| Description |
The environments of deposition of two major coal seams and t h e ir enveloping rock strata at the Trapper Mine near Craig , Colorado, are delineated. The strip mine takes a 4 foot (1.2 m) thick seam and an 8 foot (2.4 m) thick seam from the upper member of the Williams Fork Formation (Upper Cretaceous / late Campanian-early Maestrichtian ). The data for this study are a combination of 208 exploration or production drill holes located throughout the mine and 13 measured sections of one strip pit's high wall . The area of investigation is approximately 5 square miles (13 square km) and encompasses the entire mine. Environments were deduced from the combined study of facies relationships, petrographic and paleontologic data, coal characteristics, geophysical bore hole logs, and geometry and lithology of the coal-bearing strata. The environments observed in this study were swamp, marsh, pond, crevasse splay, levee and distributary channel. The coal-bearing rocks were deposited in an interdeltaic swamp belt complex situated between a more landward fluvial coastal plain belt and a more seaward marsh and lagoon belt. Coal beds developed from extensive peat swamps which thrived in the warm temperate to subtropical paleoclimate. The other rock bodies represent influxes of clastic river-derived sediments which locally interrupted peat-forming conditions at several times. Later, when the channel complexes shifted to adjacent regions due to avulsions upstream, local swamp conditions became re-established with concomitant renewed peat development. Isopach maps of the coal seams and isopach lithofacies maps of the rock interburden between coal seams were produced to show coal thickness trends and paleochannel systems. The coal seam geometry was found to be controlled by the interburden rock. When the maps are analyzed sequentially in ascending stratigraphic order, the significant effect of differential compaction is readily apparent: thicker portions of a coal seam overlie previous paleomarsh/pond environments (thin carbonaceous clay stone units), while thinner portions of a coal seam overlie previous paleochannel/crevasse splay environments (thick sandstone units). Knowledge o f the environments of deposition of coal-bearing strata is important to the coal geologist as it can aid in the prediction of coal seam correlation, geometry and quality. The variations and trends in coal seam and interburden thicknesses seen here can be best understood through application of depositional analysis. Results may benefit surface mining operations by developing insight into local conditions of coal wants, coal rolls, aquifer geometry, and high wall stability. Underground mining may benefit from knowledge of roof and floor conditions, coal wants, rolls and splits, and the potential minability of coal seams. |
