An abandoned delta distributary of the Bethel-Sample Formations, a very small part of the Michigan River system that supplied sand and mud to and across the Illinois basin during the Late Mississippian, has been delineated for 15 mi in west-central Indiana by outcrop mapping and shallow drilling.

This elongate sandstone body has sharp and relatively straight boundaries, averages 2 mi in width, and has a maximum thickness of 145 ft. Along its axis of maximum thickness the sandstone body fills a shallow channel cut in the underlying thick limestone sequence. Cross-bedding and ripple marks constitute its two principal facies and form as many as five fining-upward cycles. The ripple-bedded facies is very fine grained and much less permeable than the generally coarser grained cross-bedded facies. Cross-bedding dips uniformly southeast except in the very basal part of the sandstone body where the orientation is northwest--considered to be the product of up-channel tidal currents that introduced sparingly glauconitic marine sand into the shallow channel prior to deposition of the flu ially derived sand.

Decementation by groundwater percolating downward into the underlying carbonate has greatly increased the permeability of the sandstone body. The cross-bedded facies has an average permeability of 400 md except within 20-30 ft of the base where permeability values in excess of 2,000 md occur. Intra-sandstone-body channeling plus the distribution of the low permeability ripple-bedded facies modify permeability.

Comparable buried reservoirs should occur where an ancient distributary system is truncated by an unconformity with 100-200 ft or more of relief. Permeability will be enhanced greatly in such reservoirs if they are underlain by limestone and if the limestone-sandstone contact was above stream level during the development of the unconformity.

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