UAE Grapestones


The adjacent facies map indicates that grapestone sands accumulate and cover the shallower and intertidal portions of the coastal terraces flanking the southern shore of the Khor al Bazam (Kendall and Skipwith, 1969a). They are commonest in the intertidal sand flats present on the eroded surface of ancient reef flats and wave cut benches of pre Holocene rocks (Alsharhan and Kendall, 2003; Kendall and Skipwith, 1969b). These intertidal flats occur on shoals, the offshore bank, and coastal terrace (as for instance at Quala). They often have a gradient of approximately 1:5,000 and vary in width from .5 km to 8 km. They are generally covered by varying thicknesses of unconsolidated sand of which grapestones are not an uncommon compontent. They are often underlain by alternating layers of poorly cemented limestone (1 to 3 cm thick) and unconsolidated carbonate sand (3 to 6 cm thick)(see the oblique view of Quala). The cemented layers have flat upper surfaces and irregular lower surfaces. Some of these layers are formed as contemporaneous beachrock derived from the unconsolidated sediment of the sand flat. They are probably cemented by calcium carbonate precipitated in response to the photosynthetic processes of cyanobacteria (Nesteroff, 1956) and the evaporation of capillary water during low tide (Ginsburg, 1953b). This latter process is probably especially active during the height of summer. Before additional sediment accumulates, the upper surface of the beachrock is often truncated by storm action. Sugden (1963) and Purser and Loreau (1973) have observed similar crusts in western Abu Dhabi.

Angled Bars


Qala Bay


Qala Bay Oblique


Qala Bay Tertiary Hills


Not all the layers are contemporaneous and some are of different composition to the loose sand above them. These particular horizons often extend from 2 m below the low water mark where in the east Khor Al Bazam they form megapolygons tens of meters across (Assereto and Kendall, 1977) to a few meters above the high water mark (in the latter case they are often covered by beach and algal sediments). These limestones probably represent sediments deposited as the sea transgressed across the shelf during the last Holocene rise in sea level. They were cemented to form beachrock contemporaneous with this change in sea level.

Megapolygons are found on the intertidal sand flats south of Abu al Abyad. These polygons may average 400 m across and are best developed at the head of the Khor al Bazam where they are marked by weed growth (Assereto and Kendall, 1977). The polygonal outlines are believed to be the surface expression of cracks in the hardened surface that lies beneath the veneer of sediment, and the weed lines are due to preferred growth in the loose sediment that fills the cracks. Small polygonal cracks were seen where strong currents keep the bottom clear of loose sediment. Polygon formation may be connected with cementation of the sea floor during subaerial exposure and desiccation, or from accumulation of gases in the sediment beneath the hard layer; the size of the polygons probably reflects the thickness and competency of the layer. These polygons and the tepees of the back reef of the Upper Permian of the Guadalupe Mountains may have the same form and origin (Alsharhan and Kendall 2003; Kendall, 1969).


The surface of the intertidal sand flat is ripple marked or covered by growing seaweed, break point bars, and runnels and bars, all of which are controlled in their distribution by the direction of wave approach (Kendall and Skipwith, 1969a). The landward edge may be delimited by intertidal spits, beaches or by algal flats or mangrove clumps, or both (Kendall and Skipwith, 1969a).

The surface may be divided on the basis of fauna into Cerithium flats on the lower area and crab flats on the upper (Kinsman 1964b). Scopimera sp., the crab that characterizes the latter zone, produces radial patterns of feeding balls. Crabs, worms, and molluscs extensively burrow the sediments of the flats so the primary sedimentary structures are destroyed. Ginsburg (1957) reported the same phenomenon in carbonate sediments of Florida.

Grapestones are aggregates composed of sand sized particles bound together by an aragonite cement. Component grains protrude, giving the appearance of a bunch of grapes. The components are predominantly aragonite, with up to 20 percent quartz. Illing's (1954, p. 30) description of the Bahamian examples fits the Khor al Bazam grapestone.

The cement that joins the grains is finely divided aragonite of varying texture. It forms first around the points of contact, and is friable and chalky white, in contrast to the greasy or matt textured grains. It can easily be scraped off with a pin point, and is composed of aggregated particles of mud dimensions. From its occurrence, it is clear that it is being precipitated from seawater, yet the particles show no recognizable crystalline shape and are similar to the material that forms the matrix of the grains themselves.

Externally, the chalky white cement is restricted to the crevices between the protruding grains. As cementation proceeds, the cement beneath this surface layer becomes firmer and matt textured. Additional grains or small lumps may be joined by the same sequence of stages. However, the forces of mechanical disintegration prevent unlimited growth and finally all traces of chalky white texture are lost.

In the Khor al Bazam, grapestones form on the grassy lagoon floor and shoal areas. During periods of minimum wave activity in the lagoon, surface sediment is bound initially by an organic slime forming a thin crust (Nesteroff, 1956). On intertidal shoals, the initial binding agent is either organic mucilage or incipient aragonite beach rock cement; the latter is precipitated at low tide during hot summer months (Ginsburg, 1953). In both areas, if the cementation process continues long enough, photosynthesis by blue green algae precipitates aragonite. It cements and fuses the carbonate grains and is indistinguishable from the grains. The acicular crystals of beachrock cement are altered and the whole grain ultimately becomes homogeneous. The longer the surface is undisturbed by wave action, the better developed the binding.

Once formed, the surface crust may be broken into small lumps by wave action. The size of the lumps depends on the strength of the cement and the force of the waves. With a heavy sea and lightest of bindings, the crust may be separated into its constituent grains, but with gentle action and strong cementation, aggregates are produced. Thus on the average, aggregates are larger and more irregular in the protected grass areas of the lagoon than those of the exposed shoal regions.

In the intertidal region of Quala bay, aggregates are composed of 20 percent quartz, 60 percent aragonite pellets and 20 percent aragonite cement. The sediment in which they are present contains a ratio of aggregate to quartz to pellet of approximately 3:3:1. If the sediment were cemented unselectively, the aggregates would be expected to contain more quartz than they do. The low percentage of quartz in aggregates may be explained in three ways.

Algally induced alteration of carbonate

As with other Holocene shallow water carbonate settings, some of the sediments of the Khor al Bazam show a distinctive type of alteration that affects all carbonate grains, irrespective of their origin (Kendall et al., 1966a and b). The texture of the grains is altered to a homogeneous microcrystalline fabric of aragonite. In the final stage of development, the grains are virtually indistinguishable from each other. In many instances the external shape may be the only indication of the grains' origin. This process has been observed in the Bahamas (llling, 1954; Purdy, 1963a and b), Portuguese Timor (Wolf, 1965b), the Arabian Gulf off Qatar (Houbolt, 1957) and in British Honduras (Purdy, 1965).
Illing (1954) attributed the alteration to bacteria or algae. In contrast, Newell et al. (1960) ascribed altered zones in ooids to the effects of decaying colonies of boring algae. Later, Purdy (1963a and b) concluded that organic matter trapped within the grains promotes the process. Bathurst (1966) ascribes this alteration to algae.
In Holocene sediments of the Khor al Bazam, foraminifera and red calcareous algae seem particularly susceptible to alteration. In contrast, thick walled shells and shells with a more coarsely crystalline fabric are less susceptible to alteration. Alteration in these shells generally is confined to the surface, but a few lobes of fine grained carbonate extend deep into the shell.
Alteration also occurs in aggregates of the microcrystalline carbonate that forms fecal pellets and grapestones. These aggregates are initially poorly bound, open textured grains that are rapidly hardened and cemented as they alter. oolites are similarly affected.
This process of alteration can be seen in the peneroplids of Quala Bay. Peneroplid foraminifera are particularly susceptible to alteration (Murray, 1966). Detailed study of specimens collected from the Khor al Bazam showed all stages of alteration. The tests show a series of surface changes, the ranging from translucent and porcelaneous on fresh tests to opaque and saccharoidal on altered specimens. These changes are accompanied by rounding and pitting of tests. Many of the pits are seen to contain blue green algae. Alteration may be so pronounced that tests are almost indistinguishable from aragonite ovoids of fecal and accretional origin. Both Illing (1954) and Purdy (1963a and b) noted similar changes in the foraminifera of the Bahamas.
Thin sections of fresh peneroplid tests under plane polarized light show a brown body color. They contain no crystal shapes apart from a faint structure parallel with the walls and a very faint granulation under high power. In reflected light they are milky white. Through crossed nicols the tests are seen to be composed of parallel sheaths of crystals which show low polarization colors and lie parallel with the curvature of the walls. The sheaths are embedded in a fine, granular matrix of crystals which show pin point polarization in grays and yellows of the first order.
This process may be accompanied by the infilling of the chambers by microcrystalline aragonite. The altered areas are characteristically microgranular and lack the brown body color shown by fresh tests. Under crossed nicols the altered areas show high polarization colors, but do not extinguish.
Hand picked peneroplids from the medium sand fraction of one of the samples were separated into five groups on the basis of the extent of external alteration. X ray analyses showed that the five groups progressively changed from high magnesian calcite to aragonite.
The algae growing on some of the peneroplids also were found by John Twyman (personal commun., 1964) on the oolites from Abu Dhabi. They were identified by Dr. Stewart of Westfield College, who found them to be Entophysalis deusta (Menegh), Drouet and Daily. Newell et al. (1960) found this genus common in the Bahamas.
Algae collected in Abu Dhabi survived 3 years in storage jars. Upon exposure to sunlight they started growing vigorously.
The algal cells and filaments were found to lie just beneath the surface of the altered tests, as shown by (1) progressively dissolving the carbonate of the tests in 5 percent acetic acid or 0.2 percent HCL, and (2) rendering the surface transparent with 40 percent hydrofluoric acid. Algae could be made even more apparent by staining with malachite green, which stains algal cells and the mucilage with different intensities.
If the unaltered tests are completely decalcified, they leave behind a thin, diaphanous, soft elastic membrane that retains the original form of the foraminifera. This is the "tectin" of Hyman (1940) and is believed to be the original organic material of the test which in life, occurs in conjunction with the calcite.
Decalcified altered foraminifera leave behind a much more translucent material, which also molds the test and fills its chambers, but is markedly different in appearance from the "tectin." In all ways it resembles the mucilage that is present in association with the blue green algae. It contains algal cells and small quantities of minute mineral grains which probably adhered to the mucilage as the test rolled on the sea floor.
If foraminifera are treated with 40 percent hydrofluoric acid for more than 10 minutes, all the calcium carbonate is replaced, molecule by molecule, by transparent calcium fluoride (Grayson, 1956). The mucilage envelope and algae resist solution (Fischer, 1897) and consequently stand out as translucent and green in altered areas. Treatment with 40 percent hydrofluoric acid for 5 minutes makes the surfaces of the foraminifera transparent and reveals the network of radiating filaments and globular algae. Fresh foraminifera become completely transparent. The best optical results with the calcium fluoride specimens are obtained by viewing them under water.
Partially altered foraminifera examined in thin section show the algal filaments penetrating both the unaltered and altered areas. The filament "bores" generally are filled with microcrystalline aragonite, which preserves the filament shapes. Thus the unaltered test may be penetrated by tubes of altered material. The boundaries separating the unaffected high magnesian calcite areas of the test from the aragonitic altered areas commonly resemble the boring of algal filaments (Bathurst, 1966). Under plane polarized light, parts of the test adjacent to the bore shapes may show faint granulation, which is believed to be the first sign of alteration.
Staining thin sections of the altered material with aqueous malachite green results in a patchy effect. Deeper colors are present at the outside edges and in the chambers devoid of carbonate. Etching thin sections with weak acid solutions of malachite green dissolves the calcium carbonate, leaving the stained insoluble algae and mucilage. The deeper stain is usually at the edge of the former test and in some chambers, showing algal cells and filaments both at the surface and penetrating the interior. Generally, the parts of the chambers not filled with algal cells are stained less intensely, an indication that they are filled with mucilage. Dissolution and staining of the altered walls of the foraminifera leave only a trace of mucilage that stains lightly. This most likely results from the displacement of mucilage by the formation of microcrystalline aragonite crystals. The presence of this finely dispersed mucilage is confirmed by gently dissolving the whole tests. If the dissolution is too vigorous, the mucilage is ruptured and removed.
The intimate relation of blue green algae with alteration can be demonstrated. All plants photosynthesize and respire. Dalrymple (1965) observed the ability of blue green algae to precipitate calcium carbonate. Parks and Curl (1965) showed in the laboratory that a culture of blue green algae in sea water produces measurable change in the conductivity of the water between day and night. They inferred that, in light, photosynthesis transforms bicarbonate ions to carbonate, and in the dark, respiration causes the opposite effect.
In the peneroplids of the Arabian Gulf, the mucilaginous envelope generally creates a microenvironment within each test. Carbon dioxide given off during respiration would promote solution of calcium carbonate in this restricted environment. Conversely, the carbon dioxide utilized during photosynthesis would cause precipitation. Although high magnesium calcite is dissolved, it is aragonite that is precipitated because it is apparently the stable form of calcium carbonate under such conditions (Kinsman, 1964b). In this way, the high magnesium calcite of the foraminifera test is progressively dissolved and replaced by aragonite on a piecemeal basis. Once a foraminifer is altered, dissolution and reprecipitation of aragonite continue, ultimately destroying all evidence of its origin.
This process alone could account for alteration. Bathurst (1964) suggested that alteration is accomplished by algae boring and reboring the calcite test and that solution of the test occurs only where it is in direct contact with the algal filaments. However, the fact that the algae are restricted to the area near the surface of the test and do not necessarily extend into all the affected parts, argues against alteration as the result of boring alone. Also, the mucilaginous envelope secreted by the algae confines the carbon dioxide. This carbon dioxide can react at any point within the envelope.
The alteration of carbonate grains could be controlled by several factors: the length and frequency of exposure of the grains to direct sunlight, the dimensions of the grains and the size of their component crystals. For example, if the grain is buried quickly, it may not have time to alter or will only alter if it has a thin, delicate shell and small component crystals.
The shoal areas of the Khor al Bazam are rippled where the altered foraminifera were collected. If the ripples migrate slowly, the foraminifera being altered will be exposed only occasionally to direct sunlight. Consequently, the alteration process may be slow. Alteration is most effective on the weedcovered shoals of the coastal terrace and parts of the offshore bank. skeletal grains from those areas have abundaformformformformformnt algal filled pits. The prolific plant growth must upset the carbon dioxide balance of the sea water and alteration therefore may be more rapid.
Wolf and Conolly (1965, p. 108) pointed out that algae are generally good indicators of shallow water because photosynthesis is depth controlled. Thus, alteration would not be expected to take place below the photic zone. Off Qatar, bioclastic material is rounded and the surface character obliterated in water less than 20 m deep (Houbolt, 1957).
In rocks from similar ancient carbonate environments, the microcrystalline aragonite contained within the altered material generally is replaced by a mosaic of microcrystalline calcite in which algal alteration is still recognizable (Shearman and Skipwith, 1965). The altered material commonly forms Bathurst's (1966) micritic envelope.


As in the Bahamas where Illing (1954) recognized them the following sand sized accretionary grains have been found in the Khor al Bazam (Kendall and Skipwith, 1969b):
Friable aggregates, grapestones, botryoidal grains, encrusted lumps, and shell infillings. These aggregates accumulate and probably form on the lagoon floor and on some of the adjacent shoal areas. Material from the shoal areas tends not to be transported far into the lagoon.
Interestingly samples collected across the algal mats show that, although aggregates are forming seawrd of them, the aggregates are not transported across the algal mats. The percentage of aggregates decreases sharply landward, but the older buried, frontal algal mats contain a higher proportion of aggregates.

Aggregate Grains of the Khor al Bazam

Grain Type Composition Locality
Friable aggregates Discrete silt sized particles bound by organic fibers Lagoonal grass areas
Lumps; Grapestones Discrete sand sized particles bound by, and protruding from, an aragonite cement Lagoon floor, coastal terrace and offshore bank
Botryoidal lumps Similar to grapestones but with polished aragonite envelope which disguises components Coastal terrace
Encrusted lumps Similar to grapestones but with surface so bored by blue green algae and altered, as to be unidentifiable except in thin section Lagoon floor, coastal terrace and offshore bank
Worm tube Discrete sand sized particles bound by aragonite cement and enclosed or partially enclosed by mollusk shells Coastal terrace and offshore bank
Shell infillings Discrete sand sized particles bound by aragonite cement and enclosed or partially enclosed by mollusk shells Lagoon, coastal terrace and offshore bank

Tuesday, March 26, 2013
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