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Mesozoic and Tertiary Eras 11 W. John Nelson INTRODUCTION Triassic Period This chapter covers a long span of Earth’s history dur- As outlined in Chapters 3 (Tectonic History) and 4 ing which many geological events occurred in Illinois, but (Structural Features), all of the world’s continents came for which little direct evidence remains. Mesozoic and Ter- together to form a single giant landmass, Pangea, near tiary strata occur only as scattered erosional remnants in the end of the Paleozoic. The shallow seas that previously western and southernmost Illinois. To deduce what was covered Illinois and surrounding regions withdrew dur- happening in Illinois during this time, rocks of other U.S. ing the Permian. Situated close to the equator and isolated regions must be considered. from ocean breezes by the newly formed Appalachian and The Mesozoic includes the upper part of the Absaroka Ouachita Mountains (Figure 11-2), Illinois undoubtedly sequence (Chapter 10, Pennsylvanian and Permian) and had a torrid desert climate as the Permian passed into the most of the Zuni sequence (Sloss 1963, 1988; Sloss et al. Triassic; erosion, rather than deposition, prevailed. 1949). The Tertiary comprises the uppermost part of the During Late Triassic time, Pangea began to break apart Zuni and the overlying Tejas sequence, which extends (Chapter 3, Tectonic History). Asia and Australia drifted through the Quaternary to the present day. Because the re- westward away from the Americas as Europe and Africa cord of these sequences in Illinois is so fragmentary, this drifted eastward, creating the Atlantic Ocean. The Gulf of chapter is organized according to the U.S. Geological Sur- Mexico began to develop. A series of fault-bounded basins vey, Geologic Names Committee (2007). along the Atlantic and Gulf Coastal Plains (Figure 11-2) were filled with thick successions consisting largely of red, terrestrial sediments and basaltic lava flows (King 1977, ESOZOIC RA M E Byerly 1991, Salvador 1991). The Mesozoic (from Greek meaning “middle life”) Era Many faults in southern Illinois probably became ac- was the period of Earth’s history between approximately tive. Normal faults in the Wabash Valley Fault Zone may 250 and 65 million years ago when the dinosaurs flour- have developed in response to Gulf Coast rifting. Normal ished. The era is popularly known as the “Age of Reptiles” faulting also took place in the Rough Creek-Shawneetown and is divided into three periods: Triassic (oldest), Jurassic, Complex and Fluorspar Area Fault Complex some time and Cretaceous. Only the Upper Cretaceous rocks are pre- during the Mesozoic; the Triassic is the most logical time served in Illinois (Figure 11-1). for such faulting to have occurred (Nelson and Lumm 1987, Kolata and Nelson 1991). CHRONOSTRATIGRAPHIC UNITS SOUTHERN ILLINOIS In the western United States, the ancestral Rocky NORTH (south of 40° North latitude) GLOBAL AMERICAN Mountains were being worn down. Streams and wind car- ried and deposited red clay, silt, sand, and gravel. The east- Formation and thickness Group STAGE STAGE SERIES SERIES SYSTEM SYSTEM Sequence H. H. CHRONOSTRATIGRAPHIC Glacial drift, loess, and alluvium 0-200' UNITS NORTHERN ILLINOIS QUATER- NARY PLEISTO- CENE PLEISTO- CENE QUATER- NARY NORTH (north of 40° North latitude) P./ M. P./ M. Mounds Gravel 0-70' Grover Gravel 0-50' GLOBAL AMERICAN Tejas Tejas Claiborne/Wilcox Fm 0-300' EOCENE EOCENE Formation and thickness Group STAGE STAGE SERIES SERIES SYSTEM SYSTEM Sequence TERTIARY Porters Creek Fm 0-150' PALEO- CENE PALEO- CENE Midway Clayton Fm 0-20' HOLO- CENE HOLO- CENE Glacial and postglacial Owl Creek Fm 0-10' ? ? deposits including till, water-laid sediments, loess, and paleosols Zuni McNairy Fm 0-450' Tejas 0-500' QUATERNARY QUATERNARY UPPER MAASTRICH- TIAN GULFIAN PLEISTOCENE PLEISTOCENE Post Creek (Tuscaloosa) Baylis Fm 0-100' CRETACEOUS TERTIARY CP. CRETACEOUS Z. Baylis Fm 0-100' CT. CT. CP. Fm 0-75' So. IL W. IL UP. GF. Figure 11-1 Stratigraphic column of the Mesozoic and Tertiary strata of Illinois (Kolata 2005). Abbreviations: CP., Campanian; CT., Cretaceous; GF., Gulfian; H., Holocene; M., Miocene; P., Pliocene; Z., Zuni; Fm, Formation; So., southern; UP., Upper; W., western. 206.
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