Small sized charophyte gyrogonites in the Maastrichtian of Coll de Nargó, Eastern Pyrenees: An adaptation to temporary floodplain ponds
Introduction
Floodplain facies are very common non-marine depositional settings in the Upper Cretaceous of Eurasia, North America and Asia. This kind of deposits are accumulated after inundation events and consist predominantly of suspended load i.e. silt and mud, although fine sand may also be present in areas where the peak flood currents are stronger (Einsele, 2000). In floodplains, small and unstable ponds may develop where charophytes can thrive for short periods of time. Many palaeontological studies have been performed on such deposits, including the characterisation of fossil biotas near the K–Pg boundary in the non-marine domain. In floodplain settings of the Upper Cretaceous, charophytes are an important biostratigraphic tool (e.g. Peck & Forester, 1979 in North America; Bhatia and Rana, 1984, Wang et al., 1985, Van Itterbeeck et al., 2005 and Khosla, 2014 in Asia; Jaillard et al., 1993 in South America; Galbrun, Feist, Colombo, Rocchia, & Tambareau, 1993 and Vicente, Martín-Closas, Arz, & Oms, 2015 in Europe); however, their palaeoecology has been poorly understood up to now. It is possible that the low diversity of charophyte floodplain assemblages in comparison with assemblages from lacustrine facies might explain the limited palaeoecological interest they have aroused to date. Small sized fructifications could be a second reason why floodplain charophytes remained unnoticed in laevigates from floodplain sediments, despite their remarkable value as regards dating this type of deposit in the Upper Cretaceous (Galbrun et al., 1993, Vicente et al., 2015).
The reason why fossil charophytes from temporary ponds of floodplains generally bore small gyrogonites is completely unknown. Although this feature has previously been remarked upon by other authors such as Musacchio (2010), only a few studies have attempted to determine the parameters which control gyrogonite size in extant charophyte species (Pedrola and Acuña, 1986, Soulié-Märsche, 1989), whereas more data are available about this relationship in oospores. Thus, Casanova (1997) concluded that species occurring in temporary habitats show a consistent selection in the oospore sizes, in contrast with plants growing in permanent water bodies, which show more variable fructification sizes. According to Soulié-Märsche (1989), the size, shape and morphological characters of gyrogonites in extant Chara, Lamprothamnium, Lychnothamnus and Nitellopsis vary within a species, depending on the degree of calcification. The same author also concluded that the gyrogonite size may vary depending on the environment, i.e. Lamprothamnium populosum shows significantly larger gyrogonites in perennial water bodies than in temporary ponds. This phenotypic plasticity has also been reported in fossils, for instance in the Palaeogene species such as Lychnothamnus vectensis, Lychnothamnus pinguis and Chara artesica (Sanjuan and Martín-Closas, 2014, Sanjuan and Martín-Closas, 2015) and in the Late Cretaceous species Microchara nana (Vicente et al., 2015). Intraspecific variation in fossil and extant fructifications appears to be a response to certain environmental conditions. Thus, Casanova (1997) has shown that environmental factors such as water temperature, water chemistry or the allocation of resources to oogonia during their development could control the size and shape of oospores. However, the relationship between causes (environmental conditions) and effects (e.g. oospore and gyrogonite size) is still poorly understood.
Here, we conduct a palaeoecological analysis of charophytes occurring in small water bodies of floodplain deposits from the Upper Cretaceous Coll de Nargó depocentre, Tremp Basin, in the Eastern Pyrenees (Catalonia, Spain), and explore how palaeoenvironmental features influenced gyrogonite size. This case study will be compared with three other examples: (1) the Lower Cretaceous of the Iberian Chain (Central Spain), (2) the Upper Cretaceous of America, with two case studies from Argentina and Alaska (United States), and (3) the upper Eocene–lower Oligocene of the Ebro Basin (Catalonia), all of them with charophytes occurring in temporary ponds of the floodplain facies. The aim is to provide a palaeoecological framework which will contribute to understanding the distribution of charophytes in a floodplain environment in other northern hemisphere basins with extensive development of similar facies.
Section snippets
Material and methods
Three representative sections of the Grey and Lower Red units in the Upper Cretaceous of the Coll de Nargó depocentre were systematically sampled to identify charophyte assemblages (Fig. 1, Table 1). Up to 30 samples were taken in the field, 6 at Mirador del Cretaci section, 21 at Santa Eulàlia lignite pit mine section and 3 at the Sallent River gauge station. Charophyte fructifications were obtained after disaggregating a standardised weight of 3 kg of marls and claystones, screen-washing the
Geological setting
The Pyrenees were formed by the convergence and oblique collision of the European and Iberian plates from the Campanian to the Oligocene (Muñoz, 1992). This collision progressed south to north and displayed an anticlockwise rotational pattern (Ardèvol et al., 2000, Capote et al., 2002). As a result, the south-central Pyrenees now consist of three superimposed thrust sheets running east to west which are called, from north to south, Bóixols, Montsec and Serres Marginals (Muñoz, Martínez, &
Palaeobotanical setting (charophytes)
The Late Cretaceous–early Palaeocene transition is an important time span in the evolution of charophytes. Relict species of the clavatoracean and the porocharacean families finally became extinct after a long period of decline, and the characean family diversified (Grambast, 1974). Despite this, only a few studies have focused on characterising European Maastrichtian charophyte assemblages, most of which are from southern France, i.e. Provence, Languedoc and the northern Pyrenees, and also
Sedimentology, charophyte taphonomy and palaeoecology
The well-exposed Upper Cretaceous succession of the Coll de Nargó depocentre is useful for characterising the depositional environment of the charophyte-bearing rocks. The combined results of sedimentology and taphonomy are intended to shed light on the palaeoecology of these ancient charophytes. From east to west, the three sections studied were the Sallent River gauge station, the Santa Eulàlia lignite pit mine and the Mirador del Cretaci palaeontological site (Fig. 4).
Comparative case studies
The gyrogonite size selection found in charophytes from floodplain ponds of the Maastrichtian of Coll de Nargó is not exclusive to this basin. Floodplain pond deposits from different basins and ages display a similar dominance pattern of species bearing small gyrogonites. Four examples belonging to three different time periods are presented below.
Discussion
Charophyte assemblages from the Maastrichtian Lower Red Unit in the Coll de Nargó depocentre within the Tremp-Graus Basin (southeastern Pyrenees) show significant variation in their composition, depending on the environment in which they grew. Species-poor charophyte assemblages are related to temporary ponds on fluvial floodplains and are represented by extremely small gyrogonites of Microchara, particularly M. cristata and M. nana, and less frequently M. punctata. This pattern has been found
Conclusions
This study sheds new light on the sedimentology and charophyte palaeoecology of the Maastrichtian Grey and Lower Red units of Coll de Nargó, Tremp-Graus Basin, Eastern Pyrenees. Stratigraphic and sedimentological analyses show that the Grey Unit displays a strong lateral change in facies from east to west. Brackish marls build up most of the unit in the western part of the depocentre (Mirador del Cretaci section), while charophyte limestone intercalated with lignite and marl is the rule to the
Acknowledgements
This study is a contribution to projects CGL2011-27869 of the Spanish Ministry of Economy and Competitiveness (MINECO) and 2014SGR-0251 of AGAUR, the Catalan Research Agency. Alba Vicente's research is partly funded by a fellowship from the same Ministry (BES-2012-057837). The field work was authorised and partly financed by the Department of Culture of the Catalan Autonomous Government (Reference 2014/100927). We acknowledge the corrections and comments of Dr. Ingeborg Soulié Märsche (Univ. of
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