Within deltaic successions, discrimination between incised valley deposits and distributary channels can become complicated when the shoreface body being incised is a delta-front sandstone that is substantially thicker than normal shoreface sandstone bodies. The Iles Formation (Upper Cretaceous, Mesaverde Group) of northwestern Colorado provides large, three-dimensional outcrops exposing examples of such incised, over-thickened shoreface sandstones.

The paleotopographic lows incised into the thick shorefaces in the Iles Formation exhibit channel-complex incision commonly exceeding the thickness of average shoreface heights as determined from studies of modern shoreface examples. Also, these channels contain multiple levels of through-going, concave-up erosion surfaces pointing to repeated incision and aggradation. These observations are frequently cited criteria used in the identification of incised valleys However, these criteria may not be specific to an incised valley interpretation if a scenario is accepted where recurrent reoccupation of distributary channels in a single locality leads to the creation of a distributary-channel complex which is genetically related to a delta front progressively eroded as the channel complex progrades.

Along with previously suggested criteria such as the occurrence of estuarine deposits within an otherwise fluvial fill, several points to consider when discriminating between incised valleys and distributary-channel complexes are: 1.) incised valleys are frequently larger in size than distributary channels, 2.) is vertical stacking of distributary channels during delta-front progradation a reasonable scenario?, and 3.) shoreface-sandstone segments which are time-equivalent with valley incision should display near horizontal to falling shoreline trajectory and a reduction in thickness.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas