River‐dominated and tide‐influenced shelf‐edge delta systems: Coarse‐grained deltas straddling the Early–Middle Jurassic shelf–slope break and transforming downslope, Lajas–Los Molles formations, Neuquén Basin, Argentina

Abstract

The three‐dimensional facies and architecture variability of shelf‐edge deltaic units cropping out at the transition between the Lower–Middle Jurassic Lajas and Los Molles formations of southern Neuquén Basin, Argentina, is presented here, as well as their stratigraphic relationship to uppermost deep‐water slope channel systems. Deep‐water, slope mudstones with thin turbidite beds merge upward with prodelta mudstones and thin sandstones, which are truncated by delta‐front to mouth‐bar sandstones. The latter sandstones are then downcut by large‐scale, trough cross‐stratified coarse‐grained sandstones and conglomerates of distributary channel systems and along‐strike, amalgamated with cross‐bedded sandy units showing evidence of tidal reworking. Proximal–distal facies and architecture variability within a shelf‐edge deltaic succession demonstrates that distributary channel‐complexes become wider and deeper basinward, forming channelized river‐dominated distributary fairways separated by tidally reworked inter‐distributary sand belts at the shelf edge. Evidence from depositional‐dip oriented outcrops shows a lack of collapsed and slumped strata at the shelf edge, and that the coarse shelf‐edge distributary channel fills continue far down the deepwater slope, and conglomerates transform to become high‐density turbidites to mainly thick‐bedded, sand‐matrix‐supported debrites. The interplay between flood tides and river currents is interpreted to have primarily modulated the focusing of river drainages, and consequently coarse‐grained sediment transport, along preferential routes on the outer‐shelf to shelf‐edge and down onto the slope. This contribution documents a unique example of coarse‐grained (mostly conglomeratic) shelf‐edge delta systems, tying bed‐scale facies and architecture data to a seismic‐scale shelf‐margin morphology, thus providing outcrop analogue data for the characterization of shelf‐edge delta systems in the subsurface.

Prograding early to middle Jurassic margin, Neuquén Basin: Topset process stratigraphy and morphodynamic sediment partitioning

“…(<100 m thick) is commonly sand prone and consists of a set of sequences that overlie and interfinger downslope with the muddy Los Molles Formation (Figure 7a,b). Lower Lajas is dominated by two main facies associations; the first consists of upward‐coarsening, medium to coarse‐grained, sheet‐like (10–100 s of m wide) sandstone units (Table 1) with capping channelized conglomerates (Facies Association 3 in Table 2, column 1) (Figure 7a, left‐hand log) that incise the shelf edge in places up to 30 m (Almeida Junior et al, 2020; Gan et al, 2020). They are interpreted as progradational delta‐front units and are commonly overlain abruptly by 5–10 m‐thick, marine mudstone (level 55 m in Figure 7a, left‐hand log).…”

“…The long-term 'rise' in the Lajas shelf-edge trajectory (Figure 1a), itself indicating continuous, subsidence-driven aggradation, was sometimes interrupted by shorter time-scale irregularity reflecting sediment flux or sea-level changes. These trajectory changes were accompanied by an interfingering of Los Molles and Lajas formations, with fluvial and shallowmarine conglomeratic tongues of Lajas Formation extending basinward onto the upper slope of Los Molles Formation., as documented in the outcrops of La Jardinera (Almeida Junior et al, 2020;Gan et al, 2020;Olariu et al, 2020;Paim et al, 2011). The La Jardinera outcrops are the ideal workshop locality for examining margin-scale clinoformal continuity between fluvial and shallow-marine topset deposits with deepwater slope and basin-floor fan deposits (Olariu et al, 2020).…”

“…The detail of the process stratigraphy in several areas has been differently interpreted by researchers, though the age of the deposits may also differ somewhat. Some areas have been interpretated as tide-dominated deltas, tidal flats and estuaries, because tidal signals can be discovered at most levels through Lajas Formation (McIlroy, 1999;McIlroy et al, 2005); in other areas, strong river influence is seen from the relatively coarse grain size of the sediments (Almeida Junior et al, 2020;Paim et al, 2008), and the identification of shelf-delta lobes built by sandy hyperpycnal flows (Canale et al, 2015;Steel et al, 2018). Yet other evidence of river domination on the deltas is the documentation of river-mouth bars (Kurcinka et al, 2018) and that the Lajas topsets display tidal signals mainly in the inter-flood stratigraphic intervals (Gugliotta et al, 2015).…”

“…The lower member of Lajas Formation (Figure 3a,b) is sandstone-rich and marine as it contains shallow-marine traces (e.g., Ophiomorpha). Thick blocky sands below the cored interval suggest channel fills, encased in Los Molles Formation mud, directly connecting into the deep-water slope foresets (see Almeida Junior et al, 2020). Such channel fills are a good indicator of potential reservoirs in lower Lajas: (1) if channel fills are buffered basinward by Los Molles Formation mudstones, they represent reservoirs by themselves (as they are also underlain and overlain by mudstone rich units), (2) if channel fills correlate basinward with base-of-slope fans in Los Molles Formation, then they represent predictors of reservoirs in the basin.…”