Coal facies interpretations based on lithotype and maceral variations in Lower Cretaceous (Gates Formation) coals of Western Canada

Abstract

Maceral analyses of lithotype samples from Lower Cretaceous coal seams have been used to define compositional characteristics of the various lithotypes and to assess coal facies changes within the seams based on maceral and lithotype variations. The mean petrographic compositions of the lithotypes show a general trend to decreasing vitrinite, increasing inertinite and increasing mineral matter contents from the bright to dull varieties. The bright lithotypes are considered to have been formed in wet and densely forested swamps, whereas the banded lithotypes were formed under somewhat drier conditions. Within the dull lithotype two distinct types were recognized based on differences in petrographic composition and the occurrence or absence of facies diagnostic macerals. It is suggested that these differences are due to formation in distinct mire facies, under the influence of open moor or raised mire conditions. The lithotype variations within the seams, together with the data on their petrographic composition, were used to define facies changes, which occurred during accumulation of the peats. Three types of seam development are recognized: (a) seams containing a high proportion of clastic beds indicative of frequent floodings of the swamps. These seams are characterized by the dominance of oscillatory sequences of bright lithotypes and dulling upward sequences below clastic beds. The brighter lithotypes were formed in forested swamps and fens, whereas for the duller sequences influences of open moor conditions are indicated; (b) seams containing only minor thin clastic beds indicating relative stability of water levels during formation of the peat. These seams are characterized by the predominance of duller lithotypes with common repetitions of dulling upward sequences. The dulling up sequences indicate transitions from wet and very wet forest swamps to drier conditions with lower tree density; (c) transitional seams characterized by a few clastic beds. These seams formed under intermediate conditions of stability in respect to flooding events. The seams are characterized by the predominance of banded lithotypes, dull lithotypes near partings and brightening and dulling up sequences. The brightening up sequences represent transitions from dry forest swamp to wet and very wet forest swamps, while a reverse trend is indicated for the dulling up sequences. The overall petrographic characteristics of full seam sections indicate that the seams are in general relatively low in vitrinite and relatively rich in inertinite macerals. Liptinite content is negligible and mineral matter contents are low. Based on petrographic indices obtained from facies diagnostic macerals, an accumulation of the ancient peats under prevailing wet to very wet forest swamps is suggested. Vicinity to more open moor conditions is indicated for one seam by the higher input of dispersed macerals.