Stratigraphic relationships between shallow-water carbonates and primary gypsum: insights from the Messinian succession of the Sorbas Basin (Betic Cordillera, Southern Spain)

The lack of modern analogues for the thick primary selenite gypsum deposits formed in the Mediterranean area limits the understanding of the Messinian (Miocene) evaporitic systems and particularly of their lateral relationships, their evolution through time and their related ecosystems. Some complex stratigraphic relationships of these extreme environments are revealed in the Messinian Carlos Formation along the northern flank of the Sierra Aihamilla at the southern boundary of the Sorbas Basin (southern Spain). Here, the bottom-grown selenite gypsum beds pinch-out against graded elastic carbonate beds (ooid grainstones, rudstones and mudstones) emplaced by gravity flows. We infer that these tabular bodies formed along a low gradient ramp, derived from the periodic erosion of coeval shallow-water to coastal carbonate factories located on top of the Sierra Aihamilla ridge. The carbonate deposits bear many similarities with the Terminal Carbonate Complex described in the northern, steeper margin of the basin; both deposits show a cyclical pattern given by the repetition of couplets consisting of coarser- and finer-grained deposits. This rhythmic fades organization recalls the precessional-controlled cyclic pattern of the gypsum unit. Clastic carbonates and gypsum were deposited during the same precessional hemicycle but appear to be mutually exclusive. In fact, the elastic carbonates represent the lateral equivalent of the gypsum beds, or are occasionally found at their base or at both their base and top, but never interstratified with them. Such an arrangement suggests that during the peak of the evaporitic phases a strong pycnocline acted as the top boundary for gypsum and the lower boundary for carbonate-bearing gravity flows, with the exception of debrites, high-density gravity flows and slumps. These fades architectures appear to be mainly controlled by the interplay between the variable depth of the pycnocline and the density of the brine within the evaporitic cycle. (C) 2020 Elsevier B.V. All rights reserved.