Lithofacies distribution of invertebrate and vertebrate trace-fossil assemblages in an Early Mesozoic ephemeral fluvio-lacustrine system from Argentina: implications for the Scoyenia ichnofacies

MELCHOR, R. N., BEDATOU, E., DE VALAIS, S. Y GENISE, J. F

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

Año: 2006 vol. 239 p. 253 - 285

0031-0182

This paper discusses the lithofacies distribution and ichnotaxonomic assignment of invertebrate, vertebrate and plant tracefossils in an Early Mesozoic ephemeral fluvio-lacustrine system from northwest Argentina. The overall palaeoenvironment isenvisaged as a distal, low-gradient fluvio-lacustrine setting under semi-arid climate, similar to a terminal fan. Depositional lobes(facies association A) are composed of distributary ribbon channels (facies A1), and associated pond and sheetflood deposits(facies A2). Lowlands between depositional lobes (facies association B) contained poorly drained floodplain zones similar towetlands (facies B1) and shallow closed lakes (facies B2). Depositional lobes were fringed by playa-lake settings (faciesassociation C), including mudflats (facies C1) that are partially eroded by ephemeral fluvial channels (facies C2). The ichnofaunaof the Santo Domingo Formation displays high ichnodiversity (30 trace fossil types) and includes 5 trace fossil assemblages that arelinked to sedimentary facies. These trace fossil assemblages are highly distinctive as half of the recognised trace fossil types arerestricted to a single assemblage. The ephemeral fluvial channel assemblage (facies A1 and C2) displays the lower ichnodiversity(n=6) and includes Skolithos linearis, Diplichnites sp., Palaeophycus tubularis, Taenidium barretti, Dicynodontipus sp. andfootprints in cross-section. The floodplain pond assemblage (facies A2) shows the higher ichnodiversity (n=15) and is composedof Diplichnites sp., Helminthoidichnites tenuis, P. tubularis, Rusophycus carbonarius, S. linearis, Spongeliomorpha carlsbergi, T.barretti, appendage marks, burrows with brush-like features, epichnial bilobed traces, hypichnial bilobed ridges, bird-like footprinttype A, tridactyl footprints, footprints preserved in cross-section, and small epichnial rounded pits. The poorly drained floodplainassemblage (facies B1) is of moderate ichnodiversity (n=10) and consists of H. tenuis, S. carlsbergi, ?Spongeliomorpha sp.,imbricated backfilled burrow fillings, root trace type A, Tetrasauropus sp., bird-like footprint type A, large pentadactyl footprints,tridactyl footprints, and footprints preserved in cross-section. The high ichnodiversity (n=13) nearshore lacustrine assemblage(facies B2) is the only assemblage that lacks vertebrate traces, and is composed of Cochlichnus anguineus, Cruziana problematica,H. tenuis, Palaeophycus heberti, P. striatus, P. tubularis, Scoyenia gracilis, S. carlsbergi, T. barretti, epichnial bilobed traces,hypichnial bilobed ridges, root trace type B, and scratch marks. The intermediate ichnodiversity (n=9) mudflat assemblage (faciesC1) is typified by the largest ichnodiversity of vertebrate tracks and includes H. tenuis, P. tubularis, S. carlsbergi, Dicynodontipussp., Tetrasauropus sp., bird-like footprints of types B and C, large pentadactyl footprints, and tridactyl footprints.The trace fossil assemblages from Santo Domingo Formation are typical representatives of the Scoyenia ichnofacies. Thedistinctiveness of the studied trace fossil assemblages suggests that a future division of the Scoyenia ichnofacies would be possible,provided that occurrences of invertebrate, vertebrate and plant trace fossils in definite sedimentary facies are considered. Thesubdivision of the Scoyenia ichnofacies into recurrent groups of trace fossils (ichnosubfacies) with a palaeoecological and37 palaeoenvironmental meaning can be based on two approaches: distinction of associations of vertebrate tracks and recognition of38 ichnocoenoses related to substrates with different degrees of water saturation and firmness. The first approach can be based on an39 evaluation of the recurrent vertebrate ichnoceonoses used to recognise vertebrate ichnofacies. In this way, some of the candidate40 vertebrate ichnofacies are proposed as subdivisions of the Scoyenia ichnofacies. The second approach is based on the identification41 of crosscutting relationships between trace fossils and between trace fossils and sedimentary structures within some trace fossil42 assemblages. Using this methodology, it is possible to define distinct assemblages of trace fossils: a “pre-desiccation suite” with43 structures lacking ornamentation and developed in a soft substrate and a “desiccation suite” typified by striated traces that crosscut44 the former. These trace-fossil assemblages help to characterise the changes in organism–sediment interaction during progressive45 desiccation of a water-lain substrate. Additional neoichnological studies are necessary to assess the exact meaning of different trace46 fossil features in terms of substrate consistency.