Ichnology of ephemeral fluvial systems: an example from the Late Triassic Santo Domingo Formation, Nortwest Argentina.

RICARDO NESTOR MELCHOR; EMILIO BEDATOU,; SILVINA DE VALAIS,

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Congreso; Ichnia 2004, First International Congress on Ichnology; 2004

Museo Paleontológico Egidio Feruglio

The upper part of the Late Triassic Santo Domingo Formation from Northwest Argentina (Ischigualasto-Villa Unión Basin) contains a diverse vertebrate and invertebrate ichnofauna. The traces appear in a 200 m-thick interval deposited in a terminal fan setting. This interval contains four trace fossil assemblages occurring in distinct facies associations. The deposits of ephemeral fluvial channels, proximal sheetfloods and associated ponds (A) contains Taenidium barretti, Spongeliomorpha milfordensis, Palaeophycus tubularis, Helminthoidichnites isp., Dicynodontipus isp., Nereites isp., Skolithos isp., probable arthropod locomotion traces, highly bioturbated horizons, bird-like footprints, footprints preserved in cross-section and medium-sized theropod footprint. The facies association of intermediate to distal silty sheetflood deposits (B) includes Helminthoidichnites tenuis, Palaeophycus tubularis, Spongeliomorpha milfordensis, Tetrasauropus isp., bird-like footprints, tridactyl footprints, and abundant footprints preserved in cross-section. The facies association of evaporitic mudflats and sandy ephemeral channels (C) have yielded traces attributed to Planolites beverleyensis, Skolithos linearis, Taenidium isp., Diplichnites isp., Tetrasauropus isp., Dicynodontipus isp. and small lacertoid footprints. The fourth trace fossil assemblage is the less diverse and occurs in shallow lacustrine deposits that suffered frequent water-level changes (D). Recorded trace fossils include Cochlichnus anguineus, Nereites macleayii, Palaeophycus tubularis, Palaeophycus cf. striatus, Palaeophycus heberti, Scoyenia gracilis, Spongeliomorpha carlsbergi, Spongeliomorpha milfordensis. Some trace fossil assemblages exhibit distinctive compositional and taphonomic features. Proximal to distal sheetflood deposits (A & B) display the highest ichnodiversity and common presence of Taenidium, Nereites, bird-like footprints. Distal to intermediate silty sheetflood deposits (B) contains common footprints in cross-section and shallow lacustrine deposits (D) is dominated by burrows with striations.