Southwest Mallorca Island: A cool-water carbonate margin dominated by drift deposition associated with giant mass wasting

Abstract

By means of new high-resolution Parasound, multi-channel reflection seismic and multibeam swath bathymetry data, we provide new insight into the sedimentation processes at the margin of a cool-water carbonate ramp system off southwestern Mallorca Island, western Mediterranean Sea. Here, we mapped 7 depositional units which we tied to the Messinian unconformity. The oldest units (1–3) are deposited from late Messinian to late Pliocene during phases of extreme sea-level fluctuations. 2.4Ma ago with the global climate cooling, when the estuarine water mass circulation switched into an anti-estuarine pattern in the Mediterranean Sea and sea-level highstands reached the today level, bottom currents started to shape the distal deposits of the cool-water carbonate system off southwestern Mallorca, represented by depositional units 4–7. Although, sediment supply from the carbonate shelf to the slope is low due to the insignificant input of terrigenous clastics and slow reef growth, slope sediments reach thicknesses >140m. Our study displays that the accumulation of sediments at the southwestern insular margin was mainly controlled by long-lived bottom currents and giant submarine landslides. The bottom currents formed elongated mounded drifts that contemporaneously developed in alongslope and upslope direction. They are probably controlled by an isobath-parallel bottom current in a present water depth of 350–550m which we associate with the Levantine Intermediate Water (LIW); and an upslope current acting in 250–600m associated with the Algerian mesoscale gyres that enter the Balearic straits from SE. Several scarps at the present sea-floor indicate the predominance of slope failures in the study area. Additionally, mass transport deposits occur basinwards in almost all depositional units. Accordingly, giant mass wasting events took place from the Pliocene into recent times.Locally, above the alongslope drift a field of sediment waves occurs in a water depth of 170–310m. They are oriented slightly oblique to the contour lines with their steep flank facing upslope, showing wavelengths of 400–800m and heights of 10–15m. We suggest that the waves migrate in upslope direction perpendicular to the main isobath-parallel flow.Furthermore, the results of this study indicate that sea-level fluctuations have only a minor influence on drift deposition; instead submarine landslides caused the creation of new drift units. They significantly altered the relief of the sea-floor and thereby initiate the realignment of the prevailing current pattern. The fault scarps and detachment surfaces left behind were the nucleolus for the development of new drift units.