We have identified four classes of amalgamation, strongly amalgamated, moderately amalgamated, poorly amalgamated, and isolated fluvial sandbodies, and four smaller-scale architectural components that stack to form one of the above categories. Studied examples of strongly amalgamated fluvial sandbodies include the lower Castlegate Formation (Book Cliffs, UT) and the Canyon Creek Member of the Ericson Formation (Rock Springs Uplift, WY). The Trail Member of the Ericson Formation (Rock Springs Uplift, WY) provides an excellent example of moderately amalgamated fluvial sandbodies. For our examples of loosely connected and isolated fluvial sandbodies, we compiled data sets from the middle Castlegate Formation (Book Cliffs, UT) and the Iles Formation (Sand Wash Basin, NW Colorado) respectively.

Within these four classes of amalgamation, four smaller-scale architectural components are seen as the key to variations in alluvial architecture. We measured lengths and thicknesses of thalweg fills (1), supraplatform bars (2), floodplain silts/muds (3), and abandoned channel fills (4) for several outcrop faces in each of our four amalgamation categories. The manner in which these four components stack produces a continuum of amalgamation scenarios of which we have identified four.

In cases of moderately amalgamated fluvial sandbodies, multistorey thalweg fills (1) (all that remained after repeated channel scouring in a belt) stack with the occasionally preserved supraplatform bars (2) (preserved due to an avulsion event) and remnants of floodplain silts/muds (3) or abandoned channel fills (4). Strong amalgamation preserves little of components 2-4, resulting mainly in stacked, irregularly preserved thalweg fills, and does not allow avulsion events to be easily recognized. In alluvial successions of poorly amalgamated or completely isolated fluvial sandbodies, the biggest change is the dramatic increase in preservation of floodplain silts/muds (3), which stack with single storey thalweg fills (1), supraplatform bars (2), and abandoned channel fills (4).

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah