The New Madrid seismic zone of the Central United States - Intraplate Earthquakes - page 172

Figure 7.6 A west-east cross-section through the Mississippi River flood plain alluvium and the
ancient Mississippi River flood plain sand and gravel (Upland gravel) located 20 km north of

Memphis in Figure 7.1 (from Van Arsdale et al ., 2007 ) .

groups reflect alternating shallow marine, near shore, and fluvial environments as the Gulf
of Mexico transgressed and regressed within the Mississippi Embayment. The last marine
excursion into the Mississippi Embayment was during the Oligocene (34-24 Ma).
During early Pliocene (5.5-4.5 Ma) the ancestral Mississippi River valley looked very
much like it does today in the NMSZ region. A major river system flowed south within
a vast flood plain that covered portions of the same states that the Mississippi River
flood plain covers today. However, the Pliocene ancestral Mississippi River flowed at a
higher elevation, perhaps as much as 100 m higher than today's Mississippi River (Van
Arsdale et al ., 2007 ) . Evidence for this high-level Pliocene river is the sand and gravel
Upland Complex (Mississippi River terrace) discontinuously preserved on drainage divides
east of the modern Mississippi River from western Kentucky to southern Louisiana and
west of the Mississippi River on Crowley's Ridge in Arkansas ( Figure 7.6 ) (Autin et al .,
1991 ; Saucier, 1994 ) . The Upland Complex varies in thickness from 1 to 100 m primarily
because it has an unconformable base and top. The Upland Complex overlays Eocene and
Oligocene sediments and is overlain by Pleistocene (1.8 Ma-10 ka) loess (Clark et al .,
1989 ; Markewich et al ., 1998 ) . This loess consists of up to five loess units that together
reach a thickness of 30 m beneath Memphis, Tennessee.
Entrenchment of the ancestral Mississippi River valley began at 4 Ma apparently due

to growth of the Antarctic and Greenland ice sheets and resulting sea level decline. The
valley has experienced a complex erosional and depositional Pleistocene history tied to
the advance and retreat of perhaps 20 Laurentide ice sheets (Easterbrook, 1999 ) . During
the Pleistocene, the ancestral Mississippi River flowed down the Western Lowlands and the
ancestral Ohio River flowed down the Eastern Lowlands and the two rivers merged south
of Helena, Arkansas (Saucier, 1994 ; Van Arsdale et al ., 2007 ) . During the Pleistocene
their point of convergence jumped northward several times until reaching Cairo, Illinois, at
10 ka.