Architecture of coarse grained (conglomeratic) deep water lobes at the base of a sandstone dominated fan, Jurassic Los Molles Formation, Neuquén Basin, Argentina
The complex structural and stratigraphic framework of the Neuquén Basin developed by Triassic-Jurassic extensional processes formed a deep basin and accumulated coarse-grained gravity flow deposits on slope and basin floor. The Los Molles Formation exposes the succession of gravity flow deposits from conglomerates to mudstones over a 9 km outcrop belt in southern Neuquén Basin. The Los Molles Fm. is over 1000 meters thick and its basal part is ~200 meters thick consisting of two fan units capped by 0.5-3 m of conglomerate beds. The initial deepwater fan units start with unusual pebble- and cobble-rich conglomerate beds at their base. To characterize the conglomerate lobes and their link with the correlative and overlaying basin-floor lobe complexes, satellite images, DEM (Digital Elevation Model), photomosaics (a few km), and 19 measured sections (30-190m thick) have been collected and interpreted. In all units measured, each lobe contains, from bottom to top, very coarse, poorly sorted, and erosional-based conglomerates (1-3m) overlain by amalgamated, normal graded turbidite sandstone beds (20-30m), and silty mudstone beds (up to 15m). Each of these three facies associations forms a succession (about 30-40 m thick) of lobe complexes with an overall fining upward trend. The conglomerate thickness and lateral extent decreases upwards as the third (uppermost) conglomerate layer demonstrates rather discontinuous, lenticular bodies. In contrast, the sandstone beds increase upward in thickness with finer grain size and better sorting. The conglomerate beds are interpreted as debris flow deposits based on their structureless and poorly sorted texture. However, some conglomerates are at times erosional at the base, poorly sorted throughout, but others are capped by normal grading for up to a third of their thickness. Normal grading suggests debris flow transforming into turbidity flow vertically. Flute marks associated with sandstone and conglomerate beds indicate paleoflow toward the east, in contrast to younger sandy fans that shows progradation dominantly north-northeastwards. The modern Var River system in southern France has a similar morphology with pebbles and cobbles transported to deepwater via steep gradient slope. As in the Var system, coarse sediments in the Los Molles Fm. bypass the shelf and steep slope to build the initial base of the fan. In summary, the earliest Los Molles conglomeratic fans were linked with high relief of the basin margin. Later, this margin relief decreased, and sandstone dominated fans.