Genetic facies analysis based on bed-by-bed correlations from sheet-like lobes to the basin plain in some Hecho turbidite systems demonstrates that at least 50% of the flows building the sheet-like lobes kept moving downcurrent to the closely related basin plain and underwent flow transformations, interpreted to have resulted from interaction with topography at the basin margin(s), that gave rise to specific facies. These facies form a new facies tract that replaces the fine-grained group of turbidite facies (very fine sand to mud) and characterizes the basin-plain beds. Beds in the sheet-like lobes evolve downcurrent in a way that is predictable by the existing turbidite facies tract models, whereas 36% of basin-plain beds, which account for 78% of the basin-plain volume, do not. The latter have deposits from high-density turbidity currents at their bases and typically complete basin-floor coverage. The new facies tract developed when the flows obliquely encountered the southern foreland-margin ramp. At the ramp, the lower, sand-laden and high-density part of the larger flows was deflected, evolving downcurrent along the ramp trend. The upper part of the flow, more dilute and thicker, was reflected from the foreland margin as a train of declining undular bores (moving hydraulic jumps). Subsequent reflections generated against the flanking margins in the closed basin led to ponding, which resulted in an overall sheet-like stacking pattern across sheet-like lobes and the basin plain and is the diagnostic feature of the distal element in the lower, sand-rich stages of the turbidite systems. Calcilutites on top of beds, interpreted until now as hemipelagites, show field evidence of having a turbiditic origin (hemiturbidites), thus forming the facies capping the new facies tract.

This study aims to unravel the impact of diagenetic alterations on porosity loss of foreland-basin turbiditic hybrid arenites and associated siliciclastic sandstones of the Eocene Hecho Group (south-central Pyrenees, Spain). In this succession, hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement (δ18OVPDB = −10.0‰ to −5.8‰; Th, mode = 80° C; salinity mode = 18.8 wt% eq. NaCl), Fe-dolomite (δ18OVPDB = −8.5‰ to −6.3‰) and trace amounts of siderite. The extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains, which provided nuclei and sources for the precipitation and growth of carbonate cements. Other diagenetic alterations, such as pyrite and albitization, had no impact on reservoir quality. Scarce early diagenetic cements, coupled with abundant ductile carbonate and siliciclastic framework grains, have led to rapid porosity loss owing to compaction. Conversely, abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction. Reservoir quality was affected by mesogenetic cementation by quartz overgrowths, calcite and dolomite intergranular pressure dissolution of quartz grains, and formation of fracture-filling calcite cement (δ18OV-PDB values from −10.4‰ to −7.8‰; Th temperatures of ≈ 150° C), which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism. The results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition, particularly the proportion and types of extrabasinal noncarbonates, extrabasinal carbonates, and intrabasinal carbonate grains. These insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones. Degradation of reservoir quality makes these hybrid arenites, calclithites, and sandstones suitable as tight gas reservoirs, but only if fracture porosity and permeability develop during tectonic deformation.