Criteria for recognizing shelf‐slope clinoforms in outcrop; Jurassic Lajas and Los Molles formations, S. Neuquén Basin, Argentina

Abstract

Seismic‐reflection data show that most deepwater (>200 m water depth) basins are filled by sand and mud dispersed across clinoformal geometries characterized by gently dipping topsets, steeper foresets and gently dipping bottomsets. However, the entire geometry of these ubiquitous clinoforms is not always recognized in outcrops. Sometimes the infill is erroneously interpreted as “layer cake” or “ramp” stratigraphy because the topset‐foreset‐bottomset clinoforms are not well exposed. Regional 2‐D seismic lines show clinoforms in the Lower to Middle Jurassic Challaco, Lajas, and Los Molles formations in S. Neuquén Basin in Argentina. Time equivalent shelf, slope and basin‐floor segments of clinoforms are exposed, and can be walked out in hundreds of metres thick and kilometres‐wide outcrops. The studied margin‐scale clinoforms are not representing a continental‐margin but a deepwater shelf margin that built out in a back‐arc basin. Lajas‐Los Molles clinoforms have been outcrop‐mapped by tracing mudstones interpreted as flooding surfaces on the shelf and abandonment surfaces (low sedimentation rate) in the deepwater basin. The downslope and lateral facies variability in the outcrops is also consistent with a clinoform interpretation. The Lajas topset (shelf) is dominated by fluvial and tidal deposits. The shelf‐edge rollover zone is occasionally occupied by a 40–50‐m‐thick coarse‐grained shelf‐edge delta, sometimes incising into the underlying slope mudstones, producing oblique clinoforms expressing toplap erosion on seismic. A muddy transgressive phase capping the shelf‐edge deltas contains tidal sandbodies. Shelf‐edge deltas transition downslope into turbidite‐ and debris flow‐filled channels that penetrate down the mud‐prone Los Molles slope. At the base‐of‐slope, some 300m below the shelf edge, there are basin‐floor fan deposits (>200 m thick) composed of sandy submarine‐fan lobes separated by muddy abandonment intervals. The large‐scale outcrop correlation between topset–foreset–bottomset allows facies and depositional interpretation and sets outcrop criteria recognition for each clinoform segment.

Facies and architectural variability of sub‐seismic slope‐channel fills in prograding clinoforms, Mid‐Jurassic Neuquén Basin, Argentina

“…Single‐storey channels in the study area suggest a lack of time for channels to evolve into complex meandering terraced systems before being filled up or abandoned. The strongly progradational shelf edges of the main distributary fairways mapped from the same outcrop (Olariu et al, , this volume) resemble those associated with sediment supply dominated margins where multiple contemporaneous slope channels form and fill in relatively short timescale due to large flood‐generated sediment supply from the shelf. This short timescale can be less than or equal to a 4th order sea level cycle, even in Jurassic greenhouse time, as suggested by high‐frequency shore line transits in Wilcox formation (Zhang, Steel, & Ambrose, ) that was also severely modulated by variable sediment discharge.…”

“…Since the slope channels hold volumetrically insignificant volumes of sandy deposits, the majority of the sediments, both fine and coarse‐grained fractions, bypassed the shelf edge and ended up on the Los Molles basin floor, and produced thick sandstone bodies with even conglomeratic occurrence (Figure b) (Table ) (Paim et al, , Olariu et al, this volume, Giacomone et al, ), despite the relative constant thickness of the slope section (~300 m) and the small dimensions of the slope channel conduits (less than 50‐m thick).…”

Clinoforms and clinothems: Fundamental elements of basin infill

“…Olariu et al, 2019 , document excellent Lower‐Middle Jurassic outcrops of basin margin shelf‐edge clinoforms from S. Neuquén Basin (Argentina). Based on outcrop measurements and remote sensing data, this contribution demonstrates sedimentary processes and sediment‐facies partitioning from shallow to deep‐water segments of the clinothems.…”

“…Amorosi et al, 2019; Plink‐Bjorklund, 2019) to shelf‐edge scale (e.g. Bryn et al, 2019; Gamberi et al, 2019; Gan, Steel, Olariu, & Almeida Jr, 2019; Olariu et al, 2019; Patruno et al, 2019; Steel et al, 2019) and continental‐margin scale (Chen, Steel, Wang, Zhao, & Olariu, 2019; Midtkandal et al, 2019). Continuous and repeated erosion is commonplace in the topset areas, as highlighted by frequent downcutting of fluvial and delta‐distributary channels (Gan et al, 2019) and by shelf‐edge incisions (Olariu et al, 2019).…”

“…Clinothems record incremental sedimentary basin filling over a wide range of temporal and spatial scales (Magyar, Krezsek, & Tari, 2019; Olariu et al, 2019; Patruno & Helland‐Hansen, 2018; Zhang, Olariu, Steel, & Kim, 2019). In particular, three major scales of clinoforms can be differentiated on the basis of vertical relief of the foresets: delta scale (tens of metres relief); shelf‐edge scale (hundreds of metres), common in noncontinental margin settings, and continental‐margin scale (thousands of metres; Figure 1).…”