REVISED STRATIGRAPHY OF THE LOWER COLORADO GROUP (ALBIAN TO TURONIAN), WESTERN CANADA

The sedimentology, biostratigraphy and geochemistry of the Albian to m iddle (?) Turonian interval of the Colorado Group in Western Canada in dicate that four, regionally mappable lithostratigraphic units are pre sent in this marine shale succession. In ascending order, these are: t he Westgate Formation, the Fish Scales Formation, the Belle Fourche Fo rmation and the Second White Specks Formation. The Westgate Formation is a laminated-to-bioturbated, heterolithic mudstone to siltstone whic h thickens from 20 m on the Manitoba-Saskatchewan border to over 120 m in northwestern Alberta. This formation contains an agglutinated fora miniferal assemblage characteristic of the late Albian Miliammina mani tobensis Zone. The total organic carbon (TOC) is dominantly Type III ( terrestrially derived) organic matter and comprises 2 wt% or less of t he rock mass. The Fish Scales Formation is less than 20 m thick and co nsists of mudstone to claystone with associated sandstone and conglome rate beds. Bioturbation is sparse to absent and foraminifera are not p resent; the age is bracketed as Early Cenomanian. TOC abundances are v ariable up to 8 wt% and comprise a mixture of Types II and III. The Be lle Fourche Formation is a non- to slightly-calcareous mudstone to sil tstone which thickens westward from 20 m to 150 m. Bioturbation is spa rse to moderate and in general, grain size coarsens upward culminating in persistent, thin, siltstone to fine-grained sandstones near the to p. The foraminiferal assemblage is dominantly agglutinated and charact eristic of the middle Cenomanian Verneuilinoides perplexus Zone. TOC a bundances are generally less than 2 wt% dominantly consisting of Type III organic matter, but more elevated TOC values and Type II organic m atter occur in facies transitional into the overlying Second White Spe cks Formation. The Second White Specks Formation (SWS) is a non-biotur bated, calcareous, organic-rich claystone to siltstone. The SWS thicke ns from about 25 m in eastern Saskatchewan to over 90 m in northwester n Alberta with local thickening to 55 m in southeastern Alberta. This unit is distinctive due, primarily, to the abundance and diversity of marine bioclasts, the foraminiferal assemblage, the predominance of Ty pe II organic matter, and the high TOC content (up to 12 wt%). The dom inantly planktonic assemblage is characteristic of the late Cenomanian to Turonian Hedbergella loetterlei Zone. Another characteristic of th e SWS is the common occurrence of nannofossils, primarily coccoliths.