Active synsedimentary tectonism on a mixed carbonate-siliciclastic continental margin: third-order sequence stratigraphy of a ramp to basin transition, lower Sekwi Formation, Selwyn Basin, Northwest Territories, Canada

Abstract

The lower part of the Early Cambrian Sekwi Formation in the Selwyn Basin of the Northwest Territories, Canada, is composed of two regional, unconformitybounded sequences, S0 and S1, which record the first widespread carbonate deposition during the initial Palaeozoic transgression onto the western margin of Laurentia. These Early Cambrian sequences are unique to the western North American Cordillera, representing the only record of primarily deep-water deposition on a tectonically active, mixed carbonate-siliciclastic ramp during this period. More specifically, the geometry of the Sekwi ramp changed during deposition of S0 and S1, from a shallowly dipping homoclinal ramp during the S0 transgressive systems tract to a steeply dipping tectonically modified ramp during the early highstand systems tract of S0. The steeply dipping ramp profile of S0 was preserved into the early transgressive systems tract of S1. The Sekwi ramp returned to a gently sloping ramp during the late highstand systems tract of S1 and remained so throughout the remainder of Sekwi deposition. The evolving shape of the Sekwi ramp is attributed to syndepositional 'down to the basin' faulting during deposition of both S0 and S1 and is recorded by: (i) the westward thickening, irregular geometries of S0 and S1; (ii) geographical restriction of deep-water facies (including sediment gravity flow deposits); (iii) the presence of large allochthonous blocks; and (iv) the clast composition of sediment gravity flow deposits. Sediment gravity flow deposits play an unusually important role in the sequence stratigraphic interpretation of the lower Sekwi Formation, as they delineate depositional packages, including the maximum flooding zone, the transitions between portions of systems tracts, and the inferred locations of syntectonic extensional faults. Syntectonic faults increased accommodation basinward of an extensive ooid-shoal complex that developed along the Sekwi ramp crest, greatly influencing sequence geometry and initiating the downslope motion of sediment gravity flows. The syndepositional faulting probably was a continuation of extension that began during the latest Neoproterozoic rifting of western Laurentia. The composition of sediment gravity flow deposits track changing accommodation space on the 513 lower Sekwi ramp and can be used to differentiate systems tracts that probably were related more to tectonism than eustasy.The study area (Fig. 2) is located at the southern end of the Mackenzie Mountains, ca 50 km 514 K. A. Dilliard et al.

Buenellus chilhoweensisn. sp. from the Murray Shale (lower Cambrian Chilhowee Group) of Tennessee, the oldest known trilobite from the Iapetan margin of Laurentia

“…For example, attempts have been (and continue to be) made to correlate the late Dyeran "Hawke Bay Regression" around Laurentia and further afield (e.g., Palmer and James, 1979;Palmer, 1981;Landing et al, 2002Landing et al, , 2006Bordonaro, 2003;Landing, 2012;Geyer and Vincent, 2015;Webster and Landing, 2016). Sequence stratigraphic interpretations and sometimes sea level curves are being developed for Montezuman and lower Dyeran strata of both the Iapetan (Whisonant, 1974;Mack, 1980;James et al, 1989;Barnaby and Read, 1990;Lavoie et al, 2003;Landing, 2007Landing, , 2012Tull et al, 2010;see above) and Cordilleran margins (Hollingsworth 2005(Hollingsworth , 2007(Hollingsworth , 2011Dilliard et al, 2007Dilliard et al, , 2010English and Babcock, 2010;Webster and Bohach, 2014). Discovery of Buenellus chilhoweensis n. sp.…”

The eogenetic karst of the Cambrian Longwangmiao Formation in the Yangtze Platform: Implications for the eustatic and stratigraphic correlations of Toyonian (Cambrian)

“…Worldwide, the regression in the top boundary of the Longwangmiao Formation (Toyonian in Stage 4 of Cambrian) is identified in many places, such as the Appalachians (Barnaby & Read, ), Mackenzie Mountains (Dilliard, Pope, Coniglio, Hasiotis, & Lieberman, ) and Caledonian in Laurentia (Raine & Smith, ), and Scandinavia in Baltoscandia (Nielsen & Schovsbo, ) controlled by multiple signals (for example tectonic, eustatic, and climate). Among those locations, the regressions in Scottish Caledonian in Laurentia and Scandinavia in Baltoscandia are generated by the sea‐level fall in the shallow carbonate shelf (Nielsen & Schovsbo, ; Raine & Smith, ).…”

New information on olenelline trilobites from the Cambrian Sekwi Formation in northwestern Canada

“…1), the Nevadella zone in the Cambrian Series 2, Stage 3, from the Sekwi Formation, Mackenzie Mountains, Northwest Territories, Canada. The material of interest hails from a locality in the Selwyn Basin that has facies of a distinctive lithology relative to other localities in the Sekwi Formation: they are principally red siltstones, whereas the Sekwi Formation regionally is composed mainly of shallow-to-deep water carbonate interbedded with black shale (Fritz 1976a(Fritz , 1976bKrause and Oldershaw 1978;Dilliard et al 2009). The facies appears to represent a proximal, nearshore, shallow-water, peritidal environment (Dilliard et al 2009); this facies is low in diversity, and was not sampled in previous studies of olenelline trilobites from the region.…”