Stratigraphic change in flow transformation processes recorded in early post‐rift deep‐marine intraslope lobe complexes

Abstract

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.

Unconfined gravity current interactions with orthogonal topography: Implications for combined-flow processes and the depositional record.

“…Solitons and internal bores recognised in 2D experiments have been linked to the generation of an oscillatory flow component and the inception of combined flow (e.g., Pantin and Leeder, 1987;Edwards et al, 1994;Kneller et al, 1997). These observations have been invoked to explain the presence of combined flow bedforms, such as hummock-like structures and symmetrical megaripples above topographic slopes in deep-water settings following flow interactions with seafloor topography (e.g., Privat et al, 2021;Tinterri et al, 2022;Martínez-Doñate et al, 2023;Siwek et al, 2023). A This is non-peer reviewed preprint submitted to EarthArxiv 45 new model for the generation of combined flow in unconfined density currents has implications for interpreting the degree of flow confinement and topographic containment in deep-water systems.…”

“…These oscillations are postulated to be generated by the interaction of turbidity currents with seafloor topography, leading to the formation of internal waves. Previous field-based outcrop models (e.g., Tinterri et al, 2016Tinterri et al, , 2022Privat et al, 2021;Martínez-Doñate et al, 2023) have invoked this model to interpret sedimentary structures. However, the model is based largely on semi-quantitative (Edwards et al, 1994) and quantitative (Kneller et al, 1997) observations from 2D, non-ponded flume tank experiments.…”

“…This is non-peer reviewed preprint submitted to EarthArxiv 48 Hummock-like structures have been documented in a range of deep-marine settings, including basin-plain lobes (e.g., Mulder et al, 2009;Bell et al, 2018), channel-lobe transition zones (e.g., Hofstra et al, 2018), and intraslope lobes (e.g., Privat et al, 2021;Martínez-Doñate et al, 2023).…”

“…Such bedforms include hummocky crossstratification (HCS) (e.g., Arnott and Southard, 1990;Duke et al, 1991;Dumas and Arnott, 2006;Wu et al, 2023) and sigmoidal-cross lamination in small-and large-scale ripples (e.g., Yokokawa, 1995;Dumas and Arnott, 2006;Tinterri, 2006Tinterri, , 2007. Hummock-like structures, large asymmetric ripples, biconvex ripples, and symmetrical megaripples have been documented in several deepwater systems (e.g., Privat et al, 2021;Tinterri et al, 2022;Martínez-Doñate et al, 2023;Siwek et al, 2023;Taylor et al, 2024), and are typically postulated to have formed as a result of the This is non-peer reviewed preprint submitted to EarthArxiv 5 generation of combined flows (cf. Mulder et al, 2009).…”

“…Observations from 2D experiments of turbidity currents rebounding against topographic slopes (e.g., Pantin and Leeder, 1987;Edwards et al, 1994;Kneller and McCaffrey., 1995;Kneller et al, 1997) have been used to support outcrop-based models for the formation of combined flows and the formation of hummock-like structures in deep-water systems (Fig. 1) (e.g., Tinterri, 2011;Tinterri et al, 2016Tinterri et al, , 2022Privat et al, 2021;Martínez-Doñate et al, 2023). Tinterri (2011) suggests that flow transformations following the deceleration of flows upon incidence with slopes produces a hydraulic jump, akin to bores described semi-quantitatively with time-lapse photography and particle tracking by Edwards et al (1994).…”