Astrobiochronological Constraints on Margin to deep basin correlations across the Balearic Promontory and the Valencia basin

Resumen

During the late Miocene, tectonic-driven processes promoted the closure of the Atlantic-Mediterranean gateways, leading to severe restriction of the seawater exchange and to the formation of thick evaporite successions across the basin. Such change in the configuration of the Miocene marine gateways had a direct impact on the exchange of heat, salt, and nutrients between the Atlantic and the Mediterranean. Therefore, it affected the heat transport and regional climate, as well as the global ocean circulation patterns. This event, known as the Messinian Salinity Crisis (MSC), lasted less than 650 kyrs and was a remarkable episode of the recent Earth´s history, in which more than 5% of the world ocean´s salt precipitated. The MSC affected the entire Mediterranean basin, however, it has mainly been explained on the basis of sediments from marginal settings. For those successions, biostratigraphic chronology has played a fundamental role in constraining the evolution of this crisis. Lack of complete and stratigraphically connected intermediate- and deep-water successions has inhibited establishing basin-wide correlations, as well as understanding the basin evolution during the crisis. As consequence, responses of intermediate and deep marine environments to the MSC and their chronostratigraphic relations with marginal regions are still poorly understood. Thereby, scenarios explaining the causes, progression and timing of the MSC remain controversial, as they are largely untested. The purpose of this Thesis is building high-resolution chronostratigraphic frameworks for sediments from intermediate-depth basins belonging to the Balearic Promontory (offshore Betic Range) and the adjacent deeper Valencia Basin. The goal is to provide reliable time frames for pre- and post-crisis successions by integrating biostratigraphic and well-logging data with seismic profiles. Establishment and improvement of the chronostratigraphic framework for these offshore sections is critical for (i) correlating the offshore extension of the Betic gateway with well-known marginal sedimentary successions, (ii) assessing the environmental settings before and after the MSC, (iii) allowing establishment of marginal-deep basin correlations, hampered until now by the absence of complete and/or geometrically connected successions, and (iv) characterising the origin and nature of the Mediterranean cyclical beds. The main outcomes of this Thesis are outline in the next paragraphs, as follows: - Oscillations displayed by various downhole logs provide an excellent opportunity for evaluating the origin and nature of the orbital driven cyclicity from the Western Mediterranean during Pliocene times. The regular alternations shown by the sonic and resistivity logs from the Muchamiel-1 drill-site (Balearic Promontory) are interpreted as orbitally driven clay changes. Therefore, log alternations are used for building a high-resolution model, derived from tuning well-log data to the La2004(1,1) solution, using planktic foraminiferal bioevents as independent age constrains. Well-logs demonstrate that precession and eccentricity mainly controlled sedimentation (clay supply) throughout the Pliocene. By contrast, the expression of obliquity is mainly recorded when (i) the amplitude of the precession-related component in insolation is reduced, especially at times of ~2.4-Ma eccentricity minima, and (ii) after the onset of major Northern Hemisphere Glaciation at ~2.8 Ma. There is no evidence supporting an interglacial/glacial control on the hydrological conditions and subsequent formation of the carbonate cycles between 5.33 and ~2.8 Ma. - Log patterns and location of the Muchamiel-1 drill-site allow to infer that cyclic log variability resulted from insolation-driven oscillations of the Atlantic westerly depressions, which may exert a principal control on river runoff of the Iberian Peninsula. The striking similarity between the Muchamiel-1 record and other sedimentary records from the Atlantic margin and the Western Mediterranean (Capo Rossello section, Southern Italy) allows establishing bed-to-bed correlations on a regional scale, indicating that the same climatic forcing may have been responsible for carbonate cycles across the Western Mediterranean and adjacent Atlantic areas. Hence, clay supply and development of cyclical carbonate-rich/carbonate-poor beds during Pliocene times was directly controlled by the intensity of peri-Mediterranean precipitation controlled by the North Atlantic System. These findings highlight the potential importance of peri-Mediterranean precipitation on the hydrology of the basin by providing freshwater required for seawater stratification and subsequent sapropel formation. Consequently, the origin of the Mediterranean cyclical bedding, with the possible inclusion of marl/sapropel alternations, results from the effect of precipitation changes driven by the Atlantic system and/or in combination with the in-phase influence of monsoonal runoff from North Africa. - Astrobiochronological study of two well-sites drilled along the SW margin of the Balearic Promontory indicate that the sedimentary record of this area includes late Tortonian–Messinian marls and evaporitic beds from the first phase of the MSC (i.e., Primary Lower Gypsum-PLG stage). Sedimentary characteristics and stratigraphic patterns observed in these offshore successions largely resemble those from marginal sequences, which allows to establish, for the first time, a reliable chronostratigraphic correlation between onshore marginal and offshore intermediate-depth sequences. Furthermore, the observed offshore PLG beds correspond well with reflectors from an MSC-related unit identified as the Bedded Unit (BU). Identification of those BU reflectors as PLG deposits, occurring along the Messinian continental shelves indicates that gypsum precipitation and/or preservation was not always limited to 200-meter water-depths and could occur in non-silled basins. Finally, these successions provide new insights into the PLG timing, precipitation/preservation settings, as well as into the land-sea correlations of MSC units, and thus could potentially help refine current MSC models. - The Valencia Basin is a deeper basin closely located to well-exposed onshore MSC-related successions, for this basin a high-resolution age model is presented based on integration of well-logs, biostratigraphy and cyclostratigraphy. Age models are built for two deep drill-sites (i.e., Ibiza Marino and Cabriel wells), covering pre- and post-crisis sedimentary record. Pre-crisis sediments span from the Tortonian up to Messinian (~6.4 to 8 Ma), whereas post-crisis sequences cover the entire early Pliocene (5.33 to 3.6 Ma). Sedimentation in both deep-marine records is characterized by successive meter-scale alternations that reflect consecutive changes in clay/organic matter content. As for the Balearic Promontory, sedimentation appears to be orbitally controlled, although a reduced clay input is observed due to a more distal location. Based on established age models, an important hiatus of at least 500 kyr, starting from ~6.4 onwards, is found in the Valencia Basin. Well-to-seismic ties performed in one of the studied drill sites (Ibiza Marino) allow to define and characterise the lithological nature of two MSC-related seismic units, known as Upper Unit (UU) and Complex Unit (CU). The established chronostratigraphic framework, although missing an astrobiochronologic control for the latest Messinian, provides a solid basis for establishing well-constrained correlations between marginal records and the deep Mediterranean seismic records. - Finally, throughout the development of this Thesis, it is demonstrated that well-logs are excellent tools for establishing basin-wide correlations. Furthermore, it is shown that downhole logging data may be valuable climate archives given their continuous record, high vertical resolution, and extended spatial distribution. Therefore, they can be used both for understanding long-term climate variations as well as for building high-resolution astrobiochronologic frameworks.