika Graben adjacent to the Muskwa High. Basin initiation in the late Early Cambrian coincided with the reactivation of pre-existing regional faults. High rates of subsidence during the initial phase of extension were accommodated on these faults, which provided a locus for fan delta deposition. Continuing high rates of subsidence limited basinward fan delta development. Deposition of the formation ended with base-level transgression in the early Middle Cambrian that drowned the fan deltas and the adjacent Muskwa High and allowed development of the Kechika Trough above the older graben system. Résumé :Située à proximité des lacs Muncho et Moose dans le nord-est de la Colombie-Britannique, la bordure est du graben de Kechika contient des strates, ici nommées la Formation de Mount Roosevelt (Cambrien moyen), qui sont reliées à la distension. La formation repose en concordance sur un ensemble de quartzite équivalent à l'unité supérieure, clastique, du groupe de Gataga, informel, datant du Cambrien précoce (zone à Bonnia-Olenellus); elle est recouverte en concordance par une épaisse séquence de carbonate, sans nom, datant du Cambrien moyen (zone à Plagiura-Poliella). La Formation de Mount Roosevelt est subdivisée en trois membres. Le membre inférieur est caractérisé par des siltstones et des grès à oolites, interstratifiés avec des dolomies, des calcaires, et des conglomérats hématitiques. Par-dessus, en concordance, le membre du centre est composé d'une épaisse séquence de conglomérat grenu, hétérogène et caillouteux. Le membre supérieur comprend de la dolomie karstifiée, du conglomérat et du grès à ciment calcaire, et du calcaire. Dans son ensemble, la Formation de Mount Roosevelt est le reflet de la progradation d'un delta alluvionnaire en un environnement marin de transgression. La déposition a eu lieu dans un bassin contrôlé par des failles actives situé sur la bordure est du graben de Kechika, voisi...

Geophysical investigation and reconstruction of lithospheric structure and its control on geology, structure, and mineralization in the Cordillera of northern Canada and eastern Alaska

“…Geological maps of the Liard Lowlands and Rabbit Plateau (Figure ) at the northern end of the Rocky Mountains and Kechika trough show a change in structural style from dominantly N striking thrusts [ Post and Long , ] in the southeast to ENE to NE striking normal faults [ Ferri et al ., ] to the northwest. The subdued topography and limited geological exposure (<1%) of the Liard Lowlands and Rabbit Plateau is interpreted to be associated with a reduction in weathering resistive carbonates and quartzites north of Terminus Mountain (TM, Figure ) [ Ferri et al ., ].…”

Alluvial facies evolution during the Palaeozoic greening of the continents: case studies, conceptual models and modern analogues

“…Pedogenic horizons were only recorded in the periodically emergent tidal facies of the Herrería Formation. Calcrete was absent from all the successions visited (although it is present in Cambrian successions elsewhere: for example, Post & Long, 2008).…”

“…Calcretes are present (rarely) from the Precambrian to the Ordovician (e.g. Post & Long, 2008) and vertic palaeosols should not be dependent on vegetation provided that enough smectite‐rich clay is available for their formation (Hallsworth & Beckman, 1969; Love & Williams, 2000). The field localities from VS 4 to VS 6 represent subtropical to mid‐latitudes, which may partially account for the increased prevalence of these features.…”

Early Paleozoic post-breakup magmatism along the Cordilleran margin of western North America: New zircon U-Pb age and whole-rock Nd- and Hf-isotope and lithogeochemical results from the Kechika group, Yukon, Canada

“…The Kechika graben of northern British Columbia was a precursor to the Kechika trough (Figs. 1B and 3) that developed during middle to late Cambrian extensional faulting and local uplift (e.g., Douglas et al, 1970;Fritz et al, 1991;Ferri et al, 1999;Post and Long, 2008;Pyle, 2012). Late Cambrian to Early Ordovician extension was associated with regional volcanism in the Ospika embayment and adjacent MacDonald platform (Figs.…”

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