Clay Mineralogy of the Bottom Sediments, Rappahannock River, Virginia

CLAY MINERALOGY OF THE BOTTOM SEDIMENTS, RAPPAHANNOCK RIVER, VIRGINIA by BRUCE W. NELSON Virginia Polytechnic Institute, Blacksburg, Virginia ABSTRACT Bottom sediment clay mineral assemblages from the Rappahannock River and estuary contain kaolinite, illite, unorganized illite, dioctahedral vermiculite, 12.4A montmoril­ lonite, 14.2 A montmorillonite, chlorite, feldspar and quartz. The x-ray diffraction characteristics of these species and their thermal modifications at 25-500°C are described and illustrated. Progressive changes in the mineralogical composition of bottom sediments occur between the fresh-water and saline-water portions of the Rappahannock system. Chlorite and feldspar occur exclusively in the saline portions of the estuary. Illite shows progressive increase in crystallization quality. There appears to be less kaolinite, pro­ portionately, in the estuarine sediments of the most saline zone. x-Ray· diffraction diagrams showing these progressive changes are illustrated. INTRODUCTION This report describes the mineralogy of clay-sized fraction of bottom sedi­ ments from the Rappahannock River and estuary of northeastern Virginia. The data are interesting chiefly because they describe sediments of the fresh-water-marine transition zone. The mineralogy of the coarse fraction, the mineralogy of suspended sediments, and certain chemical measurements will be reported elsewhere. This is the first of several papers that will deal with problems of sedimentation in the Rappahannock River area. ACKNOWLEDGMENT Financial support for the field and laboratory studies was obtained through the courtesy of the American Petroleum Institute under Grant-in-Aid 21. This support is gratefully acknowledged. GEOGRAPHY The Rappahannock River drainage basin is a relatively small one, about 2400 miles in area. Headwaters of the Rappahannock rise on the east side of the Blue Ridge Mountains, and its tributaries drain parts of the Blue Ridge and Piedmont...