Alastair H.F. Robertson2


Miocene shallow-water limestones of the Eratosthenes Seamount add considerably to the picture of widespread and heterogeneous Mediterranean Miocene reef development. Shallow-water limestones were cored at two sites on the Eratosthenes Seamount during Ocean Drilling Program Leg 160, one on the plateau area and one on the upper northern slopes. The limestones at Site 966 are dated as Miocene by use of benthic foraminifers, whereas those at Site 965, although lacking diagnostic fossils, are inferred to be of late Miocene age from the evidence of preliminary strontium isotopic dating. At Site 966, shallow-water limestones are underlain by upper Eocene bathyal pelagic carbonates and overlain by a mass-flow unit of early Pliocene age; by contrast, at Site 965, paleosols, of assumed Messinian age, overlie the Miocene limestones. The limestones at both sites are mainly composed of calcareous red algae (often as rhodoliths), coral (e.g., Porites), large benthic foraminifers, bivalves, echinoderm plates, and minor sponge spicules. The frame-building material is mainly reworked by gravity (where locally coarse) and by ocean currents (where finer grained). Sediment at Site 965 is relatively coarse, which suggests a proximal reef setting. Planktonic foraminifers are common, indicating proximity to an open-marine setting. By contrast, bioclastic material at Site 966 is mainly relatively fine grained and accumulated in a lagoonal setting, characterized by lime-mud, with some in situ microbial-mat development.

Carbonates at both sites underwent a similar diagenetic history. Early void-filling cement included fibrous, acicular cement, of marine origin. Minor aragonite is preserved locally, but primary carbonate is recrystallized to low-magnesian calcite. The development of extensive dissolution porosity and calcite cement are attributed to flushing by meteoric waters during the Messinian, a period of emergence of the Eratosthenes Seamount. Common dolomite possibly formed in a mixing zone-related setting associated with marine transgression in the early Pliocene. Minor opal and microcrystalline chert may owe its origin to dissolution of sponge spicules that are still locally preserved.

Comparisons with other Miocene Mediterranean reef limestones indicate many similarities with lower Miocene reefs in southeastern Cyprus; the latter developed as scattered patch reefs on a flat, relatively tectonically stable area. There are also marked similarities with the upper Miocene reefs of the Pelagian Block in the Central Mediterranean that were isolated from terrigenous input by surrounding deep-water basins. Alternatively, the lower Miocene Eratosthenes reefs possibly formed an atoll-like structure, with coral-calcareous algal growths around the periphery, and lime mud accumulation within an internal lagoon, similar to many modern atolls. Shallow-water carbonate deposition was initiated after the Paleogene, following about 1 km of tectonic uplift, and was terminated by the late Miocene salinity crisis.

1Robertson, A.H.F., Emeis, K.-C., Richter, C., and Camerlenghi, A. (Eds.), 1998. Proc. ODP, Sci. Results, 160: College Station, TX (Ocean Drilling Program).
2University of Edinburgh, Department of Geology and Geophysics, West Mains Road, Edinburgh, EH9 3JW, United Kingdom. Alastair.Robertson@ed.ac.uk