Cycles in Alluvial Fan Successions, Hornelen Basin (Devonian), Norway

Donna S. Anderson and Timothy A. Cross

The alluvial fan facies tract of the Hornelen Basin, western Norway, contains cyclic facies successions that are linked to the expansion and contraction of the adjacent lake and braidplain facies tracts. These cycles were previously described as mostly asymmetrical coarsening-upwards conglomerate sequences. Mapping beds along depositional profiles shows that seven alluvial fan facies occupy specific positions along the profile. Two traction-load conglomerate facies occupy the highest-gradient positions. They are succeeded progressively down gradient by five sediment gravity flow facies: two disorganized, clast-supported conglomerate facies and three conglomerate-sandstone facies. Lateral facies transitions are represented by vertical successions as predicted by Walther's aw. Regular, recurring vertical facies successions define small-scale cycles (2-4 m thick). Intermediate (4-10 m thick) and long-term (100 m thick) cycles are defined by stacking patterns of smaller scale cycIes and by general tendencies toward up or down gradient facies. Both the general facies successions and the stacking patterns reflect unidirectional tendencies toward limits of increasing and decreasing A/S. Cycles defined using this approach are symmetrical in contrast to those defined using the traditional approach of grain size.

At long-term maximum A/S, the alIuvial fan facies tract contracted toward the basin margin as the Iake facies tract flooded the entire basin. At long-term minimum A/S, the alluvial fan facies tract reached its maximum expansion toward the basin axis. Full cycle symmetry at all scales is corroborated by a lack of paleosols and erosional unconformities in the alluvial fan facies tract.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California