THE DEPARTMENT OF ENVIRONMENTAL QUALITY IDISSISSippl• • • • geology Office of Geology P. 0. Box 20307 Volume 14, Number 3 Jackson, Mississippi 39289-1307 September 1993 USE OF GEOPHYSICAL WELL LOGS TO DETERMINE LOESS THICKNESSES AND CORRELATE LOESSES AND GEOSOLS IN THE MEMPHIS, TENNESSEE - NORTHERN MISSISSIPPI AREA William S. Parks U.S. Geological Survey, Memphis, Tennessee ABSTRACT the presence of geosols in the upper parts of these units. Total thicknesses of loess of Pleistocene age in the INTRODUCTION Memphis, Tennessee- northern Mississippi area were deter­ mined from the geophysical logs made in test holes and Total thicknesses of loess of Pleistocene age were deter­ ground-water wells. Total loess is thickest on the Mississippi mined generally in the Lower Mississippi Valley by Wascher River bluffs that form the eastern boundary of the Lower and others (I 948), who made field observations of the loess Mississippi Valley. Maximum loess thickness measured in at more than 1,200 localities in Arkansas, Kentucky, Louisi­ that area was about 70 feet. Eastward from the bluffs, total ana, Mississippi, and Tennessee. From these data, Wascher loess thins rapidly in the first several miles, but thereafter the and others prepared a map showing the regional relations of rate of thinning decreases. Minimum thickness measured at the maximum thickness of "Peoria loess" or "total loess." a distance of about 70 miles east of the bluffs was 4 feet. A East of the Mississippi River, loess thickness is shown on this plot of total loess tbickne5ses shows that this rate of thin rung map by eight areal zones trending generally northeast-south­ follows a logarithmic curve. west and paralleling the bluffs. In western Tennessee and Correlations of gamma-ray logs made in several ground­ northern Mississippi, loess was shown to be thickest (mostly water observation wells in the bluff areas indicate the occur­ more than 15 feet thick) along the Mississippi River bluffs in rence of four loess uru ts (from oldest to youngest): the western Tennessee and northwestern Mississippi and thin­ Crowleys Ridge Loess, the "Loveland/Sicily Island loess," nest (mostly less than 2 feet thick or absent) in central the Roxana Silt, and the Peoria Loess. Correlation of these Tennessee and northeastern Mississippi. In the bluff areas, loesses on gamma-ray logs was based primarily on strati­ Wascher and others recognized three loess uruts (from oldest graphic position, differences in natural gamma-ray emission, to youngest): the "Third loess," the "Sangamon loess," and and thicknesses reported for the loesses in previous investi­ the "Peorian loess." gations. Increases in gamma-ray emission observed on the Snowden and Priddy ( I 968) determined total loess thick­ gamma-ray logs at the tops of the Crowleys Ridge Loess, the nesses specifically in the Vicksburg area of Mississippi by "Loveland/Sicily Island loess," and the Roxana Silt indicate measuring loess thickness at 11 localities on the ridge tops 91. 90. 89" section ) )-..._ Dyersburg ' / } """'· ( ' GIBSON CO ,1.-/ -........ ....... ARKANSAS -~{ ---) I ( ---.......... --........ '/ ------ -l ' I ,-../ i ...... ~-~-"- I I ~ ·S Ld:r -g "'-< TENNESSEE Old River -;__ TIPTON CO I, section I '----.--- 0-"".L_ ::·::. .. ·. ::·.:_.:·:·::::::::~::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::2:~::::~:.. :.?~~~ I ·~ . ~ CROSS co I ( I ~Wittsburgquarry ~ - -- -- ___ J __l I 35' .--- . i' ·---- -- - ----~ . ' ~(/.)' 0 -.....; L. 0 10 20 MILES 0 1 0 20 KILOMETERS EXPLANATION Area where total loess thicknesses were determined from geophysical logs Figure I. Area where total loess thicknesses were determined from geophysical logs and location of the Wittsburg quarry, Old River section, Finley section, and wells Sh:H-23. Sh:T-17, and Ld:F-9. east of the Mississippi Ri ver bluffs from Vicksburg to Jack­ included a plot of the relation ofloess thickness (an aggregate son. Snowden and Priddy plotted their measurements on a thickness of the "Roxana loess" and the "Peoria loess") to graph to show the relation between ridge-top loess thickness distance from the Mississippi River bluffs. This plot indicates and distance from the Mississippi River bluffs. This plot a logarithmic relation between total loess thickness and dis­ resulted in a logarithmic curve showing that in the first few tance from the bluffs. In the Dyer and Gibson counties area. miles east of the bluffs the loess thinned rapidly. but the rate Huntley and others recognized three loess units (from oldestto of thinning decreased thereafter. In the Vicksburg area, youngest): the "Loveland loess," the "Roxana loess," and the Snowden and Priddy recognized three loess units (from oldest "Peoria loess." to youngest): the "pre-Farmdale loess," the "Farmdale loess," In their detailed investigation of loess in Mississippi, and the "Peorian loess." Snowden and Priddy (1968) measured natural gamma-ray Buntley and others (1977) determined loess thicknesses emission as one of many physical. chemical, and mineralogi­ by using power augers to drill auger holes in the Dyer and cal characteristics they studied in search of criteria for Gibson counties area of northwestern Tennessee. They subdividing the total loess section and forrecognizinggeosols 42 MISSISSIPPI GEOLOGY, V. 14, No. 3, SEPTEMBER 1993 T --- .,.. I --1 • THICKNESS FROM GEOPHYSICAL LOG OF J GROUND-WATER WELL t- TEST HOLE OR w .....w 60 .J ~ vi - ~ Vl w z 40 -- ~ :.£: I u I :r I t- Vl Vl I w 20 0 .....J l _J I • • __ j 0 20 40 60 80 DISTANCE FROM BLUFFS, IN MILES Figure 2. Relation of total loess thicknesses detennined from geophysical logs to distances of· test holes and wells from the Mississippi River bluffs in the Memphis. Tennessee- northern Mississippi area. (paleosols in their report). Gamma-ray logs were made in these wells were selected from a file of more than 600 several test holes drilled through thick loess near the Missis­ geophysical logs available for the Memphis, Tennessee - sippi River bluffs at Vicksburg. Snowden and Priddy ob­ northern Mississippi area. Most of the test holes and ground­ served that the fresh loess, weathered loess, and geosols water wells for which logs were used for this study were penetrated by the test holes had sufficiently different gamma­ located in Shelby County. Tennessee, and DeSoto County, ray radioactivity to provide a lithologic record. However, they Mississippi (Figure I). reported that the record of the gamma-ray logger was erratic Selection of geophysical logs was based on ( I) topo­ and that most of the variations of gamma-ray emission oc­ graphic positions of wells for which logs were available, (2) curred over short intervals. Comparisons between gamma-ray areal distribution of log control. and (3) completeness of the emjssion and the other characteristics measured apparently near-surface part of geophysical logs that show the loess. were not useful in djstinguishing loess units for their study. Most logs selected were made in wells drilled at or near ridge tops or on hills in topographically high areas so as to PURPOSE AND SCOPE presumably measure in-place loess and to avoid including reworked or colluviated loess. The logs were selected to This paper presents information on total loess thick­ include areas as far east of the Mississippi River bluffs as the nesses determined from geophysical logs of test holes and loess could be identified on geophysical logs. This distance ground-water wells in the Memphis, Tennessee - northern was about 70 miles. Mississippi area. ft also provides correlations of loesses and Although many electric logs were correlated, natural geosols on gamma-ray logs made in several ground-water gamma-ray logs proved to be more useful for the determina­ observation wells in western Tennessee. tion of loess thicknesses. The gamma-ray logs were made in This study is an extension of work conducted from 1971 uncased test holes or cased ground-water wells, and the near­ to 1978 by the U.S. Geological Survey (USGS), in coopera­ surface parts of the logs that showed the loess generally were tion with the Tennessee Division of Geology, to map the recorded. The electric logs were made only in uncased test seven 7 1/2-minute geologic quadrangles (scale I :24,000) holes drilled by hydraulic-rotary methods. that include most of the Memphis urban area. Two of these Making electric logs of the near-surface parts of test geologic quadrangles have been published (Parks 1978, holes drilled by hydraulic-rotary methods required that the 1987). drilling fluid be maintained in the upper part of the bore hole. Unless a special effort was made, this generally was not TOTAL LOESS THICKNESSES FROM achieved because the fluid level commonly fell in the bore GEOPHYSICAL LOGS hole as a result of water loss to deeper sand formations after circulation was stopped for geophysical logging. Therefore; Geophysical logs made in 72 test holes and ground-water the near-surface parts of the logs that would have shown the wells were used to determine total loess thicknesses for this loess commonly were not recorded. In addition, recognition study. The 52 gamma-ray Jogs and 38 electric logs made in of the contact between the loess and fluvial deposits generally MISSISSIPPI GEOLOGY, V. 14 , No.3, SEPTEMBER 1993 43 Ld:F-9 450 l - Altitude 437 feet Peo ria Loess I - Ro-------xana Silt ---- " Love la nd/Sicily 400 l Is land loess" ....J Crowleys Ridge w > I Lo ess / l..oJ ...J ! ! Fluvial <( l..oJ i depos its Vl 350-! --- 100 feet >l..oJ Tota l depth o f 0 we ll 403 feet m <( Sh:T- 17 ~ w - Aititud e l..oJ Sh:H-23 I... 336 feet - Altitude . ;3ij 305 feet Peoria Loe s s Pe oria Lo ess ~ r Ro xana Silt geosol 1 ~ geos ol ~ Roxana Silt ;>-- g eo s ol - - y---=-- geosol " Lov e land/Sicily "Loveland/Sicily Is land loesr_> s 250 - Island loess" ,- Fluvial f"luv iol $ d e posrt s deposits 1 1 00 feet - -Tota l depth of well 89 feet Total depth of well 650 f ee t 200 -.:._.