Geomorphology and sedimentology of the lower delta plain and subaqueous delta plain of the Athabasca River Delta, N.E. Alberta

Abstract

The Athabasca lower delta plain and subaqueous delta plain (lower Athabasca Delta) are mainly a product of fluvial influence and a shallow receiving basin. The geomorphology of the lower Athabasca Delta reflects these conditions, and is characterized by bifurcating, low-sinuosity distributary channels, muddy interdistributary basins, distributary mouth bars and a gently sloping offshore profile. The major sandy bodies within the lower Athabasca Delta are the product of deposition within the distributary mouth bar, the crevasse-splay and the infilling distributary channel. Distributary mouth bars form thin (less than 4 m), lenticular bodies which tend to coalesce and produce a semi-continuous sandy sheet along the delta front. Crevasse-splays similarly form lenticular bodies of coarse material, however, they are smaller in scale than distributary mouth bars and tend not to merge laterally with each other. Distributary channel-fills tend to consist of very clean sands, and may exceed 5.5 m in thickness. They form a bifurcating to anastomosing network, and display a broad Ushaped cross-section which may be up to 600 m wide. Channel infilling occurs due to vertical aggradation as well as through the process of limited lateral accretion. Vertical aggradation is due to bedfonn migration along the channel thalweg, and produces a uniform sand sequence. Limited lateral accretion involves the process of step-wise bar formation, and occurs diagonally as opposed to strictly vertically or laterally. This style of infilling produces three types of vertical sequence: rhythmic sand, mud and heterolithic.