The Mississippi River drains the entire continental interior of North America. This has been so until the late Mesozoic. The rapid changes in sea level due to glaciations during the Pleistocene created high terraces and deep valleys. The glaciers also provided large amounts of melt water the channeled out through the Mississippi River to create great landforms. This report will discus the general properties of the basin and it geologic history.
The Mississippi Embayment stretches longitudinally from southern Louisiana to Illinois. The basinís origin lies in the Precambrian during the break up of the supercontinent Rodinia. During Rodiniaís brake up, there were several smaller rift zones that formed adjacent to the major rift, which split off North America. One of the smaller rift zones existed below the modern Mississippi Embayment. Sediment accumulated above this small rift zone to form a proto-Mississippi Embayment. The modern embayment is attributed to the rise of sea level in the Cretaceous. With the break up of the supercontinent Pangea and higher rates of sea floor spreading, the global sea level was considerably higher. The old rift zone was a natural downwarped basin and allowed a natural course for sea level to enter the continental interior as well as exit. When the ocean retreated, the Mississippi River was born. Since the Cretaceous, large amounts of sediment were deposited in the river valley. Reoccurring activity still exists today along the riftís faults. The 1811-1812 New Madrid Earthquakes were associated with deep faults in this area and are still the largest earthquakes ever recorded in North America.
An overall trend since the initial rifting is for the whole embayment to subside. Since the end of the Paleozoic the basin has subsided to a structural trough allowing cyclic transgressions and regressions of the sea. The subsidence and cyclic deposits continued throughout the Cretaceous and Tertiary. Each invasion progressively stopped farther to the south during the Tertiary Period.
The deposits of Quaternary age blanket the Coastal Plain as sand, gravel, and clay occurring mostly as alluvial and terrace deposits. There are some deposits of loess east of the Mississippi River alluvial plain. These deposits make up the primary aquifer for the Mississippi Embayment region. The thickness of the alluvium varies but has a general range from 100 to 200 feet. The lower portions generally are composed of gravel and coarse sand, and the upper portion is typically made up of fine to medium sand and clay. (Cushing, et al. 1964). The structural history during the Quaternary Period includes the a few examples of general doming, faulting, and subsidence.
During the Pleistocene Glaciations, the sea level had great fluctuations. The lowest level in sea level allowed the Mississippi River to entrench deeper into Cretaceous and Tertiary sediments. The lower base level along with the abundant meltwater from melting glaciers produced large volumes of water over 100 feet deeper than the present day surface of the Mississippi Alluvial Plain (Arthur and Taylor, 1998). These lower units are gravels course sands. After the sea level rose, stream gradient decreased and the valley was sedimented in to its present level.
Much of the continental interior's of buried valleys and terraces is similar to the embayment's terraces and burried channels. These of course can be correlated through out river systems. The general idea is the change in base level to create these terraces and buried valleys. The idea that the valleys are buried is attributed to the sedimentation associated with the raise in base level while the river system maintains the same cannel. Although deeper and older formation produce some groundwater to the region, it is the Quaternary sediments that produce the high yielding aquifers and is more abundantly used in the region.
The New Madrid Seismic Zone is an area at the northern end of the Mississippi Embayment and extends 120 miles. This is the remnant zone of the Reelfoot Rift during the break up of supercontinent Rodinia. This area is still active today causing multiple earthquakes on a weekly basis, however these earthquakes are quite small. This zone is most familiar with three earthquakes from 1811 - 1812 which were so powerful that they caused the Mississippi to flow backwards. The largest one on February 7, 1812 had a magnitude exceeding 8 on the Richter scale. Activity today includes a . The Reelfoot Rift is named after Reelfoot Lake, which was created by the 1811-1812 earthquakes. The New Madrid Seismic Zone is a prime study area for neotectonics.
Through analysis of bed rock topography maps in Ohio, Indiana, and Illinois, there is a large system of buried valleys that alludes to an extensive proto-Ohio River system. During the Pleistocene Glaciations of the area, these large stream networks became buried and diverted. The remnants of the Teays River is preserved as flat-bottom valleys in hills south of the glacial limit. Deep valleys now filled in with sediment and capped with till are all that is left. Prior to the Pleistocene glaciation, the Teays River and Missouri River were the largest tributaries to the Mississippi River.
Geologically, the Mississippi Embayment is a syncline that plunges into the Gulf of Mexico. The thick accumulation is attributed to gradual basin subsidence. The Mississippi River basin was a massive meltwater drainage channel during the Pleistocene. During the lowest level of sea level, the Mississippi carved deep channels with the abundant water released from the Glaciers. Also, at the highest level of the sea before the Pleistocene, high terraces were left with the raised base level. These sediments are used today to pump large amounts of ground water. Neotectonics occur at the New Madrid Seismic Area at the far north end of the embayment. Activity in this area is a large potential hazard and is monitored for future earthquakes.
Arthur, J. K. and R. E. Taylor. (1998). Ground-Water Flow Analysis of the Mississippi Embayment Aquifer System, South-Central United States. USGS Professional Paper 1416-I. 48 p.
Cushing, E. M.; Boswell, E. H.; Hosman, R. L. (1964). General Geology of the Mississippi Embayment. USGS Professional Paper 448-B. 28 p.
Hansen, M. C. (1995). THE TEAYS RIVER. GeoFacts. No. 10. Ohio Department of Natural Resources, Division of Geology. <Web link> Accessed Nov. 31, 2004.
USGS. (2000). The Mississippi River's Alluvial Plain: A Tapestry of Time and Terrain <Web link> Accessed Nov. 31, 2004.
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