SEQUENCE STRATIGRAPHY OF THE UPPER SEKWI FORMATION: IMPLICATIONS FOR THE EARLY CAMBRIAN OF WESTERN LAURENTIA

The Early Cambrian Sekwi Formation in the Selwyn Basin of the Northwest Territories, Canada, represents a well-exposed carbonate platform that records the Early Cambrian transgression of the Cordilleran continental margin. The Sekwi Formation is well-exposed in easterly-directed thrust sheets emplaced during the Late Paleozoic. Ten measured sections were described in a bed-by-bed fashion, examining surfaces and stacking patterns, with several sections up to 1 km in thickness. These detailed measured sections in conjunction with trilobite biostratigraphy and chemostratigraphy enable the construction of a sequence stratigraphic framework across these closely spaced thrust sheets.

The Sekwi Formation is composed of two 2^nd-order cycles that help to differentiate an informal lower and upper part. The lower Sekwi Formation records predominantly deep-water deposition in a basin influenced by syndepositional faulting, whereas the upper Sekwi Formation is composed mostly of shallow-water deposition on a homoclinal ramp with little to no evidence of tectonism. The upper Sekwi Formation, typical of other Lower Cambrian carbonates of western Laurentia, includes a series of facies deposited on either side of a ramp crest composed of ooid shoals and/or algal-archeocyathan mounds that separates the inner ramp from basinward open marine conditions. Deposits of the inner ramp include mixed siliciclastic-carbonate lagoonal sediments, siliciclastic and carbonate tidal flat deposits, and terrestrial siliciclastics. Outer ramp facies include skeletal wackestone/packstone, interbedded shale and calcisiltstone, and black carbonate mudstone. Many of these facies contain distinct trace fossils. The upper Sekwi Formation consists of multiple 3^rd-order sequences consisting of sandy bases, deep ramp transgressive systems tracts capped by shoal water or tidal flat deposits in high-stand systems tracts. 4^th- and 5^th-order high-frequency sequences, or parasequences, are common in the Sekwi Formation but they are difficult to correlate across the platform.