Kvartérní sedimenty a nálezy rostlinných fosilií v Salvadoru, Střední Amerika

Abstract

Development cooperation projects implemented by the Czech Geological Survey in the area of Central America, focused on the evaluation of natural hazards, took place in the years 1997–2009. They started in Nicaragua (since 1997), continued in El Salvador (since 2003) and then also in Costa Rica (since 2006). The results of these projects were always handed over to local organizations (INETER, SNET and MINAE) in the form of final reports, and summarized last year in the book by Hradecký et al. 2021. As a supplement to this book, we present here some other findings of projects in El Salvador (from the years 2003–2005) that did not fit into its scope. They relate to the geology of Quaternary sediments and interesting findings of plant fossils preserved in the Neogene volcanosedimentary complexes. In the 2003 stage, work took place in the Gulf of Fonseca in south- -eastern part of El Salvador, with the dominant volcano Conchagua and the important port of La Unión. In addition to classic Quaternary sediments, the following phenomena were described: oyster hems (rims) around the islands of Periquito and Perico, volcaniclastic rocks (“tobas”) fallen into the lake environment near the Los Chorros lagoon (Břízová et al. 2004), and polygenetic sediments on the slopes of the formerly active volcano Conchagua, showing the existence of debris flows and lahars. Plant fossils have been found at one locality only, in fine-grained ash deposits of the maar sequence near El Naranjo (Figs 1 to 3). In the 2004 stage, work took place inland, in an area north of the capital city San Salvador, with the dominant volcano Guazapa. In addition to the classical Quaternary sediments, the following phenomena were described: polygenetic sediments of the depressions north of the town of Aguilares (Figs 4 to 9) and outcrops of lake sediments, exposed in the north of the studied territory on steep slopes near the Río Lempa dam (Fig. 10), south-east of the city of Colima (Havlíček – Vorel 2005). In the same area, on the bank of the dam near the town of San Cristobal, interesting consolidated (cemented) mudflows with the occurrence of fossil wood were also documented (Figs 11 and 12). In the year 2004, a paleontological expedition to the Río Sisimico canyon, about 40 km to the SE from the primarily studied area (Figs 13 to 16), was also carried out. The findings of plant remains from this locality are shown in Figs 17 to 22. Plant remains were further found at two localities in the primarily studied area: Suchitoto (Figs 23 to 29) and in the area south of El Molino (only indeterminable remains). Charred (and buried) tree trunks were also found in the white pumice tuffs called “Tierra Blanca” in several places of the studied area (Fig. 30). In the 2005 stage, the work took place in the north-western part of El Salvador, near the borders with Guatemala and Honduras, in the vicinity of the Metapán city. In addition to classic Quaternary sediments (Figs 31 and 32), the following phenomena were described: three generations of alluvial fans (alluvial gravels and sands) near the Metapán city, “kettle holes” created by water erosion in the river bottom (Fig. 33) and aeolian (wind-deposited) sediments from the Matalapa desert. The most interesting, however, were the secondarily tilted (and not yet fully consolidated) gravels and sands of the oldest generation of alluvial fans in the south-western surroundings of the city of Metapán (Figs 34 to 39). In the open cut along the CA12 road to Metapán (La Pedrera area, Fig. 40), small, although indeterminable remains of flora and possibly also fauna (fish scales?) in diatomite-like rocks were found. However, the locality is promising for future research. All the findings published in this paper were also mentioned in the final reports which were handed over in Spanish translation to the Salvadoran SNET institution (Hradecký et al. 2003, 2004 and 2005).