Kentucky Agate: State Rock and Mineral Treasure of the Commonwealth

Chapter 4. Geological Setting

31 4 Geologic Setting Adescription of the geology in east-central Kentucky involves a dynamic saga of the earth’s restless movements that occurred millions of years ago. This area is part of the western flank of the Appalachian Basin, which subsided over millions of years by the accumulation of thick sedimentary rock deposits. The area is east of the Cincinnati Arch (figure 6), which is a north-trending structural hinge line that was fairly stable and did not subside as much as the basin. Faulting occurs throughout the basin and the agate areas. The Estill County seat, Irvine (see figure 2), is located along the Irvine–Paint Creek Fault System, a major northeast-trending basement fault. The agate rocks in central Kentucky are exposed on the surface, whereas the same type of agate-bearing rocks in the central part of the basin are much deeper, about 3,000 feet below the surface. Parts of the area were mapped geologically during the 1960s and 1970s by Simmons (1967), Greene (1968), Haney (1976), Haney and Rice (1978), Hoge and others (1976), and Rice (1972). These geologic maps provide a good foundation for interpreting and understanding the basic geology of the area; they can also be used to locate agates (figure 7). In addition to geologic mapping, much research has been done in the area of stratigraphy, structure, and tectonics (Peterson and ky agate.indd 31 7/27/13 4:47 PM 32 Kentucky Agate Kepferle, 1970; Chaplin, 1980; Dever and others, 1980, 1999, 2002; Ettensohn and others, 1984, 2002; and Ettensohn and Chesnut, 1989). These reports discuss the clastic and carbonate sedimentology of the area, faulting and tectonics, mineralization, and the formation of the Borden Delta Front, an important sedimentary feature that preceded the deposition of shallow marine carbonates over the area and was influential in the development of agates. Depositional Environment Rocks and agates were formed during a time when large deltas and subsea river systems deposited sediment in the central part of the state, much like the Mississippi River deposits sediment in its delta near New Orleans, Louisiana, today. In Kentucky the Borden Delta flowed westward over east-central Kentucky before the area was inundated by a shallow sea, an event referred to as a marine transgression. Over time, the repetitive deposition of the silica-rich Borden sediments formed the upper part of the Nada Member of the Borden Formation (figure 8). By the end of this delta sedimentary cycle, a major marine transgression swept over the area and covered most of east-central Kentucky. These seas were widespread, and over the course of millions of years they deposited sediments that were transformed into the limestone and dolomite called the Slade Formation. Before and during this deposition of sediments, extensive faulting caused uplift and erosion in the rocks. As the shallow seas deposited limestone and dolomite, movement of silica-rich fluids ky agate.indd 32 7/27/13 4:47 PM Geologic Setting 33 through the sediments formed a complex chemical environment in which agates were deposited. These rocks have a gradational contact in the Borden and Slade formations, where they sometimes form an intertongued sequence and overlie one another. It is within this gradational contact that most of the agates occur (figure 9). Millions of years after the rocks were deposited, modern-day erosion dissected the Cumberland Plateau in eastern Kentucky, which liberated these agates from the grasp of nature, releasing them into the hillsides and streambeds. Stratigraphy and Structure of Agate-Bearing Rocks The principal agate-hosting formations in east-central Kentucky are the Slade and Borden formations of the Mississippian subperiod, about 300 million years old. They can be subdivided in descending order into the Renfro Member of the Slade Formation and the Nada and Cowbell members of the Borden Formation (figure 8). Agates occur along the contact between the Renfro Member and the Nada Member in Estill, Lee, Madison, and Jackson counties. The area near the Irvine–Paint Creek Fault System in Estill County (figures 7 and 8) is structurally complex. The Irvine–Paint Creek Fault is a large, deep-seated basement fault transected by two north-trending faults. The occurrence of fault systems near the agate locations may be more than coincidence; it may suggest that faulting played a role in agate formation. Subsurface fluids often travel through rocks along faults and fractures, which may explain the deposition of agates in this...