The Messinian record of the outcropping marginal Alborán basin deposits: significance and implications

Abstract

The Messinian record of marginal Alboran basins, such as the Sorbas Basin in southern Spain, consists of a shallow-marine succession with intercalated evaporites. The pre-evaporite sequence comprises a bryozoan-bivalve, temperate-carbonate unit overlain by tropical carbonates. The latter, in turn, consists of two superimposed units: a bioherm unit with coral (Porites, Tarbellastraea, and Siderastraea) and algal (Halimeda) mounds, and a coral (Porites)-stromatolite fringing reef unit. Climatic fluctuations in the Alboran area, linked to the Neogene glacial-interglacial oscillations, are thought to be responsible for the change from temperate to tropical conditions. Evaporites are mainly selenite gypsum deposits. The first post-evaporite unit is a mixed siliciclastic-carbonate marginal deposit, with small coral (Porites) patches and huge microbial (stromatolite and thrombolite) domes, changing basinward to silts and marls containing planktonic foraminifers. An incised erosion surface was scoured on top of the pre-evaporitic fringing reef unit. This erosion surface formed during drawdown and desiccation of the Mediterranean Sea, when huge masses of salt were deposited in its center. Deposition of gypsum at the very margin of the Alboran Sea took place later in small, barred, satellite perched basins. In these silled basins marine incursions became more and more frequent until a full connection with the Mediterranean Sea was established by the end of the Messinian. Reflooding was completed during the Messinian, as demonstrated by the marine marls with planktonic foraminifers found on top of the evaporites. This situation is comparable to that of the western Mediterranean (DSDP Site 372; ODP Site 975), where the upper evaporites are directly overlain by Messinian marls with planktonic foraminifers. During the initial stages of marine recolonization, microbes coexisted with, but outcompeted, the normal marine biota. This resulted in the widespread proliferation of microbial carbonates (stromatolites and thrombolites).