MISSISSIPPI RIVER AND GULF COAST TERRACES AND SEDIMENTS AS RELATED TO PLEISTOCENE HISTORY--A PROBLEM

摘要

Direct evidence of low and high sea levels accompanying Pleistocene glacial and interglacial Stages is being sought in marine terraces on the East and West Gulf coast, in fluvial terraces and deltaic plains in the valley of the Mississippi River, and in sediments of the northern Gulf Coastal Area and adjacent Continental Shelf. Marine terraces on land in Georgia, Florida, Alabama, and Mississippi suggest interglacial stages when sea level was high. The shorelines are generally considered to range from the highest and oldest to the lowest and youngest. A net uplift of the land of the East Gulf Coastal Plain relative to sea level during the Pleistocene is indicated. This is thought to have been caused by tectonic forces rather than by progressively decreasing degrees of deglaciation. However, much difference of opinion exists concerning the number and elevations of old shore lines and especially concerning their ages from Pliocene to Thermal Maximum. Shore features are notably lacking on land along the Louisiana and Texas coasts. Pleistocene subsidence on this part of the Gulf Coast is evident. Four alluvial terraces and recent alluvium in the lower Mississippi Valley are thought by Fisk and Russell to record stream deposition in Aftonian, Yarmouth, Sangamon, Brady, and Post-Mankato times, and deep erosion during intervening glacial intervals. However, Horberg, Leighton, and Willman consider that three of these four terraces are pre-Pleistocene and that the fourth is a valley train deposited during the Mankato glacial Subage. Sedimentary formations on the Gulf Coast give some evidence of repeated emergence and erosion followed by submergence and deposition, but again the number of formations and disconformities and their Pleistocene ages are uncertain. Known occurrences of sand and gravel and of bioherms in deep water near the outer edge of the Continental Shelf suggest one or more low stands of sea level, but their ages are unknown. It seems doubtful that any part of the Mississippi Valley and Gulf Coast area remained stable tectonically during the Pleistocene. Future complete cores from present sedimentary surface to Tertiary on coastal land or tideland may yield a complete record of Pleistocene history.