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Oölitic films on low energy carbonate sand grains, Bimini Lagoon, Bahamas
Marine Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
OOLITIC FILMS ON LOW ENERGY CARBONATE SAND GRAINS, BIMINI LAGOON, BAHAMAS R. G. C. BATHURST
Department of Geology, University of Liverpool, Liverpool (Great Britain) (Received September 19, 1966)
SUMMARY
Grains with thin (2/a) oolitic coats are particularly abundant in the southeastern part of Bimini Lagoon. Turbulence is low and sediment movement is restricted by a dense benthos of Thalassia, Algae, and a subtidal gelatinous mat that binds the carbonate grains together at the sediment surface. The region has the shallowest water in the lagoon, the highest content of fine sand, and grains are well-rounded and intensively bored by Algae.
INTRODUCTION
On the warm sunlit floor of the lagoon at Bimini (Fig.l), sheltered from the trade winds by low tree-clad islands of Pleistocene limestone, many of the carbonate sand grains bear on their outer surfaces thin (about 2/~) oolitic coats (films). These films have been found on grains from all over the lagoon with a frequency that varies from almost zero coated grains per sample to nearly 100 ~. Coated grains are commonest in the southeast (Fig.2). The setting is tranquil and the movement of grains by currents is impeded by a rich benthos of Thalassia (turtle grass) and Algae, and by an extensive, subtidal, organic, gelatinous mat that binds the surface of the sand. The scene here is radically different from the wild turbulence of the well known oolite shoals to the south around Browns Cay. It is necessary to ask how, in such calm surroundings, so many grains have come to be encrusted with a film of oriented aragonite. Despite the acute observations of EARDLEY(1938), ILLING (1954) and NEWELL et al. (1960) we still know almost nothing of the physical chemistry of o0id growth. The recognition, in Bimini Lagoon, of a previously undescribed morphological type of marine o0id, with an unusually thin coat, deposited in an environment unlike either the tidal oolite shoals of Browns Cay CNEWELLet al., 1960) or the Laguna Madre (FREEMAN, 1962), extends our knowledge in an area where additional information is urgently needed. Marine GeoL, 5 (1967) 89--109
90
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Fig.1. Bathymetric setting of Bimini.
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Fig.2. Bimini Lagoon: stations and frequency indices for coated grains. Martne Geol., 5 (1967) 89-109
I
OOLITICFILMSON BAHAMASANDGRAINS
91
THE PROBLEM This study arose from two observations by the author in the laboratory in England. On looking at thin sections of impregnated lagoonal sediments it was apparent first that there are o/51itic films on many grains and, second, that there is a very high concentration of these coated grains at all the stations in the southeastern part of the lagoon (later named the East Lagoon, Fig.2). These observations led to a comparative study (unhappily without the opportunity of further field work) of the East Lagoon and the rest of the lagoon, in the hope that any differences found might provide some guidance as to how the films were deposited. Bimini Lagoon is not the only place where o/31itic films have been found. Other calcarenites in the author's possession show o61itic films with dimensions similar to those in Bimini Lagoon. They come from the following localities: (1) Off the north point of North Bimini. (2) Beach, west coast of North Bimini, by Lerner Marine Laboratory, southwest of lagoon station 5. This is a coarse polished calcarenite. (3) About 2 miles south of the southeast end of South Bimini (collected by Dr. H. Fiichtbauer). (4) Off the west coast of Andros Island, Bahamas, from its southern tip as far north as Williams Island. (5) Frazers Hog Cay (northwest coast), Berry Islands, Bahamas. (6) Cockroach Cay (north coast), Berry Islands, Bahamas. (7) Batabano Bay, Cuba; in the "ovoid grain", "composite grain" and "o61itic skeletal" lithofacies of DAETWYLERand KIDWELL(1959). (8) Campeche Bank, Yukatan; in the finest sand.
HELDAND LABORATORYMETHODS In the summers of 1961-1963, three periods, each of two weeks, were devoted to field work in Bimini Lagoon. The presence of oolitic fdms on the grains was discovered only after the field work had ended. Thus the field observations and samples were not collected with this study in view. Data from 68 stations are used in this paper. They are of five kinds: (1) An index of frequency of coated grains. (2) The density of Thalassia on the floor. (3) Water depth referred to mean sea level. (4) Mechanical analysis of the sands. (5) Roundness of grains in sieve fraction 150-90/l. The methods by which the data were determined are now described.
Marine GeoL, 5 (1967) 89-109
92
i~.
o. c. BATttURST
Frequency index
The distribution of coated grains on the lagoon floor was estimated by determining, in one sample of sand from each station, the frequency index (Fig.2). This index is presumed to reflect not only the frequency of coated grains in the sand but also the tendency to growth of o61itic films. Higher frequencies are taken to imply environments better suited to oolitic growth. Simple, uncorrected comparison of sands in terms of the proportion of coated grains in each is, of course, not a sure indication of the ease of o61itic growth at any place. Grain shape, which varies greatly between stations, influences the perfection and even the existence of the o61itic film. The larger grains and especially the more irregular grains, tend to have either poorly developed films or none at all. It is necessary, thelefore, to ensure that only grains of similar shape and surface texture are compared. Fortunately there is one type of grain that has a relatively constant shape. This is the micritized skeleton (BATHURST, 1966) described briefly on p. 105. Most of these grains have cross-sections that are oval to approximately circular, though they may vary considerably in roundness. The important point is that they do not have the extreme variation of shape and surface texture that typifies many broken, un-micritized skeletal particles. They resemble the "grains of aragonite matrix" of IeHyo (1954, p.27) which make up the cryptocrystalline grains in the "bahamite" of BEALr!S (1958, p. 1846). They are within the class "peloids" of MCKEE (1966, chapter 3, section on carbonate rocks), which are grains of microcrystalline or cryptocrystalline carbonate irrespective of origin. The term peloid is adopted here. Other peloids in the lagoon are faecal pellets. All statements of frequency, therefore, refer solely to the presence or absence of an o61itic film (complete or patchy) on peloids. All sizes of grain in the sands were examined in preparing the index (Fig.2). Attempts to count coated grains (peloids with films) in thin sections of sands (in Marco Resin) were not successful. Sorting during preparation of the artificial rock caused the sections to be so unrepresentative that the accuracy introduced by counting was vitiated. Nor was it possible to distinguish coated grains from other grains in loose dry mounts. Eventually a much simplified method, using thin sections, was chosen whereby the frequency of coated grains was estimated visually in terms of four readily distinguishable classes. This gave consistent results for repeated analyses of the same sample and for analyses of replicate samples. The classes are: (l) Abundant: peloids without films are detectable only with difficulty. (2) Common: more than half the peloids have films. (3) Scarce: (a) Jew: less than half the peloids have films; (b) rare: grains with films are detectable only with difficulty. As samples with few or rare coated grains are uncommon these two attributes are, for statistical purposes, combined under scarce. In quantitative terms abundant probably means that more than 95 ~ of the peloids are coated while rare probably means that less than 5 ~ are coated. These estimates are based on counts. Marine Geol., 5 (1967) 89-109
93
OOLITIC FILMS ON BAHAMA SAND GRAINS
\.Xo +
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Fig.3. Bimini Lagoon:
Thalassia
density.
Thalassia density Bearing in mind the possible influence of leaf baffles on sediment movement (GINSBURG and LOWEr~STAM,1958), the density of the dominant vegetation, Thalassia, on the lagoon floor was estimated (Fig.3). At each station the area of floor obscured by the blades of this plant was assessed in terms of the following classes: (1) Plentiful: (a) dense where the floor is totally obscured; (b) more than half the floor obscured; (c) haifthe floor obscured; (d) less than half the floor obscured. (2) Thin where it is just detectable.
(3) Absent. As analyses of the data showed that the four denser classes differ little in their relation to other factors, they are combined as plentiful (Fig.3).
Water depths The depth of water was measured with a weighted string and corrected for mean sea level with the help of British Admiralty Tide Tables. The times and ranges of high and low tides at the Standard Port (Bermuda) were corrected for the nearest Secondary Port (Cat Cay, 16 km south of Bimini). The 0.5 m contour is shown in the figures. Marine
Geol., 5 (1967) 89-109
94
R.G.C.
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Fig.4. Representative sieve analyses of sands from North Sound and Main Lagoon, excluding channel and beaches.
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Fig.5. Representative sieve analyses of beach sands, Bimini Lagoon.
Mechanical analysis Mechanical analyses of dried sub-samples, prepared from the station samples, were made with sieve intervals of about 0.25 • (Fig.4-6). Marine Geol., 5 ( 1 9 6 7 ) 8 9 109
O~LITIC FILMS ON BAHAMA SAND GRAINS -1 D
0
4"4
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Eost L o g o o n (Excluding beaches)
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Sieve diorneters : Log. scole Fig.6. Representative sieve analyses of sands from the East Lagoon.
Roundness An indication of the roundness of the finer grains was found by comparing the silhouettes of grains in the sieve fraction 152-124 # with PETTIJOHN'S (1949) fig.24. It was felt that comparison among grains of similar size and shape must be an essential precaution. These conditions are best fulfilled by the finer grains. It is also these same grains that most readily grow o61itic films. Fifty grains randomly selected from each sample were described. Inspection of the frequencies in the five roundness classes (A-E) led to a convenient index of roundness in terms of the number percentage of grains in the combined three classes C, D, E (Fig.7). The higher the index number, the greater the roundness of grains in the fraction 152-124/z.
PETROGRAPHY OF THE COATED GRAINS
The nucleus All the types of sedimentary grain in the lagoon act as nuclei, the commonest being Foraminiferida, Halimeda, fragments of Mollusca and Corallinaceae, faecal pellets and other peloids. As the o61itic films are so thin (Fig.8) a good idea of the true size distribution of nuclei can be gained from Fig.6. Films are more completely developed on the smaller, more rounded nuclei, tending to enclose them entirely. They are poorly developed on unaltered fragments of Soritidae, Halimeda or on any other grain where the surface is highly irregular or the particle diameter exceeds 1 mm. Marine Geol., 5 (1967) 89-109
96
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Number'/,
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Fig.7. Bimini Lagoon:roundness indices.
They are generally, though not everyhwere, best developed on peloids that have diameters less than 250/t. These are mostly fragments or Soritidae, Halimeda or faecal pellets in various stages of micritization (BATHURST, 1966), a process that is discussed on p.105. Many of the peloids have no recognisable internal structure (Fig.10).
The oOlitic film The pale brown oolitic film (Fig.8) has a thickness that varies from grain to grain, achieving a maximum of about 8 #, but usually falling within the range 1-3/~. This range of thickness is also given by NEWELL et al. (1960, p.490) for the "single lamellae" of the Browns Cay oOids, The internal fabric of the Bimini film was compared with the "oriented aragonite lamellae" in oOids collected by the author from Browns Cay: no petrographic difference could be detected. The film consists of a mosaic of crystals with diameters up to about 2 #. The smallest detectable crystals were about 0.5/t, but smaller ones may exist. The mosaic was examined at a magnification of 900 × (oil immersion) with a gypsum first order red compensator under crossed-nicols. It showed the familiar orientation of the slow vibration direction normal to the surface of the coat (SORBY, 1897, p.74; ILUNG, 1954, p.38; NEWELL Marine Geol., 5 (1967) 89-109
O()LITIC FILMS ON BAHAMA SAND GRAINS
97
Fig.8. OOlitic coat on peloid, East Lagoon. Slice; in Marco resin.
Fig.9. Test of a foraminifer partly micritized. Slice; in Marco resin.
Marine GeoL, 5 (1967) 89-109
98
m (;. ( . BATHURS1
et al., 1960, p.490). The separate crystals are best seen in the region of the black cross where the coat is mostly in extinction. Here the slight differences in optical orientation are most readily apperciated. A film may be complete, enveloping the whole surface of the nucleus, or it may be patchy. The surface of the nucleus is commonly irregular and the film, whether complete or not, tends to fill the smaller embayments. The film has a dull surface, lacking the shiny polish of the Browns Cay o6ids. The film is so thin that its recognition in ancient limestones seems unlikely, particularly where it changes during diagenesis to the structureless micrite seen in some fossil o6ids.
THE LAGOON ENVIRONMENT
Bimini Lagoon lies 25°44'N, 79°16'W, and 60 miles east of Miami, Florida (Fig.l). It is bounded on the west, north and east by the horseshoe of the islands of North and East Bimini and to the south by South Bimini (Fig.2). From a narrow entrance between North and South Bimini a tidal channel runs northward as far as Mosquito Point. A broad expanse of the shallowest water, between East and South Bimini, lies open to the east. Bimini stands on the western edge of the Great Bahama Bank. To the west of the Bimini islands a platform of Pleistocene limestone, covered with ripple-marked Recent carbonate sands and patches of Thalassia, extends down to about 40 m where the surface plunges steeply to the floor of the Florida Straits 800 m below. To the east stretch the extensive shallow waters of the Bank, rarely more than 2 m deep. The lagoon floor consists of incoherent Recent carbonate sands (Fig.10, 11) overlying Pleistocene limestone. A bore 0.5 km southwest of station 49, between the channel and the shore, showed that the sand is there separated from the limestone by a mangrove peat which, in common with other similarly situated peats on the Florida-Bahamas platform, gave a 14C age of about 4,000 years (NEWELL et al., 1959, p. 192). The thickness of the sands varies from zero in places where limestone is exposed to a least 1.5 m in the East Lagoon. The warm northward-moving Gulf Stream enables a tropical West Indian biota to flourish (Fig.12, 13). Full details are given by NEWELLet al. (1959). The mean sea depth is mainly from 0.7-0.3 m. Patches of floor are exposed at low tide off the southwest edge of Pigeon Cay, also just north of Tokas Cay, and to the east of Tokas Cay between the southwest lobe of East Bimini (west of Bone Fish Hole) and South Bimini (Fig.2). The tidal channel against North Bimini, partly dredged, has depths from about 6 m at the southwest entrance to 3 m near Mosquito Point. Current velocities were not measured, but sand movement along the tidal channel was inferred from the presence of ripple-mark and scour around boulders. Nowhere else in the lagoon was ripple-mark seen nor any sign of scour. Even the loose sand of the mounds is undisturbed. Yet this is a misleading picture of the cornMadne Geol., 5 (1967) 89-109
OOLITICFILMSON BAHAMASANDGRAINS
99
Fig.10. Typical sand of the East Lagoon. Mainly peloids formed by micritization of skeletal debris; one large faecal pellet. Slice; in Marco resin.
petence of the water to move grains. The top 0.25 cm of sediment is bound by a transparent, colourless gelatinous material to form a subtidal mat which stabilizes the grains. Wherever the subtidal mat was scraped away ripple-mark formed, often in a matter of minutes. There is probably no part of the lagoon floor that is not subject to tidal currents strong enough to move sand. Nevertheless, so great is the influence of the various sediment binders that some 23 km 2 of sand is immobilized.
PLACE AND TIMEOF GROWTHOF THE COATEDGRAINS In considering the seemingly paradoxical situation, where oSlitic films grow but sediment movement is restricted, it is necessary first of all to decide whether the films grew in the places where they are now found. The types of grain in the lagoon could all be derived from organisms that live there. They cannot be confused with o~ids eroded from the Pleistocene limestones, because these have characteristically thick coats and many of them are parts of limestone clasts. The conclusion is that the Marine GeoL, 5 (1967) 89-109
100
R, t~. ('. BAI'HURST
Fig.11. Typical sand of the North Sound and Main Lagoon. Little altered skeletal debris with a few peloids. Slice; in Marco resin.
sands spread over the major part of the lagoon floor were formed close to where they are now. Two objections that may be raised with regard to the local origin of coated grains in the East Lagoon are first that they appear at the surface now only as a result of bioturbation; and second that they never grew in the lagoon but were washed in from some outside source. It is true that the burrowing animal that makes the ubiquitous conical mounds (about 20 cm high) probably ensures vertical mixing of the sands throughout their entire thickness. But, if the more recently formed grains had no o61itic films, then mixing with older, the coated grains could not yield the high concentration of coated grains, more than 95 ~o, that is known to exist at the surface. To ensure such a high concentration of coated grains in the uppermost sands, new coated grains must be forming now. Considering the possibility of an outside source, there was no sign whatsoever, in the summers of 1961-1963, that bottom traction of sand grains was going on in the East Lagoon. The floor seemed totally immobilized by Thalassia and the mucilaginous subtidal mat. The obvious direction from which coated grains could be delivered Marine Geol., 5 (1967) 89-109
OOLITIC FILMS ON BAHAMA SAND GRAINS
101
Fig.12. Thalassia and mound, East Lagoon. Upper surfaces of leaves encrusted withepifauna. Length of Coke can is 12.5 cm. Under water.
Fig.13. Penicillus (bulbs on stalks), Thalassia (blades), Porites (branching coral), Ircinia (massive sponges), fish, floor of mat-bound calcarenite. Southwest of Pigeon Cay, East Lagoon. Length of Coke can is 12.5 cm. Under water. Marine Geol., 5 (1967) 89-109
102
R. (;. C. BATHURST
from without the lagoon is from the east. If winter gales (November, December) appreciably increase bottom traction, then as they blow from the northwest they are not likely to sweep grains in from the east. Moreover, the nearest oOids, lying just east of East Bimini, are distinctive in that they have thick oolitic coats. Such forms are rare in the sands of the East Lagoon. Dr. H. Fiichtbauer has sent me samples of oOids with thin films like those in the East Lagoon which he found about 2 km south of South Bimini. To reach the East Lagoon these would either have to have been blown over the 1 km of tree covered South Bimini or carried 5 km northeastward before moving westward into the lagoon. Neither hypothesis is attractive. There cannot have been movement of grains inward through the southwest entrance, because the grains of the shelf are much coarser than those in the lagoon. Indeed, large scale lateral traction seems unlikely on the grounds that the lobate carpets of megarippled oolite which characterise the Bahamian oolite shoals are absent in the lagoon. Also, it must not be forgotten that stations 20, 18 and 80 in the North Sound have abundant frequency indices. These coated grains cannot have been supplied from outside the lagoon, because the North Sound is isolated from the open sea except for a narrow channel that wanders through the mangroves between North and East Bimini. Finally, it is again necessary to emphasise that the high concentration of coated grains in the East Lagoon could not have been caused by the addition of extra-lagoonal coated grains to a non-oOlitic indigenous calcarenite produced by the local skeletal organisms. Examination of two cores (1.25 m long) taken in the East Lagoon near station 37, which penetrated the whole thickness of the unconsolidated sediment, supports these conclusions. Samples examined at vertical intervals of 25 cm all have an abundant frequency index. It is clear that oolitic films have been deposited on grains continually since the Pleistocene floor was first covered by Recent calcarenite. Altogether the evidence points to the continual growth of oolitic films on peloidal nuclei throughout the Recent depositional history of the East Lagoon.
COATEDGRAINS, TEMPERATURE,SALINITY~DEPTH Frequency indices at the 68 stations are shown in Fig.2. If a line be drawn west of Alec and Pigeon Cays, from East to South Bimini, then the region east of this line has a frequency index abundant for all its stations. It will be referred to as the East Lagoon. In Fig.2 it can also be seen that the frequency indices near the channel and along the west coast of the lagoon are scarce, but elsewhere vary from scarce to abundant. On the basis of this distribution it is assumed that the most intensive growth of oolitic films is taking place in the East Lagoon. Alternatively, it is, of course, possible that the production of peloids is going on at a faster rate outside the East Lagoon. It can be shown, in fact, that this is not so. Faecal pellets and micritized skeletal grains are in general more numerous in samples from the East Lagoon than in samples from elsewhere in the lagoon. Marine Geol., 5 (1967) 89 109
103
O O L I T I C FILMS ON B A H A M A SAND G R A I N S
Water temperatures and salinities were recorded in the lagoon by TUREKIAN (1957) during the late spring and early summer of 1955 (Table I). He concluded that there are three distinct water masses occupying the North Sound, the Main Lagoon and the region east of Pigeon Cay. This last region closely resembles in extent the East Lagoon described in this paper. Within the Main Lagoon he found little lateral change. In the North Sound both temperature and salinity increased northward, although in such shallow water they can, at times, decrease northward as a result of rainfall. The Sound is less subject to mixing with open sea water than other parts of TABLE I DISTRIBUTION OF SALINITY AND TEMPERATURE IN BIMINI LAGOON (SPRING AND SUMMER 1955)
(After TUREKIAN,1957)
Salinity (p.p.m.)
Temperature (°C)
North Sound
increases northward from 40.4--41.9, with a drop to 40.5 in the far northeast corner
increases northward from 29.3-29.8, with a rise to 31.3 in the far northeast corner
Main Lagoon
36.0; no trend except for an increase to 39.4 near Mosquito Point
27.9-29.2 no trend
East Lagoon
36.6-37.5 no trend
28.3-30.7 no trend
the lagoon, and after rain the salinity there may fall as low as 31 p.p.m. In the East Lagoon, temperature and salinity are mostly high east of Pigeon Cay. It is of interest that the highest temperatures and salinities have been found in the North Sound. They do not coincide with the concentration of abundant frequency indices in the East Lagoon. On the other hand, dilution with rain water reduces the mean salinity of the North Sound, if this be taken over a period of time. A more detailed study of the effects of rain, and of run-off from the mangroves, on the salinity and the titration alkalinity was made by SEIBOLD (1962). His stations are, however, too few to be directly useful in this discussion. The water depths show that the East Lagoon is unique in having a shallower floor than any other part of the lagoon away from the beaches, much of it lying at less than 0.5 m (Fig.2) and parts being exposed at low tide. Summarising the information, the temperatures and salinities recorded in the East Lagoon are unremarkable, but this area is distinguished from the rest of the lagoon by its extreme shallowness.
Marine Geol., 5 (1967) 89-109
104
I~. (i. C. BA'rHURS1
COATED GRAINS, THALASSIA, FINE SAND
The importance of Thalassia and fine sand The distribution of Thalassia is regarded as particularly important (Fig.3) because, more than any other organism, it seems to influence the movement of water near the bottom. The density of the plant is therefore a convenient indication of the nature of the bottom. The unconsolidated Recent sands in which the Thalassia grows overlie a cemented Pleistocene limestone and are generally less than 1.5 m thick. Thus, where the water is deep, as in the channel, the Recent sands are either thin or absent. Colonisation of the sands by Thalassia is inhibited by two things. Thin sand cannot support the roots that commonly go down 60 cm or more, and fast turbulent water prevents growth, as is seen on the bare beaches. In general, Thalassia flourishes in the shallow, quieter water away from the beaches and channel, where the Recent sands are thicker. The term "fine sand" is applied here to the range of sieve diameters 150-90/z. An examination of the mechanical analyses from all stations showed that one parameter above all characterises the sands from the East Lagoon. They have, on the average, a higher content of fine sand, as here defined, than the sands in the other parts of the lagoon (Fig. 14). The ranges of grain diameters differ little from those in the rest of the lagoon and, although the means and skewnesses are peculiar to the East Lagoon, these only reflect in a less precise way the content of fine sand. The beaches and channel are poor in fine sand because they are high energy environments, but the low content in the rest of the Main Lagoon and in the North Sound, compared with the East Lagoon, must be a result of some other still unknown process.
Distribution of Thalassia andfine sand The relation between Thalassia density and the mean weight ~ of fine sand was examined for the whole lagoon (Table II). Only 51 stations are included, the other having no Thalassia. The results for the North Sound and Main Lagoon differ so little that they have been combined. The division of Thalassia records into the four denser classes (p.93) yielded no information since they only involve eleven samples. These classes are combined under the term plentiful. Two relationships are apparent in Table II. Both in the combined North Sound and Main Lagoon, and in the East Lagoon, the denser grass is associated with the greater quantity of fine sand. At the same time, the mean weight ~o of fine sand is higher in the East Lagoon irrespective of the Thalassia density. These differences are not statistically significant and it is conceivable that they have arisen by chance. On the other hand, the number of samples (stations) is small and a larger number might well reveal differences that are significant. The tendency for Thalassia to trap fine sand fits well with the experience of other workers and the higher content of
Marine Geol., 5 (1967) 89-109
105
OOLITIC FILMS ON BAHAMA SAND GRAINS
TABLE II MEAN WEIGHT PERCENT OF FINE SAND, FOR THE EAST LAGOON AND FOR THE REST OF THE LAGOON 1
Main Lagoon and North Sound Thalassia d e n s i t y thin
9.8
SE = -4- 1.6
East Lagoon 15.6
n=7
Thalassiadensityplentiful
11.9
S E = ± 1.7 n = 15
SE = + 3.2 n=8
23.7
S E = 4- 1.5 n =21
1 S t a t i o n s w i t h Thalassia densitiesplent(ful a n d thin. SE = s t a n d a r d e r r o r o f t h e m e a n ; n = n u m b e r of stations.
fine sand in the East Lagoon is clearly apparent from a study of the thin sections of impregnated sands.
COATED GRAINS A N D BORING ALGAE
Examination of the fine sand with a microscope reveals two striking differences between the fine sands in the East Lagoon and those in the rest of the lagoon. The fine sand grains differ in their degree of alteration to micrite and in their roundness (Fig. 10, 11). In the North Sound and Main Lagoon the fine grains consist largely of angular broken skeletons. Many of these are unaltered Halimeda and Soritidae boring Algae are rare in these skeletons. In the East Lagoon the finer grains are intensely altered to a greyish-looking micritic aragonite (Fig.10)with crystal diameters up to about 2/t. Inside the grains there are many dark tubes and blebs, about 6 / t in diameter, similar to those in the Browns Cay orids that are believed to be Algae (NEWELLet al., 1960, p.492). The greater roundness of the fine grains in the East Lagoon is also plain (Fig.7, 10, 11). The alteration of the skeleton to micritic aragonite is itselfa result of the activity of the boring Algae. When the Algae die the tubes are filled with micritic aragonite. By repeated boring, dying of Algae, and filling with micrite, the skeleton is changed to a structureless mass of micrite. This process of micritization, and some of its implications in ancient limestones, are examined in detail by BATHURST(1966). At the same time micritic (cryptocrystalline) aragonite fills the pores of Halimeda and the chambers of Foraminiferida and the pores of Echinoidea. The resultant grain is a peloid. Probably in the course of micritization the grains become rounded, by abrasion of the delicate fabric of the surface where this has been extensively bored but has yet to be filled. The abraiding agent could be any of a number of sand eating animals. It is important here to record that only a small and insignificant increase of roundness is caused by the deposition of the oSlitic film. The roundness of the coated grain is almost entirely a reflection of the roundness of the nucleus. Marine Geol., 5 (1967) 8 9 - 1 0 9
106
R. (i. c . B A T H U R S T
DISCUSSION
As a first step towards understandingwhy oOliticfilms grow so much more readily in the East Lagoon it is necessary to search for other things that distinguish this part of the lagoon from the remainder. Four such factors have been found: (l) the East Lagoon is the shallowest of the three regions; (2) the sediment on the floor has the highest content of fine sand; (3) the fine sand is peculiar in being intensely micritized; (4) the fine sand has an exceptionally high roundness. There is a further matter that should not be ignored. This is the general tendency throughout the lagoon for fine sand to be more plentiful where the Thalassia is plentiful. Taking the last point first, the role of Thalassia as a baffle, it should be noted that there is very little fine sand where the grass is absent, on the beaches and in the channel (Fig.3, 14). These are high energy environments and fine grains will not have been deposited. Yet the converse proposition, that Thalassia traps the fine grains, is impossible to demonstrate with the available data. For example, it would be wrong to suppose that the greater amount of fine sand in places where Thalassia is thin, compared with the bare beaches and channel, can be attributed to a reduction of water velocity near the bottom caused by the Thalassia blades. The description thin means that Thalassia is only just detectable, certainly less than one blade in 0.5 m 2. Under these circumstances its quantitative effect on water velocity must be unimportant. On the other hand, Thalassia grows more densely where the Recent sands are thickest and, in Bimini Lagoon, it happens that these are also the areas of quieter water. Thus the tendency for fine sands to be associated with the denser Thalassia may be a result, in part, of a local coincidence whereby both accumulate in the quieter water, but for reasons that are unrelated. Yet it is difficult to believe that the denser Thalassia does not reduce water movement and so trap fine sand to some extent. The data available are unfortunately insufficient to test this hypothesis. The shallowness of the East Lagoon is in agreement with the widely held view that oOids grow best in the shallower waters. Yet this region, unlike the Browns Cay shoals, is cooler and less saline than adjacent areas, according to TUREKIAN'S (1957) figures. Nor are the grains in the East Lagoon strongly agitated. The binding effects of the Thalassia and the Algae and, above all, the ubiquitous gelatinous subtidal mat, cause the day-to-day movement of the sand grains to be negligible when compared with the intense agitation at Browns Cay or off the beaches of Laguna Madre. Nowhere in Bimini Lagoon has there been even enough turbulence to polish the grains. The sorting of the sands here is also poorer than in the other Bahamian and Laguna Madre oolites. Low turbulence is further indicated by the high content of fine sand. Conditions in the East Lagoon must be less suitable for the growth of oolitic coats than in the other oolite forming areas, because the coats are so thin. It does not necessarily follow, though, that the coats grew more slowly while they were in the growth-promoting micro-environment. Until a good deal more is known about the process of growth it will not be possible to decide to what extent the Bimini Marine Geol., 5 (1967) 8 9 - 1 0 9
OOLITIC FILMS ON BAHAMA SAND GRAINS
107
\
,I, q ----Z-----
~ +
\o
.t.
BONE FISH HOLE
O
o.
\
"1
•
+
o
0
I
+ 0
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o •
// iiiiiiiiii~: CA T
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/ •
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Weight Percentage Fine Sand •
0
2 0 - 38
0 10-19
+0-9
0 0
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Nautical miles r
Km
2 i
Fig.14. Bimini Lagoon: weight per cent of fine sand (150-90 p).
coated grains (1) spend less time in a growth-promoting environment or (2) grow more slowly when in that environment. Moreover, as many of them spend a large part of their existence embedded in the gelatinous mat, this substance may influence ionic exchange between the oolitic coat and the water. At this point it is as well to consider how much movement of an o6id there must be so that it can grow with an even, overall development of oriented aragonite. It could be that the comparatively small amounts of movement that occur in the East Lagoon are, after all, enough to keep an o6id turning so that different parts of its surface are, from time to time, exposed to the growth environment. Causes of grain movement in the lagoon vary from the gentle browsing of fishes to the rare paroxysm of a hurricane. The fine sand with its extreme roundness would be expected to be the most mobile sediment in the lagoon. Tests in the laboratory have confirmed that these fine grains have the lowest erosion velocities in the calcarenite. This mobility may, in turn, make them more susceptible to oolitic accretion than larger grains. Yet peloids of the same roundness and small size occur elsewhere in the lagoon, but they do not have o61itic films. Moreover, the films in the East Lagoon are present on all grains, large and small. There must, therefore, be something other than diameter or roundness that causes the grains in the East Lagoon to be preferentially coated.
Marine Geol.,
5 (1967) 89-109
108
R.G. ('. BATHURST
The difference between the unaltered fine sand outside the East Lagoon and the micritized fine sand inside may be an indication not only that conditions for algal boring are more favourable east of Pigeon Cay, but that these same conditions, presumably chemical or biochemical, also favour the growth of oolitic aragonite. After all, they certainly seem to encourage the deposition of micritic aragonite in the empty algal bores and in the pores and chambers of skeletons. Some data by TAFT and HARBAUGH (1964, p.23) may be relevant. They found that, around the time of low tide in Florida Bay, conditions on the sea floor can be, for a short time, extreme. Water which, as a result of its shallowness, suffers a big loss of volume by evaporation, while being unable to mix with the surrounding deeper water, may increase its salinity by as much as 7 p.p.m. Such conditions may exist in the shallows of the East Lagoon. There is certainly a need for chemical and physical measurements throughout a complete tidal cycle. Nocturnal cycles should be included because of possible short term effects of photosynthesis and respiration. This seems to be about as far as it is possible to go with the available data. Many loose ends remain, but perhaps, now that the oolitic deposits have been described, other workers will be encouraged to carry out detailed chemical and physical investigations in this readily accessible lagoon.
ACKNOWLEDGEMENTS
I am deeply grateful to Professor John Imbrie of Columbia University, New York, for inviting me to take part in his Bahamian field programme from 1961-1963. This was supported by Pan American Petroleum Corporation, Phillips Petroleum Company and the National Science Foundation. I was enabled to use the excellent facilities of the Lerner Marine Laboratory, Bimini, as a result of the generosity of the Director, Dr. Robert Mathewson, who, with his staff, gave willing and sympathetic help. A Leitz microscope was purchased with a Department of Scientific and Industrial Research Grant for Special Research. Photomicrographs are by Mr. Wilfred Lee, Central Photographic Service, and drawings by Mr. Joe Lynch, Department of Geology, both of this University. Valuable criticism of the text was given by my wife, by Dr. John Glover of the University of Western Australia, and by Dr. Terry Scoflin, Dr. Alan Oldershaw of this Department, and Professor R. Siever.
REFERENCES BATHURST, R. G. C., 1966. Boring algae, micrite envelopes and lithification of molluscan biosparites. Geol. J., 5 : 15-32. BEALES, F. W., 1958. Ancient sediments of Bahaman type. Bull. Am. Assoc. Petrol. Geologists, 42 : 1845-1880. DAETWYLER, C. C. and KIDWELL, A. L., 1959. The Gulf of Batabano, a modern carbonate basin. WorldPetrol. Congr., Proc., 5th, N.Y., 1959, 1 : 1-21.
Marine Geol., 5 (1967) 89-109
OOLITIC FILMS ON BAHAMA SAND GRAINS
109
EARDLEY, A. J., 1938. Sediments of Great Salt Lake, Utah. Bull. Am. Assoc. Petrol. Geologists, 22 : 1305-1411. FREEMAN,T., 1962. Quiet water oolites from Laguna Madre, Texas. J. Sediment. Petrol., 32 : 475-483. GINSBURG, R. N. and LOWENSTAM,H. A., 1958. The influence of marine bottom communities on the depositional environment of sediments. J. Geol., 66 : 310-318. ]LLING,L. V., 1954. Bahamian calcareous sands. Bull. Am. Assoc. Petrol. Geologists, 38 : 1-95. MCKEE, E. D., 1966. The history of the Redwall Limestone of Northern Arizona. Geol. Soc. Am., Mem., in press. NEWELL, N. D., IMnRIE, J., PURDY, E. G. and THURBER, D. L., 1959. Organism communities and bottom facies, Great Bahama Bank. Bull. Am. Museum Nat. Hist., 117 : 177-228. NEWELL,N. D., PURDY,E. G. and IMnmE, J., 1960. Bahamian oolitic sand. J. Geol., 68 : 481-497. PETruonN, F. J., 1949. Sedimentary Rocks. Harper, New York, N.Y., 526 pp. SEIBOLD, 1:[., 1962. Untersuchungen zur Kalkf~illung und KalklOsung am Westrand der Great Bahama Bank. Sedimentology, 1 : 50-74. SORnY, H. C., 1897. The structure and origin of limestones. Proc. Geol. Soc. London, 35 : 56-93. TART, W. H. and HARBAUGH,J. W., 1964. Modern carbonate sediments of Southern Florida, Bahamas, and Espiritu Santo Island, Baja California: a comparison of their mineralogy and chemistry. Stanford Univ. Publ., Geol. Sci., 8 : 133 pp. TUREKIAN, K. H., 1957. Salinity variations in sea water in the vicinity of Bimini, Bahamas, British West Indies. Am. Museum Novitates, 1822 : 1-12.
Marine Geol., 5 (1967) 89-109
OOLITIC FILMS ON LOW ENERGY CARBONATE SAND GRAINS, BIMINI LAGOON, BAHAMAS R. G. C. BATHURST
Department of Geology, University of Liverpool, Liverpool (Great Britain) (Received September 19, 1966)
SUMMARY
Grains with thin (2/a) oolitic coats are particularly abundant in the southeastern part of Bimini Lagoon. Turbulence is low and sediment movement is restricted by a dense benthos of Thalassia, Algae, and a subtidal gelatinous mat that binds the carbonate grains together at the sediment surface. The region has the shallowest water in the lagoon, the highest content of fine sand, and grains are well-rounded and intensively bored by Algae.
INTRODUCTION
On the warm sunlit floor of the lagoon at Bimini (Fig.l), sheltered from the trade winds by low tree-clad islands of Pleistocene limestone, many of the carbonate sand grains bear on their outer surfaces thin (about 2/~) oolitic coats (films). These films have been found on grains from all over the lagoon with a frequency that varies from almost zero coated grains per sample to nearly 100 ~. Coated grains are commonest in the southeast (Fig.2). The setting is tranquil and the movement of grains by currents is impeded by a rich benthos of Thalassia (turtle grass) and Algae, and by an extensive, subtidal, organic, gelatinous mat that binds the surface of the sand. The scene here is radically different from the wild turbulence of the well known oolite shoals to the south around Browns Cay. It is necessary to ask how, in such calm surroundings, so many grains have come to be encrusted with a film of oriented aragonite. Despite the acute observations of EARDLEY(1938), ILLING (1954) and NEWELL et al. (1960) we still know almost nothing of the physical chemistry of o0id growth. The recognition, in Bimini Lagoon, of a previously undescribed morphological type of marine o0id, with an unusually thin coat, deposited in an environment unlike either the tidal oolite shoals of Browns Cay CNEWELLet al., 1960) or the Laguna Madre (FREEMAN, 1962), extends our knowledge in an area where additional information is urgently needed. Marine GeoL, 5 (1967) 89--109
90
a.
aoow
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,
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Fig.1. Bathymetric setting of Bimini.
~ Z
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Fig.2. Bimini Lagoon: stations and frequency indices for coated grains. Martne Geol., 5 (1967) 89-109
I
OOLITICFILMSON BAHAMASANDGRAINS
91
THE PROBLEM This study arose from two observations by the author in the laboratory in England. On looking at thin sections of impregnated lagoonal sediments it was apparent first that there are o/51itic films on many grains and, second, that there is a very high concentration of these coated grains at all the stations in the southeastern part of the lagoon (later named the East Lagoon, Fig.2). These observations led to a comparative study (unhappily without the opportunity of further field work) of the East Lagoon and the rest of the lagoon, in the hope that any differences found might provide some guidance as to how the films were deposited. Bimini Lagoon is not the only place where o/31itic films have been found. Other calcarenites in the author's possession show o61itic films with dimensions similar to those in Bimini Lagoon. They come from the following localities: (1) Off the north point of North Bimini. (2) Beach, west coast of North Bimini, by Lerner Marine Laboratory, southwest of lagoon station 5. This is a coarse polished calcarenite. (3) About 2 miles south of the southeast end of South Bimini (collected by Dr. H. Fiichtbauer). (4) Off the west coast of Andros Island, Bahamas, from its southern tip as far north as Williams Island. (5) Frazers Hog Cay (northwest coast), Berry Islands, Bahamas. (6) Cockroach Cay (north coast), Berry Islands, Bahamas. (7) Batabano Bay, Cuba; in the "ovoid grain", "composite grain" and "o61itic skeletal" lithofacies of DAETWYLERand KIDWELL(1959). (8) Campeche Bank, Yukatan; in the finest sand.
HELDAND LABORATORYMETHODS In the summers of 1961-1963, three periods, each of two weeks, were devoted to field work in Bimini Lagoon. The presence of oolitic fdms on the grains was discovered only after the field work had ended. Thus the field observations and samples were not collected with this study in view. Data from 68 stations are used in this paper. They are of five kinds: (1) An index of frequency of coated grains. (2) The density of Thalassia on the floor. (3) Water depth referred to mean sea level. (4) Mechanical analysis of the sands. (5) Roundness of grains in sieve fraction 150-90/l. The methods by which the data were determined are now described.
Marine GeoL, 5 (1967) 89-109
92
i~.
o. c. BATttURST
Frequency index
The distribution of coated grains on the lagoon floor was estimated by determining, in one sample of sand from each station, the frequency index (Fig.2). This index is presumed to reflect not only the frequency of coated grains in the sand but also the tendency to growth of o61itic films. Higher frequencies are taken to imply environments better suited to oolitic growth. Simple, uncorrected comparison of sands in terms of the proportion of coated grains in each is, of course, not a sure indication of the ease of o61itic growth at any place. Grain shape, which varies greatly between stations, influences the perfection and even the existence of the o61itic film. The larger grains and especially the more irregular grains, tend to have either poorly developed films or none at all. It is necessary, thelefore, to ensure that only grains of similar shape and surface texture are compared. Fortunately there is one type of grain that has a relatively constant shape. This is the micritized skeleton (BATHURST, 1966) described briefly on p. 105. Most of these grains have cross-sections that are oval to approximately circular, though they may vary considerably in roundness. The important point is that they do not have the extreme variation of shape and surface texture that typifies many broken, un-micritized skeletal particles. They resemble the "grains of aragonite matrix" of IeHyo (1954, p.27) which make up the cryptocrystalline grains in the "bahamite" of BEALr!S (1958, p. 1846). They are within the class "peloids" of MCKEE (1966, chapter 3, section on carbonate rocks), which are grains of microcrystalline or cryptocrystalline carbonate irrespective of origin. The term peloid is adopted here. Other peloids in the lagoon are faecal pellets. All statements of frequency, therefore, refer solely to the presence or absence of an o61itic film (complete or patchy) on peloids. All sizes of grain in the sands were examined in preparing the index (Fig.2). Attempts to count coated grains (peloids with films) in thin sections of sands (in Marco Resin) were not successful. Sorting during preparation of the artificial rock caused the sections to be so unrepresentative that the accuracy introduced by counting was vitiated. Nor was it possible to distinguish coated grains from other grains in loose dry mounts. Eventually a much simplified method, using thin sections, was chosen whereby the frequency of coated grains was estimated visually in terms of four readily distinguishable classes. This gave consistent results for repeated analyses of the same sample and for analyses of replicate samples. The classes are: (l) Abundant: peloids without films are detectable only with difficulty. (2) Common: more than half the peloids have films. (3) Scarce: (a) Jew: less than half the peloids have films; (b) rare: grains with films are detectable only with difficulty. As samples with few or rare coated grains are uncommon these two attributes are, for statistical purposes, combined under scarce. In quantitative terms abundant probably means that more than 95 ~ of the peloids are coated while rare probably means that less than 5 ~ are coated. These estimates are based on counts. Marine Geol., 5 (1967) 89-109
93
OOLITIC FILMS ON BAHAMA SAND GRAINS
\.Xo +
\
•
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•
0
~,
o •
'~o.s m •
\
0
i.
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/
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MOSQUITO POIN T
•
LEC ;
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.
.
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Thalassia Density Plentiful
•
0 Thin
-I'- Absent 0
Neuticol
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,,~Lynch.
Fig.3. Bimini Lagoon:
Thalassia
density.
Thalassia density Bearing in mind the possible influence of leaf baffles on sediment movement (GINSBURG and LOWEr~STAM,1958), the density of the dominant vegetation, Thalassia, on the lagoon floor was estimated (Fig.3). At each station the area of floor obscured by the blades of this plant was assessed in terms of the following classes: (1) Plentiful: (a) dense where the floor is totally obscured; (b) more than half the floor obscured; (c) haifthe floor obscured; (d) less than half the floor obscured. (2) Thin where it is just detectable.
(3) Absent. As analyses of the data showed that the four denser classes differ little in their relation to other factors, they are combined as plentiful (Fig.3).
Water depths The depth of water was measured with a weighted string and corrected for mean sea level with the help of British Admiralty Tide Tables. The times and ranges of high and low tides at the Standard Port (Bermuda) were corrected for the nearest Secondary Port (Cat Cay, 16 km south of Bimini). The 0.5 m contour is shown in the figures. Marine
Geol., 5 (1967) 89-109
94
R.G.C.
-1
0
{3
North
(J 03
+1
+2
+3
Sound and Main Lagoon
+5~
. . . .18 ."
( Excluding Channel and Beaches) I
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BATHURSI
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i
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0.64ram
Fig.4. Representative sieve analyses of sands from North Sound and Main Lagoon, excluding channel and beaches.
0
-1
+1
+2
/
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Sands
~
:
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Fig.5. Representative sieve analyses of beach sands, Bimini Lagoon.
Mechanical analysis Mechanical analyses of dried sub-samples, prepared from the station samples, were made with sieve intervals of about 0.25 • (Fig.4-6). Marine Geol., 5 ( 1 9 6 7 ) 8 9 109
O~LITIC FILMS ON BAHAMA SAND GRAINS -1 D
0
4"4
+3
+2
+1
95
Eost L o g o o n (Excluding beaches)
+sq~
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~
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i
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Sieve diorneters : Log. scole Fig.6. Representative sieve analyses of sands from the East Lagoon.
Roundness An indication of the roundness of the finer grains was found by comparing the silhouettes of grains in the sieve fraction 152-124 # with PETTIJOHN'S (1949) fig.24. It was felt that comparison among grains of similar size and shape must be an essential precaution. These conditions are best fulfilled by the finer grains. It is also these same grains that most readily grow o61itic films. Fifty grains randomly selected from each sample were described. Inspection of the frequencies in the five roundness classes (A-E) led to a convenient index of roundness in terms of the number percentage of grains in the combined three classes C, D, E (Fig.7). The higher the index number, the greater the roundness of grains in the fraction 152-124/z.
PETROGRAPHY OF THE COATED GRAINS
The nucleus All the types of sedimentary grain in the lagoon act as nuclei, the commonest being Foraminiferida, Halimeda, fragments of Mollusca and Corallinaceae, faecal pellets and other peloids. As the o61itic films are so thin (Fig.8) a good idea of the true size distribution of nuclei can be gained from Fig.6. Films are more completely developed on the smaller, more rounded nuclei, tending to enclose them entirely. They are poorly developed on unaltered fragments of Soritidae, Halimeda or on any other grain where the surface is highly irregular or the particle diameter exceeds 1 mm. Marine Geol., 5 (1967) 89-109
96
H,. ( ; .
('.
BATHURST
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i
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~.
Roundness Index
I
c~ ~ \
Number'/,
•
Km.
rl
2
Fig.7. Bimini Lagoon:roundness indices.
They are generally, though not everyhwere, best developed on peloids that have diameters less than 250/t. These are mostly fragments or Soritidae, Halimeda or faecal pellets in various stages of micritization (BATHURST, 1966), a process that is discussed on p.105. Many of the peloids have no recognisable internal structure (Fig.10).
The oOlitic film The pale brown oolitic film (Fig.8) has a thickness that varies from grain to grain, achieving a maximum of about 8 #, but usually falling within the range 1-3/~. This range of thickness is also given by NEWELL et al. (1960, p.490) for the "single lamellae" of the Browns Cay oOids, The internal fabric of the Bimini film was compared with the "oriented aragonite lamellae" in oOids collected by the author from Browns Cay: no petrographic difference could be detected. The film consists of a mosaic of crystals with diameters up to about 2 #. The smallest detectable crystals were about 0.5/t, but smaller ones may exist. The mosaic was examined at a magnification of 900 × (oil immersion) with a gypsum first order red compensator under crossed-nicols. It showed the familiar orientation of the slow vibration direction normal to the surface of the coat (SORBY, 1897, p.74; ILUNG, 1954, p.38; NEWELL Marine Geol., 5 (1967) 89-109
O()LITIC FILMS ON BAHAMA SAND GRAINS
97
Fig.8. OOlitic coat on peloid, East Lagoon. Slice; in Marco resin.
Fig.9. Test of a foraminifer partly micritized. Slice; in Marco resin.
Marine GeoL, 5 (1967) 89-109
98
m (;. ( . BATHURS1
et al., 1960, p.490). The separate crystals are best seen in the region of the black cross where the coat is mostly in extinction. Here the slight differences in optical orientation are most readily apperciated. A film may be complete, enveloping the whole surface of the nucleus, or it may be patchy. The surface of the nucleus is commonly irregular and the film, whether complete or not, tends to fill the smaller embayments. The film has a dull surface, lacking the shiny polish of the Browns Cay o6ids. The film is so thin that its recognition in ancient limestones seems unlikely, particularly where it changes during diagenesis to the structureless micrite seen in some fossil o6ids.
THE LAGOON ENVIRONMENT
Bimini Lagoon lies 25°44'N, 79°16'W, and 60 miles east of Miami, Florida (Fig.l). It is bounded on the west, north and east by the horseshoe of the islands of North and East Bimini and to the south by South Bimini (Fig.2). From a narrow entrance between North and South Bimini a tidal channel runs northward as far as Mosquito Point. A broad expanse of the shallowest water, between East and South Bimini, lies open to the east. Bimini stands on the western edge of the Great Bahama Bank. To the west of the Bimini islands a platform of Pleistocene limestone, covered with ripple-marked Recent carbonate sands and patches of Thalassia, extends down to about 40 m where the surface plunges steeply to the floor of the Florida Straits 800 m below. To the east stretch the extensive shallow waters of the Bank, rarely more than 2 m deep. The lagoon floor consists of incoherent Recent carbonate sands (Fig.10, 11) overlying Pleistocene limestone. A bore 0.5 km southwest of station 49, between the channel and the shore, showed that the sand is there separated from the limestone by a mangrove peat which, in common with other similarly situated peats on the Florida-Bahamas platform, gave a 14C age of about 4,000 years (NEWELL et al., 1959, p. 192). The thickness of the sands varies from zero in places where limestone is exposed to a least 1.5 m in the East Lagoon. The warm northward-moving Gulf Stream enables a tropical West Indian biota to flourish (Fig.12, 13). Full details are given by NEWELLet al. (1959). The mean sea depth is mainly from 0.7-0.3 m. Patches of floor are exposed at low tide off the southwest edge of Pigeon Cay, also just north of Tokas Cay, and to the east of Tokas Cay between the southwest lobe of East Bimini (west of Bone Fish Hole) and South Bimini (Fig.2). The tidal channel against North Bimini, partly dredged, has depths from about 6 m at the southwest entrance to 3 m near Mosquito Point. Current velocities were not measured, but sand movement along the tidal channel was inferred from the presence of ripple-mark and scour around boulders. Nowhere else in the lagoon was ripple-mark seen nor any sign of scour. Even the loose sand of the mounds is undisturbed. Yet this is a misleading picture of the cornMadne Geol., 5 (1967) 89-109
OOLITICFILMSON BAHAMASANDGRAINS
99
Fig.10. Typical sand of the East Lagoon. Mainly peloids formed by micritization of skeletal debris; one large faecal pellet. Slice; in Marco resin.
petence of the water to move grains. The top 0.25 cm of sediment is bound by a transparent, colourless gelatinous material to form a subtidal mat which stabilizes the grains. Wherever the subtidal mat was scraped away ripple-mark formed, often in a matter of minutes. There is probably no part of the lagoon floor that is not subject to tidal currents strong enough to move sand. Nevertheless, so great is the influence of the various sediment binders that some 23 km 2 of sand is immobilized.
PLACE AND TIMEOF GROWTHOF THE COATEDGRAINS In considering the seemingly paradoxical situation, where oSlitic films grow but sediment movement is restricted, it is necessary first of all to decide whether the films grew in the places where they are now found. The types of grain in the lagoon could all be derived from organisms that live there. They cannot be confused with o~ids eroded from the Pleistocene limestones, because these have characteristically thick coats and many of them are parts of limestone clasts. The conclusion is that the Marine GeoL, 5 (1967) 89-109
100
R, t~. ('. BAI'HURST
Fig.11. Typical sand of the North Sound and Main Lagoon. Little altered skeletal debris with a few peloids. Slice; in Marco resin.
sands spread over the major part of the lagoon floor were formed close to where they are now. Two objections that may be raised with regard to the local origin of coated grains in the East Lagoon are first that they appear at the surface now only as a result of bioturbation; and second that they never grew in the lagoon but were washed in from some outside source. It is true that the burrowing animal that makes the ubiquitous conical mounds (about 20 cm high) probably ensures vertical mixing of the sands throughout their entire thickness. But, if the more recently formed grains had no o61itic films, then mixing with older, the coated grains could not yield the high concentration of coated grains, more than 95 ~o, that is known to exist at the surface. To ensure such a high concentration of coated grains in the uppermost sands, new coated grains must be forming now. Considering the possibility of an outside source, there was no sign whatsoever, in the summers of 1961-1963, that bottom traction of sand grains was going on in the East Lagoon. The floor seemed totally immobilized by Thalassia and the mucilaginous subtidal mat. The obvious direction from which coated grains could be delivered Marine Geol., 5 (1967) 89-109
OOLITIC FILMS ON BAHAMA SAND GRAINS
101
Fig.12. Thalassia and mound, East Lagoon. Upper surfaces of leaves encrusted withepifauna. Length of Coke can is 12.5 cm. Under water.
Fig.13. Penicillus (bulbs on stalks), Thalassia (blades), Porites (branching coral), Ircinia (massive sponges), fish, floor of mat-bound calcarenite. Southwest of Pigeon Cay, East Lagoon. Length of Coke can is 12.5 cm. Under water. Marine Geol., 5 (1967) 89-109
102
R. (;. C. BATHURST
from without the lagoon is from the east. If winter gales (November, December) appreciably increase bottom traction, then as they blow from the northwest they are not likely to sweep grains in from the east. Moreover, the nearest oOids, lying just east of East Bimini, are distinctive in that they have thick oolitic coats. Such forms are rare in the sands of the East Lagoon. Dr. H. Fiichtbauer has sent me samples of oOids with thin films like those in the East Lagoon which he found about 2 km south of South Bimini. To reach the East Lagoon these would either have to have been blown over the 1 km of tree covered South Bimini or carried 5 km northeastward before moving westward into the lagoon. Neither hypothesis is attractive. There cannot have been movement of grains inward through the southwest entrance, because the grains of the shelf are much coarser than those in the lagoon. Indeed, large scale lateral traction seems unlikely on the grounds that the lobate carpets of megarippled oolite which characterise the Bahamian oolite shoals are absent in the lagoon. Also, it must not be forgotten that stations 20, 18 and 80 in the North Sound have abundant frequency indices. These coated grains cannot have been supplied from outside the lagoon, because the North Sound is isolated from the open sea except for a narrow channel that wanders through the mangroves between North and East Bimini. Finally, it is again necessary to emphasise that the high concentration of coated grains in the East Lagoon could not have been caused by the addition of extra-lagoonal coated grains to a non-oOlitic indigenous calcarenite produced by the local skeletal organisms. Examination of two cores (1.25 m long) taken in the East Lagoon near station 37, which penetrated the whole thickness of the unconsolidated sediment, supports these conclusions. Samples examined at vertical intervals of 25 cm all have an abundant frequency index. It is clear that oolitic films have been deposited on grains continually since the Pleistocene floor was first covered by Recent calcarenite. Altogether the evidence points to the continual growth of oolitic films on peloidal nuclei throughout the Recent depositional history of the East Lagoon.
COATEDGRAINS, TEMPERATURE,SALINITY~DEPTH Frequency indices at the 68 stations are shown in Fig.2. If a line be drawn west of Alec and Pigeon Cays, from East to South Bimini, then the region east of this line has a frequency index abundant for all its stations. It will be referred to as the East Lagoon. In Fig.2 it can also be seen that the frequency indices near the channel and along the west coast of the lagoon are scarce, but elsewhere vary from scarce to abundant. On the basis of this distribution it is assumed that the most intensive growth of oolitic films is taking place in the East Lagoon. Alternatively, it is, of course, possible that the production of peloids is going on at a faster rate outside the East Lagoon. It can be shown, in fact, that this is not so. Faecal pellets and micritized skeletal grains are in general more numerous in samples from the East Lagoon than in samples from elsewhere in the lagoon. Marine Geol., 5 (1967) 89 109
103
O O L I T I C FILMS ON B A H A M A SAND G R A I N S
Water temperatures and salinities were recorded in the lagoon by TUREKIAN (1957) during the late spring and early summer of 1955 (Table I). He concluded that there are three distinct water masses occupying the North Sound, the Main Lagoon and the region east of Pigeon Cay. This last region closely resembles in extent the East Lagoon described in this paper. Within the Main Lagoon he found little lateral change. In the North Sound both temperature and salinity increased northward, although in such shallow water they can, at times, decrease northward as a result of rainfall. The Sound is less subject to mixing with open sea water than other parts of TABLE I DISTRIBUTION OF SALINITY AND TEMPERATURE IN BIMINI LAGOON (SPRING AND SUMMER 1955)
(After TUREKIAN,1957)
Salinity (p.p.m.)
Temperature (°C)
North Sound
increases northward from 40.4--41.9, with a drop to 40.5 in the far northeast corner
increases northward from 29.3-29.8, with a rise to 31.3 in the far northeast corner
Main Lagoon
36.0; no trend except for an increase to 39.4 near Mosquito Point
27.9-29.2 no trend
East Lagoon
36.6-37.5 no trend
28.3-30.7 no trend
the lagoon, and after rain the salinity there may fall as low as 31 p.p.m. In the East Lagoon, temperature and salinity are mostly high east of Pigeon Cay. It is of interest that the highest temperatures and salinities have been found in the North Sound. They do not coincide with the concentration of abundant frequency indices in the East Lagoon. On the other hand, dilution with rain water reduces the mean salinity of the North Sound, if this be taken over a period of time. A more detailed study of the effects of rain, and of run-off from the mangroves, on the salinity and the titration alkalinity was made by SEIBOLD (1962). His stations are, however, too few to be directly useful in this discussion. The water depths show that the East Lagoon is unique in having a shallower floor than any other part of the lagoon away from the beaches, much of it lying at less than 0.5 m (Fig.2) and parts being exposed at low tide. Summarising the information, the temperatures and salinities recorded in the East Lagoon are unremarkable, but this area is distinguished from the rest of the lagoon by its extreme shallowness.
Marine Geol., 5 (1967) 89-109
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I~. (i. C. BA'rHURS1
COATED GRAINS, THALASSIA, FINE SAND
The importance of Thalassia and fine sand The distribution of Thalassia is regarded as particularly important (Fig.3) because, more than any other organism, it seems to influence the movement of water near the bottom. The density of the plant is therefore a convenient indication of the nature of the bottom. The unconsolidated Recent sands in which the Thalassia grows overlie a cemented Pleistocene limestone and are generally less than 1.5 m thick. Thus, where the water is deep, as in the channel, the Recent sands are either thin or absent. Colonisation of the sands by Thalassia is inhibited by two things. Thin sand cannot support the roots that commonly go down 60 cm or more, and fast turbulent water prevents growth, as is seen on the bare beaches. In general, Thalassia flourishes in the shallow, quieter water away from the beaches and channel, where the Recent sands are thicker. The term "fine sand" is applied here to the range of sieve diameters 150-90/z. An examination of the mechanical analyses from all stations showed that one parameter above all characterises the sands from the East Lagoon. They have, on the average, a higher content of fine sand, as here defined, than the sands in the other parts of the lagoon (Fig. 14). The ranges of grain diameters differ little from those in the rest of the lagoon and, although the means and skewnesses are peculiar to the East Lagoon, these only reflect in a less precise way the content of fine sand. The beaches and channel are poor in fine sand because they are high energy environments, but the low content in the rest of the Main Lagoon and in the North Sound, compared with the East Lagoon, must be a result of some other still unknown process.
Distribution of Thalassia andfine sand The relation between Thalassia density and the mean weight ~ of fine sand was examined for the whole lagoon (Table II). Only 51 stations are included, the other having no Thalassia. The results for the North Sound and Main Lagoon differ so little that they have been combined. The division of Thalassia records into the four denser classes (p.93) yielded no information since they only involve eleven samples. These classes are combined under the term plentiful. Two relationships are apparent in Table II. Both in the combined North Sound and Main Lagoon, and in the East Lagoon, the denser grass is associated with the greater quantity of fine sand. At the same time, the mean weight ~o of fine sand is higher in the East Lagoon irrespective of the Thalassia density. These differences are not statistically significant and it is conceivable that they have arisen by chance. On the other hand, the number of samples (stations) is small and a larger number might well reveal differences that are significant. The tendency for Thalassia to trap fine sand fits well with the experience of other workers and the higher content of
Marine Geol., 5 (1967) 89-109
105
OOLITIC FILMS ON BAHAMA SAND GRAINS
TABLE II MEAN WEIGHT PERCENT OF FINE SAND, FOR THE EAST LAGOON AND FOR THE REST OF THE LAGOON 1
Main Lagoon and North Sound Thalassia d e n s i t y thin
9.8
SE = -4- 1.6
East Lagoon 15.6
n=7
Thalassiadensityplentiful
11.9
S E = ± 1.7 n = 15
SE = + 3.2 n=8
23.7
S E = 4- 1.5 n =21
1 S t a t i o n s w i t h Thalassia densitiesplent(ful a n d thin. SE = s t a n d a r d e r r o r o f t h e m e a n ; n = n u m b e r of stations.
fine sand in the East Lagoon is clearly apparent from a study of the thin sections of impregnated sands.
COATED GRAINS A N D BORING ALGAE
Examination of the fine sand with a microscope reveals two striking differences between the fine sands in the East Lagoon and those in the rest of the lagoon. The fine sand grains differ in their degree of alteration to micrite and in their roundness (Fig. 10, 11). In the North Sound and Main Lagoon the fine grains consist largely of angular broken skeletons. Many of these are unaltered Halimeda and Soritidae boring Algae are rare in these skeletons. In the East Lagoon the finer grains are intensely altered to a greyish-looking micritic aragonite (Fig.10)with crystal diameters up to about 2/t. Inside the grains there are many dark tubes and blebs, about 6 / t in diameter, similar to those in the Browns Cay orids that are believed to be Algae (NEWELLet al., 1960, p.492). The greater roundness of the fine grains in the East Lagoon is also plain (Fig.7, 10, 11). The alteration of the skeleton to micritic aragonite is itselfa result of the activity of the boring Algae. When the Algae die the tubes are filled with micritic aragonite. By repeated boring, dying of Algae, and filling with micrite, the skeleton is changed to a structureless mass of micrite. This process of micritization, and some of its implications in ancient limestones, are examined in detail by BATHURST(1966). At the same time micritic (cryptocrystalline) aragonite fills the pores of Halimeda and the chambers of Foraminiferida and the pores of Echinoidea. The resultant grain is a peloid. Probably in the course of micritization the grains become rounded, by abrasion of the delicate fabric of the surface where this has been extensively bored but has yet to be filled. The abraiding agent could be any of a number of sand eating animals. It is important here to record that only a small and insignificant increase of roundness is caused by the deposition of the oSlitic film. The roundness of the coated grain is almost entirely a reflection of the roundness of the nucleus. Marine Geol., 5 (1967) 8 9 - 1 0 9
106
R. (i. c . B A T H U R S T
DISCUSSION
As a first step towards understandingwhy oOliticfilms grow so much more readily in the East Lagoon it is necessary to search for other things that distinguish this part of the lagoon from the remainder. Four such factors have been found: (l) the East Lagoon is the shallowest of the three regions; (2) the sediment on the floor has the highest content of fine sand; (3) the fine sand is peculiar in being intensely micritized; (4) the fine sand has an exceptionally high roundness. There is a further matter that should not be ignored. This is the general tendency throughout the lagoon for fine sand to be more plentiful where the Thalassia is plentiful. Taking the last point first, the role of Thalassia as a baffle, it should be noted that there is very little fine sand where the grass is absent, on the beaches and in the channel (Fig.3, 14). These are high energy environments and fine grains will not have been deposited. Yet the converse proposition, that Thalassia traps the fine grains, is impossible to demonstrate with the available data. For example, it would be wrong to suppose that the greater amount of fine sand in places where Thalassia is thin, compared with the bare beaches and channel, can be attributed to a reduction of water velocity near the bottom caused by the Thalassia blades. The description thin means that Thalassia is only just detectable, certainly less than one blade in 0.5 m 2. Under these circumstances its quantitative effect on water velocity must be unimportant. On the other hand, Thalassia grows more densely where the Recent sands are thickest and, in Bimini Lagoon, it happens that these are also the areas of quieter water. Thus the tendency for fine sands to be associated with the denser Thalassia may be a result, in part, of a local coincidence whereby both accumulate in the quieter water, but for reasons that are unrelated. Yet it is difficult to believe that the denser Thalassia does not reduce water movement and so trap fine sand to some extent. The data available are unfortunately insufficient to test this hypothesis. The shallowness of the East Lagoon is in agreement with the widely held view that oOids grow best in the shallower waters. Yet this region, unlike the Browns Cay shoals, is cooler and less saline than adjacent areas, according to TUREKIAN'S (1957) figures. Nor are the grains in the East Lagoon strongly agitated. The binding effects of the Thalassia and the Algae and, above all, the ubiquitous gelatinous subtidal mat, cause the day-to-day movement of the sand grains to be negligible when compared with the intense agitation at Browns Cay or off the beaches of Laguna Madre. Nowhere in Bimini Lagoon has there been even enough turbulence to polish the grains. The sorting of the sands here is also poorer than in the other Bahamian and Laguna Madre oolites. Low turbulence is further indicated by the high content of fine sand. Conditions in the East Lagoon must be less suitable for the growth of oolitic coats than in the other oolite forming areas, because the coats are so thin. It does not necessarily follow, though, that the coats grew more slowly while they were in the growth-promoting micro-environment. Until a good deal more is known about the process of growth it will not be possible to decide to what extent the Bimini Marine Geol., 5 (1967) 8 9 - 1 0 9
OOLITIC FILMS ON BAHAMA SAND GRAINS
107
\
,I, q ----Z-----
~ +
\o
.t.
BONE FISH HOLE
O
o.
\
"1
•
+
o
0
I
+ 0
0
~'-o-/k -T 4-
o •
// iiiiiiiiii~: CA T
~o\. 0
c--" \'...o~
/ •
i\ //o
Weight Percentage Fine Sand •
0
2 0 - 38
0 10-19
+0-9
0 0
~Lv~ch.
Nautical miles r
Km
2 i
Fig.14. Bimini Lagoon: weight per cent of fine sand (150-90 p).
coated grains (1) spend less time in a growth-promoting environment or (2) grow more slowly when in that environment. Moreover, as many of them spend a large part of their existence embedded in the gelatinous mat, this substance may influence ionic exchange between the oolitic coat and the water. At this point it is as well to consider how much movement of an o6id there must be so that it can grow with an even, overall development of oriented aragonite. It could be that the comparatively small amounts of movement that occur in the East Lagoon are, after all, enough to keep an o6id turning so that different parts of its surface are, from time to time, exposed to the growth environment. Causes of grain movement in the lagoon vary from the gentle browsing of fishes to the rare paroxysm of a hurricane. The fine sand with its extreme roundness would be expected to be the most mobile sediment in the lagoon. Tests in the laboratory have confirmed that these fine grains have the lowest erosion velocities in the calcarenite. This mobility may, in turn, make them more susceptible to oolitic accretion than larger grains. Yet peloids of the same roundness and small size occur elsewhere in the lagoon, but they do not have o61itic films. Moreover, the films in the East Lagoon are present on all grains, large and small. There must, therefore, be something other than diameter or roundness that causes the grains in the East Lagoon to be preferentially coated.
Marine Geol.,
5 (1967) 89-109
108
R.G. ('. BATHURST
The difference between the unaltered fine sand outside the East Lagoon and the micritized fine sand inside may be an indication not only that conditions for algal boring are more favourable east of Pigeon Cay, but that these same conditions, presumably chemical or biochemical, also favour the growth of oolitic aragonite. After all, they certainly seem to encourage the deposition of micritic aragonite in the empty algal bores and in the pores and chambers of skeletons. Some data by TAFT and HARBAUGH (1964, p.23) may be relevant. They found that, around the time of low tide in Florida Bay, conditions on the sea floor can be, for a short time, extreme. Water which, as a result of its shallowness, suffers a big loss of volume by evaporation, while being unable to mix with the surrounding deeper water, may increase its salinity by as much as 7 p.p.m. Such conditions may exist in the shallows of the East Lagoon. There is certainly a need for chemical and physical measurements throughout a complete tidal cycle. Nocturnal cycles should be included because of possible short term effects of photosynthesis and respiration. This seems to be about as far as it is possible to go with the available data. Many loose ends remain, but perhaps, now that the oolitic deposits have been described, other workers will be encouraged to carry out detailed chemical and physical investigations in this readily accessible lagoon.
ACKNOWLEDGEMENTS
I am deeply grateful to Professor John Imbrie of Columbia University, New York, for inviting me to take part in his Bahamian field programme from 1961-1963. This was supported by Pan American Petroleum Corporation, Phillips Petroleum Company and the National Science Foundation. I was enabled to use the excellent facilities of the Lerner Marine Laboratory, Bimini, as a result of the generosity of the Director, Dr. Robert Mathewson, who, with his staff, gave willing and sympathetic help. A Leitz microscope was purchased with a Department of Scientific and Industrial Research Grant for Special Research. Photomicrographs are by Mr. Wilfred Lee, Central Photographic Service, and drawings by Mr. Joe Lynch, Department of Geology, both of this University. Valuable criticism of the text was given by my wife, by Dr. John Glover of the University of Western Australia, and by Dr. Terry Scoflin, Dr. Alan Oldershaw of this Department, and Professor R. Siever.
REFERENCES BATHURST, R. G. C., 1966. Boring algae, micrite envelopes and lithification of molluscan biosparites. Geol. J., 5 : 15-32. BEALES, F. W., 1958. Ancient sediments of Bahaman type. Bull. Am. Assoc. Petrol. Geologists, 42 : 1845-1880. DAETWYLER, C. C. and KIDWELL, A. L., 1959. The Gulf of Batabano, a modern carbonate basin. WorldPetrol. Congr., Proc., 5th, N.Y., 1959, 1 : 1-21.
Marine Geol., 5 (1967) 89-109
OOLITIC FILMS ON BAHAMA SAND GRAINS
109
EARDLEY, A. J., 1938. Sediments of Great Salt Lake, Utah. Bull. Am. Assoc. Petrol. Geologists, 22 : 1305-1411. FREEMAN,T., 1962. Quiet water oolites from Laguna Madre, Texas. J. Sediment. Petrol., 32 : 475-483. GINSBURG, R. N. and LOWENSTAM,H. A., 1958. The influence of marine bottom communities on the depositional environment of sediments. J. Geol., 66 : 310-318. ]LLING,L. V., 1954. Bahamian calcareous sands. Bull. Am. Assoc. Petrol. Geologists, 38 : 1-95. MCKEE, E. D., 1966. The history of the Redwall Limestone of Northern Arizona. Geol. Soc. Am., Mem., in press. NEWELL, N. D., IMnRIE, J., PURDY, E. G. and THURBER, D. L., 1959. Organism communities and bottom facies, Great Bahama Bank. Bull. Am. Museum Nat. Hist., 117 : 177-228. NEWELL,N. D., PURDY,E. G. and IMnmE, J., 1960. Bahamian oolitic sand. J. Geol., 68 : 481-497. PETruonN, F. J., 1949. Sedimentary Rocks. Harper, New York, N.Y., 526 pp. SEIBOLD, 1:[., 1962. Untersuchungen zur Kalkf~illung und KalklOsung am Westrand der Great Bahama Bank. Sedimentology, 1 : 50-74. SORnY, H. C., 1897. The structure and origin of limestones. Proc. Geol. Soc. London, 35 : 56-93. TART, W. H. and HARBAUGH,J. W., 1964. Modern carbonate sediments of Southern Florida, Bahamas, and Espiritu Santo Island, Baja California: a comparison of their mineralogy and chemistry. Stanford Univ. Publ., Geol. Sci., 8 : 133 pp. TUREKIAN, K. H., 1957. Salinity variations in sea water in the vicinity of Bimini, Bahamas, British West Indies. Am. Museum Novitates, 1822 : 1-12.
Marine Geol., 5 (1967) 89-109