Browsing by Subject "Los Molles Formation"
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Item Architecture of coarse grained (conglomeratic) deep water lobes at the base of a sandstone dominated fan, Jurassic Los Molles Formation, Neuquén Basin, Argentina(2015-05) Shin, Moonsoo; Steel, R. J.; Olariu, Cornel; Mohrig, DavidThe 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.Item Facies variability in deep water channel-to-lobe transition zone : Jurassic Los Molles Formation, Neuquen Basin, Argentina(2014-05) Tudor, Eugen Petrut; Steel, R. J.; Olariu, CornelThis study focuses on the facies changes from the lower slope to toe-of-slope to basin floor over a 10 km outcrop belt, in down-dip and oblique-strike directions to the basin margin. The Jurassic Los Molles Formation in Neuquen Basin, Argentina represents the slope and basin floor of basin margin clinoforms, coeval with the shallow water and fluvial deposits named Las Lajas and Challaco formations respectively. The shallow and deep water deposits are diachronously linked in an Early-Mid Jurassic source-to-sink system developed in a back-arc basin during the incipient development of the Andes Mountains. Satellite images, high resolution panorama pictures and measured sections were used to correlate and interpret the spatial variability and overall geometry of the base of slope to basin floor units. The observations of this study refine the model for the channel-to-lobe transition zone with increase recognition and quantification of facies and architecture variability. The Los Molles basin margin was coarse grained and was ideal to observe changes in the geometry and depositional facies of channel-to-lobe deposits from updip to downdip continuous over an 8 km outcrop belt. The described channel-to-lobe transition zone clearly shows a downdip change in bed boundaries from dominantly erosive to non-erosional (bypass) to depositional and with a range of distinct facies changes. In the transition zone the sand to shale ratio is high (N:G: 65-70 %), with gutter casts and deep scours, with a high degree of amalgamation, gravel lags, mud rip-up clasts and laterally migrating beds. Within the same depositional unit (deep water lobe), at the base of the slope, the dominant sandstone beds change from amalgamated structureless and normal graded sandstone beds in the channelized lobe axis to parallel laminated and normally graded in the channelized lobe off-axis areas. Similar facies changes have been observed along proximal to distal direction. The lateral change of the dominant structures in the beds indicates changes in the flow regime and depositional style.