Jason M. Lavigne1, Murray K. Gingras2, S. George Pemberton3
(1) Talisman Energy, Calgary, AB
(2) University of New Brunswick, Fredricton, NB
(3) University of Alberta, Edmonton, AB
ABSTRACT: Deltaic Distributaries in the Basal Horsehoe Canyon Formation, Alberta, Canada. Autocyclic, Estuarine Channels, Not Incised Valeys
Channel fills in the Horseshoe Canyon Formation near Drumheller, Alberta, demonstrate many of the sedimentological aspects considered to be characteristic of tidally-influenced, marginal marine depositional systems. Previous researchers have interpreted the deposits as being indicative of deltaic processes, multiple incised valley complexes and alternatively, as tidally-influenced estuaries.
At the Hoodoos Recreation Area, the basal section is composed of heterolithic, coarsening upwards, prodeltaic fines. These sediments are sharply overlain by delta front sandstones and this surface has previously been interpreted as an unconformity. The upper portions of the prodelta, however, are characterized by large-scale ball and pillow structures resulting from rapid loading of sand onto a muddy, unconsolidated substrate. This, coupled with the more gradational nature of this surface basinward, suggests that there is no stratigraphic discontinuity at this horizon. In fact, these outcops exemplify the predictable vertical succession of facies reflected in a prograding, tidally-influenced deltaic setting.
In surface-orientated stratigraphic frameworks, caution must be taken not to artificially subdivide genetically-related sequences. While there is a basinward shift in facies across this surface, it represents an autocyclic shift within a deltaic setting. This discounts it as an unconformity candidate.
The delta front sandstones are erosively cut out by a series of tidally-influenced distributary channel deposits. The base of this channel complex is characterized by an intraformational conglomerate. The channel units incise one another and show a successive decrease in marine influence to the north (in the direction of younging) and reflect positions relatively further from the eastward-prograding shoreline.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado