Abstract

The Lower Colorado Group (Late Albian–earliest Cenomanian) has been allostratigraphically divided on the basis of regional unconformities and transgressive surfaces, resulting in recognition of an informal Lower Colorado allogroup, comprising the Paddy, Joli Fou, Viking, Westgate and Fish Scales alloformations. The Paddy alloformation forms an eastward-thinning wedge up to 125 m thick, composed of nine allomembers that progressively onlap the basal unconformity PE0 from west to east. Paddy rocks are mainly alluvial in the west, grading into marginal marine in the east and north. Paleo valley-fills are present at the tops of most allomembers. The Joli Fou alloformation transgressively overlies nonmarine Mannville Group rocks, and forms a relatively sheet-like blanket (average 20 m) of marine mudstone that coarsens in the north, where it is assigned to the lithostratigraphic Viking Formation, and in the far south, where it is assigned to part of the lithostratigraphic Bow Island Formation. The Viking alloformation erosively overlies the Joli Fou alloformation at surface VE0 and consists of regional allomembers VA, VB and VD, separated by unconformities VE0, VE1, VE3 and VE4. Allomembers VA and VB (mean 30 m thick) are intensely bioturbated shallow marine silty fine sandstones whereas allomember VD is weakly bioturbated, and includes sandy shoreface deposits in the SW and thick (>50 m) offshore marine mudstone in the north (part of the lithostratigraphic Hasler Formation). Allomember VC is confined to paleovalley deposits below VE3. All regional Viking allomembers can be traced into the lower Bow Island Formation in the south. Marine mudstones of the Westgate alloformation form a wedge thinning from >600 m in the NW to <40 m in the far south, comprising informal units WA, WB and WC from base to top. Westgate strata onlap southward onto VE4 such that only unit WC persists to southern Alberta, where it passes into marginal marine facies of the middle and upper Bow Island Formation. The Fish Scales alloformation (earliest Cenomanian) erosively overlies the Westgate alloformation at surface FE1 and comprises two units FA and FB, the former being confined to a depocentre in the NW. The Fish Scales alloformation is characterized by abrupt introduction of fine sand into the basin and sea-floor erosion which formed uranium-enriched phosphatic lags which give a characteristic highly radioactive log signature. An absence of benthic fauna and high organic content indicate deposition below anoxic water. The top of the Fish Scales alloformation is the Fish Scales Upper (FSU) marker which is a highly radioactive condensed section and downlap surface below prograding clinothems of the early–mid Cenomanian Dunvegan alloformation. Allomember FB is locally coarse-grained in the far south, forming the lower part of the Barons Sandstone, whereas the upper Barons is fine grained and equivalent to allomember C of the Dunvegan Formation.

The Paddy alloformation is entirely, or almost entirely older than the Joli Fou alloformation, and hence is much older than the Viking alloformation, with which it is traditionally equated. The new allostratigraphic correlations make it much easier to understand temporal and spatial relationships between the many lithostratigraphic components of the Lower Colorado Group, and of equivalent strata exposed in the Rocky Mountain Foothills. The new allostratigraphy reveals a complex interplay between eustatic changes and more localized flexural tectonic events.

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