Depositional dynamics of middle Devonian deposits in the Elk Point Basin: Gilwood Mbr (Watt Mountain Fm), Nipisi Field, northcentral Alberta and Yahatinda Fm (Rocky Mountain front ranges).

摘要

Sedimentologic, pedologic and geochemical data are integrated to interpret the Gilwood Mbr delta system in Nipisi field and its stratigraphic outcrop correlative, the Yahatinda Fm. Five papers are presented with an emphasis on the mudstones and carbonates comprising interfluve paleosols, lacustrine, shallow marine and peritidal deposits. Because paleosols are laterally extensive and mark significant time periods of subaerial exposure, they are useful for correlations, paleogeographic reconstructions and sequence stratigraphic analysis. Proving that these buried paleosols can be used for correlation purposed necessitates determining the conditions and time periods during which they formed. For the Gilwood Mbr this required using relative degradation of organic matter to determine maturity as standard geochemical methods showed paleosols as relatively homogeneous. Enigmatic dolocrete and calcrete within mudstone and sandstone host rock were interpreted as due to the interaction of pedogenic, vadose and phreatic processes. Lateral distribution of carbonates reflect paleogroundwater tables and show that groundwater flow was centripetal, into a fault controlled low within Nipisi field. The faulting associated with this low was related to resurgent tectonism of the Peace River Arch, which was a sediment source and possibly dissolution of salt in the underlying strata. This central low contained an ephemeral lake and created accommodation space for thicker sediments prior to and during Gilwood Mbr deposition. Tectonism, sediment supply and accommodation space within a transgressive systems tract are reflected by four progradational-aggradational parasequence sets overlain by a retrogradational parasequence. Each parasequence is comprised of a coarsening upward sandstone succession overlain by marine mudstones and paleosols. The delta system had a: (1) low slope delta plain, with shallow ephemeral, sandy braided channels; (2) dissipative, shoreline zone; and (3) delta front dominated by hyperpycnal flows. Correlation of paleosol chronosequences within the peritidal deposits of the Yahatinda Fm support its correlation to Gilwood Mbr deposits to the north. Deposition within a transgressive systems tract is supported by the observed sedimentology, however it is dissimilar to predicted idealized models of paleosol chronosequences.