Geology of the County

In Marion County, water is obtained from consolidated sedimentary rocks of Ordovician, Silurian, Devonian, and Mississippian ages, and from unconsolidated sediments of Quaternary age. The oldest rocks found on the surface in Marion County, in the Clays Ferry Formation, were deposited in shallow seas 490 million years ago during the Ordovician Period. During the Late Ordovician, the seas became relatively shallow, as indicated by the amounts of mud (shale) in the sediments. When the waters were clear and warm, a profusion of animal life developed, particularly brachiopods and bryozoa. Lying on top of the Ordovician rocks are the Silurian rocks, which were also deposited in warm seas, 430 million years ago. In Kentucky, the Silurian seas were commonly warm and clear, although the presence of some shale beds suggests that muddy conditions prevailed at times. Locally, numerous corals and brachiopods can be found in the Silurian limestones and dolomites. Above the Silurian lies the New Albany Shale, also called the black shale. The black shale was formed 400 million years ago during the Devonian Period, when the deep sea floor became covered with an organic black muck. The muck is now hard black shale (an oil shale) and is one of the most distinctive of all geologic formations in Kentucky. The Mississippian sandstones and siltstones are the result of a great influx of mud, silts, and sands brought in by rivers and streams from uplands many miles away and deposited as a great delta. The Mississippian limestones found in Marion County were deposited 350 million years ago in the bottom of a warm, shallow sea. Over the last million years, unconsolidated Quaternary sediments have been deposited along the larger streams and rivers.

Geologic Formations in the County
Unconsolidated deposits
Alluvium (Qa)

Ste. Genevieve Limestone (Mgl)
St. Louis Limestone (Mgl)
Salem, Harrodsburg Limestones (Msh)
Borden Formation (Muldraugh (Mbf)
Lexington Limestone (Ol)
High Bridge Group (Ohb)

Knox Group (Okx)

Interbedded clay shales, siltstones, and sandstones
Borden Formation (Halls Gap, Nancy, New Providence Members) (MDbb)

Fractured shales
New Albany Shale (MDnb)

Interbedded limestones and shales
Laurel Dolomite (Slb)
Osgood Formation (Slb)
Brassfield Formation (Slb)
Drakes Formation (Saluda Dolomite, Bardstown, Rowland Members) (Od, Odc) & Bull Fork Formation (Ob)
Grant Lake Limestone, Calloway Creek Limestone (Oaf)
Clays Ferry Formation (Okc)

For more information, see the definitions of geologic terms and rock descriptions, a geologic map of Marion County, a summary of the geology of Kentucky, and a discussion of fossils and prehistoric life in Kentucky.

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