Significance of lower Pliocene mass-flow deposits for the timing and process of collision of the Eratosthenes Seamount with the Cyprus active margin.

Abstract

On the crestal area of the Eratosthenes Seamount at Site 966 an important “chaotic” interval composed of clast-rich, matrixsupported sediments (here named the Mass-Flow Unit) was recovered in five boreholes, situated up to 70 m apart. These sediments overlie shallow-water limestones of Miocene age and are, in turn, overlain by deep-water nannofossil ooze and sapropels of early Pliocene-Pleistocene age. Nannofossils and planktonic foraminifers indicate an early Pliocene age for the matrix, and a late Miocene to early Pliocene age for clasts within the Mass-Flow Unit. Clasts of shallow-water limestone and nannofossil chalk are both present. The matrix is mainly nannofossil ooze admixed with silt-sized reworked carbonate grains, minor quartz, and terrigenous clay. Whole-rock X-ray diffraction reveals variable quantities of calcite, dolomite, and aragonite, together with minor quartz and pyrite. Fibrous carbonate is seen in several thin sections. Limestone clasts have undergone extensive dissolution, followed by variable meteoric water cementation, presumably during Messinian emergence. Comparable Miocene-Pliocene settings onshore in Cyprus are the following: (1) formation of lower Pliocene carbonate debris flows, similar to those at Site 966, related to extensional faulting; (2) Messinian erosion, karstification, and talus formation on the flanks of a graben undergoing active crustal extension during the late Miocene; (3) formation of deep channel and related debris flows of late Pliocene age. Of these, the first and second show similarities with the Eratosthenes Mass-Flow Unit. The Florence Rise also shows some similarities. A depositional-tectonic model is proposed for the Site 966 Mass-Flow Unit, in which the Miocene limestone of Eratosthenes Seamount was subaerially exposed and diagenetically altered during the Messinian salinity crisis. This was followed by marine transgression and accumulation of lower Pliocene nannofossil ooze. Extensional faulting was active during the early Pliocene (and possibly earlier), resulting in subaqueous mass wasting of Miocene limestones and large-scale gravity reworking of nannofossil oozes as multiple debris flows. Tilting continued during emplacement of debris flows, resulting in interstratal shearing, slumping, and minor high-angle faulting. Formation of the Mass-Flow Unit is interpreted to relate to the initial stages of collision of the Eratosthenes Seamount with the Cyprus active margin to the north. In this interpretation, the Eratosthenes Seamount was flexurally loaded by the advancing plate and underwent initial block faulting, followed by collapse and subsidence.