Dune or Dune-Like Cross-Stratification in Deep-Marine Sandstones of the Neoproterozoic Windermere Supergroup, Cariboo Mountains, British Columbia, Canada
Al-Mufti, Omar; Arnott, Bill
Although ubiquitous in fluvial strata, dune cross-stratified sandstone is comparatively rare in the deep-marine sedimentary record. In basin floor deposits of the Neoproterozoic Windermere turbidite system, dune cross-stratified sandstones, although generally uncommon, are most abundant in the axial part of distributary channel fills. Typically, these strata overlie graded structureless sandstone (equivalent of the Bouma Ta division) and consist of well-sorted, coarse-grained sandstone with a distinctive red colour related to a pervasive ferroan dolomite cement. Moreover, these sandstones commonly show only minor compaction, suggesting early (near seabed) cementation. Cross-stratified bed tops typically comprise a single set that generally is of the order of a few up to several decimeters thick. Note that multiple sets are observed also. Set bases show negligible angle of climb, but more commonly angle downward into the underlying Ta part of the bed whereas the top of the set remains more or less horizontal. Of particular note too is that dune cross-stratification commonly transitions laterally into planar-laminated sandstone. Here, individual planar lamina abruptly dip downward forming a single cross-lamina, which with aggradation of the planar laminated units forms a downward thickening (wedge-shaped) dune cross-stratified set in the direction of sediment transport. In some cases the dune cross-stratified unit thins and becomes replaced laterally by planar laminated sandstone, which in many cases terminates abruptly in an overturned fold ("swirl" structure).
The formation of planar and dune cross-stratified units is interpreted to be controlled, not so much by flow conditions, but rather spatial variations in the rheology of the underlying structureless Ta bed, specifically, the resistance of the bed material to withstand loading stresses applied by an aggrading layer of planar laminated sand. Areas of the bed that had sufficiently dewatered supported the weight of the aggrading plane bed layer. In weaker (less dewatered) areas bed-load sediment abruptly foundered and then formed a prograding wedge of cross-stratified sand, superficially resembling dune cross-stratification, that thickened by deepening along its basal contact. The eventual lateral termination of dune-like cross-stratified or planar laminated sand, commonly in the form of swirl structures, suggests the bed was so weak that is was unable to support any bed surface sediment load.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013