Eruptive evolution and 3D geological modeling of Camp dels Ninots maar-diatreme (Catalonia) through continuous intra-crater drill coring
Introduction
Maar-diatreme structures are common in monogenetic volcanic fields forming as a result of phreatomagmatic episodes caused by magma-water interactions (Lorenz, 1973). Several models exist on their formation and evolution (Lorenz, 1975, Lorenz, 1986; Geshi et al., 2011; Valentine et al., 2011; Valentine and White, 2012; Graettinger et al., 2014, Graettinger et al., 2015; Ross et al., 2017). Maar eruptions are mostly investigated through the study of the deposits forming their tuff or tephra rings (e.g., Pedrazzi et al., 2014; Valentine et al., 2015). Moreover, intra-crater successions also provide valuable information to understand the formation of such particular volcanoes and their diatreme architectures, where most of the studies come from kimberlite pipe exploration (e.g., Sparks et al., 2006; Lorenz and Kurszlaukis, 2007; Brown et al., 2009; Seghedi et al., 2009) and from old exhumed maars or eroded structures (e.g., Suoana maar in Miyakejima Volcano, Japan (Geshi et al., 2011); Hopi Buttes, USA (White, 1991; Vazquez and Ort, 2006; Lefebvre et al., 2013; Latutrie and Ross, 2019, Latutrie and Ross, 2020), Missouri River Breaks volcanic field, USA (Delpit et al., 2014); Coombs Hills, Antarctica (McClintock and White, 2006; Ross and White, 2006), etc.). Numeric and analogue modeling of maar formation has contributed to the understanding of these processes and their diatreme architectures (Valentine et al., 2011; Ross et al., 2013; Graettinger et al., 2014; Le Corvec et al., 2018). However, only scarce research drilling has been carried out in maar intra-diatreme facies, commonly performed to calibrate geophysical prospections (e.g., Pirrung et al., 2003; Schulz et al., 2005; Martín-Serrano et al., 2009; Bolós et al., 2012; Elliott et al., 2015; Jones et al., 2017), hence, the deepest regions of diatremes are still not well-understood. Despite the similarities in the eruptive processes involved in the formation of such these volcanoes, not all the resulting structures have similar shapes and dimensions. Measured diatreme dimensions range from deep narrow structures found in kimberlite pipes (e.g., Pittari et al., 2008) to shallower and wider funnel-shaped structures found in basaltic to andesitic maar volcanoes, such as the completely exposed Suoana maar-diatreme in Japan (Geshi et al., 2011). The diatreme morphology is influenced by differences in magma composition (e.g., volatile content), in the way magma/water interaction is produced, the mechanical characteristics of the substrate on which these volcanoes form (Bolós et al., 2014; Németh and Kósik, 2020), and by the lateral migration of explosion locations during eruptions (Ort and Carrasco-Núñez, 2009; Jordan et al., 2013; Blaikie et al., 2014; Saucedo et al., 2017; Graettinger and Bearden, 2021).
The Camp dels Ninots maar-diatreme (CNMD) is an example of an eroded tuff ring from which the external structure has been completely dismantled (Vehí et al., 1999). Its recognition as a maar structure is constrained by geophysical evidence (Oms et al., 2015) and the study of the intra-crater deposits, which were drilled by nine boreholes (total depths between 15 and 80 m) made for research and hydrological purposes (Jiménez-Moreno et al., 2013; Rodríguez-Salgado et al., 2021). In particular, its lacustrine sedimentary infill contains one of the richest fossil records of Pliocene fauna in Europe (3.06 Ma) (Campeny and Gómez de Soler, 2010; Gómez de Soler et al., 2012; Claude et al., 2014; Campeny et al., 2015; Přikryl et al., 2016). Two more recent boreholes (CP1 and CP2) (Fig. 1d) were drilled inside the maar crater to a depth of 113 and 145 m, respectively, aiming to obtain a volcanic record of the diatreme infill succession as well as constraining the necessary data to generate a 3D geological model. Continuous core recovery (recovery wireline ratio > 95%) was carried out for both boreholes, thus providing the required information to study the evolution of the CNMD and its internal architecture.
This study is one of the few cases in which a non-kimberlitic maar-diatreme structure is drilled to sufficient depth to study its structure and evolution. These observations provide the necessary evidence to state the main differences between the well-known diatreme structures of kimberlite pipes and other maar types such as the CNMD. We have integrated the existing borehole records with surface geology and geophysical data in a 3D geological model, which helps to define the geometry and orientation of the main fault that controlled the magma's rise to the surface, the morphology of the diatreme, its internal facies, and the layout and thicknesses of the country rock basement.
Section snippets
Geological setting
The CNMD Pliocene lacustrine infill has been dated from 3.3 to 3.1 Ma (Jiménez-Moreno et al., 2013; Carrancho et al., 2012), and therefore the eruption is slightly older. It is located in the town Caldes de Malavella (Girona province, Catalonia) and forms part of the Catalan Volcanic Zone (CVZ), one of the volcanic provinces of the European Cenozoic Rift System (Martí et al., 1992; Martí and Bolós, 2019) (Fig. 1a). The tectonic evolution of this volcanic region was controlled by the Neogene
Methodology
The 2015 coring campaign was planned in order to log the lacustrine deposits and the inner diatreme of the CNMD. The CP1 borehole reached a total depth of 113 m and was located near the inferred center of the diatreme, as determined by the minimum gravity anomaly and electrical resistivity data (Oms et al., 2015) (Fig. 1c). This drill core shows the complete post-maar lacustrine sequence, and part of the eastern wall of the diatreme, penetrating more than 10 m into the basement granites (Fig. 2
Volcanic-facies descriptions
The description of the CP1 and CP2 drill cores permits the identification of three groups of lithofacies. The first lithofacies (L1) consists of a tuff-breccia intra-diatreme deposit (Fig. 3a, b, c), which is the most predominant in the drill cores. In CP1 it forms 91.1% of volcanic succession and 9.9% corresponds to Paleozoic granitic basement (Fig. 2), while in CP2 volcanic successions are identified from 63.2 to 115.2 m (i.e., 52 m in total), accounting for 63.5% of the core (Fig. 2). The
Discussion
The continuous intra-crater succession recognized inside the CNMD provides the clues to reconstruct the eruption that generated this maar structure (Fig. 7). The onset of the eruptions (i.e., the opening crater phase) was marked by an intense phreatomagmatic phase resulting from the fuel coolant-interaction of basaltic magma with water contained in the highly fractured and faulted Paleozoic granitic rocks of the basement (Fig. 7a — Stage 1). The deposits generated by this initial explosion
Conclusions
The compilation of all the available geophysical data combined with surface geology and 11 boreholes including two new drill cores (113 and 145 m depths), extracted near the inferred center of the diatreme of CNMD, allows us to construct a robust 3D geological model of the inner diatreme fill and the eruptive evolution of this partially eroded volcano. The final 3D morphology of the diatreme confirms a geometry that results from the mechanical behavior during the eruption of a mixed soft and
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
We would like to thank the landowners, especially Mr. Jaume (Met), for their permission and help to work in the area, and the Town Council of Caldes de Malavella. We also would like to the Departament de Cultura of the Generalitat de Catalunya for the economic funding of the drill coring campaign under the project CLT009/18/00052. This study was partially funded by the project of the Spanish Government and the SGR2017-859 and SGR-2017-1666 projects of the Generalitat de Catalunya. X. Bolós was
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