Architecture and morphodynamics of a 1·6 Ga fluvial sandstone: Ellice Formation of Elu Basin, Arctic Canada

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Precambrian fluvial deposits have been traditionally described as architecturally simple, forming shallow and wide braidplains with sheet-like geometry. The varied architecture and morphodynamics of the 1·6 Ga Ellice Formation of Elu Basin, Nunavut, Canada, are examined from detailed studies of section and planform exposures along coastal platforms and stepped cliffs. The Ellice Formation overlies older Proterozoic sandstones and Archean crystalline rocks, recording sedimentation in fluvial, aeolian, coastal and nearshore-marine environments. The fluvial deposits display palaeoflow towards the west/north-west, while overlying shallow-marine deposits record transgression towards the east/south-east. The Ellice Formation displays dispersed palaeoflow at its base, and also at higher stratigraphic levels, where fluvial and aeolian deposits are associated. Elsewhere, mainly unimodal palaeoflow points to extensive low-sinuosity fluvial deposition. Within the terrestrial deposits, fluvial, fluvial–aeolian and coastal architectural elements are recognized. Fluvial elements comprise cross-bedded sandstone and minor conglomerate, exhibiting an overall fining-upward trend with associated decrease in preservation, dimension and amalgamation of channel bodies. These motifs are interpreted to portray a shift in depositional environment from proximal trunk rivers to distal alluvial plains. Low-sinuosity fluvial elements are the most common, and include major channel bodies, elongate side bars and mid-channel bars with well-developed scroll topography. High-sinuosity channel-bar complexes exhibit upbar-flow rotation and yield evidence of bar expansion coupled with rotation and translation. Fluvial–aeolian elements are composed of aeolian dunes juxtaposed with isolated channel bodies and bank-attached bars. Minor mixed fluvial–aeolian sheets record local deposition in unconfined settings (possibly floodbasins) or inter-distributary highlands. Finally, coastal elements comprise small deltaic complexes composed of sand-rich distributary-channel bodies feeding heterolithic mouth bars. Overall, the sedimentary record of the Ellice Formation demonstrates an example from the Precambrian where alluvium was locally characterized by a higher geomorphic variability than previously recognized.

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