ABSTRACT

Kaolinite, a 14 A^o clay mineral, and illite were identified in 23 samples collected from five Carolina Bays in southern North Carolina. The 14 A^o clay mineral does not have the characteristics of any of the usual 14 A^o clay minerals and can with some justification be called expanded illite, vermiculite, chlorite, or montmorillonite. White Lake and Singletary Lake, both near Elizabethtown, have a kaolinite - 14 A^o clay mineral - illite assemblage; and three small sediment-filled bays near Laurinburg have a kaolinite - 14 A^o mineral assemblage. The available facts are consistent with the conclusion that the clay minerals in the bay sediments were washed or blown into the bays from surrounding surficial Pleistocene (?) sediments and that they have undergone little alteration since deposition.

INTRODUCTION

The clay minerals in 23 samples of Carolina Bay sediments were identified by X-ray diffraction methods in order to determine:

(1) the clay minerals that characterize these bay sediments, and

(2) whether or not the clay minerals were altered during or after deposition.

Carolina Bays are elliptically shaped, northwest - southeast oriented depressions that are very numerous on many parts of the Coastal Plain from Virginia to Georgia [240k]. The origin of these bays has been attributed to a number of causes: meteorites, Pleistocene winds, solution, artesian springs, eddies, and others.

Most of these bays are now sediment-filled shallow depressions, but some have not been completely filled and contain lakes. In each of the Carolina Bays so far examined, there is a layer of blue or light gray clay between the most recent black mud and the lower-most lake sediment. In Singletary Lake, Bladen County, North Carolina, Frey (1953) found that the blue clay contained pollen of cold climate vegetation including spruce. Material from just above the blue clay had a radiocarbon date of 10,000 years. Thus the blue clay in all the bays examined was deposited during Pleistocene time. There is considerable similarity between the blue clay layers although the bays examined were located up to a hundred miles apart.
 
There may have been a dominant role of wind and wind-driven lake waves in the final formation of the bays as suggested by several authors. The blue clay may have been wind-blown or reworked during the period of ice advance and maximum winds and lake waves (Odum, 1952). The blue layer may be a stratigraphic key horizon marking a regime of cold strong winds and dust over North Carolina. A 7 inch blue clay layer also underlies Lake Mendota, Wisconsin, (Murray, 1955), and blue clay occurs in lakes of Pleistocene age in New Jersey (personal communication, Dr. Paul Pearson, Department of Zoology, Rutgers University) . 

METHODS

Short cores of sediments were taken in two of the bay lakes and in three of the sediment-filled bays (Fig. 1). The bay are White Lake and Singletary Lake, both located near Elizabethtown, Bladen County, North Carolina. Here Pleistocene surficial sands overlie the Cretaceous Black Creek formation. The three sediment-filled bays are located near Laurinburg, Scotland County, North Carolina. Here Pleistocene surficial sands overlie the Cretaceous Middendorf (Tuscaloosa) Formation.

The nature of the clay minerals and the general of these clay minerals were determined by studying the X-ray photographs of the untreated clay-size material from each of the 23 samples. In order to make more positive identifications and to study the properties of the clay minerals, more detailed studies were made of a few selected samples. For these selected samples x-ray photographs were made after Ca⁺⁺ saturation, NH4⁺ saturation, ethylene glycol or glycerol solvation, and heating at various temperatures up to 550^o C. Some of the samples were sealed in glass capillary tubes after heating and X-rayed immediately in order to minimize rehydration effects. X-ray patterns were made with a Hayes diffraction unit equipped with 114. 6 mm dianleter cameras. Samples, placed in either thin-walled cellulose acetate or glass tubes 0. 5 mm in diameter, were exposed to filtered radiation.
 

RESULTS   Mineralogy Kaolinite, illite, and a 14 Ao clay mineral were identified.  Illite -  Illite is present in all of the samples from the lakes. Figure 2 shows the lattice spacings of Sample S8 compared with the lattice spacings of illite. Because of the small amount of illite present, only the stronger lines are seen. Because of the presence of quartz, kaolinite, and a 14 Ao mineral in this sample, only a few lines (10. 0, 3. 20, and 2. 98 Ao) are produced entirely by illite.

Kaolinite -

Kaolinite, which is present in all of the samples analyzed, varies in its degree of crystallinity. Figure 3 shows the lattice spacings of three selected samples compared with the lattice spacings of kaolinite. Sample A2 contains moderately crystalline kaolinite as evidenced by the fair resolution of the 2. 56 - 2. 50 A^o and the 2. 34 - 2. 28 A^o doublets. The more poorly crystalline kaolinites give weaker patterns, in which the characteristic doublets tend to become broad lines. In sample B2, the 2. 56 - 2. 50 A^o doublet is not as clear as in sample A2, the 2. 50 A^o line being indicated mainly by a tail on the high angle side of the 2. 56 A^o line. In sample WLl, this doublet is replaced by a broad line at 2. 57 A^o. The 2. 34 - 2. 28 A^o doublet behaves in a similar fashion. Most of the kaolinite in the sediments of the Carolina Bays is poorly crystalline.
 

Because of the presence of a 14 A^o line that did not shift after glycerol solvation, the possibility existed that the 7 A^o line attributed to kaolinite was the second order basal line of chlorite. After heating the samples to 500-550^o C, however, the 7 A^o and other kaolinite lines disappeared. Heat treatment tests indicate that the 7 A^o line can be attributed to kaolinite.

14 A^o Clay Mineral -

A 14 A^o clay mineral is present in all the samples. This 14 A^o clay mineral does not have the properties of any of the usual 14 A^o minerals - vermiculite, chlorite, or montmorillonite. Identification and characterization of this mineral is difficult because the 14 A^o line is the only line remaining after the lines of the other minerals known to be present are eliminated Most of the lines of this mineral overlap lines of other clay minerals and quartz. See figure 4.
 

The mineral is definitely not typical montmorillonite as the 14 A^o line does not shift to 18 A^o after glycerol solvation. Tamura (1958, p. 146), however, after treating a somewhat similar 14 A^o mineral with sodium citrate, obtained a 16-18 A^o spacing following glycerol solvation. He suggests that the mineral is of a montmorillonite type. None of the Carolina Bay samples were given the Tamura treatment.

Although the pattern of the untreated material bears a close resemblance to the pattern of vermiculite, there are too many discrepancies for the mineral to be identified as typical vermiculite:

(1) Some of the most characteristic vermiculite lines are missing or else can be explained by the presence of other minerals. For example, vermiculite has a moderately strong line at 1. 53 A^o (Fig. 4), but all of the intensity of the line is needed to take care of the quartz in the samples as the 1. 82, 1. 53, and 1. 37 A^o quartz lines are of about equal intensity.

(2) This mineral does not react to chemical and heat treatments as does good vermiculite. Boiling vermiculite in a 1 N NH4Cl solution for 5 minutes causes the 14 A^o line to shift to 11 A^o; the 14 A^o mineral in the Carolina Bay sediments is not affected by the ammonium chloride treatment. Heating vermiculite in a thin-walled glass tube at temperatures above 110^o C followed by immediate sealing of the tube to prevent rehydration causes the 14 A^o line to shift to 11. 8 A^o (Walker, 1951, p. 203). Sample NS1 was subjected to this treatment (heated to 175^o C) with the 14 A^o line shifting only to 13. 6 A^o.

By the process of elimination this 14 A^o mineral should be chlorite, but many of the properties of this mineral are not those of ordinary chlorite:

(1) Some of the most characteristic chlorite lines are missing or else can be explained by the presence of other minerals. Except for the iron-rich chlorites, the first four or five order basal lines (14. 0, 7. 0, 4. 7, 3. 50, and 2. 82 A^o) of chlorite are of about equal intensity. Since the 14 A^o mineral in these samples produces a strong 14 A^o line, it should give strong lines for the higher order basal reflections. Such is not the case. The 7 A^o line is produced by kaolinite as heating to 500^o C destroys the 7 A^o and other kaolinite lines. The 4. 7 A^o line is absent or very weak. The 3. 50 A^o line is absent or so weak that it is over-shadowed by the strong 3. 56 A^o line of kaolinite. The 2. 82 A^o line is very weak or absent. In addition, chlorite has a strong 1. 57-1. 53 A^o doublet, which is absent in the Bay clays.

(2) This mineral does not react to heat treatments as does chlorite. Heating chlorites up to temperatures of 700^o C results in no shift of the 14 A^o line. The 14 A^o spacing of the mineral in the Bay clays decreases some at moderate temperatures and drops to 11-12 A^o at 500-550^o C.

Anomalous 14 A^o clay minerals similar in many respects to the one or ones from the Carolina Bays have been called various names: expanded illite, vermiculite (Brown, 1953, p. 64; Rich and Obenshain, 1955, p. 335), chlorite (Brown and Ingram, 1954, p. 198; Johns, Grim, and Bradley, 1954, p. 243; Klages and White, 1957, p. 20), and montmorillonite (Tamura, 1958, p. 146). Johnson and Jeffries (1957, p. 541) and Schultz (1958, p. 367) identified both chlorite-like and vermiculite-like minerals in 14 A^o complexes. In the above examples the decision as to the name to be given to the 14 A^o mineral apparently depended on which of the many possible pre-X-ray treatments were used and on which of the evidences were given the most weight when evidences conflicted. At this time no attempt will be made to name the 14 A^o clay mineral in the Carolina Bay sediments.

Some of the 14 A^o lines have tails on the 10 A^o side indicating 10-14 A^o mixed layer lattices.

Distribution

Estimates were made of the relative abundance of the different clay minerals in each of the samples on the basis of visual estimation of the intensities of the basal diffraction lines. Although this method of determining frequencies is not very accurate, it does give a general picture of the frequency distribution. These relative abundancies are given in Table 1.

Table 1 shows that:

(1) the bay lakes, White Lake and Singletary Lake, have a kaolinite - 14 A^o clay mineral - illite assemblage,

(2) the sediment-filled bays near Laurinburg have a kaolinite - 14 A^o clay mineral assemblage, and

(3) only minor variations in the reltive abundance of the clay minerals exist vertically in a given core.

² The abundance of the clay minerals is given by "abundance Numbers" ranging from 1 to 10. As these numbers are based on visual estimation of the intensities of the basal diffraction lines, they should not be considered to give very accurate estimates of percentage.

Origin of the Clay Minerals

Reves (1956, p. 20-22) and Heron (1958, p. 102) found that kaolinite and a 14 A^o mineral are both plentiful in the surficial Pleistocene (?) sediments at the same elevation of the bays studied (Coharie, Sunderland, and Wicomico terraces-?) and that illite is present in small amounts in some places. The formations underlying the surficial sediments are seldom exposed and have clay mineral compositions distinct from the composition of the bay clays. The clays in the Middendorf formation are composed primarily of kaolinite, and the clays in the Black Creek formation are composed primarily of a mixture of montmorillonite and kaolinite. The distribution features listed above are consistent, therefore, with the conclusion that the clay minerals in the bay sediments were washed or blown into the bays from the surrounding surficial sediments and that they have undergone little alteration since deposition.

References

Brown, Charles Q. and Ingram, Roy L., 1954, The clay minerals of the Neuse River sediments: Jour. Sed. Petrology, v. 24, p. 196-199.

Brown, George, 1953, The dioctahedral analogue of vermiculite: Clay Minerals Bull., v. Z, p. 64-70.

Frey, D. G., 1953, Regional aspects of the late-glacial and post-glacial pollen succession of southeastern North Carolina: Ecol. Mon., v. 23, p. 289-313.

Heron, S. Duncan, Jr., 1958, The stratigraphy of the outcropping basal Cretaceous formations between the Neuse River, North Carolina, and Lynches River, South Carolina: Ph. D. dissertation, University of North Carolina, 155 p.

Johns, W. D., Grim, R. E., and Bradley, W. F., 1954, Quantitative estimation of clay minerals by diffraction methods: Jour. Sed. Petrology, v. 24, p. 242- 251.

Johnson, L. L. and Jeffries, C. D., 1957, The effect of drainage on the weathering of clay minerals in the Allenwood catena of Pennsylvania: Soil Sci. Soc. America Proc., v. 21, p. 539-542.

Klages, M. G. and White, J. L., 1957, A chlorite-like mineral in Indiana soils: Soil Sci. Soc. America Proc., v. 21, p. 16-20.

Murray, R. C., 1955, The recent sediments of three Wisconsin lakes: Ph. D. dissertation, University of Wisconsin.

Odum, Howard T., 1952, The Carolina Bays and a Pleistocene weather map: Am Jour. Sci., v. 250, p. 263-270.

Reves, William D., Jr., 1956, The clay minerals of the North Carolina Coastal Plain: M. S. thesis, University of North Carolina, 46 p.

Rich, Charles I. and Obenshain, S. S., 1955, Chemical and mineral properties of a red-yellow-podzolic soil derived from sericite schist: Soil Sci. Soc. America Proc., v. 19, p. 334-339.

Schultz, Leonard, 1958, Petrology of underclays: Geol. Soc. America Bull., v. 69, p. 363-402.

Tamura, Tsuneo, 1958, Identification of clay minerals from acid soils: Jour. Soil Sci., v. 9, p. 141-147.

Walker, G. F., 1951, Vermiculites and some related mixed-layer minerals, in Brindley, G. W., and others, X-ray identification and crystal structures of clay minerals, p. 199-223: London, The Mineralogical Society.

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