Geology of Kentucky

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The oldest Precambrian igneous and metamorphic crystalline basement in Kentucky is only known from gravitational studies and boreholes and does not outcrop at the surface. The earliest rocks formed in the Proterozoic, east of the Grenville Front, as part of the continental crust of the Grenville Province, which collided with Proto-North America, kicking off the Grenville orogeny. In northern Kentucky mafic, felsic and metasedimentary rocks have similar abundances. A study in 1985 of 42 boreholes, spanning into central Ohio found 14 percent gabbro, eight percent metagabbro, 12 percent amphibolite, 12 percent hornblende gneiss and schist and two percent quartz diorite. Additionally, there is 33 percent granite, two percent granodiorite gneiss, seven percent metasedimentary rocks and 10 percent granite gneiss.

Metamorphism reached amphibolite-grade in the sequence of metamorphic facies. The northeast has slightly higher concentrations of granite as well as anorthosite.^[1]

Throughout the Ordovician, Kentucky was covered in shallow tropical seas, depositing limestone, dolomite and shale. The region was the flooded continental shelf of Laurentia, situated in the Southern Hemisphere as part of the Iapetus Ocean, based on reconstructed paleogeography. The Camp Nelson Limestone along the Kentucky River gorge between Frankfort and Boonesboro dates to the Middle Ordovician and is the oldest rock exposed at the surface in the state. Additional mudstone and shale formation from the Late Ordovician indicates a shift to a shallower sea, which supported extensive marine life. Brachiopod, bryozoan, coral, sponge, crinoid, trilobite, conodont, ostracode and nautiloid fossils are very common in stream beds and road cuts in the Bluegrass Region.

Silurian rocks are not exposed in the Bluegrass Region, but is common in the surrounding Knobs Region. High sediment loads prevailed in the shallow tropical sea at first, forming additional shale. Silt and clay which formed the shale appears to have originated from the east, forming alternating units between carbonate layers. Shelly invertebrates and corals form the bulk of limestone and dolomite from the period. The Cincinnati Arch formed during the Silurian due to gentle folding and upwarped parts of the region above the water level, separating Kentucky into two geologic basins.

The upwarping of the arch continued into the Devonian, which is supported by the lack of Early Devonian rocks in central Kentucky where the arch formed. Devonian rocks are only exposed at the surface in the Knobs Region and are not present in the Bluegrass Region, although they are known from the sub-surface throughout the rest of the state. For the most part, shallow sea conditions prevailed with areas experiencing dry land conditions. By the late Devonian sea levels increased, resulting in poorly oxygenated deep water. Organic-rich black shales formed, ultimately producing oil and gas which is now extracted. Black shales hold broadly similar fossil assemblages to Silurian and Ordovician deposits, but have particularly well preserved calcareous stromatoporoid sponge fossils, which are found at the Falls of the Ohio close to Louisville.

By the Mississippian, Kentucky remained largely submerged and still south of the equator, but related to the merged continental mass of Euramerica. The closing of the Iapetus Ocean created the narrower Rheic Ocean, as the supercontinent Gondwana moved toward Euramerica. The Pennyroyal Region has exposed Mississippian rocks. Black shale deposition continued through the Early Mississippian, but rivers and streams shifted the sediment load to terrestrial material from the Appalachian uplands further east, delivering sand, mud and silt.

When sea levels dropped in the Middle Devonian, massive limestone formations took shape which now form much of the state's karst landscape and host the world's largest cave, Mammoth Cave. Coastal plains and tidal deltas periodically displaced the sea. By the Devonian, proliferation of fishes and land life increased the variety of fossils. Fish teeth became common alongside invertebrates and scale ferns, tree ferns, calamite trees and large amphibians lived on the land in estuaries and ox-bow lakes. In 1995, a single five foot long embolomere amphibian fossil was found in sandstone, near the margin of the Western Kentucky Coal Field.

In the Pennsylvanian, shallow seas existed periodically, but the landscape was mostly swampy land. As the supercontinent Pangaea took shape, Kentucky was situated on the equator. Grasses and cordaite trees joined the fossil record and dead vegetation was encapsulated in silts and sands, capped by delta clays, preventing oxidation. This material formed Kentucky's two main coal fields.

The Late Permian, the Triassic and the early part of the Cretaceous—a span of 200 million years—is missing from the stratigraphic record. The rifting apart of Pangaea formed the Atlantic Ocean as North America shifted into the Northern Hemisphere. Small outcrops from the Cretaceous at the end of the Mesozoic are found in southwest Kentucky, near the state line with Tennessee, in the Jackson Purchase Region and parts of Mississippian Plateau. These sands, clays and gravels deposited at the flooded eastern edge of the Western Interior Seaway and never consolidated into rock.

In the Paleogene and Neogene, in the Cenozoic brackish, freshwater and marine sediments deposited in the Jackson Purchase Region, the northern limit of the Mississippi Embayment. Thin beds of lignite and carbonaceous clays are common in the eight counties of the region.

Unlike neighboring states, with landscapes heavily changed by glaciations during the Pleistocene, ice sheets only reached Covington, Kentucky. Glaciation changed the course of the Ohio River, in the vicinity of Louisville and Cincinnati where glacial melt waters filled the valley with sand and gravel. Lake bottom sediments deposited locally where the Kentucky, Green, Licking and Salt rivers were temporarily impounded by glacial debris.^[2]