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Distinction of Marine from Alluvial Facies in the Paleoproterozoic (1.9 Ga) Burnside Formation, Kilohigok Basin, N.W.T., Canada
David S. McCormick (1), John P. Grotzinger
The Burnside Formation records the shift from marine-shelf to largely alluvial conditions in the Paleoproterozoic ( 1.9 Ga) Kilohigok foreland basin in the northwest Canadian Shield. The Burnside is a thick (up to 3.5 km), northwest-tapering siliciclastic wedge representing a sandy braid-delta and braided-river system that prograded northwest across the Archean Slave craton. Criteria are developed to distinguish shallow-marine facies from alluvial facies allowing construction of a stratigraphic framework for this unfossiliferous, alluvially dominated succession.
The sandy alluvial system prograded over a storm-influenced fine-grained siliciclastic shelf. Shelf associations can be divided into siltstone-dominated and mixed siltstone-sandstone. The siltstone-dominated association comprises facies ranging from mudstones deposited below storm-wave influence to siltstones deposited under storm and fair-weather wave conditions. Mixed siltstone-sandstone facies form coarsening-upward sequences on the scale of several meters to a few tens of meters that appear to represent prograding marine sand bars. Initial alluvial influence is manifested by associations of mixed siltstone and sandstone with shallow-water wave-formed and current-formed structures, abundant soft-sediment deformation, and isolated meter-size channels filled with trough-cross-bedded andstone and associated rip-up clasts. Thick (tens to hundreds of meters) tabular-bedded fine sandstones represent a delta-platform environment between coarser-grained trough-cross-bedded braid-delta distributary channels. Unidirectional paleocurrents in the transitional alluvial facies indicate predominantly west to southwest transport.
The bulk of the Burnside comprises several associations representing sandy alluvial braid-plain facies. Conglomeratic lithologies are subordinate to sandy ones and are restricted largely to the proximal (southeastern) part of the basin. Braid-plain facies, occupying hundreds of meters of section, consist mainly of hierarchies of trough cross-bedding on scales of a few meters to many tens of meters. Unidirectional paleocurrents from the main alluvial associations cluster with a single narrow west-northwest mode.
Associations of sedimentary facies and paleocurrents indicate that initially the alluvial system was restricted to the proximal part of the basin and drained longitudinally between the orogenic hinterland and a synsedimentary flexural arch created by thrust loading. Contemporaneous shallow-marine-shelf facies were present to the northwest over and beyond the flexural arch. An abrupt shift to transverse drainage and the superposition of medial-braid-plain facies directly on marine-shelf and deltaic facies in the foreland mark an abrupt, unconformable contact representing the progradation of the alluvial system over the cratonic foreland. The area of maximum disparity between marine and nonmarine facies coincides with the area of greatest stratigraphic thinning and erosion, and marks th crest of the flexural arch. Only the transverse paleocurrent mode is recorded over the flexural arch and in the distal foreland. Stratigraphic relations suggest that marginal-marine and braid-delta facies tracts in the distal foreland shifted abruptly over large distances and were sensitive to changes in relative sea level. In contrast, braid-plain facies tracts shifted very little, and more gradually. The large lateral extent of alluvial facies suggests that the source area had a humid or monsoon climate.
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