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Architectural Variability of a Deep-Water Slope Channel Complex Above a Sequence Boundary, Isaac Formation, Neoproterozoic Windermere Supergroup, Southern Canadian Cordillera

Abstract

Isaac Channel 1 (ICC1) is one of at least seven leveed slope channel complex sets in the Neoproterozoic Windermere Supergroup at the Castle Creek study area. ICC1 is up to 250 m thick and exposed over 3km along strike, overlies a 200 m-thick, mixed carbonate-siliciclastic succession and consist of two vertically-stacked channel complexes (ICC1-L and ICC1-U) filled with siliciclastic strata marking a major eustatic lowstand and hinterland sediment delivery. Detailed bed-by-bed logging and mapping using drone photomosaics helped document and analyze lithofacies, facies associations, stratal geometries and stacking patterns for each complex. In ICC1-U, for example, which is the best exposed and up to 120 m-thick, three stratal elements are recognized: channel fills, levees, and channel abandonment. Channel-fill strata consist of thick-bedded, granule conglomerate to coarse-grained sandstone, mudstone-clast breccia, cross-stratified sandstone, and minor thin-bedded, interstratified sandstone-mudstone. In contrast, levees are dominated by thin-bedded, medium-and fine-grained sandstone interbedded with mudstone (Tbcd and Tcde turbidites). Channel-abandonment deposits overlie the top of ICC1-U and are composed of laterally continuous, thin-bedded, fine-grained Tcde turbidites. ICC1-U comprises three flat-based, up to 30 m-thick channel units: lower, middle and upper. The lower and middle channels units comprise numerous highly-amalgamated channel fills with common cut-and-fill features. The upper channel unit, on the other hand, is composed of multiple laterally-offset-stacked channels filled with coarse-grained strata that interfinger abruptly and obliquely upward with thin-bedded turbidites. The upper most part of ICC1-U is up to 30 m-thick, fine grained siliciclastic succession representing the abandonment of ICC1 capped by an ~1m-thick calciturbidite unit. Vertical changes in channel architecture of ICC1-U is interpreted to reflect differences in flow conditions and variability in channel confinement. The lower and middle channel units formed when sedimentation rate exceeded accommodation space due to topographical confinement and caused sand-rich deposits to amalgamate with a disorganized and nested stacking pattern. In contrast, the upper channel unit exhibits well-developed lateral-accretion deposits suggesting that through time the channel transformed to a more sinuous platform which was confined instead by (depositional) levees.