--> The Turbidites That Never Were: Sedimentological and Petrological Characterization of Fine-Grained Sandstones, Neoproterozoic Isaac Formation, Windermere Supergroup, British Columbia, Canada

AAPG ACE 2018

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The Turbidites That Never Were: Sedimentological and Petrological Characterization of Fine-Grained Sandstones, Neoproterozoic Isaac Formation, Windermere Supergroup, British Columbia, Canada

Abstract

Along the base of passive continental margins, like the Gulf of Mexico, extensive deep-water clastic turbidite systems are present. These deposits are often classified using the classical Bouma turbidite model; a five-layer, upward-fining succession of predictable sedimentary structures and textures commonly observed in strata deposited by turbidity currents. In the Windermere Supergroup, however, in addition to classical turbidites, strata often exhibit a succession of three layers that typically grade vertically into one another. Only rarely does a single bed include all layers. The basal layer (F1) is commonly massive or planar/wavy-stratified, upper very coarse- to lower very fine-grained sandstone that ranges from 1 – 130.5 cm (average 12 cm) thick with low matrix content (silt and clay <10%) and is consistent with the A and B divisions of classical turbidites. It then is overlain gradationally by wavy/cross- or planar-stratified, upper medium- to lower very fine-grained matrix-rich (30-40%) sandstone rhythmically interstratified with claystone containing minor silt (F2). This stratal layer, which ranges from 0.5 – 34.5 cm (average 6 cm) thick, is then capped by a 0.5 – 22.5 cm (average 3.5 cm) thick layer typically composed of three gradationally-bounded parts: matrix-rich (60-70%), subtly planar stratified, very fine-grained sandstone or siltstone, overlain by planar/wavy pinstripe laminated silty mudstone, overlain by massive claystone. F3 commonly stacks to form packages up to 16 layers thick.

These distinctive strata form a unit bounded below by channel deposits and distal levee deposits above. Stratigraphically upward, these strata form 2-5 m thick bedsets in which the basal F1 part fines and thins, the medial F2 portion thins, and the upper F3 part thickens. Also, compared to the cross-stratification in the C-division of classical turbidites, cross-stratification in F2 layers is consistently low-angle or wavy, even though the grain size should support the development of high-angle ripple cross-stratification. Moreover, strata in the overlying F3 layers are ubiquitously planar stratified with well defined, laterally continuous (>100 m) bands of silt and mud, which contrast the typically poorly-sorted, discontinuous nature of laminae that make up the D-division in classical turbidites. Collectively, these differences suggest a physical origin that differs significantly from classical turbidites, and therefore forms the basis of this research.