Growth forms and palaeoenvironmental interpretation of stromatoporoids in a Middle Devonian reef, southern Morocco (west Sahara)
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- Königshof, P. & Kershaw, S. Facies (2006) 52: 299. doi:10.1007/s10347-005-0041-1
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Growth forms of well-preserved stromatoporoids, including genera Actinostroma, Stachyodes, and Stromatopora, are described for the first time from the Devonian Sabkhat Lafayrina reef complex of southern Morocco (west Sahara), one of the best exposed Middle-Devonian stromatoporoid-dominated fossil reefs. Three facies types representing the well illuminated fore-reef, reef-core and transition to back-reef facies display the distribution and growth of stromatoporoids in a high latitude setting at 40–50° south of the palaeoequator. Stromatoporoids are largely in growth position and reflect the well-preserved reef architecture. Although outcrops are low topography, the reef's prominent profile is indicated by presence of spur and groove form and a clearly defined reef margin. Stromatoporoids are mostly laminar and domical forms, with little evidence of ragged margins, and indicate normal turbulence shallow waters, with low sediment deposition.
KeywordsDevonianFacies analysisReef environmentsPalaeoecologyPalaeobiology
Stromatoporoids are regarded by most workers as calcified sponges (e.g., Stearn 1993), and several views of their affinity are reviewed by Kershaw (1998); certainly their skeletal structure is comparable with certain modern calcified sponges (sclerosponges or hypercalcified sponges). They were exclusively marine, and grew in shallow waters in conditions of reduced clastic supply, thereby generally considered to represent clear waters and tropical–subtropical coral–algal reefs. Therefore, stromatoporoid-built reefs are interpreted as having been constructed in similar types of environmental settings as modern reefs. Stromatoporoids, like modern corals, were sessile benthic organisms that recorded sea floor events that took place in their environment (e.g., Stearn 1984; James and Bourque 1984; Kershaw 1998). They may therefore be used to develop understanding of the palaeoenvironment. Stromatoporoids are abundant in Devonian reef systems primarily as epibenthic sponges, thus in palaeoecology remarkably different from physically similar modern sclerosponges that grow virtually exclusively in cryptic cave or shelter habitats (e.g., Berry et al. 2005). Devonian stromatoporoids grew in sunlight exposed, illuminated reef facies and they obviously grew more rapid than modern sclerosponge counterparts. The very slow growth rate of sclerosponges ranging from 100 to 300 μm/yr according to the species (e.g., Lazareth et al. 2000; Berry et al. 2005); in contrast, fossil stromatoporoids grew at rates of several millimetres per year.
It is also the aim of this report to show that the Sabkhat Lafayrina reef complex is an ideal test location for continuing studies in palaeobiology, palaeoecology and taphonomy of such reef builders. The study of mechanisms for initiating reef growth (e.g., Buddemeier and Hopley 1988; Hubbard 1988) is another important aspect within this context; bryozoans occur in some lower parts of this reef complex (Ernst et al. 2005; Scholz et al. 2005) giving a unique opportunity to apply them to reconstruction of ecological settings and reef development.
Material figured and mentioned in the text is housed in the collections of the Senckenberg Museum, Frankfurt am Main, Germany. This paper is a contribution to IGCP 499 (“Devonian land sea interaction: evolution of ecosytems and climate,” DEVEC).
Devonian rocks are exposed almost continuously for some 100-km along the southern part of the Tindouf Basin (west Sahara, southern Morocco) which is framed to the south by the Precambrian shield of North Africa, and to the north and west by the West African fold belt. Besides the dominating siliciclastics, reefal complexes of various sizes are developed in southern Morocco at the northern fringes of Gondwana. They prevailed during the Givetian (mid-Palaeozoic) maximum reef growth (Fagerstrom 1994; Copper 2002) though some of them may reach into the Early Frasnian (Königshof et al. 2004). Surrounding sedimentary rocks are characterised by Givetian sandstones, siltstones and marls. Dumestre and Illing (1967), dealing with some of the larger reef complexes in the north-eastern part of the Western Sahara, indicated three reef cycles within the Givetian and earliest Frasnian interrupted by marly sedimentation. The present study focuses on one of the western reef structures of Givetian age, southeast of the town of Smara. This complex at Sabkhat Lafayrina represents a large reef structure of several km squared and is dominated by stromatoporoids and corals. Because of the lack of tectonic deformation and diagenetic alteration, and sparse vegetation, the preservation and exposure of these reef structures is extraordinary. These initial studies during a field campaign in 2002 have shown that stromatoporoids can provide useful information regarding palaeoenvironment and facies analysis.
Sabkhat Lafayrina reef complex
Overlying sediments have been eroded and most of the present reef complex seems to be in its original position. It is possible to distinguish at least three lateral reef zones. The lateral transition from one reef zone to the other is visible in the field, due to their remarkably sharp morphological boundaries (Fig. 3). These morphological differences correspond to different palaeobathymetrical positions (parts 1–3 in Fig. 2) representing shallow subtidal to intertidal environments. Bathymetric data given by Playford (1981) for the Canning Basin, Australia, suggest that most platform-building stromatoporoids lived in water depth of less than 10 m. Palaeobathymetric data from Devonian stromatoporoid/coral-dominated reefs are also known from locations elsewhere (Collins and Lake 1989; Halim-Dihardja and Mountjoy 1989; Smith and Stearn 1989). Furthermore, palaeobathymetric differences in the Sabkhat Lafyrina reef are connected to different biofacies. Area 1 (Fig. 2) is characterised by massive stromatoporoids, attaining more than 90% by volume in most parts of the reef core. Stromatoporoids show diameters of several dm up to 3 m across (Fig. 4g), representing a stromatoporoid bound surface. In area 2 (Fig. 2) the microfacies is dominated by rudstones, and framestones, e.g., wavy-laminar stromatoporoids. Most of the matrix is lime-mud but also present is coarse-grained matrix derived from tabular or domical stromatoporoids (Fig. 4e and f) and coral fragments. The matrix contains some gastropods, brachiopods, and widely scattered solitary rugose corals. Within area 3 (Fig. 2), which is the deepest part, grainstones and packstones contain irregularly shaped fragments of stromatoporoids, corals, and numerous shells (Fig. 5e). The matrix consists of variable portions of micrite, microspar, and pseudospar. A cross-section of the Sabkhat Lafayrina reef complex summarising the main palaeoecological features is presented in Fig. 6.
The succession of sedimentological units below shows mainly shallowing-upward trends. Such cycles are common in Devonian shallow-water carbonates (e.g., Brett and Baird 1996; Elrick 1996; Chen et al. 2001) and have been described also in Middle Devonian stromatoporoid and algae dominated reef limestones in Canada (e.g., Collins and Lake 1989). The summarised vertical section (Fig. 3) starts with oolitic limestones (Fig. 4a and b) overlying siliciclastic siltstones and sandstones. These are characterised by various trace fossils, reflecting strong bioturbation that show cross bedding and wave ripples suggesting a very shallow-water environment. Capping this sequence there is a remarkable change from sandstones to oolitic carbonates at the base, with reworked brachiopod material to ooids at the top. The ooids are covered by crinoidal limestones, mainly grainstones and rudstones with branching tabulate corals and, dendroid stromatoporoids (e.g., Amphipora, Stachyodes) and brachiopod shells.
The top of unit A of the section marks the first occurrence of solitary corals such as “Mesophyllum” (Fig. 4d). The overlying sequence mainly comprises corals. Flat-growing Phillipsastrea colonies and encrusting chaetetid sponges are common, and laminar stromatoporoids, bryozoans, and algae are locally present. This part of the sequence is covered by crinoidal limestones. Branching tabulate corals, dendroid stromatoporoids, and brachiopods occur, but less frequently. The top of unit B is characterised again by the occurrence of solitary corals, e.g., “Mesophyllum”. This layer is comparable to the layer in the lower part of the section (see Fig. 3, unit A). Above that horizon, carbonates of the lowermost part of unit C contain abundant Phillipsastrea colonies and encrusting chaetetids which suggest similar palaeoenvironmental conditions described in the lower portion of unit B (Fig. 3). This sequence is capped by stromatoporoid-dominated rudstones and floatstones that are locally associated with tabulate corals (Alveolites) and dendroid stromatoporoids. The large stromatoporoids generally show “high domical” growth forms and most of them are in life position. Some others are broken and may be encrusted by other stromatoporoids, tabulate corals and/or calcareous algae or cyanobacteria. Crinoids and bryozoans are present, but less frequent. The first massive stromatoporoid limestone has a thickness of more than 3 m and from the base to the top the abundance of stromatoporoids increases. Based on microfacies data this part of the reef obviously developed in moderate to strong wave energy, episodically reworked by storms. This sequence is covered by detrital crinoidal limestones, again representing sedimentological and palaeoenvironmental change. The occurrence of solitary corals at the top of unit C (Fig. 3), with some still in growth position, seems to correspond to “the Rübenriff” facies described by Struve (1961) in the Eifel region of Germany and also in the Rheinisches Schiefergebirge (e.g., Birenheide 1990). These corals can reach up to 20 cm in height (Fig. 4d).
The basal part of the overlying carbonates is very similar to those, which have been described for the lower part of unit C. This sequence is covered by massive stromatoporoid limestones, mainly rudstones to floatstones. The massive morphology of the main reef-builders indicates medium-strength water turbulence (e.g., Machel and Hunter 1994) or rapid growth. The reef building organisms, even big blocks of several decimeters thickness, are broken, but not rounded, suggesting a limited degree of transport. This feature can be seen also at the palaeoslope of this reef structure (Fig. 6b) where stromatoporoids grew on blocks which have been transported downslope (Fig. 4c). The size and growth forms of stromatoporoids are similar to those that have been described in the Lahn Syncline (Rheinisches Schiefergebirge, Germany) by Königshof et al. (1991). Middle to Late Givetian reefs in the southern part of the Rheinisches Schiefergebirge and in the Harz Mountains grew mainly on volcanic sediments, less influenced by siliciclastics (see for instance May 1987; Gischler 1992; Buggisch and Flügel 1992; Braun et al. 1994; Gischler 1995). The percentage of stromatoporoids increases rapidly towards to the upper part of unit D. The uppermost part of the Sabkhat Lafayrina reef complex is built up by a flat reef top formed by giant stromatoporoids up to 3 m across (Fig. 4g). This part represents the last stage in reef development preserved.
Stromatoporoid growth froms of the Sabkhat Lafayrina reef complex
Individual stromatoporoids show well-defined growth banding (Fig. 5a,c). Young and Kershaw (2005) discovered that in most studied examples of stromatoporoids with growth bands, the bands correlated with sediment interdigitation at the stromatoporoid margins (ragged margins of Kershaw and Riding 1978). The Morocco material has not yet been studied in sufficient detail to determine the nature of the banding but may reveal a repeated growth control. Nevertheless, the well-developed detail of growth banding in a large number of samples is observed in the field (Fig. 5a, c, and g).
Application of stromatoporoid palaeobiology to facies analysis in the Sabkhat Lafayrina reef complex
The facies setting described earlier, indicates that the reef grew in very shallow water. The reef developed on bedded bioclastic sediments that may have provided a stable base for growth (Fig. 4a). It seems likely that bryozoans also played an important role in initial reef growth (Ernst et al. 2005; Scholz et al. 2005). The existence of crinoidal limestones and solitary corals, which occur several times in distinct layers (Fig. 3), may represent short pulses of a rising sea level in the late Middle Devonian. In the Middle Devonian when reefs expanded globally, especially at higher latitudes, climate conditions especially those in the Givetian of Morocco—characterised by high seasonal temperature flux—have been obviously the primary factor for the existence of the reef complexes in higher latitudes (Copper 2002).
The rare occurrence of ragged and mamelon morphotypes of stromatoporoids is a possible indication of reduced sedimentation rate throughout the history of the reef complex, since the combined effect of stable substrate and low sedimentation rates were other controls in the development of the reef in this area. The well-preserved nature of whole stromatoporoids suggests that taphonomic processes did not much affect the assemblage, and generally reflecting medium-strength energy levels. The presence of probable spur and groove structures suggest reef growth of the Shabkhat Lafayrina complex in wave-dominated hydrodynamic regimes.
Targets for further work
The next stage of this work is to carry out more detailed field survey and better constrain the reef facies and associated facies in relation to stromatoporoid and coral morphology distribution. Thus the stromatoporoids and corals will be used to their full potential in facies analysis.
Quantification of growth forms and distribution in relation to reef facies, for example along energy transects across the reef system.
Measurement of growth bands in stromatoporoids and corals and their palaeobiological and palaoecological significance; here the new banding classification by Young and Kershaw (2005) can be applied to provide detailed quantification of banding features.
Facies analysis and facies zonations in shallow marine carbonates from below normal wave base to supratidal facies.
We thank Paul Copper and Rachel Wood for many constructive comments in the manuscript. Fieldwork has been funded by the Paul Ungerer Stiftung. This work has been done in the framework of the IGCP project 499.