Ichnofosilie panve Da Lat (okres Nha Trang, provincie Khanh Hoa, jihovychodni Vietnam)

The large area in southern Vietnam covers an extensive sedimentary Da Lat Basin. Its development was initiated during the marine transgression in the Lower Jurassic (Hettangian, ˜ 198 Ma) and ended by the regression during the Middle Jurassic (Bathonian, ˜ 165 Ma). Within the basin, two different developments can be distinguished, with the following formations - central area (Dak Bung Formation, Dak Krong Fm., Ma Da Fm., Song Phan Fm.) and marginal area (Dak Bung Formation, Dak Krong Fm., Ea Sup Fm.) (Khuc et al. 2005, 2006, see Table 1). The eastern part of the basin, in the province of Khánh Hoa, was systematically mapped for the first time by the Czechoslovakian geologists in the 80s, when a map series at a scale of 1 : 50 000 was compiled (Opletal 1990).
In 2016, a detailed paleontological research was carried out. As the target area of approx. 18 km² was selected the Hon Khoi Peninsula, situated 30 km north from Nha Trang, the capital of the Khánh Hoa Province. The sedimentary rocks of the Da Lat Basin are represented here by light to dark gray, fine-grained, thinly to thickly laminated, markedly folded siltstones and claystones of the Ma Da Formation (Aalenian-Bajocian, ˜170-174 Ma), occasionally with a slight indication of contact metamorphism (Figs 1, 2). Besides the relatively abundant fossil mollusc shells (? Myophorella sp., Fig. 2, GPS: 12°34.719’N, 109°14.660’E), numerous trace fossils and ichnofabrics were found (Skolithos, Thalassinoides, Palaeophycus and Lockeia), in high abundance (Figs 3A-E, GPS: 12°34.759’N, 109°14.680’E). The identified trace fossils belong to the Cruziana ichnofacies, revealing the sublittoral environment of the mid- to distal continental shelves, below the wave base. The paleoecological analysis is in good agreement with earlier investigations, based on the study of the ammonite faunas (Khuc et al. 2006) and simultaneously, the trace fossils described herein are for the first time published Jurassic traces in Vietnam.

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