Sedimentological and ichnological signatures of an offshore-transitional hyperpycnal system (Upper Miocene, Betic Cordillera, southern Spain)

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Highlights

  • Trace fossils allow to refine the depositional setting of marine hyperpycnites.

  • Prodeltaic lobe-to-channel-type hyperpycnites are linked to depauperate Cruziana ichnofacies.

  • Offshore channelized-lobe-type hyperpycnites are linked to Nereites ichnofacies.

Abstract

Hyperpycnal flows have been widely described in different lacustrine and marine environments but sedimentary structures and fossil content in hyperpycnites often offer limited information about the palaeoenvironmental conditions. This limitation can be improved by ichnological analysis, which has been recently used as a tool to differentiate between different type of subaqueous deposits, even though still only a few detailed ichnological studies on hyperpycnites exist. In order to bridge this gap in knowledge, a 50 m-thick package of terrestrial organic debris-rich, dominantly structureless and well-sorted sandstone bodies alternating with burrowed siltstones (Upper Miocene, Betic Cordillera, Spain) is here analyzed. This study is based on observations of a well-exposed outcrop and cores from a well drilled just behind the outcrop to bridge field-scale observational gaps. Two type of sandbodies were typified on the basis of their stratigraphic architecture, physical sedimentary structures, and ichnofacies in the fine-grained deposits embedding them: (1) Lobate to channelized-top sandstones embebbed into silty sands with dominant highly variable degree of bioturbation by Taenidium and Schaubcylindrichnus (depauperate Cruziana ichnofacies) and interpreted as proximal marine sustained hyperpycnites in prodelta settings; and (2) channelized-lobate (cut-and-fill sequence) sandstones embebbed into siltstones with dominant Nereites and Phycosiphon (Nereites ichnofacies) interpreted as distal hyperpycnites developed in offshore settings. The studied succession is interpreted to represent the progradation of a sandy hyperpycnal system along a prodelta to starved offshore setting with high variability in grain-size, benthic food and oxygen content. Results of this study suggest that a multi-scale analysis focused on trace fossils and physical sedimentary signatures is needed to get a better understanding of these river-derived sustained-flow turbidites (hyperpycnites) that are less well known than their conventional surge-type turbidite counterparts.

Introduction

Hyperpycnal deposits (hyperpycnites) have been reported from different lacustrine and marine environments from coastal (mainly prodeltaic subenvironments), shallow-water to deep-water settings (Mulder et al., 2003; Birgenheier et al., 2017; Steel et al., 2018). Hyperpycnites contain extrabasinal (e.g., organic debris) and intrabasinal (proximal to distal marine reworked fossils) components indicating a high-efficiency transport and wide range of water depths (Mutti et al., 2007). These deposits commonly occur from prodelta subenvironments to shallow-water (above storm wave base level) siliciclastic ramps where low transport efficiency of hyperpycnal flow along low slopes is compensated by wave influence (re-suspension) during oceanic floods (Wheatcroft, 2000; Bhattacharya and MacEachern, 2009; Poyatos-Moré et al., 2016). Hyperpycnites can show components indicating a wide range of particle provenance (including terrestrial organic particles) and water depths, which challenges differentiating examples from shoreline systems (estuarine, shoreface and delta-front) from those of deep-water systems (submarine lobes) (Zavala and Pan, 2018). Physical sedimentary structures and fossil components (as most of them are reworked from proximal areas) in hyperpycnites can also offer limited information about the palaeoenvironmental conditions of the original depositional setting.

During the past decades, ichnological analysis has emerged as a powerful indicator of palaeoenvironmental evolution and associated changes, becoming a pivotal elements, which supports sedimentological and stratigraphic interpretations, hence being key in sedimentary basin research (e.g., Buatois and Mángano, 2011; Knaust, 2012). The strength of ichnological studies lays in the relationship between trace fossils and palaeoenvironmental conditions: tracemakers' behaviour records the response to biotic and abiotic factors such as salinity, oxygen, benthic food content, hydrodynamic energy, rate of sedimentation, and substrate, among others. During the last years, ichnological information has been included as a criterion to differentiate between different types of deep-water deposits such as pelagites/hemipelagites, turbidites, contourites, and hyperpycnites, yet with a variable coverage. Thus, as pelagic/hemipelagic and turbiditic sediments have been profusely studied and ichnologically well-defined (e.g., Wetzel, 2000; Uchman, 2007; Uchman and Wetzel, 2011, Uchman and Wetzel, 2012; Wetzel and Uchman, 2012; Miguez-Salas and Rodríguez-Tovar, 2019a), contourites are now in the first phases of characterization, showing significant results (e.g., Wetzel et al., 2008; Alonso et al., 2016; Rodríguez-Tovar and Hernández-Molina, 2018; Dorador et al., 2019; Miguez-Salas and Rodríguez-Tovar, 2019b, Miguez-Salas and Rodríguez-Tovar, 2020; Miguez-Salas et al., 2019a, Miguez-Salas et al., 2019b, Miguez-Salas et al., 2020; Rodríguez-Tovar et al., 2019a, Rodríguez-Tovar et al., 2019b). The ichnological characterization of hyperpycnites is also still in the first steps, with only a few studies in which trace fossils have been emphasized, showing the variability of environmental context and the absence of a unique ichnological pattern (see Buatois et al., 2019). The few existing studies on hyperpycnites documenting biogenic structures cover a wide-range of environmental settings such as lakes (Buatois and Mángano, 1993, Buatois and Mángano, 1998; Buatois and Mángano, 1995), deltas (MacEachern et al., 2005; Bhattacharya and MacEachern, 2009; Buatois et al., 2011; Canale et al., 2015, Canale et al., 2016; Dasgupta et al., 2016a, Dasgupta et al., 2016b) and deep-marine systems (Ponce et al., 2007; Wetzel, 2008; Olivero et al., 2010; Carmona and Ponce, 2011; Ponce and Carmona, 2011). There is still a lack of particular ichnological signatures, trace fossil assemblages or ichnofacies characterizing hyperpycnites, besides the variability according to their different palaeoenvironmental settings (Buatois et al., 2019). More detailed ichnological studies on hyperpycnites are therefore necessary, in order to improve the characterization of the primary and secondary processes, associated palaeoenvironmental changes, and resulting deposits.

Here we present a study of a shallow-marine succession from the Upper Miocene of the Betic Cordillera (southern Spain), based on observations of a well-exposed outcrop, and tied to a well drilled just behind the outcrop. The main aim of this study is to provide a multi-scale outcrop and subsurface integrative sedimentological and ichnological analysis to identify and to outline criteria ascribable to marine hyperpycnites (and their depositional setting), and to distinguish them from other sedimentary processes non-genetically linked to the direct connection to the river mouth.

Section snippets

Methods

The integration of outcrop observations (main observation scale in this study) and complementary subsurface data collected from a well drilled just behind the outcrop (named: Francisco Abellan well / location: 37°18′42.32 N - 3°16′30.02 W) allowed to reduce the observational gap between large- and meso- (i.e. outcrop scale) and small-scale (i.e. thin section) data.

Large- to meso-scale 2D and 3D-features of the sand-body architecture were only observable in well-exposed outcrops and described

Geological setting

The Betic Cordillera (southern Spain) represents the westernmost Alpine Mediterranean cordillera and, together with the Rif and the Gibraltar Arc System, they formed during the Early to Middle Miocene main shortening phase. Later on, from the Late Miocene onwards compressional reactivation produced tightening and lengthening of the Arc (Crespo-Blanc et al., 2016) until the modern Gibraltar Arch geography formed (Fig. 1A). During the Upper Miocene compressional reactivation stage,

Stratigraphy

The 69 m-thick studied section can be divided in two parts: (1) A 62 m-thick lower part formed by a sandstone-dominated package overlying a mudstone-dominated interval and (2) a 7-m thick upper part formed by mixed carbonate-siliciclastic deposits represented by a marlstone-dominated package alternating with bioclastic sandstones) (Fig. 1D).

The siliciclastic-dominated lower part of the section consists of seven 5 to 10 m-thick bedsets each one constituted by a lower interval of

Hyperpycnite-dominated prodelta to offshore-transition depositional system

Integration of ichnological information, including ichnofacies, and other sedimentary features (e.g. texture, physical sedimentary structures), allow to interpreting variations of the sedimentary processes and of the palaeoenvironmental conditions affected by hyperpycnal flows within the studied transitional setting from prodelta to offshore setting. Ichnology helps to refine some environmental (ecological and depositional) conditions of each subenvironment (e.g., rate of sedimentation, energy

Conclusions

At a hierarchical major-order scale, the integration of sedimentologic and ichnological data coming from two different observation-scales (outcrop with some examples from core) allowed to reconstruct the sedimentation style of part of a Tortonian succession in the Betic Cordillera (Spain). Results suggest that the studied deposits belong to a sustained sandy hyperpycnite-dominated system in the transition from a distal shallow-marine (prodelta) to offshore transition setting (below fair- and

Declaration of Competing Interest

None.

Acknowledgments

Financial support is due to the research project CGL2017-89618-R (AEI/FEDER, UE) and research group RNM369 (PAI). Research by RT was funded by projects CGL2015-66835-P and PID2019-104625RB-100 (Secretaría de Estado de I+D+I, Spain), B-RNM-072-UGR18 (FEDER Andalucía) and P18-RT-4074 (Junta de Andalucía), Research Group RNM-178 (Junta de Andalucía), and Scientific Excellence Unit UCE-2016-05 (Universidad de Granada). Miquel Poyatos-Moré acknowledges Aker BP, sponsor of the ShelfSed project (

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