Sixty-one samples of suspended and sedimented clays were collected from master and tributary streams in the Great Miami River drainage basin in southwestern Ohio and southeastern Indiana. Major contributors of clay materials include Ordovician, Silurian, and Devonian shales and Pleistocene drift. These sediments were analyzed by X-ray diffraction for variations in composition and distribution pattern.

Dominant clay minerals include illite, chlorite, montmorillonite, mixed montmorillonite-illite, and mixed chlorite-vermiculite. The preponderance of illite is evident in Silurian and Devonian shales in the area and contrasts markedly with a significant proportion of chlorite in Ordovician shales. The suspended sediment in streams crossing from younger to older bedrock reflects these changes within a few miles. Variations in the degree of organization of chlorite in Ordovician shale and chlorite in highly leached till can be recognized as stream courses pass over bedrock and glacial material. Montmorillonitic components also appear to come from leached 10A clays in tills.

Significant sorting takes place between high- and low-water periods as a comparison of suspended clays and recently deposited bank clays shows. Degraded and mixed-layer clays tend to remain suspended longer whereas the larger, well-crystallized particles settle. Thus periods of sampling of suspended sediment are very critical for accurate identification of transported clays. Further, the heterogeneity of clay compositions at any particular location within the drainage basin points up the difficulty in defining completely the total mineralogy of a river system. Consideration of the data also illustrates the strong contrast between patterns of clay distribution in marine and fresh-water environments.

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