CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY
FIGURE 12.-Area of outcrop of Cretaceous and Tertiary strata in Kentucky (shaded). |
On completion of the cooperative mapping program, the 47 geologic quadrangle maps covering this region were compiled into a single map at a scale of 1:250,000 by Olive (1980). The discussion below is taken largely from that more comprehensive report. Descriptions of a number of specific localities are available in a guidebook published during the mapping program (Olive, 1972), and a discussion of the underlying Paleozoic rocks has been prepared by Schwalb (1969).
The Cretaceous and Tertiary deposits of the Jackson Purchase region form the northeastern part of the Mississippi Embayment of the Gulf Coastal Plain. The beds dip gently and thicken westward toward the axis of the embayment (see cross section A-A' on the geologic map). The thickness of this wedge ranges from about 2,050 ft at the western edge of the State (Olive, 1980) to a pinch-out edge eastward in the vicinity of the Tennessee and Cumberland Rivers. The deposits consist mainly of unconsolidated marine and continental gravels, sands, silts, and clays which are generally concealed beneath alluvium, loess, and continental deposits of latest Tertiary and Pleistocene age. This sequence has been subdivided on the Geologic Map of Kentucky into five map units, which are described below. (For a more detailed discussion of the stratigraphy and environments of deposition, see Olive, 1980.) These map units include, in ascending order, the Tuscaloosa Formation, the Clayton and McNairy Formations, the Porters Creek Clay, the Wilcox Formation, and the Jackson and Claiborne Formations.
The formation rests unconformably on a paleokarst surface having a local relief of about 100 ft and represents fluvial deposits in which the original bedding has been locally modified by leaching of the subjacent carbonate rocks. The unit apparently extends downdip into the subsurface no more than 3 or 4 mi, where it is overlapped by the McNairy Formation. The configuration of the unit suggests deposition in a narrow trough opening to the south (Olive, 1972, p. 4). The thickness of the Tuscaloosa ranges from 0 to 180 ft.
Clayton and McNairy Formations (TKcm).--The McNairy Formation (Upper Cretaceous) and the overlying Clayton Formation (Paleocene) are combined on the map as a single unit. These two formations are separated by a regional disconformity that is marked by distinct faunal and floral changes (Tschudy, 1970). However, because of lithic similarity and poor exposures of the formational boundary, they cannot be distinguished in the field in most places.
The sequence consists mainly of light to dark-gray, fine to medium (rarely coarse) sand that weathers to yellow or reddish brown. The sand is interbedded with gray, black, or brown clay, a few lenses of chert gravel and silt near the base, and sparse lignite. The sand, dominantly angular to subangular quartz, is generally micaceous and locally contains pebbles and cobbles of clay. Bedding ranges from thin to thick, and crossbedding and cut-and-fill structures are common. Ophiomorpha are locally abundant in the sand. Clay, commonly micaceous, occurs in fine laminae to thick beds which contain leaf imprints or marcasite concretions in many places. Sand is dominant in the south, but the clay content increases progressively northward, and near Paducah the unit is mostly clay. The basal contact is an unconformity with as much as 150 ft of relief on the underlying Tuscaloosa Formation or Paleozoic rocks.
Paleontological evidence indicates that the McNairy Formation was deposited in a marine to freshwater deltaic environment, and predominance of freshwater over marine palynomorphs suggest that the Clayton Formation is deltaic or lacustrine (Olive, 1980). Pollen studies indicated that the climate was warm temperate to subtropical (Tschudy, 1970).
Porters Creek Clay (Tp).--The Porters Creek Clay of Paleocene age is divisible into three units: interbedded sand and clay in the upper and lower parts separated by a thicker dominantly clay unit in the middle. The clay is light gray to black or brownish gray. It is characteristically faintly mottled and slightly to very sandy, and it contains fine muscovite flakes and rare glauconite, biotite, and glass shards. Typically, the clay is brittle and breaks with a conchoidal fracture. In the upper and lower units, the clay contains carbonized, silicified, and rare pyritized fragments of wood. The sand is olive gray, medium to dark gray, and bluish gray and weathers yellowish orange or reddish brown. It is composed of fine to medium, angular grains of quartz and minor amounts of muscovite, glauconite, magnetite, and other minerals. In places the sand is cemented with silica into a resistant orthoquartzite. The presence of glass shards suggests a volcanic origin for part of the sediments.
Vertical clastic dikes of glauconitic and micaceous sand, mostly less than 2 ft wide, occur in many places in the Porters Creek Clay. They probably resulted from local seismic activity. The high degree of weathering of these dikes, and their location at higher, drier elevations, suggest that they are pre-Pleistocene in age.
Fossils are rare except for palynomorphs, which indicate a marine environment, probably a warm, shallow epeiric sea, with a brackish, nearshore deltaic environment to the northeast, where a major river may have entered the sea (Olive, 1980, p. 2).
Wilcox Formation (Tw).--The Wilcox Formation, of early Eocene age, is composed of interbedded sand, clay, and silt. The sand is light gray and brown and weathers yellowish orange to reddish brown. It is composed mostly of fine to medium quartz with minor amounts of chert and muscovite and rare glauconite, biotite, and feldspar; sorting is poor. Bedding is thin to thick, and crossbedding and cut-and-fill structures are common. The sand is usually clayey or silty and in many places contains locally derived angular to rounded pebbles, cobbles, and boulders of clay. These coarse sediments occur mostly near the base of the formation in channels eroded into underlying units. "Sawdust sand," the most distinctive rock type in the formation, is composed of fine to medium grains of quartz, white chert, and kaolinite which impart a speckled appearance. This lithotype is dominant in some areas and absent in others.
The silty clay and clayey silt are light gray or brown to black and weather yellowish orange to white. Kaolinite and illite are the dominant clay minerals (Olive and Finch, 1969, p. 16). Marcasite and pyrite nodules are locally present.
In many places lignitized plant material is present, and lignite lenses occur sparsely. Palynomorph assemblages indicate freshwater deposition in a subtropical, humid to temperate climate (Olive, 1980, p. 2).
The thickness of the Wilcox is extremely variable because of the irregular erosion surface on which it was deposited and because of extensive postdepositional erosion which completely removed the formation in northern and eastern areas. Paleobotanical samples of the unit indicate an early Wilcox (early Sabinian) age (Olive, 1980, p. 2).
Jackson and Claiborne Formation (Tjc).--The Claiborne and overlying Jackson Formations (middle and upper Eocene) are combined on the map to form a single unit. In the Jackson Purchase, beds of Oligocene age are included in the uppermost part of the Jackson Formation (Olive, 1980). The State geologic map erroneously indicates that beds of Miocene age are present.
The Claiborne Formation is composed of sand containing lenses of clayey silt, silty clay, and lignite. The sand is white, light gray, and brown and weathers yellowish orange to reddish brown; it is composed of well-sorted, angular to rounded, fine to medium grains of quartz and minor amounts of muscovite and chert. Bedding is thin to thick, and crossbedding and cut-and-fill structures are common; in places, angular to rounded clasts of clay are present. Clayey silt and silty clay are light gray to black, brown, and olive gray, and weather yellowish orange, red, and white. Dark varieties are generally lignitic, and lignite occurs locally in beds generally less than 5 ft thick. Pollen assemblages suggest freshwater deposition and a warm and humid climate (Olive, 1980, p. 3).
The Jackson Formation consists of two facies, fine and coarse. The fine facies, composed mainly of silty clay and clayey silt, grades northward and eastward into the coarse facies, a sequence of sand and lenses of silty clay. The silt and clay are olive gray to light green and light gray to black and are locally very sandy and micaceous. Some clay samples contain pumiceous glass of volcanic origin (Olive, 1980). Sand is moderate gray to light gray and brown and weathers yellowish orange to reddish brown. It is composed of fine to very coarse grains and in places contains pebbles of quartz with minor amounts of white, light-gray, yellow, and, less commonly, black chert. Angular to rounded pebbles to boulders of clay occur locally. The sand is thin to thick bedded with common crossbedding and cut-and-fill structures.
On the basis of dinoflagellates in samples from the Claiborne and Jackson Formations, Tschudy (in Olive, 1980) suggested that, in part, sediments of these formations were deposited in brackish-water to marine environments. Subsequent examination by Frederiksen and others (1982, p. 43) of palynormorphs from Eocene rocks in the northern Mississippi Embayment, including the Jackson Formation of the Jackson Purchase region, revealed no indigenous dinoflagellates, but only reworked Late Cretaceous specimens. Fredericksen and others "***suggest that the dinoflagellates reported by Tschudy were reworked specimens and that the entire Eocene section in southeastern Missouri and southwestern Kentucky may have been deposited in nonmarine environments." The climate at this time was subtropical to warm temperate (Olive, 1980, p. 3).
FIGURE 13.--Age of stratigraphic units in the Jackson Purchase region of Kentucky. |
Frederiksen and others (1982) have studied cuttings and cores from two test wells in southeast Missouri, about 10 mi southwest of Kentucky, and have established the biostratigraphy of a section nearly 2,000 ft thick from the Upper Cretaceous MeNairy Formation to the upper Eocene Jackson Formation. The section is thicker than the corresponding interval in Kentucky and is subdivided more extensively. Much of the Wilcox Formation in these wells, unlike the formation in Kentucky, is of late Paleocene age (Fredericksen and others, 1982, p. 28).
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