The coal-bearing strata of the Lower Cretaceous Mannville Group (Western Canadian Sedimentary Basin, South Central Alberta), PART 2: Factors controlling the composition of organic matter accumulations

Abstract

This work analyzes the distribution of coal-bearing strata in the stratigraphic framework of the Mannville Group (Lower Cretaceous, Western Canada Sedimentary Basin). In the 3rd-order depositional sequence the maximum abundance of coal layers is found in the highstand system tracts (HST), whereas in the 4th-order eustatic sequences, coal deposits distribute similarly in the highstand and transgressive system tracts (TST). These trends have been related to the variation of accommodation space in the basin, which mostly influence the occurrence of favorable conditions for organic matter preservation.Petrographic analyses permit to identify variation in the coal layers composition and to relate them with the stratigraphic dynamic of the basin. The HST is characterized by a low subsidence rate, which induces low levels of the groundwater table, favoring dry conditions in the peat. By consequence, in the HST a higher abundance of inertinite maceral group is observed than in the TST. On the other hand, the TST is characterized by higher subsidence rates, thus more stable groundwater table levels, promoting the organic matter preservation. Therefore, in the TST a higher content of vitrinite and liptinite maceral groups is observed. In correspondence of the maximum flooding surface (MFS), the peat is frequently flooded, with the consequent dilution of the organic matter with terrigeneous sediment, forming carbonaceous shales deposits.By the analysis of the organic matter composition, seven depositional environments for coal were identified, located at different distance from the coast line. Each depositional environment is characterized by a specific organo-facies association, which is the reflex of different environmental conditions in the peat, in terms of water table level, type of vegetation and hydrodynamic processes. The vertical superimposition in a coal package of different depositional environments has been related to high order climatic cyclicity (5th-order transgressive and regressive cycles), which controls the groundwater table fluctuations.