Abstract

Coal-bearing strata from the Lower Cretaceous upper Mannville Group (south-central Alberta) were investigated in order to evaluate the nature of coal-bearing non-marine to marginal marine sediments developed in an intermediate accommodation setting, located centrally within the Alberta Foreland Basin. Downdip and lateral correlations to the northwest and east link upper Mannville Group strata to the Falher sequences and the Waseca to Lloydminster sequences, respectively, and indicate that a higher order of stratigraphic subdivision, controlled by transgressive–regressive cycles, must also be present in the upper Mannville.

Stratigraphic analysis of the upper Mannville Group in the study area, based on over 1200 km of borehole cross-sections and 50 cores, revealed a number of features that can be considered characteristic of intermediate accommodation in non-marine sediments. These include abundant, compound coal seams and numerous incised valleys with even distribution of sediments between incised valleys and adjacent interfluves. The incised valleys may correlate laterally into horizons within the compound coals, indicating that the coal seams contain sequence boundaries within them and therefore span relative sea level cycles. However, the occurrence of coal seam splits in multiple directions suggests that seam splitting in the upper Mannville Group was at least partially controlled by differential subsidence. This is attributed to fault reactivation on the underlying irregular Paleozoic basement, and maps of inferred fault planes indicate a horst and graben style of extensional faulting. Fault planes appear to be associated with relatively steeply dipping basement topography, and areas of thickest cumulative coal preferentially occur above horst blocks. Although these particular features may be unique to the Mannville Group, they suggest that underlying structures and paleotopography are likely to exert a strong influence on sedimentation patterns in intermediate accommodation settings, because the rates of vertical accretion are insufficient to suppress the effects of differential subsidence. Other expressions of basement control are the localization of single and compound incised valleys along structural lows.

Coal composition analysis is based on photometric and maceral analyses of three upper Mannville Group coals. The aim was to test various methods of identifying small-scale accommodation trends in the coal and use them to identify a characteristic accommodation signature for each coal. The Glauconite, Medicine River and informally named Hackett coals were shown to be significantly more complex than simple ‘transgressive’ or ‘regressive’ style coals. They comprise a number of wetting- and drying-upwards cycles representing repeated episodes of peat deposition under rising or falling accommodation conditions, with or without internal hiatuses between the cycles. These accommodation cycles are driven by changes in groundwater levels that are in turn hydraulically linked to relative sea level, and thus form the basis for identifying a characteristic non-marine sequence stratigraphic style.

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