Multiple-scale incision-infill cycles in deep-water channels from the lacustrine Transylvanian Basin, Romania: Auto- or allogenic controls?
Introduction
A multitude of hierarchical schemes have been developed for deep-water clastic systems to improve reservoir characterisation and modelling, and to connect outcrop, subsurface and recent examples (e.g., Cullis et al., 2018; Deptuck et al., 2008; Gardner and Borer, 2000; Gervais et al., 2006; Grundvåg et al., 2014; Hodgson et al., 2011; Macauley and Hubbard, 2013; McHargue et al., 2011; Mutti and Normark, 1987; Navarre et al., 2002; Pickering and Cantalejo, 2015; Prather et al., 2000; Prélat et al., 2009; Sprague et al., 2005). The hierarchical systems of channels, lobes or complete systems commonly define different levels by facies characteristics, geometry, scale, bounding surfaces relationships, stacking patterns, dimensions and timescales (Cullis et al., 2018; Gardner et al., 2003; Ghosh and Lowe, 1993; Hubbard et al., 2014; Mayall et al., 2006; Pyles, 2007; Terlaky et al., 2016). Since classifications are diverse, an all-encompassing hierarchical scheme cannot be established today (Cullis et al., 2018). An ideal scheme would be a genetical, process-based scheme that could solve the differences in scales. This is not on the horizon yet, as the governing processes of each hierarchical level cannot be consistently defined (Cullis et al., 2018; McHargue et al., 2011; Straub and Pyles, 2012).
Furthermore, the inferred processes dictating the order of hierarchy created are not widely discussed in most descriptions of hierarchical schemes and are generally assumed to be fractal above the bed scale (Deptuck et al., 2008; Prélat et al., 2010; Schlager, 2009), meaning that scale-invariant processes create the same architectural patterns on different levels. Some processes are thought to be governed differently on different scales, i.e., they are truly hierarchical and not fractal (Straub and Pyles, 2012; Sylvester et al., 2011). Bed and lobe/channel element stacking are inherently produced by different processes: the former aggrade, whilst the latter stack compensationally (Straub and Pyles, 2012). Moreover, there is some inconsistency with respect to the terminology. In this paper, the most widely used nomenclature based on Sprague et al. (2005) and developed by McHargue et al. (2011), is adopted: bed, bedset, channel storey, channel element (McHargue et al., 2011; equals channel fill of Sprague et al., 2005), channel complex, channel complex set and channel complex system.
The cut-and-fill architecture of channels is usually seen as fractal (Clark and Pickering, 1996). A build, cut, fill, and spill model is suggested by Gardner and Borer (2000) for the evolution of channel elements and channel complexes. The build deposits are absent on the upper slope but increase in volume downstream. A channel complex set is created by an initiation, erosion, lateral migration, and aggradation cycle according to Hodgson et al. (2011). Erosion, amalgamation, disorganized channel elements in moderate aggradation and well-organized channel elements in high aggradation phases are suggested by McHargue et al. (2011) for a channel complex evolution. The cut-and-fill cycles can be governed by auto-or allocyclic behaviour of channels. The waxing-waning cycles of flow energy of different scales can be superposed on each other and create a serrated curve, much like the sea-level curve (Figueiredo et al., 2013).
The share of autogenic and allogenic controls on depositional architecture is often speculative in ancient turbidite systems (Pickering and Bayliss, 2009), therefore the transmission of environmental signals into the stratigraphic record of turbidite systems is currently investigated by modelling (Burgess et al., 2019; Ferguson et al., 2020; Romans et al., 2016; Sharman et al., 2019; Straub et al., 2020). Allogenic factors include geological setting, i.e., tectonics in the hinterland, relief and topography of the slope and the basin, sea-level and climate. The cycles can also be interpreted as changes in the equilibrium profile of flows or changes in the slope gradient, and the subsequent adjustment of the channel depth (Kneller, 2003). Autogenic controls include compensational stacking and related avulsions, lateral accretion deposits or packages, bend cut-off in sinuous channels and progradation of lobe deposits. Lateral migration can fill a channel element without the need of changing flow magnitude and the changing direction of lateral migration can create a new incisional surface (Sylvester et al., 2011). Repeated autogenic cycles of gradual migration, channel avulsion and compensational stacking can be generated by simple diffusion-based numerical models (Hawie et al., 2018).
The Transylvanian Basin is an understudied natural laboratory, even though the scattered, near seismic-scale outcrops provide an insight into a diverse set of mixed sand-mud deep-water depositional environments. Channels, channel-levee systems, lobes with lobe axis, lobe fringe and distal lobe fringe deposits are present (Fig. 1; Tőkés et al., 2015). The small-sized late Miocene depocenter, the narrow stratigraphic range constrained to the lowermost upper Miocene (Krézsek and Bally, 2006; Krézsek and Filipescu, 2005), and the exposed compound channel architectures hold an opportunity to investigate the incisional and infill history of channel systems and their controls. In addition, targets of hydrocarbon exploration are similar turbidite systems both in the Transylvanian and Pannonian Basin (Krézsek et al., 2010; Sztanó et al., 2013b; Tari and Horváth, 2006; Vrbanac et al., 2010), thus the study of outcrop analogues is crucial to the understanding of subsurface reservoirs of these basins.
This study has the following aims: 1) to investigate the processes, evolution, and the cyclic nature of two deep-water channel systems in a lacustrine setting; 2) to present and assess a novel depiction method for channel cyclicity; and 3) to discuss the potential controls in the development of different scales of cyclicities.
Section snippets
Geological setting
The Transylvanian Basin, part of the Alpine-Carpathian-Dinaride orogen system (Fig. 2A; Ciulavu, 1999; Schmid et al., 2008), is bordered by the Carpathian thrust and fold belt from northeast and south, and separated from the large Pannonian Basin to the northwest by the Apuseni Mountains (Fig. 2B; Huismans et al., 1997). The basin acquired its roughly circular shape in the middle to late Miocene to Pliocene (Krézsek and Filipescu, 2005) The Transylvanian Basin is characterized by the relatively
Sedimentological field work
The southern Transylvanian Basin is a hilly upland where outcrops are the result of recent landslides or are sand and clay pits. Three large outcrops up to 300 m length and 50 m height were studied. Daia and Gușterița are 2.5 km from each other, Daia lies 100 m topographically above Gușterița towards the East. Concerning regional dip data, 2–5 degrees to the W-NW, the sandy outcrop of Daia and the clay pit of Gușterița lie along strike, thus they are coeval. Tău lies 40 km to the NW. The
Mudstone facies association - overbank-outer levee transition
The mudstone facies association occurs only at Gușterița where it makes up the whole 55 m thick outcrop. It consists of 99% laminated mudstone (F1) and bedded mudstone (F2), and only 1% is thin-laminated to thin-bedded sandstone (F3; Table 1). The ratio of laminated (F1) and bedded mudstone (F2) is not stated as their differentiation is only evident if the mudstone is wet.
The bedded mudstone facies (F2), forms 2–5 cm thick structureless, ungraded or graded beds, with sharp or faint bioturbated
Incision-infill graphs for the Transylvanian Basin channel systems
Incision-infill graphs, showing the successive depth of erosion and height of infill of channel-form surfaces and bodies were created for Daia and Tău (Fig. 11). The recorded successive events were equally spaced on the ‘time’ axis, since the duration of erosional and depositional events, bypass or quiescence is unknown.
This approach clearly illustrates the significance of lateral and vertical migrations (Fig. 11A). The values of lateral migration are not depicted, but where the successive
Conclusions
The near-seismic scale exposures of a poorly studied lacustrine turbidite system from the southern Transylvanian Basin provides insights into the complexity of channel systems. The variability of facies and bounding surfaces allows tracing the evolution of hierarchical levels.
- 1.
Facies analysis indicates hemipelagic settling, muddy low-density turbidity currents, sandy low, and low to high-density turbidity currents, pebbly high-density flows and debris flows. High-density currents left unique
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.
Acknowledgements
Field observations are based on the MSc thesis of LT (Eötvös Loránd University), and the BSc thesis of BIR (Babeș-Bolyai University). The project was supported by the Papp Simon Foundation, the Koch Antal Geological Society and the Hungarian National Research, Development and Innovation Office (NKFI) grant 116618. Lóránd Silye thanks the Alexander von Humboldt Foundation for the funding of his research stay in Bremen, where he was working on this paper. We wish to thank Samuel Etienne and an
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