The Ninyerola Gypsum unit: an example of cyclic, lacustrine sedimentation (Middle Miocene, E Spain)

Resumen

The Ninyerola Gypsum unit is a Middle Miocene lacustrine deposit, close to 200 m in thickness, which crops out mainly along an anticline structure trending NNE-SSW near the city of Valencia. This anticline structure is comprised of Keuper (Upper Triassic) diapiric materials in the core and a Miocene succession on its fl anks. The Ninyerola Gypsum unit appears at the base of this succession and is overlain by alluvial deposits. The latter deposits grade upwards to terrigenous, coastal marine sediments presumably of Tortonian age. The anticline fl anks of the Miocene succession differ markedly not only in terms of their depositional and diagenetic facies but also in thickness and cyclicity. These differences suggest that synsedimentary deformation of the Triassic basement exerted control on Miocene sedimentation. On the eastern fl ank, where the succession is thicker (>230 m) and well exposed, we distinguished the following stratigraphic intervals from base to top: nodular gypsum (a), bioturbated gypsum (b), alternation of laminated gypsum and carbonate (c), a calcareous interval (d), red lutites and bioturbated gypsum (e), and a clastic alluvial alternation (f); this assemblage unconformably underlies a clastic, coastal marine interval (g). The Ninyerola Gypsum unit constitutes the lacustrine sediments −intervals (a) to (e)− of the succession. In this unit, numerous carbonate-gypsum cycles are recorded in intervals (a) to (c), whose average thickness approaches 6 m. The lacustrine stages of this succession were interpreted as the result of the gradual development of a saline lake of the sulphatecarbonate type overlying the Triassic materials, which extruded during the Lower Miocene. Interal (c) represents the maximum extension, depth and subsidence of this saline lake. This deep lake stage (c) was preceded by a shallow lake stage (b), which initially experienced anhydritic sabkha conditions (a). Stage (c) was followed by a diluted lake stage (d) and a fi nal evaporitic lake stage (e). Subsequently, an input of alluvial clastic materials (f) and marine transgression (g) brought an end to the evaporative conditions in the area.