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Abstract

Rudist associations were normally destroyed during or shortly after their development. Consequently, exact paleobiological examinations are very difficult or impossible. The in–situ associations of Central Oman are often completely preserved and enable a precise paleobiological and paleoecological description to be made. They developed during a transgression onto the Arabian platform. Microfacies analysis based on extensive field observations reveal that vigorous turbulent conditions prevailed very frequently throughout the period. The development and preservation of in–situ rudist associations are rare. As a rule episodic turbulence prevented the development of such structures. In the same way, turbulence normally destroyed rudist associations. The model of constratal growth (Perkins 1974; Gili et al., 1995a; Skelton et al., 1995) is discussed, including the question whether thick, extensive rudist associations should be named reefs. The absence of vast rudist reefs is believed not to be a consequence of an inherent inability of vertically growing rudists to build such structures, but can be mainly attributed to exogenous, abiotic factors. The associations investigated here inhabited restrictive, shallow–marine environments. Rapid growth of a few rudist species is verified. Vertically growing rudist associations of 5–8 successive in–situ generations represent a period of only two to three hundred years.

Introduction

The Mesozoic transgressions in the Central Oman region took place over the Paleozoic Haushi–Huqf paleohigh (Platel et al., 1994). Broad shelves developed with these transgressions, which are subdivided into open shelves, intrashelf basins, and inner shelves (Murris 1980; Harris et al., 1984). During the Late Cretaceous the investigated areas were situated at the Equator. Computer–aided modeling yielded a position in an area with a strong E–W ocean current (Barron 1990). Throughout the Cretaceous Period the Arabian platform was situated off the Paleo–Indian Ocean. The distribution of continent and ocean has remained largely similar in this region since Late Cretaceous times. In the Campanian, as today, the coast of SE Arabia bordered directly on a wide Paleo–Indian Ocean.

The sedimentology and stratigraphy of the rudist–bearing sections in the Saiwan district of Central Oman are not described in this article. The profiles include parts of the Campanian (Samhan Formation) and the Maastrichtian (Simsima Formation). Details of the stratigraphy and biozonation have been published by Dubreuilh et al. (1992), Hughes Clarke (1988), Platel et al. (1994), and Philip and Platel (1995). In relation to the widespread outcrops of the Cretaceous formations, complete in–situ rudist associations are not very common. The aim of this paper is to find an explanation for this relative rareness even in thick rudist limestone profiles.

Lithological Aspects Of The Lower Campanian Samhan Formation

An important transgression took place in the Early Campanian over terrigenous sandstones. The profiles of the Samhan Formation of Saiwan represent the Globotruncana elevata zone, which, according to current knowledge, indicates a period of 4–5 million years.

Rudists are extremely rare within the lowermost few meters overlying the terrigenous sandstones. They are also missing as fragments in the microfacies. Throughout the next 30–40 m of the lower Campanian, rudists are found in all but a few horizons, usually as reworked rubble. This indicates that, as a rule, there was no primary development of dense rudist associations. The sediments of the Samhan Formation of Saiwan were interpreted in context with geodynamics and environments by Platel et al. (1994). Schumann (1995) investigated numerous sites in the vicinity of Saiwan. Six of these profiles have been published. Therefore, only a few basic comments on the facies are given here (Fig. 1A). The spectrum of the microfacies is not very broad. The very shallow–water sediments, consisting of sand and silt in the lower part of the profile, contain numerous thick–walled ostreids, cyclolitid corals, rare small cone–shaped solitary corals, some colonial corals, pectinids, and microfauna. Rudists are extremely rare in this facies.

Fig. 1.

—A) Typical lithologic section of the lower Campanian of the Saiwan area (section 2, Schumann 1995). V1/V2, Vaccinites associations; S, stromatoporoid association; D, Durania patch reefs. B) Location of Saiwan in Central Oman. C) Access from the Saiwan track to rudist localities 1 and 2.

Fig. 1.

—A) Typical lithologic section of the lower Campanian of the Saiwan area (section 2, Schumann 1995). V1/V2, Vaccinites associations; S, stromatoporoid association; D, Durania patch reefs. B) Location of Saiwan in Central Oman. C) Access from the Saiwan track to rudist localities 1 and 2.

Dense, occasionally very compact rudist associations (V1/V2) are found in marly limestones. As a rule these limestones are intensely weathered as a result of the modern climate aided by the salty air and a frequent stiff breeze from the east (slowly working salt wedging). The cryptic sediments of the rudist horizons (samples taken from the spaces between rudists) contain very sparse associated fauna. The horizons of the section between the rudist horizons V1/V2 and the striking stromatoporoid horizon (part 5) contain numerous colonial corals, stromatoporoids, and solitary rudists, which are only very rarely grouped to small lenticular structures. This facies contains numerous small and large foraminifera, and the bedding was influenced by strongly agitated water. Local accumulations of a large Durania sp. are found above the stromatoporoid horizon S. They are grouped into clusters of a few individuals or patches with dozens of individuals (Fig. 4), but also into associations of several hundred (Fig. 7) and possibly, according to a three–dimensional reconstruction of the exposure areas, even several thousand individuals. The primary deposition within this horizon was locally chaotic: even very heavy shells were toppled during temporary high–energy conditions. An extensive continuous increase in carbonate content can be found in the upper section of the profiles (parts 9 and 10). The hard and nearly 100% pure yellow–white micrites form characteristic escarpments in the landscape. These limestones show a lenticular bedding and locally have a character similar to nodular limestones (Schumann 1995, pi. 40, fig. 6). In most localities the uppermost part of the section consists of dense micrite, often with silification. According to Platel etal. (1994) the top of this horizon represents a maximum flooding surface with hardground development. These hardgrounds were not observed in the vicinity of the rudist associations south of Saiwan. In summary, the depositional conditions were generally dominated by agitated water (see section on shell size, gregariousness, and water energy later in this paper).

Fig. 4.

—Patches dominated by a giant Durania sp. (1) are common in the Saiwan area. Besides this species large colonial corals (2) and robust Torreites sanchezi milanovici (3) occur, but never Vaccinites. The percentage of colonial corals differs from patch to patch but never exceeds 10%. The depicted community is one of the smaller ones. Hundreds and thousands of individuals could participate, and a couple of generations could grow successively on top of each other. Around locality 2 (Fig. 1); compare Figure 4.

Fig. 4.

—Patches dominated by a giant Durania sp. (1) are common in the Saiwan area. Besides this species large colonial corals (2) and robust Torreites sanchezi milanovici (3) occur, but never Vaccinites. The percentage of colonial corals differs from patch to patch but never exceeds 10%. The depicted community is one of the smaller ones. Hundreds and thousands of individuals could participate, and a couple of generations could grow successively on top of each other. Around locality 2 (Fig. 1); compare Figure 4.

Fig. 6.

—Association of Vaccinites vesiculosus, 1 km W of locality 2 (Fig. 1). Compare Figure 2. Length of hammer is 32 cm.

Fig. 6.

—Association of Vaccinites vesiculosus, 1 km W of locality 2 (Fig. 1). Compare Figure 2. Length of hammer is 32 cm.

Fig. 7.

Fig 7 Association of several hundreds of individuals of the large Durania sp locality 2 horizon Dof the section Fig 1 Average diameter of the individuals is 40 cm Compare Figure 4

Fig. 7.

Fig 7 Association of several hundreds of individuals of the large Durania sp locality 2 horizon Dof the section Fig 1 Average diameter of the individuals is 40 cm Compare Figure 4

Geometry of The Rudist Associations

Among the outstanding features of the Saiwan region are the unique rudist exposures. They enable vertical and horizontal observations of all details. In contrast to most rudist associations of other localities, those of Saiwan have remarkable lateral extent. The Vaccinites vesiculosus–dominated associations completely cover areas up to about 100 km2. There is no known locality throughout the Saiwan region where these associations are more than 2 m thick. As a rule, they are no thicker than 80 cm.

The most successful species, although preserved only twice as massive in–situ associations in approximately 5 million years, is Vaccinites vesiculosus (Woodward). The slender shells grew up to 50 cm high. The associations may be bushy to very dense and can consist of several layers of in–situ generations (Fig. 2). This species is nearly 100% dominant. However, the following can occasionally be found growing within the Vaccinites vesiculosus associations: a Torreites sanchezi milanovici Grubic, a Radiolites sp., or a Biradiolites sp., a 50–cm–long and 35–cm–wide Durania, or a 70–cm– high, 30–cm–wide colonial coral. It must be emphasized that the Vaccinites vesiculosus outnumber the other species by 1000 to 1 or more! Therefore, large areas of the Vaccinites associations are monospecific.

Fig. 2.

Most of the Vaccinites vesiculosus associations of Saiwan are monospecific Up to 8 generations can settle one upon the other and form locally thick very compact constructions nearly without sediment The thickness shown here is developed only in a few localities in the Saiwan area compare Fig 6 Because of to the rapid growth of Vaccinites at least the last generation was above the sedimentary sea floor Sediment support aiding the vertical growth of the individuals was notnecessary because of the dense ccemeonted gregariousness Atsome localities not documented here tropical storms uprooted large parts of the communities leaving distinct discontinuities Around locality 2 Fig

Fig. 2.

Most of the Vaccinites vesiculosus associations of Saiwan are monospecific Up to 8 generations can settle one upon the other and form locally thick very compact constructions nearly without sediment The thickness shown here is developed only in a few localities in the Saiwan area compare Fig 6 Because of to the rapid growth of Vaccinites at least the last generation was above the sedimentary sea floor Sediment support aiding the vertical growth of the individuals was notnecessary because of the dense ccemeonted gregariousness Atsome localities not documented here tropical storms uprooted large parts of the communities leaving distinct discontinuities Around locality 2 Fig

A notable variation of the normal Vaccinites association is found in the same horizon 5 km to the north of the community shown in Figure 3. Here, Torreites sanchezi milanovici constitutes approximately 50% of the association. The shells of this species grew up to 40 cm high and are frequently found in originally compactly cemented groups of up to 10 individuals. The proportion of head–size stromatoporoids and colonial corals is remarkable. This locality is the most extensive Torrafes–dominated area in Saiwan and actually in the world. In view of the intense weathering and modern detritus production, the character of this community can be revealed only by excavations.

Fig. 3.

At very few localities only the normally Vaccinites vesiculosus-dominated associations show a differing composition. At locality 1 (Fig. 1) the ratio of Vaccinites vesiculosus to Torreites sanchezi milanovici is about 50%:50%. Approximately 5–10% of the community are represented by small and large stromatoporoids (dotted) and colonial corals. All Vaccinites and Torreites shells are completely cemented with each other forming very dense and robust constructions. This reconstruction is based on four excavations of about 1 m3 each and a distance of 2-4 m between them. Piece by piece was recorded. In most of the pieces exposed to weathering the upper valve was lost. In contrast, excavated individuals have been recorded much more often with their upper valve in place. This points to a rapid covering with sediment during storm events. The reconstruction shows only the part of the community that as a result of successive rapid growth existed above the sea floor. A couple of analogous communities existed below. It is not believed that the primary upright life position of the individuals was supported by a constratal growth (Gili et al., 1995a). Not only neighboring Vaccinitcs shells but also Vaccinites and Torreites shells are co-cemented very strongly. Almost all of the pieces are in Darmstadt.

Fig. 3.

At very few localities only the normally Vaccinites vesiculosus-dominated associations show a differing composition. At locality 1 (Fig. 1) the ratio of Vaccinites vesiculosus to Torreites sanchezi milanovici is about 50%:50%. Approximately 5–10% of the community are represented by small and large stromatoporoids (dotted) and colonial corals. All Vaccinites and Torreites shells are completely cemented with each other forming very dense and robust constructions. This reconstruction is based on four excavations of about 1 m3 each and a distance of 2-4 m between them. Piece by piece was recorded. In most of the pieces exposed to weathering the upper valve was lost. In contrast, excavated individuals have been recorded much more often with their upper valve in place. This points to a rapid covering with sediment during storm events. The reconstruction shows only the part of the community that as a result of successive rapid growth existed above the sea floor. A couple of analogous communities existed below. It is not believed that the primary upright life position of the individuals was supported by a constratal growth (Gili et al., 1995a). Not only neighboring Vaccinitcs shells but also Vaccinites and Torreites shells are co-cemented very strongly. Almost all of the pieces are in Darmstadt.

very large species of Durania was found in the upper parts of the profiles of Saiwan. In the area around the locality 2 (Fig. 1) this species forms dozens of patches of varying dimensions (Figs. 4,7).

The individuals attain a maximumheight of 80 cm and a maximum diameter of 60 cm. Large, massive stromatoporoids, cerioid corals, and massive Torreites shells are less common. No other rudists were found in these patches. A few sea urchins and many spines occur in the cryptic niches at the base. Single large individuals of Durania at the same stratigraphic level regularly appear in nearly all sites, but they do not form Durania patches anywhere. It seems that optimal ecological conditions for this very characteristic species existed in the region around locality 2. It is obvious, however, that it appears at some localities in completely different environments with very large specimens, e.g., among the in–situ Vaccinites vesiculosus communities or in the yellowish–white micrites of horizons 9 and 10 (Fig. 1A). In contrast, Vaccinites vesiculosus specimens have not been found in any of the numerous Durania patches. This indicates that this large Durania sp. had a higher ecological tolerance. Apart from the central rise, the upper valve of this Durania sp. is extremely thin, presumably translucent.

Fig. 5.

—A Maastrichtian association of Durania sp. Very rare thick–shelled Vaccinites vesiculosus, Vaccinites sp., Pironaea aff. syriaca, extremely robust Torreites sanchezi milanovici, large colonial corals, and massive stromatoporoids occur in an outcrop of about one km2 NE of locality 1 (Fig. 1). The dense settlement of larvae caused an extraordinary competition for space and in this way led to a compact, massive structure of the community that could resist high energies. A couple of generations settled on top of each other; of these only the uppermost is shown.

Fig. 5.

—A Maastrichtian association of Durania sp. Very rare thick–shelled Vaccinites vesiculosus, Vaccinites sp., Pironaea aff. syriaca, extremely robust Torreites sanchezi milanovici, large colonial corals, and massive stromatoporoids occur in an outcrop of about one km2 NE of locality 1 (Fig. 1). The dense settlement of larvae caused an extraordinary competition for space and in this way led to a compact, massive structure of the community that could resist high energies. A couple of generations settled on top of each other; of these only the uppermost is shown.

A Durania Association

A Dwranw–dominated association of the Simsima Formation covering approximately 1 km2 was found on a tectonically isolated block 2 km NE of locality 2. This Durania sp. is probably identical to the species classified as Durania Form A by Morris and Skelton (1995). The shells of this species have strong vertical ribs and grew up to 50 cm high. They formed densely interlocked and mutually cemented communities with new generations continuously growing on top of the last. The associations are up to 1.8 m thick. Solitary, notably massive Torreites sanchezi milanovici, Vaccinites sp., and, very rarely, Pironaea aff. syriaca shells occur in this associations. Apart from rudists, there are also isolated large colonial corals and stromatoporoids. The character of the basal sediment and the occurrence of sma 11 sand channels in a number of places indicate a very shallow–water environment. According to current knowledge this is the only outcrop of the Simsima Formation in the western part of the Haushi–Huqf paleohigh. Much better exposures of the Simsima Formation can be found in the Filim region, where remarkable associations of the same Durania species (Fig. 5) can be found in the Wadi Halfayn near the town of Hayy.

Sedimentation Rate, Colonization, And Spatial Requirements

The lower Campanian profiles of Saiwan are 40 m thick and represent 4–5 million years of sediment accumulation. Consequently, the average sedimentation rate is estimated at 1 cm per 1000 years. This is not an unusual value. The character of the sediments indicates that only gradual changes occurred over a period of several million years. Furthermore, fragments in the microfacies indicate that the same rudist species of the in–situ associations existed virtually throughout the entire period represented by the profiles under investigation. In spite of this, dense or compact rudist associations were able to develop and to be preserved on only three occasions. The reason for this must be sought in the nearly permanent active redeposition of material. The characteristics of the bedding in the profiles is typified by a sequence of discontinuities. The question arises why in–situ associations are so rarely preserved. In similarity to recent pelecypods, rudists probably also developed via a free–swimming larval stage. These larvae settled onto hard particles. On the Saiwan shelf the chances for growth over a period of decades were very unfavorable, because of the nearly permanent redeposition. Evidently, dense in–situ associations could develop only during a rarely occurring phase of somewhat lower–energy conditions lasting at least some weeks or months. One can assume the existence of an abundant supply of larvae for colonization from the vicinity of the Paleo–Indian Ocean. Essential for the successful development of a dense association is a massive larval colonization of a given area. In this way, during temporarily lower–energy conditions, a biogenic pavement consisting of millions of joined juvenile rudist shell stages could have developed within a few weeks.

The spatial requirements of the growing individuals were very high (compare Skelton 1979). For example, in Saiwan, a juvenile colony of radiolitid individuals developed from 60–70 larvae growing on a few square millimeters. Once the colony grew to a height of 2.5 cm, only 45 individuals survived, covering an area of 30 cm2 (Fig. 10). Thus, the spatial requirements increased by 400–500% within a few months.

Fig. 10.

—Space requirements of a juvenile radiolitid colony. A) Surface. B) Transverse section (reconstruction). C) Bottom view. Locality 2 (Fig.l).

Fig. 10.

—Space requirements of a juvenile radiolitid colony. A) Surface. B) Transverse section (reconstruction). C) Bottom view. Locality 2 (Fig.l).

Another colony of four Vaccinites individuals also started growing on a few square millimeters of substrate, and increased to 18 cm2 by the time they reached 2.5 cm in height and 195 cm2 at 15 cm height. This means that the spatial requirements increased by a factor of 4000–5000 within a few years. These examples illustrate that massive larval colonization can actually produce a biogenic pavement within a short period of time. Once such a pavement exists, much stronger water movements are required for reworking. Somewhat calmer periods are presumably represented only by the preserved, successful and extensive colonies of dense rudist associations. Their brief existence, as well as the presence of rubble layers (caused by storm events) often found within the structures document a rapid recurrence of strong water movements.

Growth And Death Of The Associations

The growth of several rudist species has been the subject of a number of investigations (Amico 1978; Schumann 1995; Steuber 1995,1996,1997). These have shown that a growth rate of around 1—4 cm per year, depending on the ontogenetic stage, is possible. The adult individuals of Vaccinites vesiculosus, the main constructors of the extensive monospecific associations of Saiwan, are estimated to be about30 years old. The thickest Vaccinites associations of Saiwan comprise 8 successive generations. Thus, such a rudist bed developed in 200–300 years.

There is no locality known throughout the whole Saiwan region where these associations are more than 2 m thick. However, in contrast to Gili et al. (1995a) this does not mean that rudists are not potentially capable of building thicker reefs. The structure allows unlimited colonial succession upon existing surfaces. This principal capability is demonstrated in many cases. The reason why such massive reef structures, as seen in coral reefs, did not develop on a worldwide scale is suggested to be solely due to exogenous factors. Colonization evidently took place during a transgression in very shallow waters with brief quieter conditions. The factors and levels of tolerance controlling growth differ widely from those influencing coral growth. Extensive flat bodies, such as the rudist associations of the Saiwan type, could develop only on low–angle open shelf margins, a relief that existed along the Arabian platform margin.

The disappearance of the rudist associations of Saiwan was due to an increase in sediment supply. The associations were pre– mortally covered with sediment. In correspondence to this, excavations usually reveal shells with the upper valve preserved. This would not be possible if individuals remained in contact with the ocean after death.

Surprisingly, only three cases of constructive growth of extensive rudist associations are known during the entire 4–5 million year period. Therefore, the chances for the development and preservation of such associations were near zero. On the Cretaceous shelves of the Arabian platform rudists usually have been producers and not constructors.

Paleocological Interpretations

Considering the large quantities of preserved organisms, the diversity in the Vaccinites associations is extremely low. Numerous cryptic niches existed. In the case of normal marine conditions such niches would be populated with highly diversified associations. The examined areas are located near the Paleo– Indian Ocean. Therefore, a permanent influx of planktonic larvae can be expected, which were unable to develop in the vicinity of the Vaccinites areas. Oysters cemented on Vaccinites vesiculosus are very rare. Serpulids are frequently found on the inner side of the upper valve. Many shells are heavily bored by clionids, and pre–mortal borings of mussels are occasionally found on rudists and corals. During the pioneer settling of Vaccinites, Torreites, and Durania sea urchins were obviously quite frequent, but they are not co–inhabitants of the rudist associations. The result remains/ however, that bryozoans, brachiopods, normally growing pele– cypods, gastropods, calcareous algae, etc., normally found in great numbers in Cretaceous shelf seas, are missing completely (cf. Skelton et al., 1995). Numerous washed samples also confirm this result for microorganisms. Foraminifera and ostracods are rare in the cryptic niches. Outside the rudist communities foraminifera are present in many layers, and miliolids are very frequently found in the yellowish–white, nearly nodular limestones (see Schumann 1995, p. 199, fig. 6). Other small foraminifera play a secondary role. Actaeonellids, known from similar facies in other regions, are very rare here.

These conditions are difficult to interpret. The low diversity and the often nearly 100% dominance of a single species indicate a very restrictive biotope. Vaccinites vesiculosus was a suspension feeder, with obvious capabilities for developing in such an extreme biotope.

The absence of nearly all other groups of organisms can be explained only hypothetically. I presume that the rudist associations of Saiwan developed in a very shallow inner–shelf zone. Diurnal heating of this zone resulted in hypersalinity and led to very low oxygen conditions. Low–amplitude tides enabled an exchange of water with the open ocean, especially in the night. Presumably this was also the main filter–feeding period of Vaccinites vesiculosus. All larvae of other organisms, settling under the more favorable night time conditions, were not able to survive the ecologically restrictive conditions of the next days. It follows that Vaccinites vesiculosus was a rudist able to adapt also to extreme ecological conditions.

The presence of a contemporary higher–diversity community only a few kilometers away (Fig. 3) indicates a local shelf zone with better water exchange with the open ocean. The hypothesis of an extreme biotope for the Vaccinites vesiculosus communities is somewhatlimited by the very rare occurrence of a large colonial coral, as well as the very rare presence of a giant Durania and a massive stromatoporoid. There is no doubt that these were contemporary members of the community. This is similar to the data of Gili et al. (1995b). However, the proportion of rudists and corals is very clear and indicates that there was no competition between these two organisms in the Saiwan communities.

The part of the profiles preceding and following the Vaccinites vesiculosus communities show a higher organism diversity and contain many corals. However, there is no evidence of a constructive coral reef development.

Were Rudists Able to Build Reefs?

In connection with rudist associations almost every author has used the term reef. In addition the terms bioherm, biostrome, buildup, bioconstruction, congregation, lithosome, and meadows have been used. It cannot be the object of this paper to deal with the overall controversy concerning this terminology. Nonetheless I would like to comment on some of the statements made in the publications of Gili et al. (1995a) and Skelton et al. (1995). On the basis of detailed explanations both sets of authors propose that the term reef is not to be used for rudist associations, as a matter of principle.

According to both publications true reefs are characterized by a robust biogenetic framework, topographical relief, clonal growth, special pattern of nutrition, occupation of substrata, greater supply of upwelling water, animal–plant symbiosis, indefinite potential for longevity and continuous growth, growth of primary framework, bioerosion, infilling sediments, burial, cementation, and some further subordinate characteristics.

According to this definition only the Holocene coral–algal reefs and their fossil analogues would be true reefs. In the sense of a comparison, rudist associations have been contrasted to coral–algal reefs. Thereafter some of the characteristic features of coral–algal reefs are not present in rudist communities. Indeed, I believe that hitherto no one working with rudists has treated rudist associations as structurally analogous to modern coral reefs. The more aspects that are included in the definition of a binding reef, the more seldom the term “reef” will be used, as a consequence. If such a definition would incorporate as an obligation e.g. that only clonal organisms are capable of building true reef structures, then from the start many organisms are not reef builders per definition. This is presumably the intention and proposal of the authors named. However, such a concept would have to gain general approval. As a part of the Priority Program “Global and Regional Controls on Biogenic Sedimentation and Reef Evolution” of the German Science Foundation (DFG), repeated and comprehensive discussions with over 150 participants took place debating the terminological definition of “reef” without achieving a generally accepted outcome. Briefly I would like to give my opinion on the following points.

Relief of Rudist Associations

Most rudist structures have had little or no positive topographic relief. There are several exceptions. I would like to mention only some of these. Outcrop photographs by Perkins (1974) show a distinct relief and display the structural organization of a reef, also interpreted by the author. Scott (1990) gives an account of the “best rudist–dominated reefs …” and “a rudist reef belt” (p. 43) and therefore reconstructed rudist reefs with relief (p. 19,21,55). Fagerstrom (1987) states that rudists were “important reefbuilders from the Late Aptian … through the Maastrichtian.” (p. 314). Kauffman and Johnson (1988) give detailed account of “reef–building rudistids”. Of course none of these reported rudist associations had a relief equal to Holocene coral–algal reefs.

Growth Fabric of Rudists

The lack of binding and limited mutual attachment of shells, which would implicate sediment support, are suggested to be important fea tures of vertica lly growing rudists (Gili et al., 1995a, p. 257). But there are numerous examples proving the opposite. My collection alone yields dozens of examples, where the entire length of the shells of Vaccinites, Biradiolites, Torreites, and Durania are massively grown together. It is absolutely certain that the generative zone of the mantle tissue of adjoining individuals have secreted (and co–cemented) their shells. Figure 8 shows a massive cluster of 27 individuals firmly intergrown. This cluster (now in Darmstadt) is a minor part of hundreds of individuals in Saiwan that are grown together in this way. These associations are packed so densely that virtually no space remains for sediment.

Fig. 8.

—Vaccinites vesiculosus Cluster of 27 co-cemented individuals 3 kmE of locality 2 Collection Geological Institute Darmstadt x 0.2

Fig. 8.

—Vaccinites vesiculosus Cluster of 27 co-cemented individuals 3 kmE of locality 2 Collection Geological Institute Darmstadt x 0.2

Fig. 9.

Three co-cemented individuals of Torreites sanchezin milanovici Locality 1 Fig 1 Compare Figure 3 Collection Geological Institute Darmstadt x 0.3

Fig. 9.

Three co-cemented individuals of Torreites sanchezin milanovici Locality 1 Fig 1 Compare Figure 3 Collection Geological Institute Darmstadt x 0.3

Constratal Growth

The term “constratal growth” was proposed by Gili et al. (1995a) and is suggested to be valid in general. Perkins (1974) was probably the first to suspect the vertically growing bouquets to remain in life position through sediment support. According to my observations vertically growing bouquets or associations are normally very densely packed. There is very little space remaining. Under the assumption of constratal growth a major problem arises if the sedimentation rate is to be calculated. Compare the growth of a vertically growing hippuritid, in our case Vaccinites, to the rate of sedimentation. If a 40–50 cm high shell of Vaccinites grew in 20–30 yearsitwouldequalanaverage sedimenta tion rate of approximately 13 meters in about 1000 years under constratal growth. Even eight succeeding generations (several special places in the Saiwan area) including the undersized individuals building structures of 180 cm height in 200–300 years would still result in a sedimentation rate of 6 meters in 1000 years. Even lateral redeposition of sediment does not lead to an understandable solution. Constratal growth in the sense of Gili et al. (1995a) implies that the growth rate is equal to the rate of sedimentation. This, however, is highly improbable. It seems more likely to me that vertically growing bouquets and associations are characterized by supers tra tal growth. Only the basal cavities were filled by a mixture of bioclastic sediment, particles of biodegradation (if taking place), and mud of the background sedimentation. Because of their population density the associations are very resistant. For large areas of the Saiwan associations I cannot support the view of Skelton et al. (1995, p. 124) that “there is no evidence for any upstanding, self–supporting framework”. However, in spite of superstratal growth the Saiwan associations did not develop a significant relief.

Local horizontal concentrations of borings on the shells could be one feature indicating steps of the presumed constratal growth. But among thousands of individuals not one horizontal mark could be observed. The shells of Torreites are frequently heavily destroyed by clionid borings without showing horizontal marks of concentration, also pointing to superstratal existence during the entire life span.

Furthermore it is questionable why a screen filtering species as Vaccinites vesiculosus, being the most prominent species in Saiwan, could be so successful in areas with high sedimentation rates as required by constratal growth.

Shell Size, Gregariousness, and Water Energy

The Torreites sanchezi milanovici of Saiwan in the Vaccinites– dominated associations are conspicuously less robust than the ones in the Durania sp. associations (see Fig. 1A, horizon D, and Schumann, 1995). The absolutely thickest shells of Torreites sanchezi milanovici occur in the Durania association of the Simsima Formation (Fig. 5). According to my assessment all three associations belong to high–energy environments. In the Vaccinites– dominated associations the average energy was slightly lower. In the mid–Cretaceous and therefore probably also in the Late Cretaceous, the shelves and intracratonic lagoons of modem Central Oman were situated in an area of E–W prevailing winds. The same holds true for the mean annual surface currents (Barron 1990). Therefore one has to account for a average high energy in shallow environments of the Saiwan shelf, including the very large lagoons. Nonetheless, years or decades of lesser energy existed, and this statement involves no contradiction. In modern equatorial regions violent storms happen on average every 10 years and hurricanes every 20 years. Applied to the Late Cretaceous rudist environments of Saiwan this would give the Vaccinites associations 10 years time to grow relatively undisturbed before they were again subjected to a violent storm. This is consistent with many field observations (Platel et al., 1994; Schumann 1995, p. 195, Fig. 1). In contrast to Gili et al. (1995a, p. 262) the larger rudists in the Saiwan area with thick, strong shells were commonly not solitary forms or formed sparse or loosely structured communities. They formed very massive patches (Fig. 4, 7) or extensive square kilometer– size structures of great solidity (Fig. 5). Therefore I tend towards the opinion that the larger species with thicker shells were exposed to higher energy than the Vaccinites–dominated associations in which the large Durania occurs extremely rarely.

The densely packed rudist associations lack dingers. In comparison they are apparent in the coral–dominated horizons above and below VI and V2 (Fig. 1A) in most cases solitary, rarely in small co–cemented clusters.

In conclusion it should be mentioned that on average 1 m of sediment in the Saiwan sections represents 100,000 years and therefore has been subjected to about 10,000 violent storms. Although this is just a statistical number it explains many of the sedimentological structures of the Saiwan sections.

Conclusions

The Late Cretaceous rudist associations of Central Oman are among the best preserved in–situ associations in the world. They developed during cyclic transgression phases in very shallow and usually agitated water, on an exceptionally wide shelf. The very extensive monospecific Vaccinites associations probably developed in a very restrictive environment. Successive growth periods of the thickest associations continued for 200–300 years. An increased sediment supply stopped the growth period.

The general validity of “constratal growth” applied to successive, vertically growing rudist generations (Gili et al., 1995a) is doubted, because it would require improbably high sedimentation rates. In doubt as well is whether rudist associations in general were not able to build reefs. Hermatypic corals are extremely rare in rudist associations, although they can grow very successfully. There were no coral reefs in the Campanian sections of Saiwan. Colonial corals were very common below and above the Vaccinites–dominated associations, but they did not build coalescing structures. In the beds containing corals only flat–lying dingers occur, mostly radiolitids. Obviously a suitable substrate for vertically growing solitary corals and rudists was lacking. Sediment character and sediment structure point to water slightly deeper than in the rudist environments.

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,
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Acknowledgments

I would like to thank cordially K. and Dr. S. Engel for introducing me to the Saiwan area and for all their hospitality in Muscat, just as much W. Herget, his family, and his colleagues from Wintershall AG (Muscat) for their warm reception and important assistance. Just so I would like to thank Mr. K. Thomas and his colleagues from Conquest Oil Exploration Company (Muscat) for their extensive logistical help. For the most valuable discussions and instructions I thank Dr. J.–M. Pons (Barcelona) and Dr. S. Feist–Burkhardt (Darmstadt). Furthermore I would like to thank M. Walker and R. Borkhataria for their help in translation. Special thanks again to Dr. J.–M. Pons (Barcelona), Dr. P. W. Skelton (Milton Keynes) and Dr. J. Southard (Massachusetts Institute of Technology) for their time–consuming review and for the many valuable and critical comments on the first version of the manuscript. The responsibility for all remaining errors rests on myself. Last but not least I would like to thank my sons Bernhard and Ralf, R. and U. Bredner (Berlin), and J. Wittmann (Darmstadt) for their help and the most pleasant and cooperative ambiance during the field work. The investigation was generously supported by the Deutsche Forschungsgemeinschaft, Priority Program “Global and Regional Controls on Biogenic Sedimentation and Reef Evolution” (Grant Schu 410/10–1,2,3).

This publication is authorized by the Ministry of Petroleum and Minerals of the Sultanate of Oman.

Figures & Tables

Contents

GeoRef

References

References

Amico
,
S.
,
1978
,
Recherches sur la structure du test des Radiolitidae
:
Universite de Provence, Travaux Laboratoire de Geologie Historique et de Paleontologie
 ,
131
p.
Barron
,
E.J.
,
1990
,
Atlas of Cretaceous Climate Model Results
 :
Pennsylvania State University
,
32
p.
Dubreuilh
,
J.
,
Bechennec
,
F.
,
Berthlaux
,
A.
,
Le M£tour
,
J.
,
Platcl
,
J.P.
,
Roger
,
J.
,
Wyns
,
R.
,
1992
,
Geological Map of Oman at 1/250 000, A1 Khaluf, Sheet NF 40–15 and Explanatory Notes
:
Ministry of Petroleum and Minerals
 ,
Muscat, Oman.
Fagerstrom
,
J.A.
,
1987
,
The Evolution of Reef Communities
 :
New York
,
John Wiley & Sons
,
600
p.
Gili
,
E.
,
Masse
,
J.–P.
,
Skelton
,
P.W.
,
1995a
,
Rudists as gregarious sediment–dwellers, not reef–builders, on Cretaceous carbonate platforms
:
Palaeogeography, Palaeoclimatology, Palaeoecology
 , v.
118
, p.
245
267
.
Gili
,
E.
,
Skelton
,
P.W.
,
Vicens
,
E.
,
Obrador
,
A.
,
1995b
,
Corals to rudists—–an enviromentally induced assemblage succession
:
Palaeogeography, Palaeoclimatology, Palaeoecology
 , v.
119
, p.
127
136
.
Harris
,
P.M.
,
Frost
,
S.H.
,
Seiglie
,
G.A.
,
Schneidermann
,
N.
,
1984
,
Regional unconformities and depositional cycles, Cretaceous of the Arabian Peninsula
, in
Schlee
,
J.S.
,
ed.,
Interregional Unconformities and Hydrocarbon Accumulation
 :
American Association of Petroleum Geologists
, Memoir
36
, p.
67
80
.
Clarke
,
M.W. Hughes
,
1988
, S
tratigraphy and rock unit nomenclature in the oil–producing area of Interior Oman
:
Journal of Petroleum Geology
 , v.
11
, p.
5
60
.
Kauffman
,
E.G.
,
Johnson
,
C.C.
,
1988
,
The morphological and ecological evolution of Middle and Upper Cretaceous reef–building rudistids
:
Palaios
 , v.
3
, p.
194
216
.
Morris
,
N.J.
,
Skelton
,
P.W.
,
1995
,
Late Campanian–Maastrichtian rudists from the United Arab Emirates–Oman border region
:
Natural History Museum [London]
 ,
Geology Series
, v.
51
, no.
2
, p.
277
305
.
Murris
,
R.J.
,
1980
,
Middle East—stratigraphic evolution and oil habitat
:
American Association of Petroleum Geologists, Bulletin
 , v.
64
, p.
597
618
.
Perkins
,
B.F.
,
1974
,
Paleoecology of a rudist reef complex in the Comanche Cretaceous Glen Rose limestone of Central Texas
:
Geoscience and Man
 , v.
8
, p.
131
173
.
Philip
,
J.M.
Platel
,
J.–P.
,
1995
,
Stratigraphy and Rudist Biozonation of the Campanian and the Maastrichtian of Eastern Oman
:
Revista Mexicana de Ciencias Geologicas
 , v.
2
, p.
257
266
.
Platel
,
J.–P.
,
Philip
,
J.
,
Bourdillon de Grissac
,
C.
,
Babinot
,
J.–F.
,
Roger
,
J.
,
Mercadier
,
C.
,
1994
,
Modalites de la transgression campanienne sur le massif du Haushi–Huqf (Oman), Stratigraphie, contexte geodynamique et paleo–enviroments
:
Society Geologique de France, Bulletin
 , v.
165
, p.
147
161
.
Schumann
,
D.
,
1995
,
Upper Cretaceous rudist and stromatoporoid associations of Central Oman (Arabian Peninsula)
:
Facies
 , v.
32
, p.
189
202
.
Scott
,
R.W.
,
1990
,
Models and stratigraphy of mid–Cretaceous reef communities, Gulf of Mexico
:
SEPM, Concepts in Sedimentology and Paleontology
 , v.
2
,
102
p.
Skelton
,
P.W.
,
1979
,
Gregariousness and proto–cooperation in rudists (Bivalvia)
, in
Larwood
,
G.
,
Rosen
,
B.R.
,
eds.,
Biology and System– atics of Colonial Organisms
 :
London
,
Academic Press
, p.
257
279
.
Skelton
,
P.W.
,
Gili
,
E.
,
Vicpns
,
E.
,
Obrador
,
A.
,
1995
,
Thegrowth fabric of gregarious rudist elevators (hippuritids) in a Santonian carbonate platform in the southern Central Pyrenees
:
Palaeogeography, Palaeoclimatology, Palaeoecology
 , v.
119
, p.
107
126
.
Steuber
,
T.
,
1995
,
Stable isotope sclerochronology of Vaccinites cornuvaccinum (Hippuritidae) from Beotia (Greece)
:
Revista Mexicana de Ciencias Geologicas
 , v.
12
, p.
307
314
.
Steuber
,
T.
,
1996
,
Stable isotope sclerochronology of rudist bivalves: Growth rates and Late Cretaceous seasonality
:
Geology
 , v.
40
, p.
315
318
.
Steuber
,
T.
,
1997
,
Hippuritid rudist bivalves in siliciclastic settings— functional adaptions, growth rates and strategies
:
8th International Coral Reef Symposium, Panama, Proceedings
 , p.
1761
1766
.

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