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Coarse, Well-Sorted, and Cross-Bedded Sandbodies Associated With Shelf Transgression, Jurassic Lajas Formation, Neuquén Basin

Abstract

Seismic data in Lower-Middle Jurassic, southern Neuquén Basin show that the Lajas-Los Molles system (the shelf and associated slope to basin floor) prograded and aggraded more than 120km. The interpreted shelf-edge trajectory shows that it was generally rising northwestwards as the shelf-margin sedimentary prism built out during a period of 18 Ma (Pleinsbachian to early Callovian). However, the rise of the shelf-break trajectory at times was discontinuous, causing significant transgression back across the pre-existing shelf, then leading to a resetting of the shelf break and continued regression at a higher and younger level, as it continued to prograde. The abrupt rise and backstep of the shoreline are of special interest and focus here. A few km wide and >100 m thick Lajas outcrop in southern Neuquen Basin was used to correlate flooding surfaces and mapped shelf sandstone architecture using digital elevation models, satellite imagery and drone photography. Multiple vertical sections describe the facies, i.e. lithology, sedimentary structures, ichnology, and paleocurrents. The outer-shelf Lajas transgressive deposits in the outcrops have been identified in the outbuilding margin clinoforms within the La Jardinera outcrop and indicate there was an enhancement of tidal currents along the shelf edge, strong enough to move medium-grained sand on this segment of the margin clinoform. It is interpreted from the development of these very clean and well sorted, thoroughly cross-bedded marine sandbodies (up to 12m thick) enveloped within transgressive marine mudstone. The orderly stacking of crossbedded sets, bi-directionality of paleo-flow indicators and occasional marine trace fossils suggest that the interval was generated by marine, subaqueous 2-D and 3-D dunes that were driven by tidal currents. Further, the 3 to 6 m thick units laterally continuous for a few hundred m to km, with cross-bedded sets that thicken and coarsen slightly upward suggest that the smaller simple dunes constructed compound dunes, large bedforms common in both estuaries and on tidal shelves. These sandstone bodies near the shelf break appear to have migrated along the shoreline and were generated by strong tidal currents on the outermost shelf during transgression. This study could be applied to better understand shelf reservoirs developed in Lajas Formation and also sediment transport to deep water Los Molles slope and fan deposits.