ABSTRACT

The fundamental concept on which sequence stratigraphy is based is that stratal architectures develop in response to the interaction between base-level change and sediment flux. A small lacustrine fan-delta at East Coulee, Alberta, illustrates these principles and demonstrates that sequence stratigraphic concepts are scale- and time-independent. This fan-delta formed in a matter of days in a roadside drainage ditch as water level rose and then fell. Changes in the subaqueous space available for sediment to fill (i.e., accommodation) resulted in the development of a succession of systems tracts analogous to those formed within sedimentary basins over much longer time frames.

Deposition began during lake-level rise with progradation of a fan-deltaic wedge. Accelerating lake-level rise resulted in a backstepping of the shoreline and deposition of a transgress systems tract. Subsequent lake-level fall resulted in subaerial exposure of the relatively steep delta front and caused incised valley formation and deposition of a lowstand delta. This stratigraphic response to base-level fall is typical of lowstand systems tract development on basin margins characterized by high-relief slopes (e.g., passive margins). Further lake-level fall resulted in minor additional valley incision and development of a second lowstand delta on a surface characterized by lower relief as lowering water level exposed a flatter physiography. This development of lowstand deltas and coa tlines during base-level fall constitutes a forced regression of Posamentier et al. (1990) and is typical of lowstand systems tracts on basin margins characterized by gentle slopes.

In spite of its small size, this example clearly illustrates that sequence stratigraphic principles apply to lacustrine as well as marine settings and at all spatial and temporal scales.