H. B. Willman and John C. Frye
The Tertiary System (Arduino, 1760) is a name retained from one of the earliest classifications of rocks.
Extent and Thickness
The Tertiary System is extensive only in the extreme southern part of Illinois (figs. K-3, T-1), but it also occurs in small widely scattered areas in western and northern Illinois (fig. T-1).
The Jackson Formation of the late Eocene occurs in Kentucky only a short distance south of Illinois, and Oligocene and Miocene deposits occur in the Coastal Plain Embayment area farther south.
Progressive sinking of the embayment area resulted in the relatively rapid southward thickening of the pre-Pliocene Cretaceous and Tertiary deposits and gave them a southward dip significantly greater than that of the Pliocene sediments. Tertiary sediments have a maximum thickness of about 400 feet in the vicinity of Cairo, Alexander County.
Tertiary sediments in southern Illinois are included with Cretaceous sediments in the Embayment Megagroup. The Paleocene and Eocene Series (fig. T-2) are present as Coastal Plain sediments in extreme southern Illinois, where they are separated from overlapping Pliocene deposits by a major unconformity. Late Eocene, Oligocene, and Miocene sediments are absent, although some sediments of these series could have been deposited and later eroded during development of the sub-Pliocene unconformity.
The Paleocene sediments are largely marine clays and sands, whereas the Eocene sands and silty clays indicate a return to nonmarine deltaic sedimentation like that operating during the Cretaceous. The Pliocene sediments are mostly fluvial deposits of a continental environment.
ARDUINO, GIOVANNI, 1760, Nuova raccolta di opuscoli scientifici e filologici del padre abate Angiolo Galogierà: Venice, Italy, tom. 6, p. 142-143.
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