Freshwater discharge and sediment dispersal — Control on growth, ecological structure and geometry of Late Miocene shallow-water coral ecosystems (early Tortonian, Crete/Greece)

Abstract

Insolation-driven oceanographic changes are well documented in the Mediterranean deep-sea record and reflect episodes of high precipitation and/or freshwater discharge. In the shallow-water record, however, changes in relative sea-level and sea surface temperature are usually regarded as prime controls on Miocene coral reefs, whereas pulses of freshwater influx represent a hitherto unrecognized factor influencing their growth, ecological structure and geometry. On the island of Crete (NW Messara Basin, Greece) early Tortonian coral biostromes occur associated with deltas along the basin margin and rimming islands formed by large uplifted tectonic blocks. In turbid delta environments (Apomarma area) coral biostromes were almost monogeneric without any lateral zonation and being formed almost exclusively by dense stands of branching Porites. Encrusting and boring biota are very rare, whereas pyrgomatid barnacles are common dwellers of the coral skeleton — a pattern interpreted to reflect complete cover of coral skeletons by tissue. Although most corals are not preserved in life position, they were not significantly displaced because coral debris was stabilized by subsequent overgrowth of encrusting, platy Porites and some nodular Siderastraea or renewed growth of toppled specimens. Burial of corals by clastic sediments episodically occurred in response to allo- and autocyclic processes (sea-level changes, sediment discharge variability and delta dynamics) resulting in a stack of alternating coral biostromes and sheet-like clastics. In settings protected from strong sediment influx along offshore islands (Psalidha–Ambelousos area), massive coral growth forms prevailed in biostromes with a higher-diversity constructor guild ( Porites, Tarbellastraea, Acanthastraea). Coral skeletons were strongly affected by boring sponges and bivalves, and exhibit some biogenic incrustations. Although rhythmic sediment influx did not occur, constructional coral growth rhythmically ceased. This growth pattern prevented the development of massive reef cores but gave rise to a vertically stacked biostrome complex. Its aggradational growth with minor progradation and the high thickness of the whole carbonate body (15 m) is unique for central Crete and a reflection of synsedimentary tectonic block uplift that kept the relative sea-level locally at a stillstand. All outcrops presented in this study underline the relevance of synsedimentary tectonics and sea-level fluctuations on sediment dispersal and accumulation in coral ecosystems. Nevertheless, the facies comparison of three contrasting reef settings reveals that rhythmic reef demise was also occurring in settings without appreciable siliciclastic flux occasionally accompanied by brackish biota. It is concluded that pulses of freshwater discharge have been a so far unnoticed regulating parameter for shallow-water carbonate production and must therefore be taken into account at the reconstruction of Tortonian nearshore ecosystems of the Mediterranean region.