The Westernmost Tethys Blog Geology mapping, basin analysis and 3D modeling

02/15/2021

Middle Eocene carbonate platforms of the westernmost Tethys

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

 

01/29/2020

Compactation in sedimentary basins

Filed under: basin analysis,simulation — Tags: , , — messinianalicante @ 8:59 AM

Subsidence analysis is an important technique in the study of sedimentary basins but the effects of compaction must be “backstripped”. The compaction of sediments is also of importance for petroleum and water reservoir research with very important economic derivations. Most methods for calculating compaction are based on empirically derived porosity-depth relationships from a variety of known sediment types. The challenge of this paper is to apply alternative methods for calculating compaction in sedimentary basins based on: physical calculation with elastic by Steinbrenner, oedometric and change of the specific weight of the sediment methods; and use of Loadcap software.

The Triassic to Lower Miocene 3025m thick succession of Sierra Espuña (SE Spain) is used as case study for the calculations. In this succession former mineralogical studies and apatite fission-track suggested an original thickness between 4 and 6km. The validity of each one of the proposed methods is discussed, as well as, compared for the whole succession compaction but also separately for hard vs soft sediments and for thick vs thin beds.

Accumulate thickness-age (My) graphic with the comparative of the measured thickness and the results of original accumulate thickness along time of the studied succession after decompaction with the whole methods. The mean thickness with the whole methods is also represented with dash line. Key: ESM: elastic by Steinbrenner; SWM: specific weight of the sediment methods; OM: oedometric method; PCM: porosity change method (Bond et al., 1983); LSM: use of Loadcap software method.

The compaction values obtained with the alternative methods are similar to those resulting with the lower-limit curves of the porosity-depth change method. The new methods have provided values slightly higher than 4km for the whole original thickness using the geotechnical software and the change of the sediments specific weigh methods; meanwhile values below 4km for other methods. So, in our opinion, the geotechnical software and the change of the specific weight of the sediment methods are compatible with mineralogical constraints and also, the input data are usually better known and easier to determinate. Otherwise, the elastic method seems only accurate for soft sediments; meanwhile the oedometric method is highly influenced by the thickness of the considered beds.

 

Cite as: Martín-Martín, M., and Robles-Marín, P. (2020): Alternative methods for calculating compaction in sedimentary basins. Mar. Pet. Geol. 113, 104132. doi: 10.1016/j.marpetgeo.2019.104132

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