Middle Jurassic trace fossils from the Ridang Formation in Sajia County, South Tibet, and their palaeoenvironmental significance

Abstract

An ichnofauna consisting of 18 ichnospecies (one of them new) in 14 ichnogenera are described for the first time from the Middle Jurassic Ridang Formation in Sajia County, South Tibet. The ichnofauna can be subdivided into two ichnoassemblages in ascending stratigraphic order: the Palaeophycus-Megagrapton ichnoassemblage in the lower and middle parts of the Ridang Formation, followed by the Cosmorhaphe-Nereites-Paleodictyon ichnoassemblage in the upper Ridang Formation. Overall, the trace fossils occur in a middle-distal turbidite fan sequence, as evidenced by both sedimentological analysis and the composition of the trace fossils. Several subenvironments of the middle-distal fan system have been recognized on the basis of the spatial distribution of the trace fossils. Typically, the channel-fill deposits in the middle part of the turbidite fan lack trace fossils, the interchannel and upper channel-fill (levee) subenvironments of the middle turbidite fan contain abundant faciescrossing trace fossils, in contrast to the distal part of the turbidite fan where deep-water trace fossils are dominant. The ichnofauna is similar to typical flysch ichnofaunas from Europe and North America in characteristics, and is interpreted to represent a typical deep-sea Nereites ichnofacies. The presence of these deep-sea trace fossils therefore would suggest that a continental slope environment existed in southern Tibet during the Middle Jurassic and the study area was located in a slope-abyssal plain setting.

Late Silurian trace fossils from the Melbourne Formation, Studley Park, Victoria, southeastern Australia

“…SHI et al ALCHERINGA environment, which affects sedimentation rate, sediment type and texture, seafloor oxygenation, and preservation of traces (Bromley 1996, Taylor et al 2003, MacEachern et al 2007, Zhang et al 2008. The repetition of Bouma sequences observed in many of the sandy strata and the repetition of sequences of fine-grained turbidite units in muddier intervals suggest that the Melbourne Formation at Studley Park was formed in a distal submarine fan environment of lower bathyal to abyssal depth (Figs 4, 5A).…”

Early Middle Triassic trace fossils from the Luoping Biota, southwestern China: Evidence of recovery from mass extinction

“…Megagrapton has been commonly observed in Permian to Cretaceous flysch strata in China (e.g. Zhang et al, 2008b). Palaeophycus Hall, 1847 3.6.1.…”

Historia de depósito de la Formación Concepción del Oro, noreste de Zacatecas, México: Implicaciones tectonosedimentarias para el desarrollo del sistema de antepaís en el noreste de México durante el Cretácico Tardío

“…Esta facies representa el depósito en masa, resultado del exceso de fricción intergranular o por el aumento de la cohesión entre los granos, en donde el transporte de sedimentos se efectuó por corrientes turbidíticas de alta concentración, ricas en lodos o por flujos de escombro arenosos. Las trazas fósiles se relacionan con organismos que se alimentan del sustrato en condiciones de moderado a alto régimen de flujo (cf., Heard y Pickering, 2008;Zhang et al, 2008).…”

“…La subfacies AL3.2 representa un depósito de corrientes turbidíticas de baja densidad o corrientes de fondo en áreas confinadas o donde se pierde la presión de confinamiento (cf., Pickering y Hiscott, 2016). Los fósiles traza están relacionados con organismos que se alimentaban del material en suspensión y del sustrato, y que posiblemente eran generadores de nichos (cf., Zhang et al, 2008;Heard y Pickering, 2008;Pickering y Hiscott, 2016).…”

“…En contraste, la caracterización de icnogéneros e icnoespecies asociados con sistemas marinos profundos es aún muy restringida y poco documentada (e.g., Seilacher, 1974;Uchman, 2009;Pickering y Hiscott, 2016). Los trabajos de Heard y Pickering (2008), Zhang et al (2008), Phillips et al (2010), Callow et al (2012; y Pickering y Hiscott (2016) documentan detalladamente las icnofacies arqueotípicas, distribución de fósiles traza e icnofábricas asociadas con el proceso sedimentario y paleoambiental, y su relación con los subsistemas profundos (e.g., lóbulos vs. canales), distinguiendo entre depósitos de abanico submarino proximal y distal, dentro de un entorno arquitectural (cf., Heard y Pickering, 2008;Phillips et al, 2010;Callow et al, 2014;Pickering y Hiscott, 2016).…”