Sedimentology and geomorphology of the modern epsilon cross-stratified point bar deposits in the Athabasca upper delta plain

Abstract

Epsilon cross-stratification (ECS) has been observed in many ancient channel point bar deposits. Investigation of ECS in modern tidal and estuarine systems has been limited in scale and/or detail of investigation. ECS has been documented to exist in only one fluvial system. The search for a modern large scale analogue led to the investigation of the point bar deposits in the fluvially-dominated, freshwater Athabasca Delta in northeastern Alberta. The channels of the upper delta plain are muddy fine-grained and highly sinuous in character with fluctuating discharge. The investigation concentrated on the determination of the overall sedimentological trends in terms of grain size, thickness, variability, continuity and geometry. Proximal to distal transects along point bars were investigated primarily by (1) coring the subaqueous sediments down to a maximum depth of 8.9 m and (2) trenching the subaerial sediments parallel and perpendicular to flow. Two scales of cross-stratification were apparent: (1) very large scale cross-stratification in the form of alternating sand/mud couplets which dipped down the point bar surface perpendicular to channel flow, interpreted to be ECS, and (2) small scale cross-stratification superimposed on the very large scale cross-stratification and deposited parallel to flow. Subaqueous deposits displayed ripple bedding and laminated bedding; subaerial deposits displayed flaser, wavy and lenticular bedding. The variability in thickness of the sand/mud couplets suggests a random cyclic nature. The mud beds were often discontinuous; erosion features and deformation structures were common. A fining downstream grain size trend was indicated by an increase in number and thickness of mud beds from proximal to distal on the point bar. A fining upward trend observed in the vertical sequence of sediments was indicated by an increase in mud in the subaerial sediments. A comparison was made with the existing literature documenting ECS in other depositional environments. A comparison with ECS in a fluvial system showed that t h e two systems had similar lithology and variability. A comparison with ECS deposits observed in tidal and estuarine systems showed contrast; the tidal ECS showed more rhythmic alternations of lithologies, as well as shell material and extensive bioturbation.