Clay mineralogy of sediments and source materials in the York River tributary basin

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Virginia Polytechnic Institute

This study was undertaken in order to learn the relationships between clay minerals of stream sediments and the clay mineralogy of corresponding source materials. The York River tributary system was selected to conduct such a study because of its moderate size and its geographical and geological setting. The tributary basin spans the entire Piedmont and most of the Coastal Plain before reaching West Point, Virginia, and it covers 2242 square miles.

River sediments and weathering products (source materials) of the basin were sampled in such a way that both were faithfully represented. Stream sediment samples were taken at closely spaced sites as cores, grab samples, and scoop samples from the channel bed. Source material samples were taken after reconnaissance of the area from road outs, cultivated fields and forests. Surface and subsurface source samples were collected. Five small tributaries of the system were similarly sampled in greater detail.

X-ray analyses of more than 700 samples were made using powder diffraction techniques and a General Electric recording diffractometer. Filtered Cu-K radiation was used. Each sample was analyzed untreated as an oriented aggregate. Further X-ray analyses where necessary included glycolated and heat treated samples.

Clay minerals of source materials and stream sediments of the York River tributary basin are naturally grouped into five categories on the basis of first-order basal spacings. Minerals identified are kaolinite, illite, expandable illite. typical vermiculite atypical vermiculite, montmorillonite and mixed-layer clay minerals involving 10 A and 14 A layers.

Kaolinite is present in all source area samples Vermiculite is the next most frequent source mineral followed by illite. Montmorillonite is highly sporadic in occurrence and is a minor constituent in the source area. The stream sediments contain al1 the minerals found in the source area. No new minerals were observed in the stream environment. The frequency distribution of most minerals is different in sediments. Kaolinite and vermiculite occur in all stream sediments. Illite occurs in all sediment samples of the Mattaponi and Pamunkey tributaries and its reflections become more intense in the lower part of the system. Montmorillonite occurs in more than 86 per cent of the sediments of the Coastal Plain portion of the system. Mixed-layer clay minerals become less frequent in the streams of the basin in contrast to their frequency in the source materials and become more pronounced downstream. The intensity of X-ray reflections for source material clay minerals is typically 2-3 times as intense as those of stream sediments.

Physical mechanisms are postulated to explain the decrease in mixed-layer structures, the increase in illite intensity downstream and the increase in the frequency of occurrence of montmorillonite in the sediments of the lower parts of the stream systems. Mixed-layer minerals become unmixed through a highly selective erosion and transportation process which results in removal of the clay in units of structure. This physical unmixing provides an explanation of the increase in illite reflections in sediments in the lower part of the stream. Montmorillonite is more frequent in the lower part of the stream because of its greater mobility than the other minerals.

Results of this study may require that previous studies of modern sediments be re-evaluated in recognition of the appearance of atypical vermiculite in the source area and the process of unmixing of mixed-layer clay minerals.