Abstract: Evaluation of Fluvial-Facies Models

Roscoe G. Jackson, II

Since Fisk's pioneering work, a widely accepted facies model for meandering-stream sediments and a diffuse proliferation of facies models for braided streams and low-sinuosity streams have appeared. Almost all such models arose from low-flow investigations which emphasized surficial sediments, commonly of minimal preservation potential. The paucity of measurements of bed-material size, bed forms, and nature of deposition and erosion has further clouded recognition of distinctive sedimentary facies in each type of stream.

Recent studies of meandering streams disclose a complexity of sedimentary facies which does not appear in the standard fining-upward models. The following features normally are deemed indicative of braided or low-sinuosity streams but typify many freely meandering streams: (1) negligible mud, (2) vertical sequences which do not denote an upward decrease in flow regime, (3) absence of natural levees, and (4) predominance of gravel. Many special criteria--such as lateral accretion surfaces, channel-fill deposits, and lack of scour surfaces--either have not been documented in Holocene meandering streams or else can be present in braided or low-sinuosity streams.

These and other considerations imply that the reconstruction of depositional environments of ancient fluvial sediments requires detailed knowledge of the three-dimensional lithofacies. A review of the post-1963 literature on ancient fluvial environments suggests that failure to appreciate this complexity is one main reason why most of these studies did not achieve an unequivocal, explicit environmental interpretation. A second factor may be the present ignorance of the extent of preservation of individual fluvial sequences when stacked on one another.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC