The Viking Formation in the Caroline, Garrington and Harmattan East Fields, Western South-Central Alberta: Sedimentology and Paleogeography

ABSTRACT The shallow-marine Viking Formation of western south-central Alberta is characterized by a coarsening-upward sequence, 20 to 35 m thick, of: (1) interbedded, bioturbated and loaded fine-grained sandstone and shale (20-25 m thick); (2) fine-grained sandstone (6-12 m thick) with low-angle, inclined cross-stratification; and (3) chert-pebble conglomerate and pebbly sandstone (0-4 m thick), most of which is massive and matrix-supported. Within the over-all coarsening-upward sequence of the Viking, the deposits in the Caroline, Garrington and Harmattan East fields record three distinct styles of shallow-marine sedimentation. The Caroline field represents the initial construction and progradation of a clastic wedge associated with a shoreline. Following this was a period of incision and infill of shelf-valleys that dissected the clastic wedge as shown by the Harmattan East field. Finally, sheet-like sandstones and conglomerates were deposited in the Caroline and Garrington fields, some of which have been reworked into ridge-and-swale deposits, 5 to 10 m thick, at Garrington. The first phase was associated with a progradational regression, at a time when sedimentation rates were greater than subsidence. The second phase of channel cut-and-fill was concomitant with a low-stand in the world-wide sea level and also with increased subsidence within the western Canada sedimentary basin, corresponding to tectonic loading of the margin by underplating in the Cordilleran foreland. The third phase of reworking occurred during a time of still-stand and rising sea level, associated with diminished tectonic activity and depleted sediment supply. LA FORMATION VIKING AUX CHAMPS HOUILLERS CAROLINE, GARRINGTON ET HARMATTAN EAST, ALBERTA CENTRAL: SEDIMENTOLOGIE ET PALEOGEOGRAPHIE RESUME La Formation Viking en Alberta central meridional, est composee de strates d'origine marine peu profonde. Cette serie a grains plus grossiers vers le haut, epaisse de 20 a 35 m, comprend 1) gres fin et argile intercalles et bioturbes (20-25 m), 2) gres fin a stratification entrecroisee a faible angle (6-12 m), 3) puissant conglomerat a galets de silex et gres grossier (0-4 m). Au sein de la serie Viking a grains plus grossiers vers le haut, les depots sedimentaires des champs houillers Caroline, Garrington et Harmattan East montrent trois styles distincts de sedimentation marine peu profonde. Le champ Caroline reflete l'initiation du prisme clastique qui prograde de la cote vers le large. Ceci fut suivi par l'incision et remplissage de vallees en platforme cotiere, qui entrecoupa le prisme clastique a Harmattan East. Une couverture greseuse et conglomeratique des champs Caroline et Garrington fut resedimentee en coteaux epais de 5 a 10 m a Garrington. La phase de sedimentation initiale fut accompagnee d'une regression a progradation, lorsque le taux de sedimentation surpassa celui de subsidence. La seconde phase a incision et remplissage de chenaux resulta d'un bas niveau marin mondial, ainsi que d'une subsidence acceleree au sein du basin sedimentaire occidental canadien, qui accompagna son enfouissement sous le piedmont de la cordillere canadienne. La troisieme phase vit une resedimentation lors d'un niveau marin stationnaire puis en elevation, et d'un tectonisme et apport de dechets sedimentaires en baisse. Traduit par A. Zolnai