Kaskaskia between the full immersion of the Silurian and the early Mississippian. Finally, the Absaroka sequence emplaced into the early Pennsylvanian. The Great Unconformity separates Proterozoic igneous rocks from the Lamotte Sandstone, the basal unit of the Sauk sequence which also includes the Bonneterre Dolomite, Davis Formation, Derby-Roe Run Dolomite, Potosi Dolomite and Eminence Dolomite from the Cambrian, and the Ordovician Gasconade Dolomite, Roubidoux Formation and Jefferson City Dolomite. The Tippecanoe comprises the St. Peter Sandstone, Joachim Dolomite, Plattin Limestone, Decorah Shale, Kimmswick Limestone, and an unconformity, followed by the Girardeau Limestone (only in the southeast), Maquoketa Shale and northeast Missouri Noix Limestone. Silurian Tippecanoe rocks include only two units: the Bainbridge Formation in the southeast and Bowling Green Dolomite in the northeast. Devonian Kaskaskia rocks include the Chattanooga Shale in the southwest, Cedar Valley Limestone, split by an unconformity and separated from Mississippian rocks by a second unconformity, with numerous small shale and limestone units in the east. A complicated sequence of shale, sandstone and limestone follows from the Carboniferous. Sedimentary petrologists have debated why dolomite is so prevalent in Missouri relative to limestone, even though marine conditions often favor limestone formation. Only a few dolomite beds formed after the Middle Ordovician and few are younger than the Joachim Dolomite. Fault fractures suggest that groundwater altered limestone to dolomite, replacing calcium with magnesium. Mesozoic (251–66 million years ago) The Zuñi sequence formed in the Mesozoic, during a fifth major marine transgression lasting from the Middle Jurassic into the Cretaceous. The Cretaceous geochronology of the state is unclear and debated, between some who believe the state experienced dryland conditions and others who suspect it was flooded. The only Cretaceous deposits are unconsolidated marine and delta sands together with clay in the Southeast Lowlands, beneath Pleistocene alluvium. Previously, small circular deposits of Cretaceous clay in the Ozarks were suspected to be the remains of sinkholes in Ordovician dolomite. However, recent fossil discoveries suggest possible small lakes and ponds. Cenozoic (66 million years ago – present) A final marine transgression called the Tejas sequence took place in the Cenozoic. Aside from clay and both delta and marine sandstone in the Crowley Ridge, deposited in the subsiding Mississippi Embayment, there are no Paleogene rocks in the state. The Midway Group deposited in the Paleocene, rich in bentonite, followed by the Eocene Wilcox Group, which contains terrestrial sandstone and clay laden with plant fossils.