Paleoecological constraints on reef-coral morphologies in the Tortonian–early Messinian of the Lorca Basin, SE Spain

Abstract

Coral reefs represent one of the main carbonate factories that contributed to the control of the stratigraphic architecture of carbonate platforms, which had a widespread development during the late Miocene in the paleo-Mediterranean area. The late Miocene reef complexes of the Lorca Basin in southeastern Spain are composed of five mixed siliciclastic/carbonate units, middle Tortonian to early Messinian in age. The development of coral reefs probably ceased when the first evaporitic event occurred in the basin centre in the early Messinian. This study mainly focuses on the response of reef communities and the modifications of reef organisation to global and regional parameters. At the platform scale, the carbonates are intermixed with terrigenous deposits related to two main types of clastic systems: torrential fans and fluvial to deltaic systems. The amount of clastic input greatly affected reef growth and coral morphologies. Three different types of stratal geometries were delineated in the reef complex: sigmoids, bioherms, and patches and carpets. The reef frameworks are mainly constructed by a poorly diversified assemblage of corals composed of poritids, faviids, and mussids. Porites is the principal reef builder of the sigmoids and carpets where it is widely distributed. Tarbellastraea is common in bioherms and Acanthrastraea appears generally associated with Porites in patches. Five basic growth forms of Porites are observed: thin branching or “finger-shaped”, thick branching to columnar, domed to hemispheric, encrusting, and platy to dish. Differences in coral morphology are used to define a relative water depth zonation in monogeneric reefs. The distribution of these growth forms was principally controlled by water depth. The reef flat is dominated by small thin branching or finger-shaped corals that are replaced towards the reef front by domed to hemispheric corals commonly encrusted by coralline algae. Downslope, columnar morphologies grade into thin branching shapes. The reef morphologies are variable throughout the five mixed siliciclastic/carbonate units at the platform scale. The first and oldest unit is dominated by bioclasts, whereas units 2, 3, and 5 are Porites-dominated, sigmoid complexes. Unit 4 is a well-developed biohermal complex mainly composed of Tarbellastraea. These units started to develop as early as middle Tortonian and stopped as late as early Messinian, and show a progradational trend, where the two latest units are well developed. Thus, carbonate production changed from grain-producing biota in the basal unit to framework-producing biota in the overlying units, consistent with evolution from a distally steepened ramp to a reef-rimmed shelf. At the scale of individual reef units, the relative water depth zonation of the corals is controlled by ecological changes (substrate, nutrients, synecologic relations, and diversification of coral species). In the transects across the carbonate platform related to the different units, the coral zonation records changes in spatial distribution of corals in response to ecological stresses and changes in regional and global environments (tectonic, relative sea-level changes, and runoff).