Sedimentology and fossil-fuel potential of the Upper Carboniferous Barachois group, western Newfoundland

Solomon, Steven M. (1986) Sedimentology and fossil-fuel potential of the Upper Carboniferous Barachois group, western Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The coal-bearing Upper Carboniferous Barachois Group of western Newfoundland consists of the youngest known rocks in the St. George's Bay Lowlands. These lowlands are part of the Bay St. George Subbasin, which, in turn, is part of a network of Carboniferous depositional areas in eastern Canada. A detailed sedimentological study of the Barachois Group was undertaken as part of a more general study (involving geology and geophysics) to assess the fossil-fuel potential of the western Newfoundland Carboniferous subbasins. -- Cores and exposures of the Barachois Group in the St. George's Bay Lowlands define a lower coarse unit and an upper fine unit. The coarse unit consists of alternating red-brown mudstone-dominated units and grey sandstone and conglomerate-dominated units. The fine unit consists of thick grey or red-brown mudstone-dominated units with interspersed sandstone units. Eleven facies (A to K) are recognized in the two units. The facies are organized into three facies associations and eight subassociations. Facies association I includes fluvial channel sandstones and conglomerates found in outcrops and the BSG£1 core (subassociation IA) and channel and minor sheetflood deposits found in the FB2-76 core (subassociation IB). Facies association II comprises sandstones and mudstones deposited in proximal crevasse channel and levee environments (subassociation IIA), in undifferentiated crevasse splay, levee, and floodplain environments (subassociation IIB), and by progradation of crevasse splay, levee, and delta environments (subassociation IIC). Facies association III includes mudstone-dominated subassociations which represent the deposits of floodplain lakes (subassociation IIIA), poorly-drained swamps (subassociation IIIB), and well-drained swamps (subassociation IIIC). -- Sandstones of the Barachois Group are classified predominantly as arkose and subarkose, with up to 65% feldspar. The sandstones include a variety of authigenic minerals such as kaolinitie, chlorite, illite, calcite, and silica. Sandstone mineralogy does not identify any distinctive provenance. Potential sources of the arkoses could have included several of the presently exposed igneous intrusive complexes in the Long Range Mountains located southeast, east, and north of the study area. -- X-ray diffraction of the <2 micrometer fraction shows that mudstones contain illite, chlorite, kaolinite and mixed-layer clay minerals. Illite predominates in non-grey mudstone; chlorite is the most abundant clay mineral in grey mudstones. Variations in clay mineralogy are due to a combination of initial detrital mineralogy and relative stability of clay minerals through a variety of depositional and diagenetic environments. -- Kerogen types in the Barachois Group are dominated by woody (type III) kerogen with lesser amounts of amorphous and herbaceous types. Thermal maturation indices and kerogen type indicate that mudstones have a fair gas potential, but potential reservoir rocks exhibit poor to fair porosity. Oil shales are lean and coals exhibit considerable variation both between and within seams in terms of ash and sulfur contents. Oil shales and coals appear to be of limited lateral extent and coals are commonly faulted.

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