The stratigraphic record of the Messinian salinity crisis in the northern margin of the Bajo Segura Basin (SE Spain)

摘要

The Bajo Segura Basin is a marginal basin of the Western Mediterranean located on the eastern end of the Betic Cordillera (southeastern Spain). Its Messinian and Pliocene stratigraphic record is divided into four allostratigraphic units, with bounding unconformities represented by erosional surfaces corresponding to palaeogeographic changes. The Messinian Unit Ⅰ (MⅠ) is composed of three depositional systems (fluvial, coastal lagoon, and shelf) which lateral facies changes, deposited in a context of sea level highstand. The upper boundary of the MⅠ Unit corresponds to the intra-Messinian unconformity, a lowstand erosional surface with palaeovalleys of 30 m deep generated by a sea level fall. Messinian Unit Ⅱ (MⅡ) comprises five correlative depositional systems (from fluvial to coastal) deposited during a new sea level highstand after a reflooding that followed the preceding lowstand. The upper boundary of Unit MⅡ corresponds to the end-Messinian unconformity, which is another lowstand erosional surface with similar palaeovalleys to those of the intra-Messinian unconformity. Messinian Unit MⅢ forms the fluvial fill of some small incised palaeovalleys, developed during the end-Messinian lowstand. Over the end-Messinian unconformity lies the Pliocene Unit (P), which consists of four depositional systems superimposed in continuity. The first two (littoral and open marine) form a transgressive assemblage that fill the palaeovalley carved over the end-Messinian unconformity. The latter two (shallow marine to coastal in the lower part, and fluvial in the upper part) form a regressive prograding assemblage deposited in a highstand context, when the sea level reached its height in the early Pliocene. In relation to the Messinian salinity crisis, we propose that Unit MⅡ is correlative to the evaporites deposited in the centre of the Bajo Segura Basin (San Miguel Gypsum Formation), corresponding to the marginal evaporites of the Mediterranean. Evaporitic sedimentation in the Mediterranean abyssal plain, which is represented in the margins by erosional surfaces, may correspond to either of the two unconformities (intra-Messinian and end-Messinian) recognized in the Bajo Segura Basin.