Diversity, growth forms and taphonomy: key factors controlling the fabric of coralline algae dominated shelf carbonates

Abstract

AbstractThe fabric of biogenic carbonate sediments can be differentiated (1) with respect to diversity of constituent components, (2) using features pertaining to their growth forms and (3) upon consideration of taphonomic aspects. These not only determine limestone fabrics, but also form the basis for facies differentiation and palaeoecological interpretation.This study is based on coralline algae dominated Lower Oligocene shelf carbonates from northern Slovenia from which seven facies (nummulitic, bivalve, foraminiferal-coralline algal, coralline algal, coralline algal-coral, coral and grainstone) are differentiated. The role of diversity, growth forms and taphonomy of coralline algae in each facies is discussed. Nine species from seven genera of coralline algae including geniculates and non-geniculates were recognized. Numerous different growth forms ranging from crusts, protuberances, lamellae to arborescent types are present. A wide range of taphonomic features including disarticulation, encrustation, fragmentation and abrasion can be observed.The determination of diversity is dependent on taxonomic identification using preserved diagnostic characters relevant to palaeontological and botanical systematics. Growth-form determination in thin section is influenced by orientation and sectioning effects. The taphonomy of red algae is highly dependent on initial growth form and the specific environment in which they are found. A number of taphonomic processes described in Recent environments (e.g. disease, shallow grazing) cannot be ascertained in fossil material, while others are readily observable. Some taphonomic processes are detrimental (e.g. fragmentation, abrasion) to the preservation and recognition of vegetative and growth-form features, while others have positive effects (e.g. encrustation).