Geology of the Hat Creek Coal Deposits

CIM Bulletin, Vol. 70, No. 782, 1977

D. D. Campbell, President, L. T. Jory, Vice-President, and C. R. Saunders, Secretary, Dolmage Campbell & Associates Ltd., Vancouver, B.C.

The Hat Creek coal deposits occur within folded and faulted Tertiary (Eocene) strata of the Coldwater Group, which comprises weakly lithified shales, siltstone and conglomerates underlying the floor and lower flanks of the valley of Upper Hat Creek. This upland valley, 16 miles in length and 2-4 miles in width, is located within the eastern foothills of the Coast, Mountains, 120 miles northeast of Vancouver, approximately midway between the towns of Lillooet and Ashcroft in the Interior Dry Belt of British Columbia. Most of the valley floor is covered by till and glacial outwash deposits ranging up to 500 feet in depth. Because of this overburden cover, the majority of the data on the coal deposit have been obtained from diamond drilling done during 1974-76 by the property owner, B.C. Hydro and Power Authority. Because of the limitations imposed by the lack of outcrop, the Tertiary geology at Upper Hat Creek is imperfectly known; however, it appears from correlation with other coal-bearing Tertiary remnants in the southern interior of the province that these remnants, including Upper Hat Creek, may represent preserved segments of the western shoreline of a Tertiary continental sea along which deltaic swamps developed and were sustained for unusually long stable periods, giving rise to the world's thickest known coal beds. The coal-bearing Tertiary rocks underlying the valley of Upper Hat Creek have been faulted down into the adjacent older formations as a graben structure which has been extensively dislocated in turn by steeply dipping longitudinal (N-S) and cross (E-W) block faults. Despite this dislocation, the principal coal layer has been traced by drilling, with local displacements, along much of the length of the valley. The main coal layer ranges in true width from 1000 to 1800 feet, of which approximately 30 per cent consists of inter-bedded waste rock. The No. 1 Deposit, at the north end of the valley, is currently being explored as an open-pit source of feed for a 2000-MW thermal-electric generating plant. The plant-feed coal averages about 28 per cent ash and 6000 Btu/lb, at 20 per cent moisture, and is sub-bituminous B to C in rank. The No. 1 Deposit is estimated to contain approximately 725 million plant-feed short tons. The inferred coal resources south of the No. 1 Deposit may exceed 10 billion tons occurring as a coal layer about 1600 feet in thickness. The layer is gently folded and block faulted and most of it lies beneath 600 feet or more of cover.

Keywords: Coal deposits, Hat Creek coal deposit, Cold-water Group, Tertiary geology, Structural geology, Drilling data, Thermal coal.