ABSTRACT

Floodplain paleosols in time-equivalent strata of the Willwood For mation differ in terms of paleodrainage and degree of pedogenic development. Ancient soils were more poorly drained and more strongly developed in one area compared to a second area about 45 km away. This difference in drainage is attributed to parent material. The more poorly drained paleosols formed in an area with less permeable, clay-rich floodplain deposits. In the stratigraphic interval that accumulated more slowly, paleosols are more strongly developed, less numerous, and thicker than in the more rapidly accumulating stratigraphic interval. These differences are attributed not only to different sediment accumulation rates, which are commonly considered to control the degree of ped ogenic development, but also to differences in avulsion frequency.

A simple model is offered to show how paleosols change with varying basin subsidence rates and the relationship between avulsion frequency and accumulation rate. When avulsion frequency increases more rapidly than sediment accumulation rate, an increase in subsidence rate causes strongly developed paleosols to be overlain by a stratigraphic interval with paleosols that are less well developed, thinner, and more densely spaced. When avulsion frequency is not related to accumulation rate, as early models of alluvial architecture assumed, an increase in subsidence rate produces a stratigraphic section in which strongly developed paleosols are overlain by paleosols that are less well developed, thicker, and more widely spaced. Study of the thickness and degree of development of floodplain paleosols can be used together with more traditional studies of alluvial architecture to provide a clearer understanding of the factors that influenced the stratigraphic architecture in a particular alluvial basin.