Geology and hydrology of the Claiborne Group in western Tennessee

Abstract

The area of western Tennessee underlain by the Claiborne Group is about 7,200 square miles and lies on the east flank of the syncline that forms the Mississippi embayment. It includes the Mississippi Alluvial Plain and part of a dissected upland plateau. The Claiborne Group dips to the northwest at 10-25 feet per mile and ranges in altitude from 600 feet above mean sea level in the outcrop area to 900 feet below mean sea level near the embayment axis. The Claiborne Group is tentatively subdivided into five units including, in ascending order, the Meridian Sand Member of the Tallahatta Formation, the Basic City Shale Member of the Tallahatta Formation, the Sparta Sand, an unnamed clay unit, and an unnamed sand unit. The two major aquifers in the Claiborne Group are the '500-foot' sand and the unnamed sand unit. The top of the '500-foot' sand is correlated with the top of the Sparta Sand; and the base, with the base of the Claiborne Group. The '500-foot' sand ranges in thickness from 200 to 750 feet and consists mainly of very fine to coarse sand or gravel. It also contains layers of white to blue, pink, gray, or brown clay, which constitute only a small percentage of the total thickness. The unnamed sand unit ranges from 0 to 210 feet in thickness and consists mostly of white, gray, or brown fine-grained lignitic sand. An estimated 75 percent of the ground water withdrawn in western Tennessee (west of the northward-flowing segment of the Tennessee River) is taken from the '500-foot' sand and the unnamed sand unit. The quantities of water available to wells from the '500-foot' sand are currently adequate for all municipal and industrial needs. The permeability of this aquifer is about 570 gallons per day per square foot. An estimated 155 mgd (million gallons per day) is pumped from the '500-foot' sand, about 140 mgd is discharged from the aquifer as the base flow of surface streams, and about 40 mgd is discharged from the report area as underflow. Water from the '500-foot' sand contains objectionable quantities of iron in the western half of the report area. Otherwise the quality of the water is suitable for most needs. Quantities of water adequate for domestic use and for small municipal systems can be obtained from the unnamed sand unit in most of the report area. The field permeability of this aquifer is probably about 270 gallons per day per square foot. About 8 mgd is discharged into adjacent formations, and about 2 mgd is withdrawn by pumping. Water from the unnamed sand unit contains objectionable quantities of iron in the western half of the report area. Otherwise the water from this aquifer is of good quality. Ground-water supplies in both the '500-foot' sand and the unnamed sand unit will be adequate for the predicted rate of municipal growth and economic development for many years to come. If the hydraulic gradient in the '500-foot' sand were increased to 19 feet per mile, the average dip of the top of the aquifer, about 578 mgd would be transmitted downdip. Similarly, the unnamed sand unit would transmit about 34 mgd downdip under a hydraulic gradient of 10 feet per mile. Furthermore, additional amounts of water could be induced into the report area as underflow from adjacent States. The anticipated effects of additional large scale development are (1) a drop in local and regional water levels in proportion to the increase in pumpage, (2) an increase in the net inflow of ground water from adjacent States, and (3) an increase of recharge to the aquifers at the expense of streamflow.