Geomorphic evolution of the Malta Escarpment and implications for the Messinian evaporative drawdown in the eastern Mediterranean Sea

Abstract

Carbonate escarpments are submarine limestone and dolomite cliffs that have been documented in numerous sites around the world. Their geomorphic evolution is poorly understood due to difficulties in assessing escarpment outcrops and the limited resolution achieved by geophysical techniques across their steep topographies. The geomorphic evolution of carbonate escarpments in the Mediterranean Sea has been influenced by the Messinian salinity crisis (MSC). During the MSC (5.97-5.33 Ma), the Mediterranean Sea became a saline basin due to a temporary restriction of the Atlantic-Mediterranean seaway, resulting in the deposition of more than one million cubic kilometres of salt. The extent and relative chronology of the evaporative drawdown phases associated to the MSC remain poorly constrained. In this paper we combine geophysical and sedimentological data from the central Mediterranean Sea to reconstruct the geomorphic evolution of the Malta Escarpment and infer the extent and timing of evaporative drawdown in the eastern Mediterranean Sea during the MSC. We propose that, during a MSC base-level fall, fluvial erosion formed a dense network of canyons across the Malta Escarpment whilst coastal erosion developed extensive palaeoshorelines and shore platforms. The drivers of geomorphic evolution of the Malta Escarpment after the MSC include: (i) canyon erosion by submarine gravity flows, with the most recent activity taking place <2600 cal. years BP;(ii) deposition by bottom currents across the entire depth range of the Malta Escarpment;(iii) tectonic deformation in the southern Malta Escarpment in association with a wrench zone; (iv) widespread, small-scale sedimentary slope failures preconditioned by oversteepening and loss of support due to canyon erosion, and triggered by earthquakes. 3We carry out an isostatic restoration of the palaeoshorelines and shore platforms on the northern Malta Escarpment to infer an evaporative drawdown of 1800 -2000 m in the eastern Mediterranean Sea during the MSC. We interpret the occurrence of pre-evaporite sedimentary lobes in the western Ionian Basin as suggesting that either evaporative drawdown and canyon formation predominantly occurred before salt deposition, or that only the latest salt deposition at the basin margin occurred after the formation of the sedimentary lobes.

The Zanclean megaflood of the Mediterranean. Searching for independent evidence

“…This argues against the possibility of multiple flooding events (Rouchy and Martin, 1992) or against calm periods intercalated during the flood. It conflicts with the interpretation of a ravinement surface in the Gulf of Lions as the result of wave erosion during the flood (Bache et al, 2009;Estrada et al, 2011) and suggests that this might have been formed during climatically-controlled changes of lake level during the drawdown phase, similar to the wave-cut surfaces in the Malta Escarpment that will be shown below (Micallef et al, 2019). The accelerating flooding rates due to inlet erosion (Garcia-Castellanos et al, 2009) precludes the stagnation of the flooding process once the water discharge has become substantial in comparison with the water budget of the Mediterranean and has significantly risen its level.…”

“…The head of this canyon has an amphitheater-shaped head, 6 km in width and 700 m high. This is very unusual for a submarine canyon and conspicuously different from the other canyons along the escarpment (Micallef et al, 2019). The strike of the Noto Canyon changes by 90°at a distance of 8 km upslope from its mouth.…”

“…5). These are interpreted as palaeoshorelines and shore platforms excavated by wave erosion related to a lowstand of the Mediterranean Sea below its present sea level (Micallef et al, 2019). An isostatic restoration of those palaeoshorelines and shore platforms on the northern Malta Escarpment suggest an evaporative drawdown of 1800-2000 m below present sea level.…”

“…The bottom of the Noto Canyon (2300-2600 m) is close to the depth of those shorelines and to the base of the megaflood deposit, allowing us to speculate that the canyon may have been excavated after a long period when the level of the Eastern Mediterranean remained at that level. After isostatic restoration by removing the weight of the Plio-Quaternary sediment and the water column (Micallef et al, 2019), the original depth of the base of the canyon the during the MSC would range between 1750 and 2050 m.…”

Deformation Pattern of the Northern Sector of the Malta Escarpment (Offshore SE Sicily, Italy): Fault Dimension, Slip Prediction, and Seismotectonic Implications

“…Accordingly, the pre-MES unit has been interpreted as a Cenozoic sediment package that includes mainly limestone and marls, locally pierced by volcanic and/or mud intrusions forming sometimes cone-shaped seamounts at the seafloor ( Figure 3A) (see also Scandone et al, 1981;Catalano et al, 2001;Barreca, 2014). According to its seismic characters (high reflectivity of the top reflector) and to the abundant literature concerning similar deposits occurring all around the Mediterranean basin (Lofi et al, 2011;Micallef et al, 2019 and references therein), the MES unit has been interpreted as Messinian in age deposits (mainly gypsum and evaporitic carbonate). The overlying Unit PQ 1 is interpreted as a part of the Pliocene succession since it correlates with the subunits "PQb" and "PQc" described by Camerlenghi et al (2019) and partly with the "Unit one" of Micallef et al (2018).…”