Distinctive thin‐bedded turbidite facies and related depositional environments in the Eocene Hecho Group (South‐central Pyrenees, Spain)

Abstract

The vertical and lateral stratigraphic relations of facies and facies associations, palaeocurrent directions, and geometry and internal organization of associated thick‐bedded and coarse‐grained bodies of sandstone provide the framework for distinguishing five thin‐bedded turbidite facies in the Eocene Hecho Group, south‐central Pyrenees, Spain. Each facies is characterized by a number of primary features which are palaeoenvironmental indicators by themselves. These features and their palaeoenvironmental significance are summarized below. The impressive regularity and lateral persistence of bedding and depositional structures, combined with the association of thin hemipelagic intercalations are typical characteristics of the basin plain thin‐bedded turbidites. Lateral variations in bed thickness, internal structures, grain size, sand: shale ratio, and amounts of hemipelagic intercalations are present in these sediments, but take place so gradually that they cannot generally be recognized at the scale of even very large exposures. The basin plain facies has a remarkable character of uniformity over great distances and considerable stratigraphic thicknesses. Thickening‐upward and/or symmetric cycles with individual thicknesses ranging from a few metres to a few tens of metres are typical of lobe‐fringe thin‐bedded turbidites. The sediments that comprise the cycles contain small but recognizable variations in bed thickness and sand: shale ratio. The diagnostic cyclic pattern can be detected in relatively small exposures. It should be noted that in absence of coarse‐grained and thick‐bedded sandstone of the depositional lobes the above cyclic pattern is diagnostic of fan‐fringe areas. An extremely irregular bedding pattern with lensing, wedding, and amalgamation of individual beds over very short distances, sharp rippled tops of many beds, and internal depositional structures indicative of mainly tractional processes without substantial fallout, are typical and exclusive characteristics of channelmouth thin‐bedded turbidites. Bundles of interbedded thin‐bedded sandstone and mudstone as thick as a few metres that are separated in vertical sequences by mudstone units of roughly similar or greater thickness are typical of interchannel thin‐bedded turbidites. The most diagnostic feature of this depositional environment is the presence of beds of sandstone filling broad shallow channels as probable crevasse‐splays. Thin, thoroughly rippled sandstone beds with marked divergence of the bedding attitude characterize the channel‐margin facies. The divergence or expansion in thickness, is consistently toward the channel axis. Small and shallow channels filled with thin‐bedded deposits, interpreted here as crevasses cut into channel edges or levees during period of severe overbanking are also characteristic.

Insight into the development of a carbonate platform through a multi-disciplinary approach: a case study from the Upper Devonian slope deposits of Mount Freikofel (Carnic Alps, Austria/Italy)

“…In this simplified model, the more distal setting is characterized by deeper slope to distal reef floor autochthonous sedimentation (MF1, MF3), temporarily influenced by distal turbiditic current (MF3). These distal turbidites are comparable to turbiditic fringe lobe developed in basin floor environment (e.g., Mutti 1977). In the intermediate to distal fore-reef-slope area, an interplay of various gravity processes (debris and turbiditic flow) dominates the depositional style and corresponds to an accumulation of various autochthonous and allochthonous sediments (MF6, MF5, MF4, and MF2); these sediments represent different portions of deposition occurring in Bouma (e.g., Ta, Tb, Tc, Td-e; Bouma 1962) or Meischner sequence; it is equivalent for carbonate environment (e.g., Zones 1, 2, and 3; Meischner 1964).…”

“…These authors interpret these facies as the result of the concurrent action of relevant seismic shocks that induced tsunamis, back currents, and related turbidity current. The m-thick lithoclastic beds characterizing MF6 combined with local occurrence of fining upward sequences passing upward to wackestone and mudstone interbeds are diagnostic features of turbiditic sequence (Bouma 1962;Mutti 1977). However, the rare occurrence of a floatstone texture might reveal a deposition rather influenced by debris flow.…”

“…The intercalation of this thin-bedded lithoclastic grainstone (MF3) horizons within the hemipelagic/pelagic setting (MF1), the abundance of lithoclasts, their relatively good sorting, the fining upward, and the lateral persistence of the layers are diagnostic features of distal slope thin-bedded turbiditic deposits (Mutti 1977;Flügel 2004). According to Mutti (1977), thin-bedded turbiditic deposits are the product of waning and relatively dilute turbidity currents that carry fine-grained sand and silt-sized particles.…”

“…According to Mutti (1977), thin-bedded turbiditic deposits are the product of waning and relatively dilute turbidity currents that carry fine-grained sand and silt-sized particles. According to Flügel (2004), the absence of shells concentration and flat top/base as well as the presence of grading allow to exclude an accumulation by distal tempestites.…”

“…In the intermediate to distal fore-reef-slope area, an interplay of various gravity processes (debris and turbiditic flow) dominates the depositional style and corresponds to an accumulation of various autochthonous and allochthonous sediments (MF6, MF5, MF4, and MF2); these sediments represent different portions of deposition occurring in Bouma (e.g., Ta, Tb, Tc, Td-e; Bouma 1962) or Meischner sequence; it is equivalent for carbonate environment (e.g., Zones 1, 2, and 3; Meischner 1964). Therefore, these microfacies could reflect a deposition in a channel lobe system in an outer fan turbiditic model (e.g., Mutti 1977).…”