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Middle Eocene carbonate platforms of the westernmost Tethys

02/15/2021 / Leave a comment

A study of the paleoenvironmental evolution of the middle Eocene platforms recognized in the westernmost Tethys has been carried out in the well exposed middle Eocene succession from Sierra Espuña-Mula basin (Betic Cordillera, S Spain). Eight microfacies (Mf1 to Mf8) have been recognized, based mainly on fossil assemblages (principally larger benthic foraminifera), and rock texture and fabric.

Environmental microfacies distribution for the Middle Eocene marine Depositional Sequence 2 (Malvariche andCánovas fms) in Sierra Espuña, arranged from proximal to distal depositional environments: Mf3, Inner ramp lagoon, upper subtidal environment; Mf5, Inner ramp seagrass, euphotic subtidal environment; Mf6 – Mf7, Inner ramp, euphotic lower subtidal environment; Mf2, Proximal middle ramp LBF accumulations (nummulitids), mesophotic environment; Mf1, Proximal middle ramp maërl, mesophotic environment; Mf8, Distal middle ramp LBF accumulations (orthophragminids), mesophotic environment; Mf4, Outer ramp lacking Large Bethic Foraminifera (LBF), oligophotic environment. Ramp subdivision is based on Burchette and Wright (1992), and photic zones are analogous to those described by Pomar et al. (2017), with a ‘mesophotic zone’ comprised between lower limit of occurrence of marine vegetation and the storm wave base (swb).

 

The fossiliferous assemblage can be asigned to the ‘subtropical’ heterozoan association or to the low-latitude ‘foralgal facies’ , which are dominated by non-framework building, light-dependent biota such as perforate larger benthic foraminifera, coralline algae, and sometimes green algae and solitary corals. Larger benthic foraminifer assemblages, corresponding from euphotic to oligophotic conditions and the large surface showed, suggest a progressive marine ramp under essentially oligotrophic conditions. Eventually, supply of detrital sediments from the continent and/or upwelling currents increasse the nutrients of marine waters. Comparision with other Tethyan sectors allows stating that coral-reef buildups (z-corals) were widespread on shallow platforms of the central and eastern Tethys Ocean, but that these were neither of great dimensions nor dominant because of the much more dominant presence of larger benthic foraminifera. Moreover, these coral constructions were completely absents in the westernmost Tethys. The dominance of larger benthic foraminifera and the absence of z-corals in the westernmost Tethys is explained by particular paleogeographic features due to the occurrence of a narrow and deep oceanic branch (i.e., the Maghrebian Flysch Basin) connecting the Tethys with the Atlantic Ocean.

Biochronostratigraphic chart with numerical time scale, magnetochrons, magnetic polarity, planktonic foraminifera and calcareous nannoplankton zones based on GTS 2012 (Gradstein and Ogg, 2012), correlated with shallow benthic zones (SBZ). Interpretations of main climatic events, trophic resources continuum, LBF specific diversity and coral events in the Tethyan domain are also represented. A synthetic column with the stratigraphic formations and the main trophic conditions and Large Bethic Foraminifera (LBF) and coral (*) events of the Sierra Espuña-Mula Basins are also included.

The various issues regarding the morphological characters and evolution of larger benthic foraminifera in the study area, such as sizes of tests, specific diversity and/or intraspecific variability, number of appearances and last occurrences during the middle Eocene are analyzed and compared with those appearing in other Tethyan sectors. In addition, the early to late Bartonian boundary is recognized in the study area as critical for the biological change as in other shallow-marine environments along the Tethys margins.

Cite as: Martín-Martín, M., Guerrera, F., Tosquella, J., Tramontana, M., 2021. Middle Eocene carbonate platforms of the westernmost Tethys. Sediment. Geol. 415, 105861. doi:10.1016/j.sedgeo.2021.105861