Sediment gravity-flow deposits along the active flank of a backarc basin: Example from the Neocomian of the Neuquén Basin, Argentina

SPALLETTI, L.A.; VEIGA, G.D.; SCHWARZ, E.

Congreso; 23rd Meeting of Sedimentology; 2004

International Association of Sedimentologists

During the Late Jurassic and the Early Cretaceous the Neuquén Basin was a restricted gulf-like trough partially isolated from the Pacific Ocean by the geographic barrier of the Andean magmatic arc. Thick deposits of black shales and marls are widespread, and a gently sloping marine ramp, showing the gradual passage from shallow marine to deep marine anoxic deposits, is envisaged for the cratonic (southern and eastern) margins of the basin. To the west, the sedimentary record of the active flank of the basin is poorly preserved because of the growth and tectonic uplift of the Andean arc. However, two areas of arc-related sedimentation (Huncal and Rahueco), characterised by sediment gravity-flow deposits, were recognised in the west of the Neuquén Basin. The Upper Berriasian ? Lower Valanginian Rahueco system comprises a more than 80 m thick succession developed upon a thick package of Berriasian anoxic shales and marls, and limited on top by the Intra-Valanginian unconformity. It mainly consists of massive mudrocks with interbedded thin (< 0.3 m) and isolated parallel- to ripple cross-laminated fine- to very fine-grained sandstones, showing delicate tool sole marks and small grouped flute casts. Seven coarse- -grained tabular lobes (1.5 to 6 m thick), stacked in thickening up and aggrading patterns (upward-thickening and aggrading trends), are intercalated within the fine-grained framework interlobe deposits. Internally, each lobe is formed by mediumto fine-grained sandstones (0.1 to 0.9 m thick) devoid of internal stratification and showing normal grading near the top of each bed, where current ripples are usually preserved. Large, both grouped and isolated, high-regime l-ridges and flutes, associated with load casts, are frequent at the bottom of each bed and indicate palaeocurrents running to the East and North. Though sandstone layers can amalgamate, they are usually separated by thin muddy or heterolithic beds. In contrast, the Huncal system comprises a 15 to 25 m interval located within a thick and uniform black shale succession. It is composed of sandy turbidites with complete, base-cut and top-cut Bouma sequences associated with thin mudstones and very fine-grained sandstones and siltstones. The whole interval is involved in an isolated slump fold accompanied by low- to moderate-angle shear zones, and showing a persistent orientation of quasi-solid deformation structures to the East. The location of the Huncal slump fold within a black shale succession, its wide areal distribution and orientation suggest a sudden displacement along a talus slope located to the West (the active flank of the backarc basin) as the result of a large Berriasian submarine seismic shock. The turbiditic succession of the Rahueco system and the slump folds of the Huncal system offer the first depositional arguments to reconstruct the western, tectonically active, margin of the Neuquén Basin for the Berriasian and Early Valanginian times. Unlike other margins of the basin, its arc- -related flank possessed a well developed talus slope and enough sediment supply to promote deep marine turbidite deposition and progradation.