Chapter 4 Mines-Lower Miocene

Chapter 4

MINES - LOWER MIOCENE INTRODUCTION

Thin, relatively pure palygorskite clay beds, 1-5 m thick, are restricted to the area slightly north of the Georgia-Florida border and south to the Lake Talquin area (see Fig. 4 and Map 1). The main commercial interval commonly contains two clay beds separated by a sand, shell, dolomite, or soil bed of variable thickness. These two clay beds (identified in the Lake Talquin well) appear to be relatively continuous throughout the north Florida area of the Trough. Thin, discontinuous beds occur above and below the main clay horizon but they generally contain abundant montmorillonite. LA CAMELIA MINE (MC-1)

A core (9 m) from the La Camelia palygorskite mipe (Engelhard Minerals and Chemical Corp.) in north Florida was studied in detail to determine the vertical variability and the relations of the various parameters. The structural, textural, mineralogical and chemical data indicate there are two major depositional cycles represented within the section. The sediments deposited during the two cycles differ in detail but in general are similar. The environments of deposition grade from shallow marine to lagoonal to tidal flat to soil. Fig. 29 is a generalized lithologic description of the core. There are two pure clay beds (0.5-1.5 m and 6.0-7.3 m) that consist of relatively pure, parallel-laminated clay. The clay beds were deposited during two periods of regression separated by a period oE
72 Meters

Marine Cloy Burrowed

Logoonol

Tidal

Marine Sand

Soil

Supra Tidal

Lagoonal

Tidal Marine Sand

Fig. 29. Lithology of MC-1core from La Camelia Mine, Florida. Blank intervals contain relatively pure homogeneous palygorskite clay beds. General environments and direction of shore-line movement is indicated. Core diameter 10 cm.

This interval is overlain by a sandy bed which is a greenish clayey sand with irregular mottles of white sand and worm tubes. Over this is a white sand containing pelecypod shells. The mixing in the lower part of the sand is apparently due t o both burrowing and current action. There is a gradual transition from sand to dolomitic clay ( 3 m). A pure clay bed containing patches of dolomite extends up to 2.4 m. Dolomite then becomes predominant and there is 0.9 m of clayey dolomite. The dolomitic bed is overlain by 1 m of pure clay. The lower one-third has an irregular, massive appearance with some irregular vertical fracture surfaces. This clay bed grades into an interval which consists largely of the same type of clay, but is heavily burrowed and infilled with a coarser clayey sand

73

Fig. 30. M u d cracks in palygorskite clay infilled with sandy montmorillonitic clay (7.6 m).

(Fig. 32). This is the top of the core. It is overlain by a clayey sand similar to the type that occurs in the burrows in the top of the core. Thus on gross lithology, there appear t o be two similar, but distinct depositional units. Both units appear to be topped by a hiatus, and both start with a sand bed of probably marine (littoral) origin. The lower unit is characterized by mud-cracked and locally reworked sediments and the upper unit by dolomitic beds.

Clay mineralogy Palygorskite is the predominant clay in the section. Montmorillonite is second in abundance, followed by sepiolite. Illite and mica are present in

74

Fig. 31. Sample from soil zone showing ped surface with clay skin (cutan) and burrow filled with sandy clay (4.9 m).

minor amounts throughout the section. The estimated relative clay-mineral content, based on X-ray patterns of oriented slides, is shown in Fig. 33. The clay-mineral suite is closely related to the lithology and thus presumably depositional environments. In addition, there are significant differences between the upper and lower depositional units. In the pebbly zone (6-5 m) the pebbles have a high palygorskite content. The clayey-sand matrix is composed largely of montmorillonite (Fig. 33). Small (2-5 mm) rounded tan clay grains are similar in composition to the large blocks of apparent mud-crack origin. Sepiolite is present throughout this interval in the clay pebbles and grains but not in the matrix. The clay

75

Fig. 32. Palygorskite clay containing burrows filled with sandy montmorillonitic clay and lightcolored fragments of palygorskite clay (0.3 m). (Top is to the right.)

suite of the matrix is similar to that of the overlying montmorillonitic sandy soil. Montmorillonite comprises over 90% of the clay suite in the soil zone. Sepiolite has a maximum concentration at the bottom of the soil zone and is not present in the overlying sediments. This is the only interval that contains more sepiolite than palygorskite. A detailed study of the bottom part of the soil zone indicates that the clay minerals are inhomogeneously distributed. The greenish sandy clay is composed almost entirely of montmorillonite, with some biotite. Some small white pebbles have a composition similar t o the underlying large pebbles (palygorskite > montmorillonite > sepiolite). Also present are some small tannish grains (2-5 mm) and a thin tannish coating on the vertical fracture surfaces. In both of these types of samples montmorillonite is the dominant clay and sepiolite is more abundant than palygorskite. Thus the ‘sepioliterich’ suite has a distinctive occurrence. The distribution suggests the clay may be secondary and has formed by postdepositional leaching of the upper

76 Meters

WILYGORSKITE

75;-j

MONTMORILLONITE

--

SEPlOLlTE

9b

5FT50

I= BURROWS M-MATRIX P i QESOLES

Fig. 33. Graph showing distribution of major clay minerals in MC-1core.

part of the soil interval and growth in the bottom. Tannish grains in the upper portion of the soil zone are composed almost entirely of palygorskite, sometimes with dolomite. These grains must have been added from adjacent overlying sediments. Burrows in the 4-4.8 m montmorillonite-rich interval are composed largely as palygorskite, indicating clay has been worked down from as much as 1.5 m above. The burrows contain no dolomite which suggests that either the animals selectively by-passed the dolomite rhombs or that the dolomite was formed at a later stage.

77

In the overlying shelly-sand zone palygorskite increases and becomes relatively abundant within the sand. However, the mottles and pebbles of greenish clayey sand in this interval have a high montmorillonite content similar t o that of the underlying interval. Thus some of the mixing is probably due to current reworking of the lower material into the upper, rather than burrowing which would cause a downward mixing. The large shells in this interval (3.3-3.6 m) have been converted to dolomite and much of the palygorskite is secondary. This interval is discussed further in Chapter 7. Palygorskite is at a maximum (90%) in the dolomite and upper clay bed. There is a slight decrease in palygorskite in the clays of the uppermost burrowed zone. The burrows are filled with a sandy montmorillonite clay derived from the overlying montmorillonite.

Texture Twenty-one samples were sieved at 0.5-4intervals. The samples were disaggregated by pounding with a vertical motion. This tended $0 break the nonclay minerals from the clay with a minimum of damage. The material coarser than 4.38 4 was considered to be the ‘sand’ fraction. Microscopic examination indicates most of the clay ‘broke down’ and passed through the finest sieve. Fig. 34 shows the amount of nonclay material and its composition. The sands at the base of each unit contain 6 0 4 0 % sand, most of which is quartz. The clay beds and dolomite bed contain from 0.3 to 7.2% sand. Sorting values, means, etc. are not too meaningful as most samples contain several minerals, some of which are detrital and some authigenic. The calcite was dissolved from the sieved fractions of several samples and frequency curves constructed for the calcite (plus phosphate) fraction and the quartz fraction (Fig. 35). The sample from the overlying reworked zone (7.6 m) contains 24.6% nonclay material which consists of 59% calcite (plus minor value for the quartz component phosphate) and 41% quartz. The sorting (q) is 0.51 which Folk (1968)classes as moderately well sorted. The value for the combined samples is 1.33 or poorly sorted. The calcite, apparently authigenic, has a coarsely skewed size distribution. The detrital quartz in the lower interval, whether it comprises 67% or less than 1% of the sample, is moderately sorted with a well-defined symmetrical mode at 3.2-3.5 9. As the depositional environments indicate a wide range of energies were operative, the source area for the quartz sand must have had a uniform texture throughout deposition of the lower unit. The series of frequency curves (5.0-3.0 m) in Fig. 36 shows the textural transition from the lower interval t o the upper interval. In the soil zone between 5.0 m and 3.0 m the proportion of the sand-mode characteristics of the lower interval decreases and a newer, coarser, but similarly sorted, quartz-sand suite increases. The amount of clay, phosphate and calcite grains is not sufficient t o account for the new mode and most of the new, coarser

78 Meters

Fig. 34. Amount and composition of the coarser than 4.4 (I material. The bars in the left graph show the total amount of greater than 4.4 (I material. The vertical connecting line indicates the amount of quartz in each sample. The right graph shows the composition, in percent, of each sample. Mean and sorting values are also listed.

mode is made up of quartz sand, presumably from a metamorphic source (as indicated by thin-section and heavy-mineral studies). At 3.8 m, where the marine mottled sand and shelly-sand zone starts, the mode characteristic of the lower interval is completely gone and the new mode at 2.8 4 is dominant; however, this is accompanied by an even coarser new mode at 2.3 4. This coarse material is made up largely of clay grains,

79 100

90

80

70

--

60

59% CALCITE 41% QUARTZ BULK

I-

= r 0

=

g 50 IY

6 40

UUARTZ

30

20

10

0

1

2

' 0

5

Fig. 35. Frequency distribution curves of the matrix sandy clay sample from 7.6 m. Broken-line curve is for total sample. The other two curves show the distribution for quartz and calcite grains.

though there is appreciable quartz in this size range. The transgressing sea reworked some of the lagoonal palygorskite deposits and formed sand-size clay grains. In the clay-rich interval which starts at about 3 m, the two quartz-sand modes characteristic of the soil interval (5.0-4.0 m) reappear and comprise the bulk of the nonclay material. Above this interval the amount of quartz is minor; also, it is much finer, being mostly in the silt range. The quartz in the upper clay bed is distinctly finer grained than that in the lower clay bed (Fig. 37); the coarser mode in the 1.1-m sample is due to calcite. Extremely fine-grained quartz also characterized the underlying

80

0 Fig. 36. Series of frequency curves of samples from the transition interval between the two major depositional units showing the systematic development of coarser modes with decreasing depth and increasing marine influence. Dashed lines show suggestive distribution of two populations.

dolomite. The coarser fraction in this interval is also calcite. The amount of quartz and the grain size increases in the top 30 cm of the upper clay bed where some burrowing is evident. The coarse mode at 3.1 4 is presumably the quartz sand which the burrowing organisms have brought down from the overlying sand bed. The 3.8-4 very fine sand is the quartz sand that was originally deposited with the clay. In the samples which show evidence of reworking (lower interval) and burrowing (upper interval) the quartz in the matrix clay tends to be coarser grained than in the indigenous or reworked material. As might be expected,

81 101

91

81

71

61 I-

r

3 2

r

5 U

51

I IE Y

41

31

21

/

11

I

Fig, 37. Frequency curves of the material from the upper and lower clay beds. The model values for the shallower samples must be in the fine siltaize range.

the currents that accomplished the reworking were probably of higher energy than those in the environment in which the mud cracks formed and carried coarser detrital material. The burrows in the more clayey material contain sandy coarser material carried down from the overlying beds. Frequency curves for the quartz, phosphate, and clay grains (latter two based on microscopic examination of the sieved fractions) indicate the mode of the clay grains is from 0.2 t o 0.5 o units coarser than the quartz mode (Fig. 38). This difference is what would be expected on the basis of the difference in specific gravity. In the soil zone (4.5 m) both the quartz and clay grains have a bimodal distribution (Fig. 39). This presumably reflects limited sorting action and

82 100

I

90

-

80

-

70

-

60

-

50

-

40

-

a RI

1 z a I

a

I

I

I

I

I

I

W

z

IL I-

z

W W

n

\:,

\

\

Fig. 38. Frequency curves for the quartz, clay and phosphate grains from 5.5 m sample.

may be due to deposition by flood waters as is suggested from other lines of evidence. One clay mode is coarser and the other finer than the two central sand modes. It is possible that the finer grains are fecal pellets or were created during the formation of the soil.

Composition of sand grains The sieved fractions of the greater than 4.44 fraction (nonclay) was studied with the microscope and by X-ray diffraction. Quartz is present in all samples and is generally highly polished though much of the quartz in the sand samples from 4.1 to 3.0 m have thin clay coating (discussed in Chapter 7). Varying amount of K-feldspar are present in all but the upper 0.6 m. Well-rounded, white to light-tan phosphate grains are present in amounts ranging from less than 1--15% of the greater than 4.44 fraction in all s a m ples but the top burrowed zone. The phosphate is two to three times more abundant in the lower clay bed than in the upper clay bed. The origin of the phosphate is not clear. The clayey-sand interval at 3 m contains hollow

83

I

2

3

8

4

5

6

7

Fig. 39. Frequency and cumulative curves for quartz and clay grains from the soil interval (4.5 m). The soil is believed to have developed on river flood-plain deposits which would account for the poor sorting.

cylinders that appear to be worm tubes, with the walls of the cylinders made of light-tan apatite. This same interval contains paper-thin flakes of material that also appears to be apatite (larger sheets of phosphate occur at 4 m). Phosphate is relatively abundant in the sandy interval (shallow marine) between the two basic depositional Qnits. Some of this phosphate is, at least indirectly, organic in origin. This suggests that some of the round phosphate grains could be locally deposited fecal pellets. Calcite, in amounts ranging from 2 to 78%, is present in nearly all but the upper 0.6 m of the section. From the bottom up to 2.7 m the calcite grains are generally subequant in shape with some clear rhombs and a minor amount of rice-shaped grains. The dolomitic unit, from 2.7 to 1.5 m, contains an abundance of distinctive rice-shaped calcite grains that are apparently authigenic in origin. The calcite in the upper part of the dolomitic zone and the upper clay bed is largely subequant in shape. Well-rounded clay grains are fairly abundant in the central sandy interval and in the uppermost burrowed interval. The clay grains are probably more

84

abundant than indicated in Fig. 34 as some probably broke up and others are so coarse as not to be included in the sieve analysis. Visual examination of the samples indicates clay grains range from silt size up t o 3 cm or more in diameter. Pyrite, in the form ,of rosettes and elongated ovals, occurs in the upper clay and dolomitic zone (2.3-0.5 m). This suggests that reducing conditions existed at some stages during deposition of this interval.

Thin-section Thin-section studies indicate the basal clayey sand has approximately 2% K-feldspar. The sand is composed largely of plutonic quartz and a minor amount of embayed quartz. Minor phosphate is also present. The sand grains

Fig. 40. a. Channel argillan in clay at base of soil (MC-1, 5.2 m). White bar equals 0.05 mm. b. Channel argillan and grain argillans from near base of soil (MC-1, 5.0 m). White bar equals 0.1 mm. c. Craze planes in sandy clay from near base of soil (MC-1, 4.9 m). White bar equals 0.1 mm. d. Omnisepic plasmic fabric near middle of soil (MC-1, 4.4 m). White bar equals 0.01 mm.

85

Fig. 41. a. Thick skew plane argillan lining void near middle of soil ( M C - 1 . 4 . 4 m). Dark vein is a void. White bar equals 0.05 mm. b. Mosepic plasmic fabric near top of soil (MC-1, 4 . 1 m). White bar equals 0.1 mm. c. Thick grain argillans (montmorillonite) coating quartz grains at top of soil ( M C - 1 , 4 . 0 m). White bar equals 0.05 mm.

are floating in a clay matrix, and there is little grain-to-grain contact. The depositional environment was probably shallow marine or tidal. The clay in the lower clay bed is very well oriented parallel to the bedding and was deposited in a low-energy environment. Near the top of the bed thin (2-4 grains thick) horizontal laminae of quartz grains are relatively common. These laminae also contain some clay and phosphate grains. The clay grains appear to be montmorillonite. The matrix of the laminae consists of unoriented palygorskite clay similar to that in the clay bed. The detritus in these laminae was presumably carried into the low-energy environment (lagoonal) by wind or relatively gentle periodic water currents. The matrix material in the pebbly (palygorskite) zone overlying the clay bed consists of clayey (montmorillonite) sand and sandy clay. K-feldspar, phosphate, and clay grains and relatively common. Large, embayed, quartz grains are common. The embayments probably indicate dissolution rather than a volcanic origin (Cleary and Conolly, 1972). A few composite, metamorphic quartz grains are present in this interval and in the overlying sands.

86

Fig. 42. Clayey (palygorskite) dolomite. Some have hollow (dark) centers and some carbonate (apparently dolomite) nuclei (MC-1, 2.1 m), White bar equals 30 pm.

These are not present in the basal sand which suggests there was a partial change in the source area. Thin-sections in the soil interval from 5.0 t o about 4.0 m show a systematic change. The lower interval contains an abundance of palygorskite clay grains (50% of rock) in a matrix of montmorillonitic sand. Upward the amount and size of palygorskite grains decreases, the amount and size of quartz increases, and the amount of montmorillonite appears to increase. Cutans (argillans) of montmorillonite are common, lining grains, channels, voids and joint planes. They appear t o have been formed by illuviation, though TEM pictures suggest some are authigenic. There is a suggestion that some of the palygorskite has been altered to montmorillonite, but in most

87

Fig. 43. Vague ped structure near top of upper clay bed (MC-1, 0.6 m). White bar equals 0.5 mm.

cases the montmorillonite fills thin veins and fractures in the palygorskite grains, SEM pictures of quartz grains show that some grains contain abundant solution pits. The plasma (reorganized colloidal material) fabric varies from mosepic t o skelsepic to omnisepic (Brewer, 1964). A selection of these features is shown in Figs. 40 and 41. They appear t o be identical to those found in modern soils (Brewer, 1964). The dolomitic, sandy clay (2.9m) contains scattered rhombs of dolomite. The rhombs are euhedral and many have dark centers that are apparently voids. A few aggregates of fine dolomite are present but most occur as isolated rhombs. Thin laminae contain quartz, phosphate, and calcite grains

88

that are rice-shaped. Similar calcite grains (-10%) occur scattered throughout the clay. The clay is relatively well oriented. In the overlying clayey-dolomite interval (1.5-2.3 m) the dolomite has a fair amount of grain-to-grain contact but the rhombic shape is maintained. In addition t o dark centers some of the larger rhombs have carbonate cores (Fig. 42), probably dolomite according t o X-ray data. The rhombs range in size from less than 0.005 mm t o 0.025 mm with most being 0.015 mm. The clay in the overlying clay bed is highly oriented in a horizontal direction except for the upper part (0.5-0.8 m) which has a patchy pattern (peds) with the clay in the patches being well oriented (Fig. 43). A few thin veins (channel argillans) of montmorillonite are also present. This interval appears to have been reworked, probably by burrowing organisms, and a minor amount of clay has been carried by water from the overlying montmorillonitic clay. The burrows at the top of the clay (Fig. 32) contain sandy montmorillonitic clay and pebbles and grains of palygorskite. Much of the burrow material and some of the palygorskite pebbles have a dark-brown stain which appears to be a thin film of montmorillonite.

Heavy minerals In order t o determine if volcanic material was a major source material, the heavy minerals were collected from the lower (7 m) and upper (1m) clay beds and the soil zone. The lower clay bed contains significant amounts of zircon, tourmaline, rutile, apatite, staurolite, kyanite, and sillimanite. The heavy-mineral suite suggests a mixed metamorphic-igneous source with, probably, relatively little volcanic material. The heavy-mineral suite from the soil zone is similar except that the metamorphic suite is relatively more abundant. Thin-section examination also indicates metamorphic quartz is more abundant in this section. This new source material presumably accounts for the occurrence of the bimodal sand in this interval (Fig. 36). Most of the heavy minerals from the upper clay bed are opaques. A few grains of tourmaline are present. There is little evidence that volcanic material was a major source of the sediments in this section.

Interpretation It is evident from the study of this one core that these Miocene sediments were deposited in a shallow-water environment near the strand line. In general the montmorillonitic sandy intervals appear t o be of shallowmarine origin. The horizontal-bedded, clay-rich palygorskite beds must have been deposited in a quiet lagoon. The dolomitic beds were deposited in a

89

similar environment, though some is replacement dolomite and probably formed epigenetically . The pebble and mud-crack beds represent lagoonal deposits that were later reworked by currents coming from either the seaward or landward direction. The vertically oriented montmorillonitic, organic, sandy clay bed is a soil formed on fluviatile sediments. Burrowing is evident throughout, but is particularly important in the sediments closing the end of each depositional cycle. The depositional cycles started off with a sand or sandy shell bed which acted as barriers. Shallow-water lagoons developed behind these relatively thin barriers. In the older cycle the lagoon was sometimes evaporated t o near dryness and mud cracks developed. During the early stage the barrier was breached and clayey marine sands were mixed with the mud clasts. Near the end of the cycle fresh-water currents probably did the reworking. This situation seemed t o be characteristic of the lower depositional unit. This lower unit is topped by a soil zone developed on fluviatile sediments, suggesting the overall unit is regressive. This regression is followed by an abrupt transgression (shelly marine sand) which might reflect only a minor lateral shift of environments. The upper depositional unit shows little reworking ,and a relatively thick lagoonal sequence (dolomite and palygorskite clay bed), which suggest a more permanent barrier plus a decrease in available Si and Al. This interval appears to end in a shallowing regressive environment (burrows and reworking), followed by a relatively abrupt marine transgression (Fig. 29). Thus, both depositional units appear t o represent a seaward migration of a shallow-water fluviatile-lagoon-barrier sequence. Whenever the migration was interrupted by a sustained marine transgression a new cycle began. The lithology indicates that the lower unit was probably more effected by physical energy (clay clast) and the upper by chemical energy (dolomite). The detailed mineral and chemical differences between the two units likely reflect only minor environmental and source differences; however, from a practical standpoint these differences can be used to identify the different clay beds. LA CAMELIA MINE (MC-2)

A second core (8.8 m) was obtained approximately 4 km northwest of the first core. The sediments in this core were deposited in a slightly different but related sequence of environments. The lower 5.8 m, apparently equivalent to the interval below the soil zone in the first core, is composed largely of montmorillonite sand (Fig. 44). The upper and lower portions have minor amounts of palygorskite and sepiolite. Sepiolite-rich pebbles are common in the lower meter. This latter interval also contains worm burrows and 5--10% phosphate. The sand has a major mode, 3.1 @, similar to the sand in the bottom of the first core; however, this sand has a minor coarse mode (2.0-

90 CLAY

MINERALOGY %

sa

I1 5

10.5 6 5

0 3 86

37 3.9

78

35

69 5

2.83

77 2

2.77

52 0 77 3

3.2 2.66

43 I

3.48

pY 1% 76

2.78

67 0

27

72.0

2 57

Fig. 44. MC-2 core, 4 km northwest of MC-1, La Camelia Mine. Blank area represents massive, relatively pure clay.

2.6 qj) suggesting slightly higher energy conditions in the northern portion of the area. The lower 1 m of sand is overlain by 0.6 m of relatively coarse sand (coarse Mo 1.5-2.0 qj predominant). Some cross-beds are present. The overlying sand is finer grained with the major Mo ranging from 3.0 t o 3.5,which is similar t o the sand in the lower clay and clay-clast interval in the southern core. This sand is fairly massive, but contains a few clay patches and clasts. Phosphate is scarce but the upper portion contains 5--10% of sand-size palygorskite grains. Thus the interval below the soil horizon in the northern well represents a higher-energy environment than that in the south. The sands may have been deposited as a beach or sandflat as compared t o the more tidal flat-lagoonal conditions to the south. The basal sand is overlain by approximately 2.1 m of complex sediments, presumably equivalent t o the soil and shell interval (3.3-5.2 m) in the southern well, The lower 0.6 m consists of irregularly mixed and burrowed sand and white clayey dolomite. Some calcite cement is present. The lower 15 cm consist of horizontally bedded white rhombic dolomite (the only bedded dolomite in the core). This may just be a lens. The white material overlying this occurs as irregular patches, clasts, and thin coatings. It contains hollow rhombs of dolomite and abundant palygorskite and sepiolite with only minor montmorillonite. SEM pictures indicate the material in the

91

Fig. 45. Incipient clay peds near top (1.8 m ) of MC-2core. White bar equals 0.1 mm.

white coatings is probably authigenic. The matrix clay is predominantly palygorskite, but appreciable sepiolite and montmorillonite are present. This interval is overlain by about 1m of horizontally laminated clay with thin laminae of coarse granular calcite in the lower portion and sand in the upper portion. Montmorillonite decreases upward in the section, but never attains the low values found to the south. The grain size is quite fine (Mo= 3.5 @), and the upper portion contains a few diatoms and sponge spicules. This interval is overlain by a 0.5-m thick clayey shell (calcite) bed. Sepiolite occurs in the lower portion of the shell bed but does not occur any higher in the section. Thus, sepiolite is restricted to the lower depositional unit as it is in the first core.

92

The cored section does not have a well-developed soil zone similar t o that in the southern core, but the mixed lithology and the development of veins and coatings of dolomite and thin laminae of granular calcite suggest some subaerial exposure (3.3-5.5 m). As we will show this zone is quite extensive in north Florida and has a highly variable character. The shell bed is overlain by 0.8 m of well-laminated, light-gray clay (commercial zone) which in turn is overlain by 2.4 m of fairly well laminated gray, partially iron-stained (goethite) clay with thin sand laminae and a few small patches of sand. Palygorskite is most abundant in the lower 0.8 m of the clay bed. Montmorillonite and kaolinite systematically increase in the overlying 1 m and then remain constant for the final 1.5 m. There are 3 m of sandy overburden. The top of the clay bed appears to contain incipient clay peds outlined by argillans (Fig. 45), suggesting mild weathering. The problem is whether this relatively low palygorskite content is due to weathering or is depositional. The sand content of the clay ranges from 6 to 12%. This, plus the sand laminae, indicates a slightly higher energy environment for this clay than its southern equivalent. The nonclay component is mostly fine quartz and a few heavy minerals. The iron-stained areas have the same clay suite as the unstained gray areas. SEM pictures d o not suggest the palygorskite fibers are altered. The decrease in palygorskite in the northern core probably is a result of deposition in a different, probably higherenergy environment. Diagenetic modification of the detrital clays did not progress as far as in the MC-1core. LA CAMELIA MINE OUTCROP

Over a period of several years several sections of the mine were sampled. The first section sampled was near the center of the mine and is similar to the first core (Fig. 46). The two clay units are separate by a 2-m interval (12-14 m). The upper 1 m of this interval is a fossiliferous, montmorillonitic sand. (In other areas a 1-m oyster bed (coquina) occurs at this interval.) This is underlain by 0.6 m of gray montmorillonitic clay and 0.3 m of brown pebbly (0.5-2 cm) clay. The upper part of this brown zone contains dark-brown montmorillonite clay pebbles or peds and a sandy matrix of similar composition. The matrix contains small rounded white grains rich in palygorskite and sepiolite. The clay in the lower portion of the tan interval has the same composition as the overlying rounded white pebbles and presumably they were derived from the underlying material. Some pebbles contain more sepiolite than palygorskite. Vertical worm burrows are present in this interval and associated with them are white, elongated (3 mm) fecal pellets composed only of quartz and palygorskite. Either the worms are very selective in their concentrating or they have transported clay from the overlying beds. This interval is equiva-

93 CLAY

MINERALOGY

L A CAMELIA

MINE

Fig. 46. Outcrop section in La Camelia Mine showing composition of clay fraction. 121 4 m interval is equivalent to the soil zone separating the two clay beds.

lent t o the clay soil in the MC-1 core. It is underlain by approximately 1m of pebbly or crumbly gray clay with tan montmorillonite films and then 1m of well-laminated gray clay. This is the lower commercial clay bed. The pebbly interval is extensive throughout this mine and several other mines. In general, the pebble size increases downward. In some instances the material looks like mud-crack clasts that have been only slightly moved. In other areas, and lower in the interval, it looks more like soil macropeds (compound particles, clusters of primary particles, which are separated from adjoining aggregates by surfaces of weakness). Further, the pebbles are commonly coated by a thin clay film (montmorillonite). These films are present in soils where they are called plasma separations. Thin-section examinations confirm the ped texture of the pebbly or crumbly interval. It is difficult t o establish if any of the montmorillonite is replacing the palygorskite, but SEM pictures suggest it is not. The shell-soil’horizon is overlain by approximately 2 m of gray laminated palygorskite clay (minor montmorillonite, no sepiolite) which contains lenses of white dolomite and thin layers of mud-cracked dolomite intimately mixed with the clay. The mud-crack structure indicates the dolomite is penecontemporaneous rather than syngenetic. Layers and veins (filled mud cracks) of grainy rice calcite are also present in this interval. These layers and veins are rich in montmorillonite and low in palygorskite. Montmorillonite was presumably the original detrital clay supplied to the mud-cracked depositional site by invading waters and the Ca/Mg ratio must have been

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high enough to allow calcite t o form and to preserve the montmorillonite. The dolomite zone is overlain by 1.5 m of well-laminated gray clay (commercial bed) which grades upward into a 0.5 m of crumbly, nonlaminated clay and then to 0.3 m of pebbly clay with a calcareous sandy matrix. In the uppermost zone some pebbles are dolomitic and contain pure palygorskite. The calcareous sandy matrix has a high montmorillonite content, as does the granular calcite filling the mud cracks. This upper sequence is similar to that in the first core and also suggests a hiatus and some form of reworking at the top of both the lower and upper clay beds. The pebbly layer comprising the top of the upper clay bed is overlain by approximately 7.5 m of clay, sand, and clayey sand with several thin beds of fossiliferous limestones. Fossils in the equivalent interval in the Gunn Farm mine are assigned a Chipola age (Olson, 1966), but may be Upper Torreya (Lower Miocene). The clay in this interval is almost entirely montmorillonite. Some palygorskite sand-size grains are present. Phosphate grains are present through most of the interval. The sands are similar texturally to the lower sands associated with the clay beds. They are well sorted with a well-defined mode ranging from 3.0 to 3.4 4. In some parts of the mine, this upper montmorillonitic interval rests on a palygorskite-rich dolomite bed equivalent to the upper clay bed. The two are separated by a 5-cm layer of fine sand which contains a relative abundance of coarse, black phosphate grains. The sand layer rests unconformably on the dolomite and appears to have been derived from it by leaching. The relatively large size of the phosphate grains suggests mild marine reworking. This unconformity is equivalent to the clay-on-clay unconformity seen in other parts of the mine. In what is apparently the western portion of the mine the brown-soil zone (Fe and organic material) can be traced for at least a quarter of a mile. Over most of this interval it is composed largely of montmorillonite with minor amounts of palygorskite and sepiolite with the sepiolite being more abundant, relative to palygorskite, than in the underlying clay. The sand content becomes relatively high and the material has a mottled appearance. The underlying clay is similar to that previously described; palygorskiteand sepiolite-rich pebbly clay grading down into laminated clay. Westward dolomite lenses become increasingly abundant both above and below the soil zone. Sepiolite is present in and below the soil interval and not above. At one point in the westernmost portion of the mine the brown interval gradually thickens from 0.3 to 1.5 m, and the brown color fades out downward. Vertical burrows, 1 cm in diameter and 0.3 m or more long, are abundant. They are filled with quartz sand and clay grains and contain only montmorillonite. The brown interval is predominantly montmorillonite with a slight increase in palygorskite and sepiolite in the lower section and in the top of the underlying gray clay.

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Starting approximately 1 m below the top of the brown zone there is a concentration of white, rounded, pebble-like particles and irregular flat particles (5-15 mm in diameter) which are composed predominantly of palygorskite and contain more sepiolite than the bedded clay. Minor montmorillonite is present but this may be contamination. Below the tan interval this whitish, pearly-appearing palygorskite is present as a thin coating on the gray clay. SEM studies show the white palygorskite-sepiolite material is a secondary clay skin. Van den Heuvel (1964)found sepiolite and palygorskite in the form of sand- and silt-size aggregates and as linings on channel walls in a calcareous soil from New Mexico. The data is fairly convincing that these clays formed in place in the soil. The white particles in the Miocene soil always contain more sepiolite than any of the other clays and occur in the lower portion of the soil. This s u g gests a secondary origin. It is difficult to visualize how these large white particles could form in a soil, but then chert nodules and concretions of various compositions form easily enough in muds. The presence of dolomite or calcite particles may maintain the high pH conditions necessary for the precipitation of sepiolite. The brown zone has a higher sand content than the ,palygorskiteclay beds. East of the thick (1.5 m) brown area the material equivalent to the lower 1.2 m is a palygorskite-rich pebbly or ped clay with a relatively low sand content. Some brown clay extends down into this interval. This suggests that the variation in thickness of the brown zone is, in part, controlled by original lithologic and environmental differences, and is related to a montmorillonitic sandy facies. It is unlikely the montmorillonite formed from the alteration of palygorskite. That the montmorillonite is largely detrital, rather than altered palygorskite, is suggested by the fact that in some areas of the mine the montmorillonitic brown sand-clay is overlain by approximately one meter of montmorillonitic light-gray sandy clay with marine calcareous fosssils. In the westernmost portion the entire upper interval is apparently represented by a montmorillonitic sandy-clay and clayey-sand facies. Unfortunately this interval was not sampled in detail. To the southeast, in the Midway Mine, the brown-soil zone (relatively rich in Fe and organic material) is only 15 cm thick, and is composed of pebbly clay containing palygorskite, much sepiolite, and only trace amounts of montmorillonite. The brown stain is independent of clay-mineral type and if it represents a soil horizon it indicates that at least in some areas weathering was not severe enough to alter the palygorskite to montmorillonite. In any event there was a widespread change of conditions at the end of deposition of the first clay bed. The upper clay bed in the La Camelia Mine is much more erratic than the lower one. In addition to being a well-laminated clay and highly dolomitic, it also contains layers (up to 0.6 m thick) of grainy calcite which contain pure palygorskite. The thin-section of this material (Fig. 47) shows the paly-

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Fig. 4 7 . Spar calcite separated by thin ribbons of optically oriented palygorskite. La Camelia Mine. White bar equals 0.1 mm.

gorskite is all in optical continuity and consists of long lenses and ribbons periodically separated by layers of spar-calcite grains (1-2 mm). Most grain boundaries are smooth but a few are serrated, suggesting replacement or interference. Some grains contain small fragments of palygorskite. Residues of the calcite grains are composed almost entirely of palygorskite, in contrast to the montmorillonite in the calcite grains associated with mudcracked clay. Though there is some suggestion of replacement it seems more likely that there was a rhythmic deposition of calcite and palygorskite. First calcite would precipitate, thus increasing the Mg/Ca ratio and allowing palygorskite to form. The other possibility is that the moist palygorskite dried and developed fractures parallel to the bedding and calcite formed in these fractures. Similar rocks occur below the lower clay bed in some areas. One sample which was collected from a clayey, very fine sand in the montmorillonite ‘overburden’ interval contained a mixed-layer montmorillonite-kaolinite A good example of lateral variations in mineralogy was observed in a pit (cut NS) in the southwesternmost part of the mine. At the northern end of the west wall of the mine there is a gray, sandy (30% greater than 48 pm)

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section, 6-9 m thick. The lower portion contains mostly montmorillonite with sepiolite (minor) being more abundant than palygorskite and illite. This grades upward into a gray, sandy clay with palygorskite more abundant than montmorillonite (20-3076). On the opposite wall of the mine (15 m away) the lower sandy, gray clay (or clayey sand) contains only montmorillonite and minor illite. This is overlain by a sandy shell bed with large worm burrows. This shell interval is the lateral equivalent of the palygorskite-rich, sandy clay on the west wall and has the same clay-mineral suite. Thus, within an interval of approximately 15 m the sandy palygorskite clay grades into a marine or brackish-water shell-rich sand which presumably acted as a barrier. Laterally (along the west wall) the palygorskite content of the upper clay increases and the montmorillonite decreases to less than 10%. The sand content decreases to a few percent. This relatively pure clay continues for approximately 100 m and changes from a very light-gray, fairly well-laminated, clay into a medium-gray blocky clay with small patches of sand. Over an interval of about 3 m this grades into a brown clayey montmorillonitic sand with small patches of calcite and with only minor palygorskite and kaolinite. This in turn grades gradually into a white sandy fine-grained calcareous material containing slightly more kaolinite. This sand may be effected by recent weathering and the calcite may have come from dissolved shells. For about 30 m the section is missing, but was probably largely sand (barrier or channel?); on the other side the palygorskite-rich upper clay bed is present and continues for at least 50 m. Thus in this area of the La Camelia Mine.the lower clay bed is quite sandy and largely montmorillonitic, the soil zone is not present, the upper clay bed grades laterally into sandy clays and sands, and the montmorillonite content is closely related to the sand content as it is in other areas. These factors strongly suggest that the clay distribution is environmentally controlled and not seriously modified by weathering. The sediments appear t o have been deposited in a coastal deltaic influenced environment. ADJACENT MINES

About 1.5 km north of the northern portion of the La Camelia Mine is a small mine with a thin, slightly sandy, palygorskite-rich clay bed. Worm burrows are abundant. A thin layer of small shells is present. Granular calcite is abundant. The calcite grains contain a montmorillonite and illite. Approximately 3 km t o the southeast of the La Camelia Mine is a mine with 1.2-1.5 m of palygorskite-rich clay with no sepiolite. The bottom 0.3 m is well laminated and contains worm trails and vertical tubes. This grades upward into pebbly and poorly laminated clay with a sandy-clay matrix. Granular-calcite grains occur in both vertical cracks and horizontal layers and are associated with a relatively high montmorillonite clay suite. The montmorillonite and quartz contents increase slightly upward. The clays and quartz contents in the worm burrows in the lower portion are similar to

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those in the overlying interval. The upper 0.3 m or so of crumbly clay has a high montmorillonite content. There is a fairly abrupt contact between the clay and the overlying montmorillonite sand. Relatively well-rounded clay pebbles, from 0.5 t o 15 cm in diameter, are abundant in the sand. They usually contain a minor amount of palygorskite in addition t o montmorillonite. The pebbles were presumably derived from the top of the underlying clay bed. MIDWAY MINE

In the Midway Mine, approximately 20 km southeast of La Camelia, the lower clay bed is well developed (Fig. 48). The base of the lower clay (base of the mine) contains thin (-15 cm) beds and lenses of soft dolomite and hard, granular calcite. Thin-section studies indicate the calcite is identical to that found in the upper portion of the La Camelia Mine (alternating layers of spar calcite and oriented palygorskite). The generally smooth boundaries of the calcite grains and the orientations of the clay suggest that there has been no replacement, only infilling. The dolomite is rhombic with most having dark centers. Grains and patches of palygorskite are abundant. The clay bed contains more sepiolite and less montmorillonite than in the La Camelia Mine. Most of the clay is massive t o laminated but the upper 0.3 m consists of angular to round clay pebbles (peds) in a tannish, silty, clay matrix. In the lower portion they range from 0.5 to 8 cm in diameter. The size decreases to 0.5-1.2 crn at the top. The upper 1 5 cm has a dark-brown to black organic stain similar to that La Camelia Mine. The clay suite itself is similar to that in the

9-

6-

3-

a

0-

Fig. 48. Cross+ection of Midway Mine, Florida. Only lower clay bed is present.

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Fig, 49. Ped structure and argillans (white) in upper portion of lower palygorskite clay bed, Midway Mine. White bar equals 0.1 pm.

underlying clay except sepiolite is more abundant. Thin films of clay and worm burrows extend down below the brown zone. It is difficult to see the pebble character in thin-sections of the brown zone but it is very evident in the underlying material where the relatively pure clay clast and peds are separated by silty clay and argillans (Fig. 49). The brown zone is thinner than in La Camelia and the clay is largely palygorskite rather than montmorillonite. Sepiolite is relatively abundant in both areas. Round phosphate grains are relatively abundant in the lower clay bed. The material overlying the brown-soil zone is quite variable. Sand and clay are mixed in wide variety of ratios and textures; clay pebbles and patches of

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calcite are present (Fig. 48).The lower 1.5 m of the interval overlying the soil zone contain roughly equal parts palygorskite and montmorillonite. Above this montmorillonite is predominant. Phosphate grains are common throughout the interval (-6 m). Thus, the sepiolite-containing lower clay bed in its structural and textural features is similar to that in the La Camelia Mine. The upper clay bed, which has an erratic distribution even in the La Camelia Mine, is missing and is replaced by a more clastic, sandier facies. GUNN FARM MINE

In the small Gunn Farm Mine (Fig. 4) about 3 km north of the La Camelia Mine the lower clay bed (3 m) contains appreciable sepiolite, in addition to minor montmorillonite. In some samples, sepiolite is more abundant than palygorskite. The upper 0.6 m of the clay bed is made up of clay clasts and contains a brown stain similar to that seen in other mines near the top of the lower clay bed. This is overlain by approximately 1.5 m of grayish, very sandy clay to clayey sand with a clay suite similar to that below. This is the highest occurrence of sepiolite and probably is equivalent to the top of the soil zone seen in other areas. This is overlain by 0.6 m of montmorillonitic sand with clay clasts and burrows. The sand is overlain by 1.5-1.8 m of palygorskite-rich clay which contains no sepiolite. Montmorillonite is more abundant than in the lower clay bed. A few oyster shells occur in the upper portion. The clay contains an abundance of relatively coarse-grained calcite. This unit is probably equivalent to the upper clay bed at La Camelia. The next interval consists of approximately 4 m of interbedded coquina (three layers) and montmorillonitic fossiliferous sand. The fauna (pelecypods, gastropods, foraminifera, ostracods, etc.) in this interval has been described as Chipola age, but may be older (Huddlestun, personal communication, 1975). It represents nearshore, brackish, shallow-water conditions (Olson, 1966). This faunal interval appears to be the same as that in La Camelia (on top the upper palygorskite clay bed). CHESEBROUGH MINE

The westernmost commercial mine occurs approximately 1.5 km south of Quincy (Fig. 4). The palygorskite clay bed (only minor amounts of montmorillonite) is approximately 5 m thick. Phosphate grains are present. Thin dolomite lenses and beds are present and most of the section contains megafossils. A thin (0.5 m) irregular fossiliferous sand bed occurs near the middle of the clay bed. This section contains more shells than sections in the other mines, suggesting it was deposited closer to a normal marine environment. It is not clear whether the whole section is equivalent to the upper clay bed or

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whether the lower portion, below the sand interval, is a more marine equivalent of the lower clay bed. The palygorskite clay is overlain by approximately 6 m of fossiliferous (pelecypods, ostracods, and manatee) clayey sands, sandy clay, and thin limestone beds. Montmorillonite is dominant with minor amounts of palygorskite. The vertical sequence suggests a transition from a tidal-lagoonal environment upward to an open-water brackish or marine environment. SUMMARY

In the southeastern portion of the mining area the lower clay bed is commonly underlain by dolomite and limestone beds which contain abundant palygorskite. Northward, near the Georgia-Florida border, montmorillonitic sand is present. The lower commercial clay bed, characterized by the presence of sepiolite, appears to be relatively extensive and continuous but grades laterally from clean palygorskitesepiolite clay to sandy montmorillonitic clay and clayey sand. Dolomite lenses are present in the first of these facies. The palygorskite-rich facies usually contains peds and/or pebbles, particularly in the upper portion, indicating some weathering and reworking after deposition. The sandy facies lies to the west of the clay facies and appears to have been a major north-south barrier sand (parallel t o the Trough and Ocala High) behind which lay a relatively large lagoon in which the clay was deposited. To the southwest (seaward) dolomite was deposited on top of the lower clay unit, indicating lagoonal conditions still prevailed. Over most of the area a brown organic soil zone was established and a pedal structure developed. To the east (landward) the soil developed on the palygorskite clay bed and there was little mineralogical change except for the development of some secondary sepiolite. Westward the top of the lagoonal clay is overlain by a montmorillonitic sandy clay with soil features. The increased sand content of this material indicates that higher-energy conditions were introduced to the area. The sepiolite distribution suggests there was some subsequent weathering of the montmorillonitic unit. Farther west the brown zone thickens and becomes more complex. The interval is represented in some areas by 1.5 m of montmorillonitic sandy clay with abundant worm burrows. This increased thickness appears to be at the expense of the underlying palygorskite clay bed. Nowhere does sepiolite extend above the brown zone. In some locations the brown color is missing and the entire lower palygorskite clay bed may be replaced by a montmorillonitic sandy clay or clayey sand. On top of the barrier sand separating the lagoon from open-marine conditions the soil interval appears to be represented by a gray, crumbly, calcareous, palygorskitemontmorillonite clay with thin secondary films of dolomite, palygorskite, and sepiolite. In other areas the brown interval grades upward into a gray sandy mont-

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morillonitic clay soil overlain by fine sand, usually containing shells. This may be overlain by a shell or sandy shell bed 0.6-4.5 m thick. The sand and shell sequence presumably represents the end of subareal or near-subareal weathering, and marks the beginning of a shallow-marine and brackish-water transgression. During deposition of the lower clay bed a large sand barrier island and lagoon apparently was present. The clay in the lagoon was fairly extensive but locally interrupted by the deposition of tidal delta sands and wash-over fans. As the lagoon filled subareal alteration occurred. This varied in character throughout the lagoon and barrier area. In the eastern portion the soil developed on the clay bed proper. To the west fluvial montmorillonitic sandy clays were deposited on top of the lagoonal deposits. This may indicate a slight lowering of sea level and/or the diversion of a nearby river into the area. The humic acids in the soil apparently were deposited with the sediments. Many of the present-day rivers in the southeast have such a high content of organic material that they are colored dark brown. The isopach map (Fig. 2) indicates an elongated thick interval in this area running perpendicular t o the shore line. This could represent a Miocene river deposit. The river would have kept the lagoonal waters relatively fresh. The suggested beach and river alignment is in general agreement with the cross-bedding inclination directions obtained from outcropping Miocene sands (Tanner, 1955). Some sands have a general northeastsouthwest pattern suggesting deposition by littoral currents. Others have a west and southwest orientation and are presumably stream deposits. The sea then reinvaded the low-relief area, a sandflat was developed, and localized shallow enclosures were formed, commonly by the development of sandy shell barriers. The resulting clay beds formed in these enclosures are of more local extent than that of the underlying lagoonal clay. The upper palygorskite clay bed (usually laminated) is well developed at Lake Talquin, near Quincy, in portions of the La Camelia Mine, and slightly east of La Camelia. It appears to extend into Georgia where it is present in the area of the Block N Mine and at Climax and Fall Caves. The lower portion usually has a high content of lenses and thin beds of dolomite. Eastward (landward) the dolomite is not present and this interval is represented by a continental mixed sand and clay facies which contains appreciable amounts of montmorillonite. To the west and northwest of the palygorskite bed the clay becomes sandier and the montmorillonite and kaolinite content increases. It eventually grades into montmorillonitic sand, presumably marine, and shell deposits. There appears to be a brief hiatus after the deposition of this upper clay bed, indicated by the presence of a burrowed thin crumbly and pebbly zone and, in some areas, a thin residual phosphatic sand layer. This was followed by an eastward movement of the open-marine conditions and the deposition of montmorillonitic sand and clays and brackish-water fauna (Chipola or

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Late Torreya age). This facies is restricted to the northwestern portion of the area (Quincy, La Camelia, and Gunn Farm areas). East and south of this area fossils are absent and the highly mixed sediments resemble continental deposits. Sepiolite is present in the soil and the lower clay bed but not in the upper clay bed. This may indicate fresher-water conditions existed during deposition of the lower sediments. This is suggested by the fact that regionally sepiolite occurs landward of palygorskite deposits. However, this could also be indicative of periodic hypersaline conditions near to shore. A'ITAPULGUS, GEORGIA

Approximately 11 km northeast of the La Camelia Mine, (Fig. 4) thbre are several small mines which contain a light-tan blocky palygorskite clay differing in general appearance from the laminated to pebbly clays to the south. Stratigraphic correlation (Fig. 5) indicates they are equivalent to the lower Miocene clay beds of Florida. At the bottom of the clay bed is a light-tan granular limestone, which in some areas is composed of interlocking sparry-calcite grains (0.5-2 cm) containing silt- and sand-size grains of quartz, K-feldspar and palygorskite. Other portions contain thin laths of parallel-oriented palygorskite units scattered among the calcite grains (Fig. 47). In the more granular material the oriented palygorskite is abundant and the calcite grains (rice-shaped) are scattered through it. The presence of worm trails suggests the original material was a carbonate mud (micrite). The quartz grains have a Mo of 2.8 $ and the lower-density clay grains 3.3 $, indicating little, if any, current sorting. This also suggests the clay grains are fecal pellets. The approximately 3 m thick clay bed was sampled at two different locations. In one section the clay bed is composed predominantly of palygorskite (-95%) with variable, but minor, amounts of montmorillonite. The uppermost portion of the clay bed has slightly more montmorillonite. The lower meter of clay contains thin sand laminae and channel-shaped patches (-25 cm). The well-sorted channel sand (Mo= 3.3) is partially cemented with calcite. The clay contains only 1.0% coarser than 400 mesh very fine sand (Mo = 3.9 $), similar to that in the Florida mines, with approximately one percent clay and phosphate grains. A few sponge spicules and diatoms are present. Some flat rounded clay clasts (-15 mm) occur associated with the sand, and one spoon-shaped patch with a concentric dehydration pattern was found The tannish clay overlying the sand interval contains irregular lenses of white clay with gradational boundaries. The white clay is nearly pure palygorskite. The amount of nonclay material (1.1%) is similar in both clays. The nonclay material in the white clay is similar to that in the tan except it contains a few sponge spicules and diatoms. The detrital material appears to have been uniform. Therefore the local variations in clay mineralogy indicate

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there may have been variations in water chemistry. The white clay may have floated in from the edge of the lagoon. Approximately 0.6 m above the top of the sand is a 2-10 cm thick bed of rice calcite with large (1-5 cm), horizontal clay chips. About 0.3 m above the calcite bed is a 0.3 m thick bed of soft white dolomite. The dolomite consists of thin-walled, partially formed rhombs. This is overlain by 1 m of clean, blocky clay. In this area there is considerable lateral variation over relatively short distances. In other portions of the mining area the entire section above the basal calcite bed is a clean blocky tannish clay with no sand laminae, calcite, or dolomite, and a relatively low palygorskite content. The sand content of the clay decreases from 34% at the base to 0.8% at the top of the section, whereas the palygorskite decreases from 90% to 40% from bottom to top. This trend (high sand-high palygorskite) is the opposite of that found in the Florida area. The sand is bimodal (3.5 4 and 4.0 4). The coarser mode value is similar to that found for the La Camelia samples. The sand represented by the finer mode is more abundant. The Mz values for the clay samples from both sections are larger (3.5 4 to 3.8 4) than for the La Camelia clays. The fact that both sand populations were present and the finer one is relatively abundant in the Block N clays suggests these clays were formed in a lower-energy environment than some of the La Camelia and other Florida clays. Mineralogically, texturally, and structurally these Georgia clays resemble the upper clay bed in the La Camelia area and have little in common with the lower clay bed. The weathered montmorillonitic clay immediately on top of the palygorskite clay (0.8% sand) contains 30.1% sand, and the sand is coarser than that in the palygorskite clay bed, suggesting the difference is a depositional rather than a weathering feature. The lower half of the clay bed in this area contains more sand than the upper half and contains phosphate grains which are not present in the upper half. This vertical change in depositional conditions is presumably equivalent to that seen in the other section where the lower portion contains sand channels and clay clasts and the upper portion dolomite. The vertical changes are not identical in the two sections, but both indicate a change to a lowerenergy environment during deposition of the upper portion of the clay bed. The thin clay beds in Climax and Fall Caves, to the north, contain the only pure palygorskite found in this study. The clay is relatively clean and does not contain phosphate grains, sponge spicules, or diatoms. It must have formed in a shallow, protected area near the edge of the lagoon.