Cronoestratigrafia amb nanoplàncton calcari del Bartonià de la Conca de Jaca (Pirineu aragonès)

Abstract

In the Lower and Middle Eocene marine sediments of the Jaca Basin, despite recent biostratigraphic and paleomagnetic studies, there is a lack of high-resolution biostratigraphic information and the establishment of robust age models to mainly calibrate existing and future paleomagnetic studies. This work proposes to carry out a biostratigraphic study based on the content of calcareous nanoplankton in Bartonian sediments, with the characterization of different types of biohorizons defined by variations in the relative abundance of different taxa of biostratigraphic importance. The studied stratigraphic section records a transition from deep-marine sediments, with the Rapitán turbidite channel at the base, through intermediate slope and shelf deposits of respectively the Larrés Marls and Sabiñánigo Formation, following upwards in the Pamplona Marls and Atarés Delta formations. The succession culminates with the continentalization of the basin, marked by the onset of delta plain facies of the lower Campodarbe Formation. A chronostratigraphic age with calcareous nanoplankton is established and validated by the Bartonian of the Central Jaca Basin, spanning from the last deep marine basin stage of the Hecho Group, specifically the Rapitán turbidite channel at the beginning of the Bartonian, to the continentalization of the basin with the onset of the lower Campodarbe Formation in the early Priabonian. In this succession, the NP16 and NP17 biozones of Martini (1971); CNE14, CNE15, CNE16, and CNE17 of Agnini et al. (2014); CP14 and CP15 of Okada and Burky (1980) have been identified. These biozones are characterized by the presence of several taxa of Reticulofenestra (Hay et al., 1966), Sphenolithus (Deflandre in Grassé, 1952), and Chiasmolithus (Hay et al., 1966). Bartonian paleoclimatic variations are inferred from the relative abundance of specific taxa: Zygrhablithus bijugatus (Deflandre in Deflandre and Fert, 1954); Chiasmolithus (Hay et al., 1966); Reticulofenestra (Hay et al., 1966); and Cyclicargolithus (Roth and Hay, in Hay et al., 1967). These variations have allowed the identification of a cold climate period at the base of the Bartonian, corresponding to the coarse-grained deposits of the Rapitán system. At the beginning of the transition to the Larrés Marls, a sharp increase in warm-water taxa is recorded, which may be related to the Middle Eocene Climatic Optimum (MECO, 40-40.75 Ma). In addition, this relative peak in warm-water taxa abundance, a relatively long period begins in which taxa related to temperate waters also show high values, encompassing the Larrés Marls, Sabiñánigo Sandstone deposits, and much of the Pamplona Marls. The latter is associated to the post-MECO stage. At the transition between the Pamplona Marls and the Atarés Delta (between 38.25 and 37.83 Ma), a relative decrease in warm and temperate water taxa has been detected, countered by an increase in cold-water taxa possibly associated with the precursor pulses of the Priabonian Oxygen δ18 Isotope Maximum (PrOM, 37.00 Ma). The paleoclimatic study suggests correspondence between the relative temperature values obtained and the Earth eccentricity cycles of 400,000 years. The type of reworked nannoplankton taxa suggests an origin in fine-grained marine sediments from the Upper Cretaceous, possibly from the thrusts forming the Central South Pyrenean Unit.

Date of Award	5 Nov 2024
Original language	Catalan
Supervisor	Eduard Remacha Grau (Director), Josep Oriol Oms Llobet (Director) & Jose Abel Flores Villarejo (Director)