NADON, G.C., Department of Geological Sciences, Ohio University, Athens, OH 45701, nadon@ouvaxa.cats.ohiou.edu and Issler, D.R., Geological Survey of Canada, 3303 33rd St. N.W., Calgary, Alta. T2L 2A7, dissler@gsc.emr.ca

Backstripping is an important step in determining the evolution of a sedimentary basin. The algorithms used in this procedure assume that marine and nonmarine deposits have similar compaction histories. An examination of typical fluvial deposits illustrates that fluvial sediments cannot be decompacted using the same empirical factors as their marine counterparts. Heterolithic fluvial sediments show none of the differential compaction expected if the lithologies compact in the same manner as their marine counterparts. For example, the tops of winged channel sandstone bodies in mud-dominated fluvial deposits are flat even though the edges overlie mudstones. The only conclusion that can be drawn is that sandstones and mudstones in fluvial settings undergo essentially equal reduction in porosity during initial compaction. This is borne out by engineering data from modern floodplains which shows that the porosity of floodplain sediments varies from 33.8%-39.4% (average 35.5%) for clay and silty soils below the liquid limit and 27%-34.6% (average 30.8%) in sandy soils. All these values are substantially lower than the assumptions used in backstripping algorithms.

The result is that the total subsidence in basins where mud-dominated fluvial deposits are common is overestimated by hundreds of meters. This error, in turn, affects estimates of the amount and rate of tectonic subsidence for a given basin. In the case of the St. Mary River Formation of SW Alberta, which is now approximately 560 m thick, original thickness estimates vary from a maximum of 887-982 m to 732-764 m (this study) depending on how the sediments are partitioned within the section.