ABSTRACT

The modern oolites, coral reefs, calcarenites, calcilutites and beach rock are cemented with a CaCO₃ cement which consists of crystals of aragonite. Experimental evidence supports the view that the crystals of aragonite are formed by organisms which are thought to be actinomycetes.

The Floridan and Puerto Rican beach rocks are formed in tropical and subtropical climate in littoral and supralittoral environments and are confined to high energy areas of diurnal water agitation and a continuing supply of fresh nutrients.

The Floridan beach rock occurs as an intermittent shallow shelf on the Atlantic Coast from Key West to Key Largo and along the east coast from West Palm Beach to Jupiter. However, the Puerto Rican beach rock is primarily confined to the north side of the island where the formation of beach rock may be related to the path of the currents which bring nutrients to the surface.

The aragonite in these sediments is probably of biogenetic origin. However, the partial to complete alteration of the cementing agent to calcite is due to penecontemporaneous solution and redeposition. In the alteration of aragonite to calcite, some of the CaCO₃ is removed in solution. Consequently, interstices remain within the matrix, and the resulting rock can be highly porous owing to this development of intergranular microporosity.

ROLE OF MICRO-ORGANISMS IN THE FORMATIONS OF LIMESTONE¹

Most of the exposed rocks in Florida consist of marine limestones, primarily calcarenites, calcilutites, calcirudites and beach rocks and range in age from the Middle Eocene to Recent. As much as 95 percent of the calcarenites consist of a debris of exoskeletons, mollusks, foraminifers, worm tubes, bryozoans, corals, microcrustaeans, and all of these are cemented by aragonite or calcite matrix. Occasionally, dolomite and dolomitic limestones are present. The environmental factors which controlled the deposition and diagensis of these beds are a major concern of paleoecologists. In order to understand the ancient sediments, similar modern phenonema must be observed. The perplexing problems caused by dolomitization of beach rock and limestones become acute, as this process also effects the porosity of the fresh-water aquifers and oil reservoirs.

Limestone-making processes in existence today result in lime muds, shell hash, oolites, beach rock and summit conglomerate.

Beach Rock

Beach rock is a littoral or supralittoral calcirudite with framework particles recemented by a matrix of a lesser grain size. The principal cementing material is aragonite in the Florida beach rock (Ginsburg, 1953) and in the American Samoa (Daly, 1924). However, in the Puerto Rican material (Kaye, 1959) the cementing agent is secondary calcite. Calcite, which is the more stable polymorph of CaCO₃ is considered by Kaye to be secondary after aragonite in the Puerto Rican beach rock (Fig. 1).

Aragonite, as a mineral, is formed in a marine environment and also occurs in the shells of mollusks in various fresh-water environments in terrestrial areas. The aragonite needles, discoasters and grape-stone gravel, which act as the cementing agent of the beach rock are thought to be secreted by marine microalgae.

Beach rock commonly occurs on the Atlantic side of Key West (Fig. 2) as a pavement about 4 feet wide. The base of this deposit is marked by low tide. The sample illustrated in Figure 2 was collected in August, 1960, near an area which was used as a rubbish heap, and contained lithified pieces of bone, glass, iron, tin cans and other rubbish recemented by black and brown oxides. X-Ray analyses show that the cementing material consists of aragonite. The rapidity of the formation of this rock is phenomenal, as it was deposited in only a few years. Similar beach rock is recorded from Puerto Rico, as a pavement near an ancient rubbish heap in which Spanish coins, glass, tin cans and other objects are cemented together (Kaye, 1959). In areas where iron is readily available, it seems that the formation of beach rock parallels the growth pattern of these microalgae, as they thrive in such an environment.

The modern beach rock occurs as an intermittent shelf on the Atlantic coast of Florida from West Palm

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FIGURE 1. [Grey Scale] Beach rock - East of Pto. Salvia au vovte 165, Puerto Rico.

FIGURE 2. [Grey Scale] Modern Beach rock from a beach rock bench, at the city dump near the harbor at Key West. A. Specimen showing agglomerate of bone, limestone, glass and tin cans, recemented by an aragonite matrix. B. Other side of the same specimen.

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FIGURE 3. [Grey Scale] A. Exposures of solution - Permeated Anastasia Formation at Blowing Rock, near Jupiter, Florida. B. Close up of the base of the exposure at sea level showing modern shells recemented in pockets of the Anastasia Formation.

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FIGURE 4. [Grey Scale] Development of beach rock in Miami Oolite (SW 13th Street and first Avenue, Miami, Florida). A. Large cobble of the oolitic limestone is embeded in a matrix of calcite, aragonite and quartz. B. Same outcrop showing a large angular block of the Miami Oolite embeded in an oolite matrix.

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Beach to Jupiter. Exposures of solution-permeated Anastasia Formation at Blowing Rock (Figure 3) show modern shell-hash recemented in the pockets of the Anastasia Formation at sea level.

Fossil beach rock commonly occurs in the Chattahoochee Formation, the Hawthorn Formation and the Miami Oolite. Parker, et. al. (1955, p. 103) reported beach shingle from the Miami Oolite in the Miami area. Here large cobbles of the oolitic limestone are embedded in a matrix of calcite (Fig. 4A). Also at the same outcrop (Fig. 4B), a large angular block of the Miami Oolite is embedded in an oolite matrix. The mineralogic composition of the oolite matrix consists of calcite, aragonite and quartz.

Summit Conglomerate

Summit conglomerate is a calcirudite formed by the winnowing wave action above the summits of bioclastic reefs. Such conglomerates commonly occur near the top of the exposures of the reefal Key Largo Limestone on Windley Key, Monroe County, Florida (Figures 5, 6). In a recent paper Stanley (1966) discussed the diagensis of the Key Largo at this outcrop. The twenty-eight percent of the constituent composition of the Key Largo is estimated to be calcilutite and on the average twenty-five percent of the mineralogic composition of the calcilutite consists of aragonite (Stanley, 1966). X-Ray diffraction analysis of the summit conglomerate at this outcrop shows that the matrix consists of calcite. This calcite matrix (Fig. 5) in the calcilutite is considered to be secondary after aragonite and has been altered by solution.

Summit conglomerate also occurs in the Grand Bahamas (Fig. 7) and consists of broken pieces of hard, microfossiliferous limestone which is held together by a matrix composed of calcite. The microporosity in this rock is thought to be formed by alteration of the original aragonite matrix to calcite.

Lime Muds

Currently consensus by most workers, is that most of the lime mud of the Bahamas and Florida is a physiochemical precipitate from the sea water. Published data on the composition of the Florida Bay sediments is summarized by Scholl (1965) from reports by Taft and Harbaugh (1964), Stehli and Hower (1961) and other workers. The average percentage of aragonite in the eastern Florida Bay (67%) is higher than the western part (59%) while the percentage of calcite is lower (35%) in the eastern Florida Bay than the western part (41%). Most of the calcareous mud banks in the Florida Bay are covered by high tide and are tidal or supratidal deposits. The aragonite, which is the principal cementing agent in this area, is most probably of bacteriological origin.

A supratidal dolomite crust is reported by Shinn (1964) from the Sugar Loaf Key, Florida (Fig. 8). This modern crust contains 15 to 25 percent of dolomite. An algal mat (Fig. 9) that covers the surface of the

FIGURE 5. [Grey Scale] Exposures of the reefal Key Largo Limestone, Windley Key. The top of exposure show summit conglomerate shown on Figure 6.

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FIGURE 6. [Grey Scale] Summit conglomerate from the top of the Key Largo Limestone, Windley quarry. A. Polished slab showing agglomerate of calcite (x-ray analyses of sample 1, 2), coral and Miami Oolite recemented by a matrix consisting of calcite (x-ray analysis of samples 3 and 4). B. Polished slab of another specimen.

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FIGURE 7. [Grey Scale] Summit conglomerate, from a water well, 30-45 feet below ground surface, Freeport, Grand Bahamas. Such conglomerates commonly occur along the top of some of carbonate banks off the Florida coast. A. Entire specimen. B. Polished slab. X-ray analysis show that areas 1, 2 and 3 are calcite.

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supratidal dolomite coast is also found here. This algal mat and microalgae also contribute to the formation of calcilutite.

An example of lime mud deposition in ancient rocks can be seen along the Suwannee River in the Hawthorn Formation where there occurs thinbedded, contorted dolomite, quartz and phosphorite and algal dolomite plates (Puri and Vernon, 1964). The occurrence of these sediments in a former beach area, with ripplemarks and mud cracks, would suggest the existance of limestone shelf areas in the Middle Miocene, similar to those that occur along the modern coast of Florida.

Worm Rock

A sabellariid reef system occurs around the Florida coasts between Cape Kennedy and Cape Florida Light on the Atlantic Coast and from Panama City to Naples on the Gulf Coast. The worm which builds the Atlantic Coast worm rock belongs to Phragmatopoma lapidosa, while the Gulf Coast reef builder is Sabellaria floridensis. Small clusters of this reef occur from Daytona Beach in the south to Fernadina Beach in the north. The reef is being maintained at this time from mid-tide to 85 feet below sea level (David W. Kirtley, Personal Communication, June, 1967).

The matrix of this rock along the Atlantic Coast at Canova Beach, intersection of Fla. 518 and U.S. A1A, 3 miles east of Eau Gallie, Broward County, Florida (Fig. 10), consists of calcite, aragonite and quartz which is agglutinated in a protein cement secreted by the animal.

Role of Marine Microorganisms in Formation of Calcium Carbonate

A synthetic culture medium (Table 1) designed for isolating certain types of marine microalgae spontaneously produces a series of crystalline growths of an unusual nature. The medium departs from synthetic seawater in several minor (quantitatively) respects. It has been found that the addition of bicarbonate content (1.0 gram instead of the normal 0.192 gram) is sufficient to demonstrate this phenomenon. Study presently in progress indicates that other minor elements also play a role in the production of crystalline growths. The crystalline structures appear in a matter of hours

FIGURE 8. [Grey Scale] Exposure at Sugar Loaf Key showing a sun-cracked supratidal storm layer with algal mats developed on the dolomite filled cracks only.

FIGURE 9. [Grey Scale] Close up of the Sugar Loaf Key showing the supratidal dolomite crusts and algae mats.

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and continue to develop into the more elaborate structures over a period of a few days (Figures 11, 12, and 13).

The crystalline types A and B, as shown on figure 11, are consistent in all fresh preparations of the medium. Figure 12 is a photograph of an early stage of the Type B; figure 13 shows the larger crystalline spheres and associated hyphae of an actinomycetes as they were grown on a microscope slide over a period of four weeks. The large domes have grown from the Type B structures, shown in their early stages in Figure 11. An x-ray spectrograph of these crystals displays the characteristics of aragonite. Further work on the crystal structure and composition of these bodies is in progress.

When 10 ml of the medium is inoculated with 1 ml of raw seawater, the crystalline structures begin to grow as soon as the microalgae contained in the inoculum grow. Figure 14 shows an association of crystalline bodies and algae, after one month of incubation, as an illuminated algal culture. These growths occur abundantly and become quite elaborate, suggesting development along the lines of grapestone gravel.

When this medium is used as the fluid for making up agars for bacterial studies, the crystalline structures will appear as "contaminants" in the solidified agar.

However, when plates free of "contaminants" are streaked with a previously flamed loop, which has been in contact with the wet surfaces of beach rock, beaches and other coastal features, prolific growth of a number of micro-organisms occurs, producing great numbers of crystals and induce these and other calcareous structures to form. Cultures of these organisms have been isolated from a variety of calcareous environments at Bimini Island, Puerto Rico, for location see figure 1, the Florida Keys, Key West beach rock locality (fig. 2) and the Sugar Loaf Key dolomite crust locality (fig. 8, 9) and the east coast of Florida (Blowing Rock, see fig. 3). Figure 15 is a photograph of a portion of an agar streak showing the many crystalline bodies produced.

In a matrix of decaying organic matter and evaporating seawater, rich in bicarbonate ion, it is conceivable that these bacteria might provide a mechanism for cementing and lithifying various sedimentary aggregates. It would be of considerable interest to have at hand a detailed knowledge of the vertical circulation of the ocean waters in the Antillean-Floridian area, since a

FIGURE 10. [Grey Scale] Specimen of modern worm rock, a sabellariid reef, consists of a calcarenite cemented by aragonite, calcite and quartz. The worms reach out of the honeycomb of tubes for food.

Table 1. Composition of CXM-67, a synthetic medium for the isolation of marine algae. This medium is not autoclaved, but is sterilized by filtration.

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Figure 11. a-e: Representative stages in the growth of crystalline structures Type A. f-i: Representative forms in the growth of crystalline structures shown in Figure 13 (Type B) I is related to the form shown in Figure 12.

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source of water rich in bicarbonate ion might well play a role in the bacterial aspect of carbonate deposition. Unfortunately sufficient data for an adequate treatment of this subject are not available.

Bacteria which produce calcium bicarbonate crystals are well known and geochemists have postulated a possible biological factor in the cementation of beach rock and aeolionite. This contribution consists of describing a medium which will provide an opportunity to study laboratory simulations of cementation and the calcifying processes, in a context hitherto unavailable. It is our plan to utilize factoral designs and computer programs, which we have recently established, for the multivariate analysis of algal media in the study of this problem. The production of crystalline structures in the presence of other algae will be of particular interest.

SUMMARY AND CONCLUSIONS

1. The modern oolites, coral and worm reefs, calcarenites, calcilutites and beach rock are cemented with a CaCO₃ cement which consists of crystals of aragonite and calcite. There is a strong correlation between the occurrence of crystals of aragonite in the matrix of these sediments and the on site presence of marine microorganisms which produce aragonite. Experimental evidence supports the view that crystals of aragonite are formed by marine bacteria which are thought to be actinomycetes.
2. The partial to complete alteration of the biogenetic cementing agent (aragonite) to calcite is due to penecontemporaneous solution and redeposition. In the alteration of aragonite to calcite, some of the CaCO₃ is removed in solution. Consequently, interstices remain within the matrix, and the resulting rock can be highly porous owing to the development of intergranular microporosity.
3. The Floridan beach rock which occurs as an intermittent shallow shelf on the Atlantic Coast from Key West to Key Largo and along the east coast from West Palm Beach to Jupiter, is formed in tropical and subtropical climate in littoral and supralittoral environment and is confined to high energy areas of diurnal water agitation and a continuing supply of fresh nutrients.

FIGURE 12. [Grey Scale] Early stage in the proliferation of a Type B crystalline structure. The double tufts are characteristic and their recurved proliferation will eventually form a spherical surface. From slide used in Figure 13.

FIGURE 13. [Grey Scale] Dome shaped crystalline structures resulting from Type B growths. Hyphae and associated actinomycetes have started spreading on the slide. Smaller Type A structures can also be seen.

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