Factors controlling the growth of stromatoporoid biostromes in the Ludlow of Gotland, Sweden

SEDIMENTARY GEOLOGY ELSEVIER

Sedimentary Geology89 (1994) 325-335

Factors controlling the growth of stromatoporoid biostromes in the Ludlow of Gotland, Sweden Stephen Kershaw

*, M i c h a e l K e e l i n g

Palaeobiology Research Unit, West London Institute, Borough Road, Is&worth, Middlesex, TW7 5DU, UK

(Received June 22, 1993;revised version accepted July 12, 19930

Abstract Stromatoporoid biostromes and stromatoporoid-algal-coral bioherms in the Silurian of Gotland display differing gross morphology and internal construction which suggests a difference in factors controlling their formation and development. Lower Wenlock bioherms of the Visby and H6gklint Formations, biotically zoned, grew during a relative sea level fall, on substrates of limited lateral extent, and sediment accumulated around and on the reefs as they grew. In contrast, the growth and morphology of middle Ludlow Hemse Group biostromes was determined by low levels of clastic supply, laterally extensive suitable substrate, relatively stable sea level and a largely flat sea bed in shallow water conditions. These are interpreted to result in dense accumulation of skeletal biota in biostrome form. In detail, the biostromes contain low- and high-profile stromatoporoid growth forms and exhibit differences from upper Ludlow reef stromatoporoids, which contain the same stromatoporoid species suite but as low-profile forms only. Controls on stromatoporoid growth form differences are unclear but may include substrate consistency, environmental energy and closer adaptation to environment by the same species between middle and upper Ludlow times. The abundance of biostromes on Gotland may be due to climate; drier episodes result in lower clastic supply, and if sea level was also stable, then biostromes could result. Hemse Group biostromes in one locality occur during dry phases of a two-state dry/wet model proposed for the Silurian, and demonstrate the potential of global controls on these reefs.

1. Introduction Stromatoporoid biostromes are common in the middle Ludlow of Gotland, Sweden, and are striking for their densely packed reef builders. Here they are regarded as reefs because of the abundance of in-place reef-building skeletal biotas, and because in some places they grade vertically into bioherms. These deposits emphasise the

* Corresponding author.

ability of stromatoporoids to form reefs. However, the factors which allowed their unusual abundance have not been clearly defined, and such study is necessary to understand how these reefs developed, which in turn may assist interpretation of stromatoporoid palaeobiology. As far as is possible to determine from the poor inland outcrops, bioherms and biostromes are broadly stratigraphically separated on Gotland. Major bioherm phases occur in the lower H6gklint Fm. (units a and b of Laufeld, 1974b), Klinteberg Gp. and Hamra Fm., while biostromes

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S. Kershaw, M. Keeling ,/Sedimentary Geology 89 (1994) 325-335

are present in the upper H6gklint Fm. (units b and c), Tofta Fm., Slite Gp. (especially unit e), middle Klinteberg Gp. and Hemse Gp. (especially unit c), and possibly the Sundre Fm., where reefs are less well exposed. Our purpose here is to identify the main controls on growth of stromatoporoid biostromes in the middle Ludlow Hemse Group on Gotland, in an attempt to de-

velop a model for their formation and growth. Such a model may also be applicable to biostromes elsewhere. We examine the likely factors in relation to a single stromatoporoid biostrome, where the fauna is known, at Kuppen (Kershaw, 1981, 1990), and we compare this firstly with a reef at Holmh/illar in the upper Ludlow Sundre Fm., which has a similar stromatoporoid

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S. Kershaw, M. Keeling/Sedimentary Geology 89 (1994) 325-335

assemblage (Mori, 1970; Kano, 1989), and secondly with biohermal patch reefs and biostromes of the lower Wenlock Hfgklint and Tofta Fms. (Riding and Watts, 1991). Such an approach enhances understanding of the Hemse Gp. biostromes. Current Gotland stratigraphy is given by Hede (1960) and Bassett and Cocks (1974), and locality details by Laufeld (1974a, b; see Fig. 1). This paper provides source data used by Kershaw (1993) in a broader discussion of the control of reef growth on Gotland.

2. Geological setting Gotland geology has been described by Bassett (1985), Laufeld and Bassett (1981) and others, as

Kuppen 5 \ ~

327

a carbonate platform on which transgressive-regressive wedges of broadly stratigraphically alternating limestone and shale are recognised (Riding, 1981). A ramp setting has been envisaged (Frykman, 1989), and southerly deepening of the Silurian sea in the Baltic area is generally recognised (Martinsson, 1967). Reefs developed extensively in the shallower parts of these wedges.

3. Factors controlling biostrome growth Given appropriate water temperature and nutrient supply, and that light intensity had no proven effect on stromatoporoids, major controls on stromatoporoid-dominated biostrome growth and biota may be expected to include the follow-

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Fig. 2. (A) Location of the Kuppen facies complex in coastal outcrop. (B) Stylised vertical section of the Kuppen complex, showing interbedded stromatoporoid biostromes, bioclastic limestones and shales. Coarse bioclastic facies occur at the south end with Silurian cliff feature. Note 10 x vertical exaggeration. (C). Typical view of vertical section of the lower biostrome, showing two basic stromatoporoid shapes: low-profile sheets and domes, and high-profile domes and columns; growth forms are related closely to species. Drawn from photographs; all fossils in this view are stromatoporoids. (D) Typical view of vertical section of the Holmh~illar reef for comparison with (C). Note the consistent low-profile forms of stromatoporoids, set in a coarse crinoidal limestone, compared to the finer sediments in (C).

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S. Kershaw, M. Keeling/Sedimentary Geology 89 (1994) 325-335

ing: (a) clastic sedimentation rate; (b) water turbulence and reef strength; (c) water depth and sea level stability; (d) reef growth rate; (e) substrate suitability (including composition and areal extent, topography and carrying capacity of the unconsolidated sediment blanket); (D water salinity. These features are examined in relation to the Hemse Group biostromes. A low clastic sedimentation rate can be proved for biostromes (densely packed stromatoporoids which lack sediment interruptions), but other parameters are less easy to assess qualitatively.

4. Biostromes at Kuppen

4.1. Description Good exposures at Kuppen continue for almost 1 km horizontally, and biostromes vary from 0.5 to ~ 5 m thick (Fig. 2); see Kershaw (1981, text-figs. 2 and 3, and 1990, text-fig. 3) and Riding (1981, figs. 37 and 38), for further illustrations. Outcrops of stacked biostromes and interbedded skeletal limestones and muds in eastern Gotland are apparently replaced to the southwest by marls, suggesting deepening water in that direction (although unconfirmed at a detailed level; L. Jeppsson, pers. commun., 1993). Biostrome upper surfaces are commonly eroded, where stromatoporoids and reef sediment are eroded alike; biostromes lithified prior to erosion. Along most of the length of the biostromes, the basal 0.5 m is normally a crinoidal grainstone, containing whole (sometimes displaced from growth attitude) and damaged stromatoporoids, and overlies muds. The grainstone appears to be a thin sheet which provided a base for prolific stromatoporoid growth. A detailed treatment of the stromatoporoid suite of one biostrome (Kershaw, 1990) is summarised here. The fauna is dominated by a single stromatoporoid species, Clathrodictyon mohicanum Nestor, present as a low-profile form, and representing ~ 40% of the number of undamaged stromatoporoids; in many places, this species

exceeded 1 m in horizontal dimension. This, and other low-profile stromatoporoids, display smooth margins, and only rarely developed ragged margins. Ragged margins formed by episodic sedimentation on stromatoporoid flanks, and the stromatoporoids at Kuppen therefore prove a low sedimentation rate throughout development of the biostrome. Other stromatoporoids, Plectostroma scaniense Mori, Parallelostroma typicum (Rosen), and Stromatopora bekkeri Nestor, developed tall profile forms. The tall forms were also not ragged and were mostly found lying on their sides, and, in many cases, damaged. In some places, symbiotic rugose and syringoporid tabulate corals are abundantly present within stromatoporoids, particularly tall forms (Kershaw, 1987a). The biostrome contains little clastic sediment, and contains mostly fine-grained carbonate as micritic geopetal fills, with sporadic coarse debris of bryozoans, brachiopods and crinoids. Dense packing of constituents is enhanced by pressure solution. Biostromes throughout eastern Gotland presently have nearly horizontal attitudes, and because the geological dip of Gotland is slight (2°), they apparently grew on an almost level sea bed. The undulating character of the sediment layers (Fig. 2B) is probably due to differential compaction. There is little evidence of a cemented framework in the biostromes at Kuppen; disruption of the fossils is common, presumably by storm action. The majority of low-profile stromatoporoids, however, are in growth attitude, and are unlikely to have been moved because they are commonly of large size. Penecontemporaneous Silurian cliffs and stacks, covered by reef debris and crinoidal gravel, occur at the south end of the outcrop in the lower biostrome and the overlying biostrome and sediments. Also, biostromes thin towards the northwest, with intercalated fine limestones (Fig. 2). There is no vertical change in the stromatoporoid assemblage of the lower biostrome at Kuppen, but a weak horizontal variation has been recognised (Kershaw, 1990), whereby large lowprofile stromatoporoids are abundant in the southern half.

S. Kershaw, M. Keeling/Sedimentary Geology 89 (1994) 325-335

4.2. Interpretation We interpret the lower biostrome as follows. Large stromatoporoids in a low-diversity assemblage throughout the biostrome's thickness sug-

SPECIES

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gest rapid reef growth. Low-profile growth forms, rarely ragged, indicate a reduced clastic sedimentation rate (Kershaw, 1981); sediment is low in siliciclastic content. The small vertical variations in facies and fossil content suggest conditions

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Fig. 3. Species and growth forms of the major stromatoporoids from Kuppen and Holmh~illar. Note the dominance of Clathrodictyon mohicanum at Kuppen, while Parallelostroma typicum and Plectostroma scaniense dominate at Holmhiillar, with different morphotypes.

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hardly changed as the reef grew, and its considerable horizontal extent (nearly 1 km) suggests persistence of a suitable substrate of crinoidal gravel. That the crinoidal sheet was unconsolidated when the fauna colonised is indicated by poorly developed early cement, which predates abundant meteoric phreatic cement seen under cathodoluminescence (Kershaw, 1990). High abundance of low-profile stromatoporoids in the southern part of the outcrop and presence of rocky shores there and not elsewhere suggests that part faced the open sea. A stable sea level is suggested during the geologically short period of its growth, and prolific reef growth with common occurrence of reef rubble suggests shallow water. The stacked nature of biostromes at Kuppen, with at least three erosion surfaces recognised in successive biostromes, indicates episodic sea level fluctuation relative to sea bed. Erosion surfaces pass laterally into contemporaneous cliffs and indicate emersion. The erosive tops of some biostromes suggest shallowing-up cycles, also known to affect stromatoporoid reefs in the Lower Devonian of New York (Busch, 1983) and early Silurian platform seas in the USA (Johnson, 1987). However, confirmation of whether the Kuppen biostromes are actually shallowing-up features is not available on present evidence. The sea bed was probably nearly horizontal. Because of the lack of framework, the physical strength of the biostromes to resist high energy is not considered to have been well developed, and severe disruption of biostrome biota in places supports this conclusion. However, the dominant low-profile stromatoporoids were ideally suited to turbulent environments, and Riding (1981) interprets high-energy conditions for biostrome growth. Nevertheless, there may be other reasons for low-profile growth, such as competition for space, and we advise caution in relating form to energy unequivocally. Many tall stromatoporoids in the biostrome had vertical height/basal diameter ratios in excess of 2; these are nearly always toppled (Fig. 2), and were clearly not adapted to high-energy conditions. Kershaw (1990) suggested that energy during growth of the lower biostrome was generally low while it grew, punctuated by storms. In general, this interpretation is also appropri-

ate for other biostromes at Kuppen and elsewhere in the Hemse Group, which rest either on previous biostromes, or skeletal debris.

5. Reef at Holmhfillar

5.1. Description This reef (containing densely packed stromatoporoids) is well described by Manten (1971), Riding (1981) and Kano (1989). A typical view of stromatoporoid frame is included in Fig. 2D. A thin outcrop (maximum 8 m thick), the reef is traversed by an irregular erosion surface separating an upper stromatoporoid reef growth phase from a lower one, both with similar stromatoporoid suites (Kano, 1989). Kano confirmed Mori's (1970) discovery of a low-diversity stromatoporoid assemblage, with dominance by two species. Comparison with Kuppen is pertinent to an interpretation of the controls on the Hemse Gp. biostromes because of the similar stromatoporoid assemblage but in Holmh~llar, Clathrodictyon mohicanum plays only a minor part, while Parallelostroma typicum and Plectostroma scaniense dominate (Mori, 1970; Kano, 1989; see Fig. 3). Kuppen and Holmh~iltar are the only reef sites on Gotland where stromatoporoid morphology and species have been comprehensively studied. The Holmh~illar reef biota displays a strong contrast to Kuppen, because nearly all stromatoporoids at Holmhfillar are low profile (Mori, 1970; Kano, 1989). Plectostroma scaniense and Parallelostroma typicum developed a low-profile form at Holmh~illar, while they grew tall forms at Kuppen (Fig. 3; Kershaw, 1981). The low-profile stromatoporoids here are more ragged than the Kuppen equivalents, indicating episodic sedimentation, but since they are very abundant ( ~ 60% of reef volume, Kano, 1989), the overall clastic sedimentation rate must also have been low for this environment. Sediments within the reef, and on the erosion surface between the upper and lower units, are commonly coarse-grained crinoidal limestones, contrasting the finer-grained sediments in the lower biostrome at Kuppen. How-

S. Kershaw, M. Keeling/Sedimentary Geology 89 (1994) 325-335

they grew, forming a subsediment frame, a concept described by Mayall (1979) for Devonian reefs in England. This frame would have given the stromatoporoids good stability, enhanced if they were also cemented to their substrates. One uncommon species at Kuppen and Holmh~illar, Lophiostroma schmidti (Nicholson), often encrusted other fossils, but the behaviour of the main species at both localities, in this respect, is unknown. Kano (1989, p. 313) interpreted the hard substrate provided by growth of earlier skeletons to have promoted lateral growth of stromatoporoids in the Holmh~illar reef; however, Kano also offered an alternative interpretation that ragged low-profile forms could have grown over soft substrate following partial burial by episodic sedimentation events. Because it is also possible that low-profile growth reflects a high-energy environment, growth of stromatoporoids with this shape can be attributed to more than one control, as noted for Kuppen, above. These points are important, because interpretation of the controls on stromatoporoid growth are pertinent to understanding the reef environments. The Holmh~illar reef formed in shallow water, but there is no evidence of frequent exposure; however, the reef has been interrupted by uplift to generate one prominent irregular erosion surface, subsequently resubmerged for further growth.

ever, Kano (1989, p. 302) reports unbroken branching rugosans and bryozoa in depressions of the erosion surface, suggesting that they may have lived there, or were transported unbroken for a short distance. Fragments of these fossil groups, together with fragmented trilobites, ostracods and crinoids were found in the lower biostrome at Kuppen, as were occasional encrusting bryozoa. Cephalopods, brachiopods, ostracods and trilobites also occur in reef hollows at Holmh~illar. Algae exist in the reef, but are missing from Kuppen.

5.2. Interpretation While being a highlight of Gotland reef geology (Manten, 1971), Holmh~illar is a difficult reef to interpret because its base and lateral margins are not exposed, and the top is eroded and missing. Thus confirmation of whether or not it is a biostrome cannot be given. The isolated nature of the outcrop means that lateral facies relationships are unknown. Riding (1981) interpreted a high-energy shallow environment for the Holmh~illar reef, suitable for low-profile stromatoporoids. An interpretation of high energy is more acceptable for Holmh~illar than for Kuppen, because of lack of tall stromatoporoids. Also, sediment accumulation on these would have partially buried them as

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Fig. 4. Schematic vertical section of a typical H6gklint bioherm, with expansion to a biostrome, and overlying Tofta biostrome. Reduced sedimentation in shallow waters allowed bioherms to expand laterally into biostromes, and provided a base for Tofta biostromes. (Frohi Riding and Watts, 1991.)

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Overall, the Kuppen and Holmh~illar reefs had comparable controlling parameters, both suited to prolific stromatoporoid growth. Differences between them largely revolve around the change of dominant stromatoporoid species and their morphology. However, in neither case has the nature of the environment been identified completely, partly because of the isolated nature of the exposures.

6. H6gidint biohermal patch reefs and biostromes Watts (1988) and Riding and Watts (1991) detailed the facies of lower Wenlock H6gklint reefs in northwest Gotland. Referred to as the Hoburgen reef type by Manten (1971) and Riding (1981), these are typified by vertically extensive patch reefs (Fig. 4). They developed in a regressive regime (Riding, 1981), from the relatively deeper water Visby Fm. to shallow water H6gklint Fm. beds, where emersion is evident. Bioherms were rooted in underlying crinoidal gravels, and bioherms did not form in areas where those are missing, emphasising the importance of substrate for reef growth (R. Riding, pers. commun., 1993). Watts (1988) proved a vertical zonation in patch reef biota from tabulates and halysitids in the lower part, laminar stromatoporoid community in the centre, followed by a domical stromatoporoid community near the top, capped by algaldominated biota; this he related to the shallowing-upwards sequence. Relevant to understanding controls on growth of the biostrome at Kuppen, Riding and Watts have shown that, in vertical section, these reefs expand upwards, and their upper parts developed into stromatoporoid-algal-cyanobacterial biostromes. Biostromes also developed in the overlying Tofta Fm., where erosional unconformities separate some of the units, with some biostromes resting on the stable base of eroded lithified H6gklint reef tops. The biota of these biostromes have not been studied in as much detail as at Kuppen or Holmh~illar, but contain abundant stromatoporoids, bryozoans, cyanobacteria and algae. Notably, the stromatoporoids commonly have a ragged wavy laminar shape (Riding and Watts, 1991).

7. Discussion The relatively thick exposed sequence in the H6gklint Fm. (~ 35 m) provides a useful perspective for examining biostrome growth controls. The upward-expanding form of bioherms indicates a reduced sedimentation rate in the upper parts which eventually allowed lateral expansion of reef facies into biostromal structures (Riding and Watts, 1991). Sediment accumulation around the patch reefs proceeded as they grew, so that a low profile was present at any one time (maximum relief 15 m, R. Riding, pers. commun., 1993). Lessening of sedimentation rate was a major control on reef shape. Upper H6gklint, Tofta, Kuppen and Holmh/illar reefs are interpreted as growing in low sedimentation regimes, and are shallow water features, but with no certainty as to absolute water depth. However, since the Wenlock sequence up to the top H6gklint is largely complete, then on a Waltherian reconstruction, the biostrome phase may have existed in shallow water, at the same time as the bioherm phase developed further offshore in deeper water (Riding, 1981, p. 71). In the case of Kuppen and Holmh/illar, bioherms may exist in the (interpreted) deeper water facies to the southwest, but there is no convincing evidence of this in the poor vertical exposures of inland Gotland, where little thickness of rock is seen. It is possible instead, that Kuppen and Holmh~illar reefs grew in times of low sedimentation rate when bioherms did not form. We view substrate as a crucial feature; H6gklint bioherms did not develop in areas lacking crinoid debris, and the Kuppen biostrome is floored by a crinoidal grainstone sheet. However, the most important aspect is not accounted for: the characteristics of the environment which allowed such prolific, low-diversity, stromatoporoid growth in the Kuppen and Holmh~illar reefs. Kershaw (1981) interpreted the strong competition known amongst modern organisms in good conditions which lead to abundant low-diversity growth, as an analogue for the Kuppen reefs. Holmh~illar particularly fits this model with low-profile growth forms and very low diversity of stromatoporoids; six species (Kano, 1989), compared with sixteen

s. Kershaw, M. Keeling/Sedimentary Geology 89 (1994) 325-335

species at Kuppen (Kershaw, 1990). At Kuppen, however, the model fits less well because of the lower-energy situation described above, the tall forms susceptible to damage, and the lack of obvious frame. Kuppen is older than Holmh/illar, and there is an alternative interesting possibility that the species which grew tall forms at Kuppen, evolved into lower-profile forms by upper Ludlow time. If the Kuppen reef grew in predominantly quieter water as we suggest, what other factors could generate such profilic growth? Because reef growth is well developed, adequate aeration can be expected, and we consider two possibilities: a sea water composition stress such as salinity variation, and extremely low clastic sedimentation rate in a low-nutrient setting. 7.1. Salinity

For Kuppen, the accessory fauna of bryozoa, brachiopods and crinoids are typically regarded as fully marine fauna. However, apart from a few encrusting bryozoa, these fossils are present as debris, and could have been imported into the environment from a fully marine situation. Rugosans and syringoporids grew within stromatoporoids as part of the fauna, and heliolitids are occasionally found, as complete specimens. Strangely, algae are absent, and there is a notable paucity of bioerosion and cryptic biota. Replacive gypsum (Kershaw, 1987b), collected from sediment in the upper part of the lower biostrome at Kuppen, is evidence of hypersalinity, but could have formed at any time after lithification and is not necessarily related to the environment of Kuppen. Had this displaced sediment laminae, indicating crystal growth prior to lithification, a case for salinity stress could be made with greater confidence. The lack of algal mats and other features of abnormal salinity also makes this possibility unlikely. For Holmh/illar, the dominant low-diversity stromatoporoid fauna is complemented by several normal marine fossils, but algal mats are present, interpreted by Kano (1989) as intertidal to supratidal. Evaporites have not been reported and algal mats are found on the erosion surfaces in the reef, not amongst the reef builders (Kano,

333

1989, p. 301). Thus for neither locality has reef growth been demonstrated to occur in conditions of abnormal salinity. 7.2. Sedimentation rate

We believe that low sedimentation rate was a most important factor in generating biostromes, and since there is evidence of stratigraphic separation of bioherms and biostromes, consideration of the mechanisms which caused variations in sedimentation is appropriate. The regression from Visby to H6gklint Fms. generated shallow water and suitable substrate for reefs to grow, but sedimentation also lessened. This could be attributed to increasing turbulence which holds fines in suspension in shallow water. Clearly, clastic supply to the Gotland area was low to allow reefs to grow, and a combined effect of shallowing and reduction in the settlement of sediment can explain the development of the Wenlock bioherms and biostromes. At Kuppen, there is no vertical zonation in biostrome biota. When compared with the Tofta and Holmh~illar reefs, ragged stromatoporoids at Kuppen are rare, and this indicates a consistent very low sedimentation rate. With a generally low environmental energy, turbulence could not have prevented sediment settling, and so a very low clastic supply is likely. However, although lack of sedimentation can help understand why the biostromes are so fossiliferous, it does not explain the low faunal diversity, and particularly why there should be a poorly developed accessory fauna, and no algae. It can be presumed either that conditions were unsuitable for such biota, or alternatively, the stromatoporoids somehow excluded other organisms. Although modern sponges grow slowly, the large size of stromatoporoids in Palaeozoic reefs strongly suggests they could have grown faster, perhaps comparable with modern reef-building corals (Wood et al., 1992, p. 152), and outcompeted other organisms. A low nutrient supply may have played a part in prolific reef growth, since nutrients are depleted in circumstances where reefs grew well (see Jeppsson, 1990). A new perspective on sedimentation rate

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S. Kershaw, M. Keeling/Sedimentary Geology 89 (1994) 325-335

comes from work on Silurian conodonts. Jeppsson (1983) showed that calcarenites above the eroded top of the lower biostrome at locality Kuppen 1 (Laufeld, 1974a) has a conodont fauna consistent with the Polygnathoides siluricus Zone, and during that zone, worldwide ocean oxygenation and sedimentation patterns produced particular conditions (Jeppsson, 1987, 1989, 1990). In that interval, reduction in carbonate deposition was recorded, with shale as the dominant lithology, deposited in relatively oxygenated deep shelf and oceanic sediments. In this location at Kuppen 1, the P. siluricus Zone fauna occurs through an interval only 0.11 m thick (Jeppsson, 1983, p. 142). Detailed mapping by one of us (MK), which will be presented later, demonstrates that sediments reported by Jeppsson (1983) as being in the P. siluricus Zone, were deposited in the interval between two successive biostromes. Jeppsson (1990) referred to the oceanic conditions generated during this zone as a P episode, one half of a two-state model. The other type of condition, the S episode, then seems to coincide with the occurrence of stromatoporoid biostromes at Kuppen. In S episodes, drier climates discourage weathering and erosion, so that clastic supply to oceans is reduced, and may therefore have encouraged lateral growth of reefs to produce biostromes with dense faunas, barely affected by sedimentation. This interpretation could apply to Kuppen and the other biostromes which are widespread in this area of the Hemse Group, but as yet there is no conodont evidence that the Holmh~illar reef and H 6 g k l i n t / T o f t a biostromes grew in S episodes in the Jeppsson model. Thus the differences between these three reef settings could reflect control processes originating outside the Gotland area.

8. Conclusions A middle Ludlow biostrome at Kuppen grew in conditions of low clastic sediment supply, in conditions of probable normal salinity, on level sea bed with suitable, extensive, substrate. Shallow water environments are indicated, and water turbulence was generally low, punctuated by

storms. This model for biostrome growth may be applicable to other stromatoporoid biostromes, and emphasises the contrasts in controls of growth of bioherms and biostromes. The differences between growth forms of the same stromatoporoid species at Kuppen and Holmh~illar reflect profound variations in response; potential controls include environmental energy, substrate control and better adaptation to environment. The formation of biostromes may be controlled ultimately by oceanic processes on a global scale.

9. Acknowledgements We are grateful to Robert Riding (Cardiff) and Lennart Jeppsson (Lund) for discussions of Gotland reefs, and for critically reviewing an earlier draft. Peter Gutteridge, Bruce Sellwood and an anonymous reviewer provided many helpful comments. Funds provided by the West London Institute, the Geologists' Association, Robertson Group and The Geological Society of London Timothy Jefferson Fund are gratefully acknowledged, as is the provision of the Allekvia field station on Gotland, currently administered from Lund University.

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