Classification and geological significance of biostromes

Summary

Biostrome and bioherm were described as terms byCumings (1932), and bioherm has become synonymous with reef because of the discrete mound or lens shape in vertical section. The phrase “reefs and biostromes” is common in the literature and emphasises that biostromes are normally regarded explicitly as not reefal structures, because of the lack of topographic relief and common absence of a framework. However, the position adopted here is that bioherm and biostrome are most usefully applied to simply describe the outline shape of an organic accumulation, and not to denote any particular inherent internal structural organisation. Furthermore, the view here is that biostromes are most usefully considered as single organic layers (i.e. beds). Observations of biostromes of numerous ages and settings indicate that a considerable variety of internal structure exists within the outline which defines biostrome. Often, the structure comprises frame-works and dense clusters of in-place organisms and is just as much “reefal” as similar constructions with a biohermal shape. In other cases biostromes consist of beds of skeletal debris consistent with the concept of biostrome used by. many workers. These differences demonstrate that classification of biostromes is needed in order to allow comprehensive palaeoenvironmental analysis, and highlight the long-standing problem of using ‘reef’ to describe organic buildups. For biostromes,autobiostrome, autoparabiostrome, andparabiostrome are introduced to describe a continuum from structures where the constructing organisms are mostly in place (autobiostromes), to mostly debris of the structure (parabiostromes), with autoparabiostrome as intermediate.Allobiostrome, describes biostromes formed of material derived from allochthonous sources, for example skeletal plankton sedimented onto the sea bed. Most biostromes are of calcareous construction and their composition is most adequately described by existing limestone classification terminology.

Other descriptive terms include: a) for biostrome geometry—ribbon and sheet; b) for internal layering—internally unbedded andinternally bedded, because some biostromes show lateral facies changes resulting in single layers becoming internally divided by bedding; c) for internal packing variation of constructors, using terminlogy introduced by R. Riding—dense (where constructors are closer together than one unit distance), andsparse (where constuctors are more distantly spaced). Biostromes are further categorised to account for thickness variations. Adaptation of terminology used for bed thickness descriptions is applied; standard bed thickness categories are not appropriate to biostromes, which are often thicker than 1m. Instead:Very thin biostromes-up to 0.1m,thin biostromes-0.1–0.5m,medium biostromes-0.5–2.0m,thick biostromes-2.0–5.0m,very thick biostromes->5.0m. Autobiostromes which form significant features in sedimentary successions may be regarded as the peak of in place benthic organic skeleta buildup (=reefsensu lato) development, and their recognition is required to permit full palaeoenvironmental analysis of facies containing them. Particular emphasis may be placed on their role in identifying sea level change in shallow water carbonate sequences, and thence their utility in models to explain such change.