The Early Jurassic Los Molles Formation in the Neuqu en Basin of western Argentina is a rare example of well-exposed syn-rift to post-rift stratigraphy. In the Chachil Graben, the onset of the early post-rift stage is marked by drowning of a carbonate system and the development of two deep-marine intraslope lobe complexes. This field-based study in the Chachil Graben involved field mapping and correlating eleven stratigraphic logs, and petrographic analysis to document how grain size and texture within intraslope lobe sandstones change from the lobe centre to their frontal pinch-out. Eight different bed-scale facies are identified and inferred to be formed by turbulent (turbidites; Type A and B beds), transient turbulent-laminar (transitional flow deposits; Type C, D, E and F beds), laminar gravity flows (debrites; Type G) and post-depositional clastic injections (injectites; Type H beds). Fifteen lobes form two stacked lobe complexes that show stratigraphic evolution from a lower argillaceous sandstonedominated lobe complex, built by transitional flow deposits, to an upper coarser-grained, sandier lobe complex largely constructed by turbidites. Petrographic analysis quantified sandstone mineralogy, matrix content, grain size and sorting, revealing that both lobe complexes are volcanic arc-sourced. This study proposes that the differences in the character of the two lobe complexes are due to maturation of sediment transport routes through progressive healing of the intraslope relief, with a concomitant decrease in substrate erosion and flow bulking. Also proposed here is a model for intraslope lobe complex development that accounts for the impact of flow-confinement on flow behaviour and transformation induced by the inherited topography. Bed type distribution suggests that high-density flows terminate more abruptly against confining slopes and produce greater depositional variability than lower-density flows. This integrated petrographic, architectural and sedimentary process model provides new insights into how post-rift intraslope lobe systems may act as hydrocarbon reservoirs, aquifers and carbon storage sites.

Submarine landslides can generate complicated patterns of seafloor relief that influence subsequent flow behaviour and sediment dispersal patterns. In subsurface studies, the term mass transport deposits (MTDs) is commonly used and covers a range of processes and resultant deposits. While the large-scale morphology of submarine landslide deposits can be resolved in seismic reflection data, the nature of their upper surface and its impact on both facies distributions and stratal architecture of overlying deposits is rarely resolvable. However, field-based studies often allow a more detailed characterisation of the deposit. The early post-rift Middle Jurassic deep-water succession of the Los Molles Formation is exceptionally well-exposed along a dip-orientated WSW-ENE outcrop belt in the Chacay Melehue depocentre, Neuquén Basin, Argentina. We correlate 27 sedimentary logs constrained by marker beds to document the sedimentology and architecture of a >47 m thick and at least 9.6 km long debrite, which contains two different types of megaclasts. The debrite overlies ramps and steps, indicating erosion and substrate entrainment. Two distinct sandstone-dominated units overlie the debrite. The lower sandstone unit is characterised by: 1) abrupt thickness changes, wedging and progressive rotation of laminae in sandstone beds associated with growth strata; and 2) detached sandstone load balls within the underlying debrite. The combination of these features suggests syn-sedimentary foundering processes due to density instabilities at the top of the fluid-saturated mud-rich debrite. The debrite relief controlled the spatial distribution of foundered sandstones. The upper sandstone unit is characterised by thin-bedded deposits, locally overlain by medium-to thick-bedded lobe axis/off-axis deposits. The thin-beds show local thinning and onlapping onto the debrite, where it develops its highest relief. Facies distributions and stacking patterns record the progradation of submarine lobes and their complex interaction with long-lived debrite-related topography. The emplacement of a kilometre-scale debrite in an otherwise mud-rich basinal setting and accumulation of overlying sand-rich deposits suggests a genetic link between the mass-wasting event and transient coarse clastic sediment supply to an otherwise sand-starved part of the basin. Therefore, submarine landslides demonstrably impact the routing and behaviour of subsequent sediment gravity flows, which must be considered when predicting facies distributions and palaeoenvironments above MTDs in subsurface datasets.

Submarine landslides can generate complicated patterns of seafloor relief that influence subsequent flow behaviour and sediment dispersal patterns. While the large-scale morphology of submarine landslide deposits, or mass transport deposits (MTDs), can be resolved in seismic data, the nature of their upper surface, and its impact on facies distributions and stratal architecture of overlying deposits, is rarely resolvable. MTD is a commonly used term in subsurface studies, covering a range of processes and resultant deposits that can not be resolved in seismic or core-based datasets. However, field-based studies often allow a more detailed characterisation of the deposit. The early post-rift Middle Jurassic deep-water succession of the Los Molles Formation is exceptionally well-exposed along a dip-orientated WSW-ENE outcrop belt in the Chacay Melehue depocentre, Neuquén Basin, Argentina. We correlate 27 sedimentary logs constrained by marker beds to document the sedimentology and architecture of a >47 m thick and at least 9.6 km long mud-rich debrite. The debrite overlies ramps and steps, indicating erosion and substrate entrainment. Megaclasts sourced from shallow-marine environments support a shallow marine origin of the mass failure. Two distinct sandstone-dominated units overlie the debrite. The lower sandstone unit is characterised by: i) abrupt thickness changes, wedging and progressive rotation of laminae in sandstone beds associated with growth strata; and ii) detached sandstone load balls within the underlying debrite. The combination of these features suggests syn-sedimentary foundering processes due to density instabilities at the top of the fluid-saturated mud-rich debrite. The debrite relief controlled the spatial distribution of foundered sandstones. The upper sandstone unit is characterised by thin-bedded deposits, locally overlain by medium- to thick-bedded lobe axis/off-axis deposits. The thin-beds show local thinning and onlapping onto the debrite, where it develops its highest relief. Facies distributions and stacking patterns record the progradation of submarine lobes and their complex interaction with long-lived debrite-related topography. These characteristics can help us understand post-depositional processes above MTDs and predict facies distributions and palaeoenvironments in subsurface datasets. The emplacement of a kilometre-scale debrite in an otherwise mud-rich basinal setting and accumulation of overlying sand-rich deposits suggests a genetic link between the mass-wasting event and transient coarse clastic sediment supply to an otherwise sand-starved part of the basin.

The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentological and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that submarine fan retreat in the Alpine foreland basin corresponds with positive δ13C excursions related to major global perturbations of the carbon cycle and cooling in the earliest Oligocene. Submarine fan retreat is suggested to be influenced by this cooling through enhanced aridity and reduced subaerial runoff from the Corsica-Sardinia hinterland. The influence of aridity was periodically overwhelmed by local environmental factors, such as hinterland uplift, which increased sediment supply to deep-water during arid periods. These results highlight that: 1) hinterland climate may play a greater role than sea-level in dictating sediment supply to deep-water and, 2) submarine fan evolution occurs through a complex interplay between climate, eustasy and tectonics, which makes robust interpretations of paleoenvironmental change from their stratigraphic record, without multi-proxy records, difficult.

No abstract