PPalaeoenvironmental significance of beds bioturbated by Haentzschelinia ottoi (Geinitz) in delta front facies, Lajas Formation (Middle Jurassic)

Abstract

The distribution of Haentzschelinia ottoi (Geinitz) is studied from the Lajas Formation (Middle Jurassic) of the Neuquén Basin in Arroyo Carreri. For this work were considered more than 200 specimens and their relation with the facies and other trace fossils, as well as the ichnofabric index (ii) and the bedding plane bioturbation index (bpbi) were determined. H. ottoi appears in fluvio-dominated delta front (FA2) and fluvio-dominated delta front to delta plain with tidal influence (FA3). In general, trace fossils correspond to deposit-feeders (H. ottoi, Planolites, ?Taenidium, ?Parahaentzschelinia), suggesting the development of an impoverished Cruziana ichnofacies. In particular, H. ottoi constitutes monoichnospecific associations with ii/bpbi = 2-3 (in FA2 and FA3) or more diverse associations with other trace fossils and ii/bpbi = 3-4 (only in FA3). The development of one association or another depends on the interplay between sediment discharge and salinity conditions. More diverse associations with ii/bpbi = 3-4 would be favoured by a rise in salinity under the influence of tides and enough time for substrate colonisation by other deposit-feeders. Lower salinity conditions or high frequency of sediment discharge would have given rise to monoichnospecific associations of H. ottoi with ii/bpbi = 2-3.