ABSTRACT: Origin of Deltaic Sandbodies within the Lea Park-Belly River Transition in Central Alberta

POWER, BRUCE A., Chevron Canada Resources, Calgary, Alberta, Canada

The Campanian Lea Park-Belly River transition in central Alberta can be subdivided into an allostratigraphic framework containing eight regressive shoreline members.

These shoreline members are dominantly composed of sediments interpreted to be indicative of deltaic depositional environments. The marine portions of Lea Park-Belly River deltaic successions are dominated by structureless or very vaguely laminated fine-grained sandstones. These structureless sandstones do not appear to have a biogenic origin, and they often comprise the dominant portion of thick, waning-flow beds that grade upward at the top into vaguely laminated to well-laminated, low-angle inclined stratified sandstone. Good evidence of wave and/or storm reworking is present at the tops of some beds, but is not abundant.

Sand-body geometries show that the shorelines were characterized by proximal, elongate shore-normal protrusions of thick sandstone surrounded by much broader and thinner lobate areas of sandstone which extend offshore for several tens of kilometers. The structureless waning-flow beds are present in both proximal and distal areas, but evidence of wave-reworking is more abundant in the distal areas.

Existing deltaic models for modern shorelines do not provide adequate explanations for either the abundance of structureless sandstone or for the odd shoreline geometries observed. The presence of the lobate sandstone bodies tens of kilometers offshore appears to indicate significant wave-reworking in the distribution of sediment. However, the facies successions do not support an interpretation of a wave-dominated deltaic system. A non-actualistic interpretation of these sandstone bodies is that they were formed by fluvially dominated deltas in which the dominant method of deposition was by sediment-laden hyperpycnal flood flows which were able to travel long distances from the fluvial feeder channels before depositing their load from suspension as a dominantly structureless bed of sa dstone. Hyperpycnal flood currents such as these have not been observed in modern deltas. Their existence and feasibility to travel the required distances is questionable, but they are presently thought to be the depositional mechanism that best explains both the observed facies successions and sediment body geometries.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)