Comparative palaeoecology of marly coralline sponge-bearing reefal associations from the Kimmeridgian (Upper Jurassic) of Portugal and Southwestern Germany.-
Proceedings, Part 2, VI Intern. Fossil Cnidaria Congress, Münster 1991, Cour. Forsch.-Inst. Senckenberg, 172, 381-397, Frankfurt a.M.
Keywords
Palaeoecology, reef associations, corals, calcareous sponges, siliceous sponges, bivalves, microbial crusts, bathymetry, Upper Jurassic, Portugal, Southern Germany
Three marly reefal associations from the Kimmeridgian of Portugal (Alcobaça, Serra Isabel) and southwestern Germany (Faulenhau), which share a high proportion of coralline sponges as a main unifying element, were studied in a comparative manner. The two Portuguese associations are dominated by corals and coralline sponges (Alcobaça), and by crinoids and corals (Serra Isabel), respectively. Both sedimentological and palaeoecological analysis evidence a very shallow environment at Alcobaça and a deeper slope setting at Serra Isabel. The association at Faulenhau, occurring within the sponge mound facies of Southern Germany, is dominated by hexactinellid and lithistid siliceous sponges, but contains an unusually high proportion of coralline sponges.
Comparison of the three associations allows the establishment of bathymetric gradients and enables interpretation of the somehow enigmatic association at Faulenhau. All three associations grew in a fully marine, low-energy, low- sedimentation environment. The compositional differences are due to increasing bathymetry (presumably Alcobaça less than 20 m, Serra Isabel 50-60 m, Faulenhau 70-90 m). This is well reflected only at the generic and species level, where several taxa from various faunal groups (coralline sponges, bivalves, cyanobacteria and microproblematica) are stenobath, whereas many others are not. The main trends towards deeper water are decrease in the diversity of corals, coralline sponges and "algae", increase in the diversity of siliceous sponges, change in the composition of encrusting bivalves, and decrease in the activity of boring bivalves. Changes in morphotypes and dominance of guilds occur among corals, sponges and bivalves, but are only partially helpful in discriminating environmental differences.
Bathymetric change is commonly, but not necessarily correlated with a change in nutrient and oxygen levels. Partial decoupling of these factors is obvious at Serra Isabel, where siliceous sponges are, at a cm-scale, replaced upwards by a hermatypic coral-crinoid association. However, the dominance of sponges at Faulenhau cannot be explained solely by slightly increased nutrient levels, since the diversity of coralline sponges is much lower than at Alcobaça and Serra Isabel. It is expected that the bathymetric position of the majority of the Upper Jurassic sponge-crust mounds grew in water deeper than for the Faulenhau example, i.e. below 70-90 m, although siliceous sponge facies could have occurred at lower depths in regions or at times of lowered oxygen contents or slightly increased nutrient levels, e.g. during rapid sea-level rise.
Last changes Nov. 2004 by Reinhold Leinfelder