The Messinian stromatolites of the Sierra del Colmenar (Western Mediterranean): facies characterization and sedimentological interpretation

A representative outcrop of the Messinian stromatolites belonging to the Terminal Carbonate Complex unit, from the northern sector of the Bajo Segura basin (Caja de Ahorros del Mediterraneosection, Sierra del Colmenar, SE Spain) has been studied. Here, we present a detailed analysis of the architecture, external morphology, and internal morphology in order to reconstruct the environmental and palaeoecological conditions for their growth. The stromatolites macrostructure consists of a continuously doming type morphology (build up and sheets areas). These developed close to the coast and acted as a palaeogeographic barrier, reducing physical stress, channeling the erosive effect of water and favoring restricted conditions. This stromatolitic macrostructure exhibits variations in its internal morphology, giving rise to seven subfacies, which are a product of the environmental changes experienced during the growth of the microbial mats. Although broadly suggesting a coastal environment, restricted and shallow during formation, the variation in internal morphology (mesostructure and microstructure) is evidence of minor changes in the physical environment that indicate a progressive shallowing.

The Messinian stromatolites of the Sierra del Colmenar (Western Mediterranean): facies characterization and sedimentological interpretation

The Messinian stromatolites belonging to the Terminal Carbonate Complex unit, from the northern sector of the Bajo Segura Basin (CAM section, Sierra del Colmenar, SE Spain) have been studied. To understand the direct relationship between the morphologies of the stromatolites and their deposition context in order to reconstruct the environmental conditions for their growth, a detailed study of their architecture, external morphology and internal morphology was carried out (macrofabric and microfabric). The stromatolites are made up of domic bodies laterally linked to each other, generating a macrostructure (bioherms) with lateral continuity. This stromatolitic macrostructure presents variations in its internal morphology, giving rise to seven subfacies product of the environmental changes experienced during the growth of the microbial bushes. The stromatolites are arranged parallel of coastline acting as paleogeographic barriers to reduce the physical stress of the environment. Although in general lines suggests a coastal environment, restricted and shallow for the formation of the whole level, the variation in internal morphology is evidence of minor changes in the physical environment.

The Messinian stromatolites of the Sierra del Colmenar (Western Mediterranean): facies characterization and sedimentological interpretation

The Messinian stromatolites belonging to the Terminal Carbonate Complex unit, from the northern sector of the Bajo Segura Basin (CAM section, Sierra del Colmenar, SE Spain) have been studied. To understand the direct relationship between the morphologies of the stromatolites and their deposition context in order to reconstruct the environmental conditions for their growth, a detailed study of their architecture, external morphology and internal morphology was carried out (macrofabric and microfabric). The stromatolites are made up of domic bodies laterally linked to each other, generating a macrostructure (bioherms) with lateral continuity. This stromatolitic macrostructure presents variations in its internal morphology, giving rise to seven subfacies product of the environmental changes experienced during the growth of the microbial bushes. The stromatolites are arranged parallel of coastline acting as paleogeographic barriers to reduce the physical stress of the environment. Although in general lines suggests a coastal environment, restricted and shallow for the formation of the whole level, the variation in internal morphology is evidence of minor changes in the physical environment.

Quaternary active tectonic structures in the offshore Bajo Segura basin (SE Iberian Peninsula – Mediterranean Sea)

The Bajo Segura fault zone (BSFZ) is the northern terminal splay of the Eastern Betic shear zone (EBSZ), a large left-lateral strike-slip fault system of sigmoid geometry stretching more than 450 km from Alicante to Almería. The BSFZ extends from the onshore Bajo Segura basin further into the Mediterranean Sea and shows a moderate instrumental seismic activity characterized by small earthquakes. Nevertheless, the zone was affected by large historical earthquakes of which the largest was the 1829 Torrevieja earthquake (IEMS98 X). The onshore area of the BSFZ is marked by active transpressive structures (faults and folds), whereas the offshore area has been scarcely explored from the tectonic point of view. During the EVENT-SHELF cruise, a total of 10 high-resolution single-channel seismic sparker profiles were obtained along and across the offshore Bajo Segura basin. Analysis of these profiles resulted in (a) the identification of 6 Quaternary seismo-stratigraphic units bounded by five horizons corresponding to regional erosional surfaces related to global sea level lowstands; and (b) the mapping of the active sub-seafloor structures and their correlation with those described onshore. Moreover, the results suggest that the Bajo Segura blind thrust fault or the Torrevieja left-lateral strike-slip fault, with prolongation offshore, could be considered as the source of the 1829 Torrevieja earthquake. These data improve our understanding of present deformation along the BSFZ and provide new insights into the seismic hazard in the area.