ABSTRACT: Siliciclastic Sequence Stratigraphic Development in Foreland Ramp-Type Basins, with Examples from the Viking Formation in South-Central Alberta, Canada

POSAMENTIER, HENRY W., ARCO Oil and Gas Company, Plano, TX, and GEORGE P. ALLEN, TOTAL Compagnie Francaise des Petroles, Paris, France

Two aspects of foreland basins that result in depositional sequence development that is distinctly different from that which develops on trailing or passive margins are: (1) total subsidence decreases in a seaward direction and (2) sea floor physiography commonly lacks a discrete shelf/slope break. These factors result in abundant development of lowstand shorelines produced by forced regression in the seaward extremity of the basins. Lowstand fluvial valley systems may not be deeply incised and therefore will not have strong preservation potential. On the proximal side of the basin, landward-increasing subsidence provokes a "backward rotation" of fluvial profiles. This results in a tectonically induced, nearly continuous increase of subaerial accommodation even when these fluvial syst ms farther downdip are subjected to a eustatically induced base level fall and consequent erosion. This model suggests that any fluvial incision that might occur as a result of base level fall would be restricted to the lower reaches of the rivers while fluvial aggradation, albeit at a slower rate, continues upstream where flexure-related tectonic activity is more important. This has implications regarding the stratigraphic expression and regional extent of Type 1 and Type 2 unconformities in foreland basins.

The Upper Albian Viking Formation, Alberta, Canada, provides examples of sequence stratigraphic development near the distal limit of terrigenous sedimentation in a foreland basin. This formation is characterized by multiple lowstand shorelines as well as incised valleys. Lowstand shorelines commonly are relatively narrow (<10 km wide) and are deposited during stillstands punctuating intervals of relative sea level fall or rise. Preserved incised valleys occur up to 30 m deep and are characterized by longitudinal profiles that have distinct deeps or hollows. The occurrence of relatively few incised valleys versus comparatively common lowstand shorelines suggests that only some relative sea level falls were of sufficient magnitude to subaerially expose significant parts of the basin. Only during these infrequent events was steeper physiography exposed so as to induce fluvial incision.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)