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Depositional and erosional bedforms can be used to reconstruct sedimentary processes and aid palaeoenvironmental interpretations. Using exhumed deep-marine strata in the Eocene Aínsa Basin, Spain, we document a 3-dimensional package of dunes, a rarely identified bedform in deep-marine environments. Our analysis shows that the dunes have curvilinear crests in planform, with smaller superimposed oblique dunes and ripples across the stoss sides. Beds containing these dunes have two main internal divisions: a lower inversely-graded (fine-to-coarse sandstone) and predominantly structureless division, and an upper coarse-grained sandstone division with well-developed cross-stratification, which is scoured and mantled with mudclasts and coarse-grains on the stoss-side. The rugose remnant relief of the bedforms controls the location of subsequent bedforms. Following recently reported direct measurements of natural turbidity currents, we interpret the basal division as recording deposition from the dense basal head of a high-velocity turbidity current, followed by the development of dunes beneath the more sustained but relatively high-velocity and unsteady flow body that reworked the initial sandy deposit into downstream migrating dunes and scours. These dune-forming beds have been identified in different deep-water environments in the Aínsa Basin stratigraphy, including channel overbank and channel mouth settings and scour-fills. These locations suggest that the dunes were intimately tied to high-velocity flows that bypassed through channel axes before expanding and depositing in less confined channel overbank or channel mouth settings. Preservation of these dunes in the Aínsa Basin was likely enhanced by tectonically-forced lateral migration of channels, which prevented cannibalisation of bypass-dominated zones, in combination with high aggradation rates due to confinement. Where identified these dune-like bedforms are considered diagnostic of substantial sediment bypass downslope to deep-water basins. This article has a related Corrigendum

Tectonic deformation and associated submarine slope failures modify seafloor relief, influencing sediment dispersal patterns and the resulting depositional architecture of deep-water systems. The exhumed Middle Eocene strata of the Banastón deep-water system in the Aínsa depocentre, Spain, allow the interplay between submarine slope confined systems, mass flow deposits, and syn-depositional compressional tectonics to be investigated. This study focuses on the Banastón II sub-unit, interpreted as deposits of low-sinuosity and narrow (2-3 km wide) channel-belts confined laterally by tectonically-controlled, fine-grained slopes. The studied succession (111 m-thick) is exposed along a 1.5 km long SE-NW trending depositional dip section and is documented here by facies analysis and physical correlation of 10 measured sections. Results show a stratigraphic evolution in which the channel axes migrated to the southwest, away from a growing structure in the northeastern part of the Aínsa depocentre. Uplift of this structure promoted the breaching of confining channel walls and slope material with mass failures and the development of sand-rich crevasse scour-fills and crevasse lobes. We show that crevasse deposits form an important component of overbank succession. These crevasse lobes are characterized by structureless thick and medium beds that form < 5 m thick packages in proximal parts and thin abruptly over 1 km across strike (NE) and along downdip (NW) into structured thin beds, similar to the heterolithic dominated overbank deposits. Although the development of crevasse lobes has been observed in multiple deep-water systems in ancient and modern systems, this study documents, for the first time, crevasse lobe development on the active compressional margin of a foreland basin rather than on the opposing, more stable and gentler inactive margin. We discuss the mechanism for forming these crevasse deposits, which exploited the accommodation generated by submarine landslides derived from the tectonically-active compressional margin.

Clinoforms develop at the hearts of prograding sedimentary systems and are an integral part in understanding basin development and sediment partioning. The Eocene Sobrarbe Formation in the Ainsa Basin in Spain, displays well-developed dipping clinoform geometries. The study observes depositional facies, mineralogy, and performs curve-fitting calculations in 2D to better understanding of factors controlling clinoform geometries. The Sobrarbe Formation is a unique prograding calciclastic delta system, in which carbonates are dominant, but depositional environments and sedimentary processes are akin to silicilclastic systems. Four clinothems (CT 1–4) were selected for detailed study. The clinothems display an overall coarsening upward trend. The lowermost clinothems comprise dominantly pro-deltaic marlstone deposited from suspension and dilute hyperpycnal flows, with occasional thin fine-grained episodic turbidite beds. The uppermost clinothems are dominated by reworked remobilized topset deposits comprising bioturbated calcarenites with a variety of trace-fossils, shells and large assemblages of nummulites, and finally proximal subaqueous stacked calcarenites. Three clinothems (1, 2, 4) are characterized by sigmoidal geometry (Gaussian distribution curve), symmetrical sediment distribution, a simple coarsening upward trend, and where calculable, ascending regressive trajectories. Clinothem 3 comprises a more complex stratigraphic development and displays an overall ascending transgressive trajectory, linear slope and asymmetric sediment distribution. The results show that sigmoidal geometries develop in both the distal pro-delta and in the coarser-grained proximal delta, suggesting the sigmoidal form develops naturally granted sufficient accommodation. While speculative it appears that linear slopes might be associated with draping during transgression, and asymmetric distribution associated with high sedimentation rates vis a vis accommodation. Accommodation exerts the dominant control on the location of the rollover and the trajectory. The topset-foreset ratio indicates the relative rate of sedimentation compared to accommodation, as high sedimentation rates increase deposition on the foreset. Results indicate that accommodation appears to be the most important factor controlling foreset slope, with steeper inclination developing in deeper water. Meanwhile sedimentation rate, grain size, and the role of turbidity currents may play a secondary role influencing slope angle. • Extraction of clinoform geometries from outcrop successions and curve-fitting. • Sigmoidal clinoforms developing in both distal pro-delta and high energy near-shore. • Accommodation space exerts dominant control on trajectory. • Accommodation exerts dominant control on foreset slope, deeper = steeper. • Linear slopes possibly associated with transgressive draping. [ABSTRACT FROM AUTHOR]

Abstract: We present field evidence from the Middle Eocene deep-marine Ainsa Basin, Spanish Pyrenees, to show channel-like features likely created by erosive subaqueous debris flows. Evidence from this basin suggests that the most erosive subaqueous debris-flows may create megascours removing up to ∼35 m thickness of sandy submarine-fan deposits from base-of-slope and lower-slope settings. This study suggests that individual debris flows may have been more erosive than turbidity currents, an observation that is opposed to many previous studies from the Ainsa Basin and other ancient deep-water clastic systems. In the Ainsa Basin, many of the debris flows deposited pebbly mudstones immediately above the basal erosion surfaces into which gouging flow-parallel grooves and pebble scours left isolated pebbles embedded in the immediately underlying sandstones. In one particularly well-exposed case, the sandstones immediately below the eroding debris flow were incorporated into it and preserved as sheared, disaggregated, brecciated, and partially liquefied sandstone beds within the pebbly mudstone. Our study suggests that erosion by large-volume debris flows in base-of-slope settings can be at least as important, if not more so, than turbidity currents in producing submarine megascours (probably chutes that, in cross section, superficially resemble submarine channels). This has important implications for understanding the erosivity of debris flows versus turbidity currents in modern and ancient environments, and it has significant implications for hydrocarbon reservoir continuity and heterogeneity, including the origin and recognition of mudstone-filled chutes or channels. [Copyright &y& Elsevier]