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Bedrock Geology of the Thebes Quadrangle Alexander County, Illinois, and Cape Girardeau and Scott Counties, Missouri

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Bedrock Geology of the Thebes Quadrangle Alexander County, Illinois, and Cape Girardeau and Scott Counties, Missouri STATEMAP Thebes-BG Bedrock Geology of the Thebes Quadrangle Alexander County, Illinois, and Cape Girardeau and Scott Counties, Missouri Joseph A. Devera, Jeremy R. Breeden, and Franck Delpomdor 2017 Prairie Research Institute ILLINOIS STATE GEOLOGICAL SURVEY 615 East Peabody Drive Champaign, Illinois 61820-6918 (217) 244-2414 http://www.isgs.illinois.edu © 2017 University of Illinois Board of Trustees. All rights reserved. For permission information, contact the Illinois State Geological Survey. Introduction Mesozoic Era sediments. The Mississippi River changed its course from flowing west at Grays Point to the south by The Thebes Quadrangle is located in extreme southwestern cutting through indurated Paleozoic bedrock at Thebes Gap Illinois and southeastern Missouri. This report covers only during the Late Pleistocene 10,000 yrs. BP (Blum et al., the Illinois portion of the Thebes Quadrangle. The Thebes 2000). The Gulf Coastal Plain sediments also blanket the Quadrangle contains rocks and sediments that were depos- southern portion of the quadrangle from east to west. Paleo- ited during the Paleozoic, Mesozoic and Cenozoic Eras. gene sediments are confined to structural grabens, whereas The oldest formation in the study area is the Kimmswick Neogene sediments cap the ridges, and terrace the valleys. Limestone (Ordovician) the youngest deposit is Holocene The bedrock hills are highly dissected, in some cases along alluvium. The Thebes Quadrangle occurs south of the Il- structurally or vertical joint controlled valleys. The bed- linoisan glacial boundary and most Quaternary deposits rock valleys are choked with Pleistocene to Holocene chert were not drawn on the map. The loess deposits that blanket gravels derived from the local thick, cherty limestones and the upland surfaces, were not depicted in order to better silicified beds of Bailey and Grassy Knob Chert. portray the bedrock. Not depicting 20 to 65 feet of loess on some hilltops creates an apparent over thickened bedrock Previous Work unit in some places. Only Holocene alluvium (Cahokia) was mapped for differentiation of stream valleys mainly for The earliest work in the Thebes area was that of Fielding aesthetic appearance on the map. The main purpose of this Meek and Amos Worthen (1866). They originally named map was to show bedrock relationships, tectonic fabric, and the Cincinnati Group which included the Thebes Sandstone stratigraphic successions. and the Cape Girardeau Formations. T.E. Savage (1909) proposed the Alexandrian Series that was composed of the The study area lies along the northern extent of the Reelfoot Girardeau Limestone and Edgewood Limestone which he Rift and includes a structural feature called the Commerce incorrectly put into the basal Silurian. These two formations Geophysical lineament (CGL). The CGL has an approximate are now considered to be uppermost Ordovician Hirnan- trend of N 40°E. Also antithetic to the CGL is the southern tian Stage (Amsden and Barrick, 1986). The Edgewood as extremity of the Ste. Genevieve and Delta Fault Zones along it occurs in the study area is currently called the Leemon with the N 70°W trending Pascola Arch. The Ozark Dome Formation (Thompson and Satterfield, 1975). Savage (1909) affected the study area during the mid-Paleozoic, and there is also proposed the Orchard Creek Shale and Sexton Creek an igneous intrusion near Elco, Illinois northeast of the study Limestone. A later work describes the faunas of the Thebes area. Deep seated Precambrian tectonic structures also trend Sandstone and Orchard Creek Shale (Maquoketa equiva- northwest in Missouri and Illinois, a local example is the lent), (Savage, 1917). The first geologic map of the area Commerce Anticlinorium. Polyphase tectonic movements by Savage was published later with revisions by J. Marvin accompanied by the rotation of the primary stress direction Weller and George E. Ekblaw (1940). Further development has created a complexly faulted region. Tracing faults over a of the stratigraphic classification was the change of the Fern- long distance was difficult due to the small offsets observed vale Limestone to the Ada Limestone by Gutstadt (1958) on many of the structures. Most faults observed yielded less but Templeton and Willman (1963) stated that the name Ada than 50 feet of displacement. Poor exposure due to thick had been taken and so they used the term Cape Limestone. A loess, up to 65 feet, also hampered the tracing of individual geologic map including the current study area was produced faults. In addition, small scale stratigraphic off-sets along by Pryor and Ross (1962). The most recent work on Paleo- structures were hard to detect in the thicker units such as the zoic stratigraphy is the Handbook of Illinois Stratigraphy, Bailey Limestone. However, erosion of the wind-blown silt (Willman et. al., 1975). The latest geologic map of the in deep ravines and valleys exposed bedrock and made it Thebes Quadrangle was published by the USGS (Harrison, possible to map the stratigraphic contacts with confidence, 1999). The latest stratigraphic work is a revision of the Moc- and project fault zones throughout the study area. casin Springs Formation into four distinct members: Greens Ferry, Sheppard Point, Lithium and Randol (McAdams, Physiography of the Quadrangle 2016). This Silurian nomenclature of McAdams is followed in this report (Figure 2 on map sheet). The highest point in the quadrangle is 720 feet above mean sea level in the northeastern corner of the quadrangle. This Methods bedrock high is a part of the Ozark Plateau Physiographic Province. The Ozark Plateau is composed of Paleozoic rocks Geologic mapping of the Thebes Quadrangle involved that are on-lapped by unconsolidated Cretaceous sediments numerous aspects: the primary source of data was obtained derived from the Gulf of Mexico and continental drain- from traversing most of the ravines, core drilling data from age older than 66 million years. The Thebes Quadrangle Union Pacific Minerals (UPM), ISGS core drilling, IDOT is bisected by the Mississippi River from north to south. engineering borings, and water well data. A secondary Thebes Gap is the area where the Mississippi River, rela- source of data was historical ISGS field notes. tively recently, incised through the Paleozoic bedrock and 1 Series Stage System Formation Member Graphic Column Thickness (ft.) Sexton Creek 40 SILURIAN Aeronian Llandovery Leemon 0-10 Girardeau 0-40 Hirnantian Orchard Creek sh. 5-30 Maquoketa Formation Thebes ss. 10-80 Upper Cape La Croix sh. 5-23 0-17 ORDOVICIAN Cape Limestone Katian Kimmswick Limestone 100-125 Figure 1 Ordovician stratigraphy in the Thebes Quadrangle. Samples were collected from measured sections and various This unit is a calcium-rich carbonate rock that is dominated outcrop locations. Samples and cores were processed for by disarticulated crinoids, with some cystoids, dasyclade conodont, chitinozoan and graptolite determinations. Carbon green algae called Fisherites oweni (Receptaculites), trilo- and oxygen isotopes and trace elements were processed for bite genera: Illaenus, Scutellum, Amphilichas, Gabricerau- biochemostratigraphy determinations because the lower rus, Thaleops, Encrinurus, Isotelus gigas, Remopleurides Bailey is thought to contain the Klonk carbon excursion and Calyptaulax. Other fauna include ostracodes, numerous (McAdams, 2017). Klonk Hill in Czechoslovakia is the trepostome bryozoans, and the brachiopod genera: Rafines- Global Stratotype Section and Point (GSSP) for the Silu- quina, Strophomena, Dalmanella, Hebretella, Sowerbyella rian/Devonian Boundary. Thin sections were also prepared and Pionodema. The gastropods Hormotoma and Maclurites, from collected samples for petrographic analysis on several along with nautiloid cephalopods, echinoderms, conodonts lithologic facies. Photos were taken of important outcrops, and rugose corals. Taphonomy of the body fossils shows structural features, fossils, stratigraphic and sedimentologic transportation with disarticulation and fragmentation at least features. in the 40 feet observed. The list of fossil genera is only a partial list of fossils found while mapping in the Thebes Stratigraphic Units and Depositional Settings Quadrangle. Larger lists of fossils from the Kimmswick have been published from outside the study area. Kimmswick Limestone (Ordovician, Upper Mohawkian to basal Cincinnatian Series, Katian Stage) The Kimmswick Limestone is a shallow platform carbonate Equivalent to the Galena and Trenton Limestones the that was deposited in the near-shore wave dominated envi- Kimmswick (Fig. 1) is a white, fossil grainstone to pack- ronment. Thin sections show a fossil grain-supported fabric stone. This unit exhibits a coarsely crystalline texture be- with sparry calcite cement filling interstitial space. Two cause of the large fossil grains present. Bedding is medium generations of cement are found: a primary drusy cement to thick and slightly wavy with lateral accretion sets contain- lining grains and a second generation blocky equant sparry ing abundant stylolites. Only the upper 40 feet is exposed in calcite cement. High biodiversity reflects a warm tropical the quadrangle along the Mississippi River from just north climate that probably existed at the time of deposition. The of Thebes, Illinois to a mile south of the village. The upper wide spread presence of the coralline, green algae is good contact is disconformable with the overlying Cape Lime- evidence of a shallow marine setting. Lateral accretion bed-
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