The Sedimentary Development of the Eratosthenes Seamount

Introduction
Lying unsuspectingly on the seafloor 100km south of Cyprus in the Eastern Mediterranean is the locality of a sub-rectangular structure, 20km long and 80km wide drowned massif of the Eratosthenes Seamount ​(Mart and Robertson, 1998)​. The intriguing geological complexity of this active, geodynamic region lies between Cyprus to the North and the Levantine Basin to the south (Kepmpler, 1998). The African-Eurasian plate boundary located in this region south of Cyprus is distinguished as the forefront of the continent-continent collision of Cyprus and the Eratosthenes Seamount since the Pliocene ​(Spezzaferri and Tamburini, 2007)​. For this reason, the seamount is currently undergoing collapse as it is being thrust beneath the flanking Cypriot margin.

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The seamount has been the focus of notable research cruises with objectives to obtain geophysical and geological data. Numerous research vessels and expeditions have been carried out in the region such as the ODP Leg 160 in 1995 and 1993 Training Through Research Program (​Robertson et al., 1995; Robertson, 1998​). This essay’s focal point is on the data acquired from the ODP drill sites which are illustrated in Figure 1.

Results stemming from the ODP Leg 160 led to the deduction that the Eratosthenes Seamount contains a succession of pre-Messinian carbonates overlain by deep water Pliocene-Quaternary sediments ​(Mart and Robertson, 1998)​. Figure 2 demonstrates the overview of the Cenozoic sedimentary succession that will be further discussed.

The dispersal and localities of different sediment deposits allowed for three sediment cores to be extracted and logged throughout different localities of the Eratosthenes Seamount (Robertson, 1998)​. Their analysis has led to the Lower Cretaceous strata of the seamount correlating with the neritic carbonates (shallow marine carbonates) followed in succession by the pelagic carbonates facies from the Upper Cretaceous and shallow Miocene marine carbonates as shown in Figure 3 ​(Coletti et al., 2019)​.

Mesozoic (Cretaceous, Jurassic and Triassic)
The Eratosthenes Seamount is assumed to have detached from the Northern African-Arabian margin in the Mid-Upper Triassic during the rifting phase which led to the birth of the southern region of the Neotethys (Robertson, 1998; ​Nicolaïdes et al., 2017)​. Continued extension and spreading occurred into the Jurassic and Early Cretaceous and upon the termination of rifting movements and consequent volcanic activities the Eratosthenes Seamount developed into a shallow-water carbonate platform with dolomite, marls and sandstones ​(Robertson et al., 1995); Coletti et al., 2019)​. Site 967 from the ODP Sites penetrated these Mesozoic carbonates (Robertson, 1998). The presence of these deposits indicates a dependence upon light, therefore, occurring within the low energy environment of the photic zone such as a lagoon ​(Coletti et al., 2019)​. The great benefit of carbonate deposits being present in the Eratosthenes Seamount is that their presence records oceanographic oscillations in sea level ​(Coletti et al., 2019)​. As mentioned previously, the evolution of the sedimentary deposits transitioning from Lower to Upper Cretaceous is an indication of sea-level transgression linking to the emergence of pelagic sediment from neritic ​(Robertson et al., 1995).
Cenozoic (Paleogene, Neogene and Quaternary)
Sediment throughout the Cenozoic era was more affected by deformation than those deposited within the Mesozoic. In summary, the Miocene epoch showed increases in sediment deformation, leading into a quieter geological phase in the Lower-Mid Pliocene followed by notable rapid uplift in the region occurring from late Pliocene-mid Quaternary with decreasing effect defined by the emergence of the Holocene epoch ​(Taylforth et al., 2014; Coletti et al., 2019)​. The uplift that occurred around the Neogene resulted in sedimentation change from a deeper water pelagic sediment occurring of the Upper Cretaceous to photic-zone depths due to the physical movement of the seamount to shallower depths ​(Coletti et al., 2019).
Results stemming from the ODP Sites 966 and 967 provide evidence of this change through the identification of carbonate rocks ​(Coletti et al., 2019)​. The Pelagic sedimentation continued its deposition right up until the Eocene in the Late Paleogene. The transition of the shallowing sequence to the photic-zone depths in the Neogene started to occur in the Miocene and the limestones have been identified as consisting of calcite and dolomite (Robertson, 1998). The composition is of coralline algae, hermatypic corals and large benthic foraminifera (Robertson, 1998; ​Coletti et al., 2019)​. Despite slight variations in sea level from the Mesozoic to the Cenozoic, there was never any significant period where the calcite compensation depth was not met deduced by the continual presence of carbonate materials ​(Robertson et al., 1995).
The seamount was submerged throughout the Paleogene and uplifted several hundred meters in the Miocene. It was precisely during the Messinian Salinity Crisis when the Eratosthenes Seamount was uncovered. Through the ODP Sites, 965 and 966 paleosols and breccia were identified as part of the core and thus evidence of the seamount’s emergence ​(Coletti et al., 2019)​. After which Eratosthenes was submerged alongside tectonically instigated debris flow ​(Robertson et al., 1995)​.
Marine conditions are overlying the Messinian deposits. This is supported by two forms of evidence. Firstly, as a result of the Zanclean flood transgressing over the region, Zanclean sediment deposits are found over the Messinian ones and they are characterised as deep-water sediments ​(Coletti et al., 2019).​ Secondly, the region is overlain by deep water hemipelagic muds and sapropels (Robertson, 1998). Both of which cement the belief of the re-emergence of marine conditions within the Mediterranean 5.33 Ma.
The formation of these carbon enriched deposits researched through studies from Wehausen and Brumsack (2000), ​Taylforth et al (2014) and ​Incarbona and Di Stefano (2019) have linked sapropel formation in the area to humid climates with higher precipitation. This is paired with increases in fluvial run-off thus contributing to an increase in organic matter production ​(Taylforth et al., 2014, ​). All of which suggests after the Messinian evaporite environment, the influx of water into the system was driven by i) the submergence of the seamount by isostatic loading and ii) influences of climate conditions (Wehausen and Brumsack, 2000; ​Taylforth et al., 2014)​.
To tie the relationship between the later transition and change of sediments from the shallower Miocene to deeper Pliocene sediments is not solely attributed to the collision and underthrusting – yet this does account for the majority. The impact of the thick Messinian evaporite deposits just below the Zanclean sediments should be considered in their effect of adding significant flexural load in a geologically short period of time between 5.8 Ma and 5.32 Ma to the seamount and is likely to have contributed to its resulting submergence (Robertson et al., 1995; Coletti et al., 2019)​.
Classifying Miocene Sediments
Using organic facies to track paleo changes in water depth and environmental conditions to explain the sedimentary development was used in studies by ​Coletti et al (2019)​. It was deduced that the presence of glaucony grains were frequently occurring above the sharp boundary between pelagic, Eocene chalk and overlying coarser shallow-water Miocene limestones. Transitioning through upper deposits glaucony grains are less frequent and smaller ​(Coletti et al., 2019)​. This would suggest that sedimentation rates were slow to allow the development of large and frequent grains, however, the study found this contradicted with already determined carbonate production of the region which suggests the opposite: active and dynamic ​(Coletti et al., 2019)​. These glaucony grains were deduced to have undergone reworking from an earlier period where the seamount was in deep water where sedimentation rate was indeed slow. During the uplifting period in the Miocene, the reworking of these grains occurred as a result of the higher energy environment and thus, upper glaucony grains became less frequent eventually being replaced by carbonates
(Coletti et al., 2019)
Coletti et al (2019), has led to improvements in the sedimentary stratigraphy within the Miocene. Lower, Middle and Upper can now be categorised by carbonate facies as shown in Figure 4. The Lower Miocene is mostly large benthic foraminifera and echinoids as a result of the uplift to photic zone depth (Robertson, 1998; Coletti et al., 2019). The Middle Miocene has carbonate deposits of rhodoliths and corals a the base and top, with seagrass and seaweed in the middle ​(Coletti et al., 2019).​ The Upper Miocene had coral reefs due to paleo lagoon environment (Coletti et al., 2019)

Conclusion
Sedimentation development of the Eratosthenes Seamount has been diverse and sediment deposits have been regulated by changes in sea level. The Early Cretaceous saw the seamount in a shallow-water carbonate platform to pelagic carbonates in the Late Cretaceous (Robertson, 1998). The Miocene was a crucial moment as a rapid uplift of the seamount emerged to shallow waters (Nicolaïdes​ et al., 2017)​. An oligotrophic environment was present in the Miocene ​(Wehausen and Brumsack, 2000)​. Research from Coletti et al (2019) has determined three major carbonate units deposited within this time period. The Miocene was ended abruptly by the onset of the Messinian Salinity Crisis as a result of the desiccation of the Mediterranean Sea between ​5.96 and 5.33 Ma (Reiche, Hübscher and Ehrhardt, 2015). Overlying the Messinian deposits of salt and gypsum are marine sediments which indicate the submergence of the Eratosthenes Seamount in the Pliocene to a bathymetric low (Major, Ryan and Jurando-Rodriguez, Mart and Robertson, 1998; Coletti et al., 2019). The first layer being deep marine sediments of the Zanclean flood followed by sapropel deposits linked to the emergence of a eutrophic environment (Wehausen and Brumsack, 2000). The succeeding submergence of the Eratosthenes Seamount has been attributed to the isostatic loading of the thick deposits of the Messinian and as suggested by Wehausen and Brumsack (2000), also by insolation cyclicity which influenced the carbonate production through a humid climate and increased water influx (Wehausen and Brumsack, 2000).
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