Trace Fossil Diversity in Hyperpycnites: Ethologic Implications and Comparison to Trace Fossil In Episodic Gravity Flows

Noelia Carmona, Juan Jose Ponce, M.I. Lopez-Cabrera, and Eduardo Olivero
Laboratorio de Geología Andina - Centro Austral de Investigaciones Científicas (CADIC-CONICET). Bernardo Houssay 200. (V9410CAB) Ushuaia, Tierra del Fuego, Argentina

1. Introduction

Hyperpycnal flows, generated by extraordinary fluvio-derived discharges, are characterized by sustained sedimentation, fluctuations in flow velocity, and usually have high concentration of phytodetrital influx. On the contrary, “classical turbidites” are generated by episodic gravity flows, after slump events or other gravitational collapses in unstable depositional slopes, and are characterized by a rapid deceleration due to the short-lived sediment influx and the growing friction with the sea floor.

In the lower Miocene of the Austral Basin foredeep, thick hyperpycnal deposits generated by hydraulic jumps were recognized at the toe of depositional slope breaks. Six sedimentary facies were identified; each one characterized by transitional and recurrent passages of sedimentary structures, without sharp rheological boundaries and with distinctive trace fossil associations. Particularly, the composition of the trace fossil suites and their distribution in these deposits, differ from the typical associations identified in episodic gravity flow deposits (e.g. classical turbidites). Consequently, the main objectives of this study are to discuss and determine the implications of these fluvio-derived flows in the spatial distribution of the biogenic structures, and to compare these trace fossil suites to those found in episodic gravity flows deposits.

2. Sedimentary facies and trace fossil associations

Six main sedimentary facies (F1-6) were recognized in the studied outcrops (Figure 1). F1 comprises thick packages, up to 3 m thick, of pebbly sandstones and sandstones, with sharp to erosive surfaces, and with coarsening-thickening trends followed by fining-thinning trends. Internally, each package consists of transitional passages between massive sandstones with deformation structures, and pebbly sandstones with cut-and-fill structures. Pebbles are usually imbricate and have their largest axis oriented normally to the flow direction. This facies has no bioturbation. F2 consists of thick packages, up to 1.5 m thick, of pebbly sandstones and medium- to fine-grained sandstones, with sharp surfaces, and with coarsening-thickening trends followed by fining-thinning trends. Internally, it consists of transitional passages between massive sandstones, and pebbly sandstones and sandstones with parallel lamination, climbing dunes, and cut-and-fill structures. Bioturbation is absent in this facies. F3 consists of fine sandstones with coarsening-thickening trends followed by fining-thinning trends. Internally, it shows parallel lamination with parting lineation, and asymmetrical ripples. This facies shows a monospecific association of Diplocraterion parallelum (Figure 2A). F4 comprises fine sandstones with climbing ripples and moderate phytodetrital content. Scolicia prisca and Nereites cf. missouriensis dominate the ichnoassemblage (Figure 2B). F5 consists of rhythmical successions of heterolithic, thinly bedded fine-grained sandstones and mudstones. These heterolithics show wavy and lenticular bedding, with high phytodetrital content. Scolicia isp. (Figure 2C) and small Rosselia-like structures are the only biogenic structures recognized in this facies. F6 consists of massive and laminated mudstones, with minor sandstone lenses. In this facies, the diversity and abundance of trace fossils show an important increase, and the suite consists of Asterosoma isp., Cardioichnus cf. planus, Chondrites cf. patulus, Thalassinoides isp., ?Phycodes isp., Protovirgularia dichotoma, Scolicia isp., Stelloglyphus isp. and Tasselia isp. (Figure 2D).

3. Depositional mechanisms

Figure 1 shows an idealized representation of the sedimentary facies tract and the ichnologic content in a proximal-distal trend. Variations in sediment concentration produced by fluctuation in flow velocity during hyperpycnal discharges, and the generation of hydraulic jumps at the toe of depositional slopes are considered the main mechanisms that control the sedimentation in the analyzed outcrops.

In this context, the thick massive sandstone packages are interpreted as deposited through the collapse and “in mass” deposition of a high concentration of suspended load during sudden flow expansions associated to hydraulic jumps. After this deposition, and due to an important decrease in sediment concentration, cut-and-fill structures, climbing dunes and parallel lamination are generated in pebbly sandstones and in medium to fine sandstones (Facies 1 and 2). Under these flow conditions, the colonization and establishment of an endobenthic community is inhibited. The last waxing phase of the hyperpycnal flow is represented by upper- and lower-flow regime tractive structures (Facies 3). These deposits are colonized by suspension-feeder organisms that produce rapidly adjusted structures assigned to Diplocraterion paralellum. The waning phase of the hyperpycnal flow is represented by Facies 4 and 5 (climbing ripples, heterolithics). In these deposits, the endofauna is mainly represented by deposit-feeder organisms (e.g. Scolicia prisca and Nereites cf. missouriensis). Facies 6, dominated by mud settling from suspension, shows the greatest ichnodiversity. It is important to note that the same trend in facies distribution is found both in parallel and normal directions of the flow.

4. Ichnological suites in hyperpycnites

In this study, the trace fossil associations are characterized by moderate to low ichnodiversity, low intensity of bioturbation, orientation mainly parallel to the bedding plane (poor development of the tiering structure) and normal sizes. The ichnofauna mostly records equilibrium/adjustment structures and simple feeding structures (mainly mobile deposit-feeders), with absence of ichnotaxa reflecting complex behavioral patterns.

These features differ from those usually invoked for episodic gravity flow deposits. One of the most important distinctions relates to the type of infaunal communities developed in these hyperpycnal deposits. In “classical turbidites” the pre- and post-event suites are clearly distinctive, whereas in these hyperpycnites, particularly in more proximal areas (facies 3 and 4), the trace fossils are generated during the depositional event. In addition, these hyperpycnites show an endobenthic community with poor development of the tiering structure, whereas in classical turbidites the ichnoassemblages can reach deep tiers. This feature could be reflecting a high frequency between hyperpycnal depositional events, and therefore, a limited time for the organisms to develop a complex-tiered community.

5. Conclusions

The analysis of trace fossil suites in hyperpycnal deposits reflects important clues to understand the dynamic of these flows. These ichnoassemblages, associated to environments with sustained and rapid deposition of sediment, are characterized by a moderate to low diversity, low intensity of bioturbation, ichnotaxa with normal sizes and absence of complex behavioral strategies. In general, the trace fossil suites are mainly dominated by deposit-feeder organisms, showing a paucity of suspension-feeding ethologies (except in more proximal areas). Compared to the trace fossil suites of episodic gravity flow deposits, the ichnofauna of these hyperpycnites show dominance of facies-crossing forms, absence of especialized-feeding ethologies and communities with less-developed tiers. In addition, most of these structures were developed during the depositional event.

The evaluation of the ichnologic characteristics in these hyperpycnites helps to recognize the major paleoecological parameters operating in these flows. In particular, the pulsating and sustained character of the hyperpycnal flows seems to be one of the main stresses influencing and controlling the development of the endobenthic communities in these environments.

Figure 1. Idealized scheme showing the spatial distribution of sedimentary facies and their trace fossil associations (not to scale).