An analogue of dominance of tectonic over climatic forcing in intermontane coal-bearing basins: Padul (SE Spain)
Introduction
Lacustrine basins with peat and/or sapropel accumulations are of economic interest. They have often been drilled and studied for coal or oil reserve prospection (Cabrera and Saez, 1987; Wehmeyer et al., 1986; Whateley, 1986; Crowley et al., 1993; Diessel et al., 2000; Carrol and Bohacs, 2001; Sáez et al., 2003; Erdenetsogt et al., 2009). In this regard, the Padul Basin (PB) has attracted geological interest after the first World Oil Crisis of 1974, when the basin was intensely prospected for coal (peat) reserves (Enadimsa-Endesa, 1981).
In the case of lacustrine deposits, tectonics may have played a crucial role in their development. Examples of such deposits are found worldwide and include the Kathmandu Basin in Nepal (Dill et al., 2001), the Tibetan Plateau (Zhan et al., 2018), the Intra-Sudetic Basin in the Czech Republic (Cabrera and Saez, 1987; Oplŭstil et al., 2013), Lake Mucubají in Venezuela (Carrillo et al., 2008), the Natuna Basin in Indonesia (Morley et al., 2003), the Fucino Basin in Italy (Cavinato et al., 2002), the Soma-Isikar Basin in Turkey (Whateley, 1986), the Dzhazator River Valley in Russia (Agatova et al., 2017), and many others. The geological structure of the PB has been linked to the convergence of the Eurasian and African plates during the Alpine Orogeny (Argus et al., 1989). Thus, sedimentation in the PB was controlled mainly by the still active Padul-Nigüelas Fault (Doblas et al., 1997; Sanz de Galdeano and López-Garrido, 1999; Sanz de Galdeano and López Garrido, 2001). This normal fault provided continuous subsidence at its western footwall, where the alluvial fan apices are aligned along the fault trace at 800–900 m a.s.l. (García et al., 1998; Alfaro et al., 2001a, Alfaro et al., 2001b) (Fig. 1) and fault-drag alluvial fan deposits are visible. On the western boundary of the PB, the Albuñuelas Fault throw is small and the basin can be described as an asymmetric graben.
Furthermore, organic-rich lacustrine deposits are excellent archives of environmental changes. However, long Quaternary records covering more than 300 ka are scarce: Velay (Southern France), with ca. 450 ka (Reille et al., 2000; de Beaulieu et al., 2001); Valle di Castiglione (central Italy), with ca. 300 ka (Follieri, 1988); Ioannina (Western Greece), with ca. 480 ka (Tzedakis, 1994); Kopais (Southeastern Greece), with ca. 500 ka (Tzedakis, 1999; Okuda et al., 2001); Tenaghi Philippon (Northeastern Greece), with 1.35 Ma (Tzedakis et al., 2002, Tzedakis et al., 2006; Pross et al., 2015); and Guadix-Baza (Southeastern Spain), with ca. 1.8 Ma (Ortiz et al., 2006). In this regard, in the southwestern Mediterranean realm an exceptional combination of subsidence and sedimentation resulted in one of the few examples of long palustrine/lacustrine sequences with a unique palaeoclimatological record. The PB contains one of the longest terrestrial records in the world, with more than 100 m covering ca. 1 Ma (Ortiz et al., 2004, Ortiz et al., 2010, Torres et al., 2020). The relevance of the PB record was recognised in the pollen studies of Menéndez-Amor and Florschütz, 1962, Menéndez-Amor and Florschütz, 1964, Florschütz et al. (1971) and Pons and Reille (1988). In Dekkers, 1997, in the framework of the EU-funded project EQUIP and Paleoclima (funded by the Spanish Agency for Radioactive Waste Management and the Spanish Nuclear Safety Council), a new 107-m borehole (SPD) was drilled in the depocenter of the basin, in which the largest sedimentary record was recovered. Thus, the organic geochemistry and carbon stable isotopes of the sedimentary record of SPD allowed the paleoenvironmental reconstruction of the PB (Nestares and Torres, 1998; Ortiz et al., 2004, 2010), and a palynological study of the SPD core by Torres et al. (2020) provided the paleoclimatic evolution of the PB over the last 800 ka.
Recently, Camuera et al., 2018, Camuera et al., 2019 studied a shorter core (Padul-15-05) (42.64 m deep) covering 200 ka in a marginal area of the basin, although the chronological model used by those authors should be taken with caution, as it was built using only two ages above 50 ka.
Therefore, given its exceptional characteristics, including a consistent chronological model including diverse numerical datings (14C, Th/U, amino acid racemization and paleomagnetism) (Ortiz et al., 2004; Torres et al., 2020), the PB can be considered as an ideal model to infer the sedimentary evolution of intermontane coal-bearing basins and to study the interplay between tectonic and climatic forcings. For this purpose a multiproxy study based on sedimentology, mineralogy, magnetic susceptibility, organic geochemistry and fossil content was used to infer the tectonic influence and climatic control of sedimentation and facies distribution in these type of basins. Thus, the aims of this study are to: 1) analyze the geometry of the PB and its facies distribution through the detailed study of 18 borehole cores; 2) model the sedimentary evolution of the basin through a multiproxy study based mainly on the “section type” (> 100 m); and 3) determine the influence of recent tectonics and paleoclimate variations on facies arrangement.
Section snippets
Geographical and geological settings
The PB catchment covers a total area of 44 km2, while the peatland covers 4.6 km2 and is found 720 m a.s.l. (Fig. 1). The basin is a closed flat depression 12 km south of Granada (Fig. 2A). The village of Padul lies on its northern border.
The western boundary is formed by the elevations of the Albuñuelas range (1000 m a.s.l), while the eastern one is the bald and craggy Sierra Nevada, which reaches its highest point at the Mulhacén peak (3482 m a.s.l.), only 20 km east of Padul. The northern
Material and methods
To evaluate coal reserves, in 1981 Enadimsa-Endesa drilled a total of 16 borehole cores with a conventional drill pipe and continuous core recovery (Fig. 1). These cores were used here for basin analysis and stratigraphic correlation purposes, together with the Padul 15–05 core described in Camuera et al. (2018). The drilling operations of the SPD Padul core were performed by the Geological Service of Spain, using a drilling ring with a conventional drill pipe and direct flow of bentonite-water
Results
The lithological description of the PB core is provided in Fig. 5. Coarse materials were predominant in the lower part of the core (from 107 to 70 m) with scarce peat intervals, whereas in the middle part (between 70 and 50 m) peaty muds (gyttja) were dominant. In the uppermost 50 m, peat was dominant, with some marly and sandy interbeds at 37–33 m and 7–5 m respectively. These observations were in good agreement with the magnetic susceptibility values, which were relatively high in the lower
Sedimentary units
On the basis of the sedimentological record, combined with data on the mineralogy, fossil content, magnetic susceptibility and total organic carbon (TOC, Ortiz et al., 2004), three main sedimentary units in the PB were identified.
Of note, magnetic susceptibility may indicate the input of detrital sediments into the basin. In this regard, high values of magnetic susceptibility coincided with gravel and coarse sand presence (Fig. 5). In contrast, low of magnetic susceptibility were obtained in
Conclusions
The PB is an intermontane coal-bearing basin with exceptional characteristics that makes it ideal for modeling the sedimentary evolution and facies arrangement of similar areas.
The fence correlations of 18 cores, allowed to identify the facies distribution and to model the sedimentary evolution. The PB had more subsidence at its northeastern boundary, in which a series of alluvial fans with their catchments in the Sierra Nevada developed. The sedimentation characteristics and thickness
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Funding was obtained through the projects “Evidence from Quaternary Infills Palaeohydrogeology” (European Union, F14W/CT96/0031), “Evolución Paleoclimática de la Mitad Sur de la Península Ibérica” of ENRESA (National Company for Radioactive Waste Management, 703238) and “Paleoclima” of ENRESA and CSN (Spanish Nuclear Safety Council). Endesa (Y. Ivanovic) gave valuable information of the cores. We thank two anonymous reviewers for their valuable and helpful comments on the manuscript.
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