ABSTRACT

We here present the first detailed published sequence stratigraphic study of the Lower Cretaceous Bow Island Formation. Two orders of sequences are identified, based on the integration of sedimentology, micropaleontology, ichnology and petrophysical characteristics. The studied interval encompasses two lower-order sequences. Shelf, shoreface, and coastal plain deposits of the lower and middle members of the Bow Island Formation form the transgressive and highstand systems tracts of the basal sequence. A major relative sea level fall caused a prolonged period of sub-aerial exposure and weathering of the deposits of the middle member of the Bow Island Formation, and farther to the east and northeast, incision of deep valleys into shoreface deposits of the Viking Formation. An overlying complex succession of tidal flat, estuarine, shoreface, and shelf deposits of the upper member of the Bow Island Formation and marine shales of the Westgate Formation represent the transgressive systems tract of the younger low-order sequence. Within the transgressive succession of the upper member of the Bow Island Formation and Westgate Formation, five higher-order sequences are identified. The high-order sequence boundaries are marked by basinward shifts of facies, incision of up to 15 m deep valleys and soil development on intervalley areas. This demonstrates that this period of overall sea level rise was punctuated by relative sea level falls, justifying subdivision into sequences rather than parasequences. Transgressive surfaces, often developed as prominent, pebble-mantled ravinement surfaces, subdivide the individual highorder sequences into lower and upper units. The lower units are comprised of tidal influenced incised valley fill, forming the main reservoirs, and tidal and storm influenced back-barrier lagoons deposited during lowstand and early transgressive periods. Foraminiferal assemblages were critical in distinguishing between these lagoonal deposits and open marine shoreface deposits. Deposits of the basal part of the upper units are characterized by a stepwise change towards more open marine conditions following each transgression, from marginal marine tidal flat and lagoonal deposits to shoreface and finally to shelf deposits of the Westgate Formation. This demonstrates the stacking in a retrogradational sequence set of the high-order sequences of the upper member of the Bow Island Formation and Westgate Formation. Erosion associated with the transgressions increases upward in intensity with each successive transgression with the greatest depth of erosion associated with the Westgate transgression. This reflects an upward increase in energy regime of the sedimentary environments that bypassed the area during the transgression. Within the overall transgressive upper member of the Bow Island Formation, an upward increase in the depth of incised valleys with each successive incision event likely reflects an upward increase in depositional relief due to the development of more open marine conditions and to the retrogradational and aggradational character of the upper member of the Bow Island Formation. The Westgate transgressive surface is a complex ravinement surface characterized by a westward, stepwise rise, indicating that it was formed during a period of gradual relative sea level rise, interrupted by phases of more rapid sea level rise. Establishment of this high-resolution sequence stratigraphic framework allows differentiation between these various generations of incision and thereby the mapping of the individual valleys and reservoirs associated with them.

* Current address: Petroleum Geology, Saskatchewan Industry and Resources, 201 Dewdney Ave East, Regina, SK, Canada, S4N 4G3; E-mail: [email protected]

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