The Effects of Sea Level Changes on the Sedimentology of the Cardium Formation Northeastern Pembina, Alberta

Abstract

<p>Examination of 32 cores and over 100 well logs in Northeastern Pembina allowed 8 facies to be distinguished. The facies within the Raven River Member comprise a shelf sequence which begins with dark bioturbated muddy siltstones, and coarsens upward into pervasively bioturbated muddy sandstones, bioturbated sandstones and nonbioturbated sandstones. The most characteristic sedimentary structure in the nonbioturbated sandstones is llumnlocky cross stratification wtlich is formed during storm reworking of offshore sediments. An erosion surface separates the Raven River Member from the overlying conglomerates of the Carrot Creek Member. Maximum erosion on this surface occurs within a drilling gap present in northeastern Pembina and off the northern boundary of the field. On either side of the drilling gap, facies 7 is present beneath the conglomerates. However within the drilling gap there is a pronounced thinning or complete absence of facies 7. Conglomerates form a very thin veneer on top of the erosion surface which may indicate a second period of erosion after deposition of the conglomerates. Off the northern boundary of the field, the Cardium sequence is partially or completely absent. The origin of the erosion surface may be due to a relative lowering of sea level in the Alberta Basin. This would cause the shoreline to advance many kilometres basinward and a new shoreface to become established on sediments which were previously offshore. Newly incised rivers could then carry sand and gravel to the new shoreface. The Carrot Creek Member is overlain by the Dismal Rat Member, the base of which is characterized by pebbly mudstones that fine upwards into laminated dark mudstones and massive dark mudstones. This sequence may represent a relative rise of sea level within the Alberta basin. Pebbly mudstones may be the result of gravel on top of the erosion surface being reworked by storms into the transgressive muds. The morphology of the datum (used for correlation of well logs) in northeastern Pembina is characterized by undulations and discontinuities. These structures were originally thought to be tectonic. However, the topography mimics that found on the erosion surface above the Raven River Member. The morphology of the datum may represent the draping of sediments on top of the erosion surface above the Raven River Member. Alternatively, the morphology of the datum may reflect erosion on this surface.</p>