TECTONIC SETTING FOR THE LlGNITE-BEARING SEDIMENTS OF THE MISSISSIPPI
EMBAYMENT-A STRUCTURAL INVERSION
	CHESNUT, Donald R., Jr., Kentucky Geological Survey, 228 MMR Bldg., University of
Kentucky, Lexington, KY 40506-0107, chesnut@kgs.mm.uky.edu.

Cretaceous and younger lignite deposits of the upper Mississippi Embayment are interbedded with clastic 
sediments deposited in coastal settings. These largely unconsolidated sediments fill a concave bedrock 
depression or basin with an axis located along the Mississippi River. The sedimentary basin is herein termed 
the Paducah-Memphis Basin; the term 'Mississippi Embayment' should be restricted to physiographic usage. 
Although the surface expression of the bedrock-sediment contact is basin shaped, subsurface cross sections 
show that the Paleozoic bedrocks form a truncated structural arch redefined here as the Pascola Arch. The 
unusual structural relationship between the Paleozoic rocks and the post-PaleozoIc sediments is an 
aulacogenic structural inversion. The following aulacogenic cycle is responsible for development of the arch 
and the overfying basin.
	During the Cretaceous, a thermal event in the area of the present Mississippi Embayment caused 
the crust to become more buoyant and the Pascola Arch was formed. The uplifted area was exposed to 
erosion that removed a large part of the Paleozoic rocks. Chert gravels and cobbles, eroded from Paleozoic 
rocks on the uplift, were redeposited along the flanks of the uplift by debris flows and braided streams (the 
'Tuscaloosa' gravel). Continued erosion reduced the uplifted area to a nearly flat plain with local relief of 
about 100 to 150 feet. As the thermal event ended, the crust cooled, became less buoyant, and subsided. 
However, because a large part of the Paleozoic rocks were eroded, the crust contained less mass, and 
accordingly did not subside to the same depth as before the uplift. After subsidence, the Pascola Arch, 
although of less structural relief than before, still existed as a structural arch. The subsidence, however, was 
sufficient to cause the erosional surface to change slope and become the Paducah-Memphis Basin. The 
newly formed basin was filled with Late Cretaceous and younger sediments that were deposited 
unconformably over the Paleozoic rocks. Alternating marine and lignite-bearing terrestrial sediments, once 
attributed solely to repeated tectonic events, may have been influenced more by eustatic changes in sea level.