Geology of the Princeton Basin (NTS 092H)
Paper 1983 - 3
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Paper 1983-3 keys on the coal and industrial mineral resources of the Princeton Basin in south-central British Columbia.
The basin is a half graben bounded on its eastern margin by a north-northeasterly trending, west-dipping extension fault that has a minimum stratigraphic separation in the order of 1400 metres. The basin is filled with Paleogene strata of the Princeton Group. The basal strata consist of flows and related volcaniclastic rocks of the Lower Volcanic Formation that attain a maximum aggregate thickness of 1370 metres. These are overlain by a minimum 1700-metre thickness of coal-bearing, nonmarine terrigenous clastic and lesser volcaniclastic sedimentary rocks of the Middle Eocene Allenby Formation.
The Allenby Formation is informally subdivided into three members. Flows and tuffs that are intercalated predominantly with basal sedimentary strata of the Allenby Formation are designated as the ‘volcanic member.’ Sedimentary strata of the Allenby Formation that outcrop in the southern part of the basin can be subdivided on the basis of coal occurrence and proportion of volcaniclastic material into a lower clastic and an upper coal-bearing member The four coal zones of historical economic importance (Princeton-Black-Blue Flame, Pleasant Valley-Jackson, Gem-Bromley Vale, Golden Glow) occur in the basal 530 metres of the ‘coal-bearing member.’ Other coal zones (Freeman, Allenby, Bethlehem) occur in the upper middle part of the preserved ‘coal-bearing member.’ Strata in the northern part of the basin appear to be a coarser and thicker lateral facies equivalent of the ‘lower member’ in the south, although precise correlation is uncertain.
Strata of the Allenby Formation have been deposited primarily in fluvial environments. The coarser northern facies probably represents a higher energy braided river depositional environment, whereas the finer southern facies probably represents a lower gradient, meandering river environment in which related backswamp, overbank/floodplain, and lacustrine environments were common. The coal probably accumulated as vegetative matter in peat (?) swamps in these latter environments. The distribution of facies, the variation in maximum clast size, the orientation of paleocurrent indicators, and the sediment provenance suggest that sedimentary strata of the Allenby Formation were derived largely from a terrane that lay to the north-northeast of the Princeton Basin, and that the Osprey Lake intrusive body supplied much of the arkosic detritus. Volcanic detritus was locally derived.
The Princeton Basin is shallower in the north than in the south. Strata of the Allenby Formation comprise a homoclinal, east-dipping panel in the northern part of the basin, whereas they outline a complex, deeper basin in the south that has been modified by faulting and folding. Gravity profiles across the basin outline variations in preserved thickness of Paleogene sediment fill, in places in excess of 1200 metres, and clearly delineate anomalies associated with the known coal zones.
Historical coal production in the Princeton Basin is in the order of 1.8 million tonnes (2 million tons) of a subbituminous product. The coal seams exhibited marked lateral variability in thickness, character, and quality. The mining operations were often plagued with economic and geologic difficulties. Nevertheless, the Princeton Basin still contains a substantial coal resource that is probably two orders of magnitude greater than that previously mined. Stratigraphic and structural relationships within the Allenby Formation show that the southwestern part of the basin is the most promising area for future coal exploration.
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