Geology and depositional setting of the late cretaceous, upper Bearpaw and lower Horseshoe Canyon formations in the Dodds-Round Hill coalfield of central Alberta : a computer-based study of closely-spaced exploration data
Authors: Hughes J D 
Published: July 2015

Coal-bearing strata of the Late Cretaceous upper Bearpaw and lower Horseshoe Canyon formations in the DoddsRound Hill area of central Alberta are examined in this report, using exploration dat&amp;lt;1 comprising geophysical logs, driller's logs and
analytical information from 1349 boreholes drilled on 400 to 1600 m centres, throughout an area of 440 km 2 These data were manipulated by computer to determine the subsurface distribution and geometry of coal seams, marker units, and lithologic units within interseam intervals. The succession at
Dodds-Round Hill grades upward from shallow marine and brackish strata of the upper Bearpaw Formation to predominantly nonmarine strata of the lower Horseshoe Canyon Formation. Deposition of the lower part of the succession occurred within northeasterly-prograding, fluvially-dominated delta
complexes along the western margin of the Bearpaw Sea. Overlying nonmarine strata were deposited in fluvial-alluvial plain settings as these delta systems migrated farther northeast and east. The succession has been subdivided into several depositional subenvironments based on the geometry,
lithologic composition and vertical and lateral lithologic relationships within specific stratigraphic intervals. Shallow marine, prodelta, distributary mouth bar, distributary channel, interdistributary bay, beach, fluvial channel, floodplain and swamp-marsh subenvironments are recognized. Deltaic
complexes comprise distributary mouth bar and channel sandstone units, which grade downward into prodelta and shallow marine deposits, upward into swamp-marsh or interdistributary bay deposits, and laterally into prodelta or interdistributary bay deposits. These sandstone units comprise
northeast-trending, linear bodies five to ten metres thick, three to six kilometres wide and more than twenty-five kilometres in length. Fluvial-alluvial plain deposits comprise linear, fining-upward sandstone units deposited within meandering fluvial channels and, laterally adjacent, finer grained
floodplain deposits which accumulated during overbank flood events. Fluvial channel sandstones comprise northeast- and east-trending units five to ten metres thick, 500 to 2000 m wide and more than 15 km in length. Coal seams within the succession accumulated within interdistributary and
interchannel areas in swamp-marsh subenvironments that were removed from active sedimentation by overbank flooding. Individual seams are variable in thickness and may coalesce laterally with other seams or pinch out. Seams with coal thicknesses of more than two metres are restricted to areas of 100
km 2 or less. Groups of coal seams, or coal zones, cover areas in excess of 400 km 2 Interseam strata within coal zones commonly comprise wedge-shaped partings, interpreted as overbankfloodplain deposits derived from source channels located in their direction of thickening. Other partings, which
are thin, bentonitic and laterally very extensive, may represent altered volcanic ash deposits. Silicification associated with the alteration of volcanic ash to bentonite may be related to a laterally extensive concretionary horizon within the Round Hill Coal Zone. The geometry of interseam elastic
sediments and hence the configuration of bounding coal seams is strongly related to the orientation and position of depositional subenvironments, which in turn are related to paleoslope direction. Paleoflow in the study area was to the northeast and east. Differential compaction between laterally
adjacent subenvironments has, in several instances, greatly influenced sedimentation patterns and the distribution of subenvironments in overlying stratigraphic intervals, and also has influenced the present configuration of coal seams. These effects are most pronounced between the linear,
distributary mouth bar and fluvial channel sandstones, and laterally adjacent, finer grained, more compactible strata. Cyclicity is evident on several scales within the succession. Small-scale vertical lithologic cycles related to specific subenvironments may reflect phenomena with periodicities of
days to several hundred years. Larger scale cycles, representing considerably longer time periods, comprise groups of subenvironments. These larger cycles include three constructional-destructional phases of delta development. Knowledge of the geometry, distribution and vertical and lateral
lithologic relationships associated with depositional subenvironments in the succession has implications for coal exploration in laterally equivalent strata. For example, borehole control necessary to adequately define the geometry of coal seams separated by fluvial channel deposits may have to be
400 m or less, whereas seams separated by more tabular, overbank-floodplain or interdistributary bay-fill deposits may be adequately defined with a much broader spacing. This knowledge is also important later in the development process for mining feasibility, geotechnical and environmental
considerations