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

Abstract

ABSTRACTThe 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.