Miocene Reef Facies of Pelagian Block, Central Mediterranean

H. Martyn Pedley

Miocene reefs outcrop in the Maltese Islands, southeastern Sicily, and the pelagian island of Lampedusa. Several rapid eustatic sea level fluctuations affected these late Tortonian-early Messinian build-ups; normal salinities appear to have been maintained during these events.

Substrate, topography, sedimentation rate, and tectonic/eustatic events controlled reef development, which can be grouped into three settings: (1) The most stable situation, the oldest Maltese and southeastern Sicilian reefs, has a ramp profile 15-30 km wide. The outermost zone consists of a broad belt of the large benthic foraminifer Heterostegina (compared with the underlying Oligocene beds rich in Lepidocyclina). Coralline algal carbonates, commonly rhodolitic, form a broad biostromal up-ramp association, kilometers in width, which commonly extends into the shallowest parts of the shelf. Scattered across the shallower ramp areas, in water depths generally less than 10 m, are coral-algal patch reefs, rarely larger than 20-50 m in diameter, commonly with truncated tops, and dominated by crustose coralline algae and the corals Porites and Tarbellastraea. (2) Synsedimentary faulting in Malta and Lampedusa created steep platform margins during the early Messinian. Patch reefs, indistinguishable from the slightly earlier ramp-associated reefs, typify the platform tops though interreef sediments are dominated by Halimeda plates. The fault-controlled slope break was the site of massive build-ups of Porites branches, each a few centimeters in diameter, but up to 1 m in length. (3) Reefs in abandoned early Messinian delta-top distributary channels are found in central northern Sicily. They occur at the shelf break and occupy the mouths of the channels. In structure, they are comparable to the Spanish Messinian reefs and have well-developed fore-reef talus and reef cores domi ated by branching and domed growth forms of Tarbellastraea and Porites. The remaining areas behind the reef-blocked channel mouth, however, are filled by carbonate debris, siliciclastic muds, and sands containing Porites biostromes.

In addition to seafloor topography, reef development appears to have been controlled by turbulence. Encruster-dominated patch reefs are typical of platform and shallow ramp situations where turbulence is high. Branching and massive coral assemblages are typical of fore-reef curtains and steep slope substrates.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.