ABSTRACT
Seven major erosion (E) surfaces are found within the 100-m-thick Cardium Formation in the subsurface of Alberta. Erosion surface E5 has up to 20 m of erosional relief in the Carrot Creek area, and is cut into originally horizontal open-marine sandstones and mudstones. The E5 surface has been correlated into the Pembina area by construction of a structure map based on 2470 resistivity logs. The E5 surface is characterized by a series of sub-parallel, southeast-trending steps, with lower areas between the steps. This topography is interpreted to have formed during erosional shoreface retreat in a period of alternating transgression and stillstand of relative sea level. Tectonic tilting of the basin floor is also suggested by the topography of the erosion surface; the tilting is related to the accretion of exotic terranes and loading the continental margin. The erosion surface is overlain by a continuous veneer of conglomerate interpreted as a transgressive lag. This conglomerate thickens to 20 m in the lower areas at Carrot Creek, where it is interpreted as a shoreface deposit rather than an offshore bar. These shoreface conglomerates form important reservoirs. The veneer also thickens to about 9 m in parts of Pembina, but the thick conglomerates occur not in the lows but on top of the steps. Reconstruction of the geometry of the steps indicates that at times of unusually high sediment supply during transgression, conglomerates were deposited as local bars in the lower shoreface.
The sandstones and mudstones below the erosion surface at Pembina shale out northeastward, partly forming a stratigraphic trap. However, these sediments are also truncated by the E5 erosion surface and buried by transgressive mudstones; these mudstones contribute significantly to updip stratigraphic trapping. The long, narrow sand body trends mapped in Pembina and adjacent fields are due to the erosive dissection of an originally extensive and continuous sheet of interbedded sandstones and mudstones, and not, as has been suggested, to an offshore bar origin for the fields.
