Memphis Area Landform Composition, Seismology & Ground-Water Resources
From GeoClasses
Summary
This landform research report will provide a general outline of the Memphis Tennessee regional geophysical structure's historical background, formation, and locally specific compositions. It will summarize two primary geological concerns significant to the area's populations; seismic activity and Memphis' positioning relative to the New Madris Seismic Zone, and the natural water resources upon which Memphis has been entirely dependent for over a century. From this research, I expect that site visitors will gain enhanced knowledge and perspectives of local earthquake potentials and necessity for individual preparedness. I also hope that site visitor's appreciation regarding our natural water resources will be enhanced and that recognition of further needs to ensure for the preservation, conservation and sustainability of these precious resources will continue to be prioritized.
News: June/July 2008
'The Mississippi River Project' A Seismic Experiment in the Mississippi Ebayment "This summer, with the help of the U. S. Corps of Engineers, a group of researchers from the University of Memphis and the University of Texas at Austin plan to lift the veil of mystery that lies underneath the stretch of the Mississippi River between Helena, Arkansas and Caruthersville, Missouri." Review the researchers' journey by their journals and posted photos on the site linked directly below. http://www.memphis.edu/riverproject/
Early Geophysical Formation
Today’s Memphis climate in Shelby County, Tennessee is an agreeably temperate, but humid with a mean annual temperature of 62.3 degrees Fahrenheit and a mean annual precipitation of 52.1 in. Early history of the region reveals that during the Proterozoic era about 600 million years ago, the Mississippi Embayment in the central United States was pushed upward to create a ‘pillow’ shaped structure by dense, molten magma from the Earth’s liquid mantle core swelling into the lower crust, creating numerous faults and the original intracontinental Reelfoot Rift. Upward movement subsided and the rift eventually sank due to the weight of rock. Erosion filled the low-lying area with sediment. Later, seas covered the area creating sequences of sedimentary layers which eventually became limestone, sandstone and shale.
In the Mesozoic Era 200 million years ago, there was major rifting along the North American coast that stretched the continent, pulling the Reelfoot Rift apart in another continental rifting, and formed plutons moving upward along old faults. The plutons cooled rather than pushing further upward to create a mountainous, volcanic area. Again the rifting failed but remained a weak zone. With the rift’s ceasing, the area sank and the ocean covered over it. Sand, clays and gravel sediments gathered, but without having been heated or compressed for long enough this time to become solid rock, remained unconsolidated. These sediments remain in the local area’s base as voluminous inverted prism about 3000 ft. thick.
Perhaps twenty times in the past 2 million years, glaciers came as far south as Carbondale, Illinois before receding back to the north. The most recent glacier covered North America about 18,000 years ago and its gradual receding melt cycle completed 12,000 to 7,500 years ago. A hypothesis is that glaciers increased the pressure on the weakened NMSZ. Although the area experiences nearly 150 measurable, minor tremors annually, the most recently recorded and major earthquake activity of the NMSZ occurred for four months beginning in December of 1811. Then, only two hundred years ago, the NMSZ shook with regularity surrounding at least three major quakes estimated to have been about magnitude 8 on the Richter scale, one creating lakes and causing the Mississippi River to flow backwards for hours.
Reelfoot Rift & New Madris Seismic Zone
The New Madrid Seismic Zone (NMSZ) is a series of faults miles below the earth’s surface, half way between St. Louis and Memphis, and historically shaking with violence about every 500 years. It is an area of frequent smaller quakes stretching along the lower Mississippi Valley northward from west of Memphis into Southern Illinois. If there had been a complete separation of the Reelfoot Rift at its emergence, the NMSZ could have been on the Atlantic coast and the present southeastern U.S., attached to the African continent. More regarding Reelfoot has been provided by Univ. of Memphis scholar, Savannah Rowe at http://lurbano-5.memphis.edu/Classes/index.php/Reelfoot_Lake
Memphis' Relative Location
Memphis’s west border is the Mississippi River Bluff, binding the western edge of the East Gulf Coastal Plain, the River of course, and the relatively flat Mississippi Alluvial Plain with a 5 to 10 ft. relief, 180 to 220 ft. altitude and gradual slope declining the north. At Memphis, the Gulf Coastal Plain’s altitude ranges from 200 to 390 ft. above sea level. The majority of the Memphis metropolitan area is in the Gulf Coastal Plain section, and about one quarter of the area is in the Mississippi Alluvial Plain section of the two physiographic subdivisions.
The Memphis area is in the northern-central part of the Mississippi Embayment basin forming a structural trough that dips southward along the Mississippi River. Well beneath it, Tertiary-age geologic units including unconsolidated sand, silt and clay slope slightly down and westward. Eocene age, major tertiary formations consisting of 650ft to 1,100 ft. thick layers of sands and clays are the Claiborne Group and Jackson Formation. The lower 75% of the Claiborne includes a 650 to 820 ft. thick Memphis Sand including the natural aquifers and primary water source. Above them are the Cook Mountain and Cockfield formations comprised of silt and fine clays, making up the Jackson-upper Claiborne confining bed between deeper artesian Tertiary age Memphis Sand and the Quaternary age, shallow aquifers.
Area Specific Structural Detail & Composition
Pleistocene and Pliocene fluvial deposits generally cover the Tertiary units, but with irregularity and paleovalley features with depths exceeding 45 feet have been found beneath terraced layers. Fluvial terrace deposits, sand, and gravel in the lower alluvium are the shallow water-table aquifer across much of the county and about 70 ft. higher than the levels of the Memphis Sand/Aquifer. Layered clay beds between aquifer levels vary in thickness from 10 ft. to nearly 200, suggesting that at least some connectivity between the shallow aquifer and the greater, underlying Memphis Sand likely exists. Nearer the surface and apart from the Loosahatchie, Nonconnah, and Wolf river tributary valley lows, between 10 and 65 vertical feet of confining loess are covering fluvial deposits, underlying the area topography from thickest points at the Mississippi River Bluffs and thinning eastward. Further eastward there is an even deeper artesian well of 1,400 ft., the Fort Pillow Sand, with its western edge beginning where the Memphis Sand’s eastern edge ends and extending to the immediate west of the Tennessee River.
The 'Saudi Arabia' of Water
In a 2003 interview with Water World' and subsequent article http://ww.pennnet.com/articles/article_display.cfm?article_id=185394 , Dr. Jerry L. Anderson, then the Director of the Ground Water Institute at the University of Memphis and a nationally known expert in the field was quoted as saying "Memphis has the ‘sweetest, most wonderful tasting water in the world,’ he says, in part because of the presence of so few minerals that the water can be utilized with little treatment when it is withdrawn from underground."
Memphis aquifer ground water is the main source of drinking water for the regional, metropolitan area. The Mid-South area’s ground water resources are critical for meeting increased demands for public supply, industrial and irrigation use. They're relied upon locally for 100% of these needs. The Memphis Sand is only a smaller part of the North Mississippi Embayment systems comprised of Tertiary and Cretaceous-age sand aquifers also vital to nearly all urban areas of west Tennessee.
Earthquake Probability
In the New Madras Seismic Zone, a 2003 forecast estimated a 7 to 10% chance, within the next 50 years, of magnitude 7.5-8 earthquakes repeating and a 25-40% probability within 50 years of one of magnitude 6 or higher. A more recent study by Northwestern University shows that “motion across the New Madrid Seismic Zone currently is very slow or at zero” but asserts “Because this motion has to accumulate for many years to cause a large earthquake, it will be at least hundreds of years, or perhaps much longer, before another large earthquake happens.” The 200 year anniversary of the 1811-12 events in the NMSZ is approaching, and scientists are closely surveying and monitoring area activity.
Groundwater Problems & Preservation Considerations
Regarding the maintained quality and sustainability of Mid-South natural water resources, the confining clay layers amidst the water table are thin at points or nonexistent in some areas of eastern Shelby County, and have been found to be permeable. Allowing downward filtration of surface and shallow groundwater, these areas are suspected to contribute to variations in water quality, causing much concern, and could potentially lead to runoff and/or infiltration contamination.
Recharge of the Memphis Sand takes place mostly east of Memphis where it is closer to the surface. The infiltration is greater with heavier rain during winter and spring and at lesser rates during the summer and fall seasons having less rain and higher levels of transpiration and evaporation.
Erosion of the confining layers that protect aquifers will now promote consequent lateral merging amongst shallow, unconsolidated water-table aquifer units and could introduce surface contaminants into enter the much larger, regional aquifers. In confining bed absent areas, water moves directly out of shallow aquifers into the Memphis Sand.
The heavy pumping of ground water by MLGW, industrial operations and other such municipalities as Bartlett, Germantown, Raleigh and Southaven, has created depressed surface cones at local surface station areas and years ago, geophysical temperature logs indicated that pumping from the Memphis Sand had caused leakage into it from the water table aquifers.
In the context scenario of a polluted reservoir, residence time refers to the lengths of time required for decontamination. A dramatic contrast of water reservoir residence times is obvious when understanding that a typical river would require about two weeks, and groundwater residence times are estimated to be 10,000 years. Because of this, crucial prevention of, and early detection initiatives for potential pollutants contaminating and eventually degrading the quality of the Memphis aquifer are major, ongoing priorities. Up to the dates of USGS report references, contamination of the Memphis Sand, if it exists, has yet to be detected.
further research & report references
Rock Hounding Arkansas; Gary Patterson. -June 30, 2008. http://rockhoundingar.com/geology/fault.html
Reelfoot –MS. Ebayment diagram http://www.geo.arizona.edu/geo5xx/geos577/projects/muhlenkamp/tectonics.htm
The Memphis Aquifer - Reelfoot Rift (sand & aquifer diagram) and article. retrieved July 8, 2008. http://showme.net/~fkeller/quake/origins2.htm
Reelfoot Rift / Mid New Madras cross section image retrieved July 8, 2008. http://en.wikipedia.org/wiki/Image:Reelfoot_rift.jpg
Memphis Water called 'sweetest in the world' and Cheapest to Deliver. A ground water expert has called the Memphis region 'the Saudi Arabia of water' because of its clear, sweet taste. Aug 25, 2003. –Water World http://ww.pennnet.com/articles/article_display.cfm?article_id=185394
Structure Contour Map -Pleistocene fluvial terrace deposits/W. TN http://water.usgs.gov/wrri/00grants/TNrecharge.html
MS, AR, & TN Regional Aquifer Study (MATRAS) Collaborative studies by U.S. Geological Survey, University of Mmephis, Ground Water Institute in conjunction with area/partner universities and the City of Memphis and Shelby County, Tennessee.
http://choctaw.er.usgs.gov/matras/index.htm
Congressional District: Ninth. Statement of Critical Regional Water Problems State Water Resources Research Institute Program; title: ‘An Investigation to Identify Sources & Quantities of Modern Recharge to the Memphis Aquifer in the Sheahan Well Field in Shelby County, Tennessee’ March 2000 to February 2001. http://water.usgs.gov/wrri/00grants/TNrecharge.html
One pg. summary of the Memphis aquifer -Mississippi State http://www.msstate.edu/dept/geosciences/CT/TIG/WEBSITES/LOCAL/Summer2003/Thomas_Donna/memphisaqu.html
USGS Statement of critical regional water problems. An Investigation to Identify Sources and Quantities of Modern Recharge to the Memphis Aquifer in the Sheahan Well field in Shelby County, Tennessee. http://water.usgs.gov/wrri/00grants/TNrecharge.html
U.S. Dept of the Interior; U.S. Geological Survey ‘Quality of Shallow Ground Water in Recently Developed Residential and Commercial Areas, Memphis Vicinity, Tennessee, 1997 by Gerard J. Gonthier. Jackson MS 2002 http://ms.water.usgs.gov/publications/WRIR_02_4294.html
Defense Technical Information Center. Accession #: ADA459554 ‘Chemical Character of Ground Water in the Shallow Water Table Aquifer at Selected Localities in the Memphis Area, TN. Geological Survey, Washington D.C. Parks, W.S.; Graham, D.D.; Lowery, J.F. http://stinet.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA459554
U.S. Geological Survey, Water Resources Investigations Report 85-4295 by D.D. Graham and W.S. Parks Potential for Leakage Among Principle Aquifers in the Memphis Area, Tennessee Sept 5, 2005. http://pubs.usgs.gov/wri/wri85-4295/
New Evidence Shows New Madrid Seismic Zone may be Cold and Dying Megan Fellman fellman@northwestern.edu ; Northwestern University. Dec 2006 http://www.eurekalert.org/pub_releases/2006-12/nu-nes121106.php
U.S. Geological Survey. Scientists Update New Madrid Earthquake Forecasts News Release January 13, 2003. ctc Eugene Schweig or Joan Gomberg http://www.showme.net/~fkeller/quake/lib/usgsjan03.htm
New Madrid Seismic Zone graphic -retrieved July 8, 2008. http://www.google.com/imgres?imgurl=http://www.showme.net/~fkeller/quake/images/crossection.jpg&imgrefurl=http://www.showme.net/~fkeller/quake/maps2.htm&h=324&w=462&sz=36&tbnid=kT6xP9yc4oAJ::&tbnh=90&tbnw=128&prev=/images%3Fq%3Dcaruthersville%2Bseismic%2Btectonics%2Bcross%2Bsection%2Bimages&hl=en&sa=X&oi=image_result&resnum=1&ct=image&cd=1
acknowledgements:
Special thanks to Dr. Hsian-Te Kung and Dr. Lensyl Urbano of the University of Memphis' Earth Science Department for sharing their experience, expertise, and enthusiasms leading associated sections of this semester's activity curriculums.
Support for this semester specific research project was largely sponsored through partnerships with FedEx and Century Management LLC.
M.Thais may be contacted via email at mathais@memphis.edu
