Architecture of Coarse Grained (Conglomeratic) Deep Water Lobes at the Base of a Sandstone Dominated Fan, Jurassic Los Molles Formation, Neuquén Basin, Argentina

Abstract

The complex structural and stratigraphic framework of the Neuquén Basin affected by Triassic-Jurassic extensional processes formed a deep basin and accumulated coarse-grained gravity flow deposits on the lower slope and basin floor. Lower Los Molles Formation exposes the succession of turbidites from conglomerates to mudstones over a 9 km outcrop belt. The Los Molles Fm. is over 1000 meters thick and its base is ~100 meters consisting of 3 units capped by 1–3m thick conglomerates. This initial deep water fan units start with unusual pebble- and cobble-rich conglomerate beds at its base. To characterize the conglomerate lobes and their link with the other basin-floor lobe complexes, satellite images, DEM (Digital Elevation Model), photomosaics (a few km), and about 20 measured sections 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 sandstones (20–30m), and silty mudstones (up to 10m). Each of these 3 facies 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 increases upward in thickness and has finer grain size and better sorting. The conglomerates are interpreted as debris flow deposits based on their structureless and poorly sorted texture. However, few conglomerate deposits are at times erosional at the base, poorly sorted throughout, but some capped by normal grading for up to a third of their thickness. Normal grading suggests debris flow transforms into turbidity flow vertically. Flute marks associated with conglomerate beds indicate paleoflow toward the east, in contrast to the younger sandy fans that prograded dominantly north-northeastwards. The Var River system in southern France is a modern analog with pebbles and cobbles transported to deep water via steep gradient slope. Similarly, coarse sediments in the Los Molles Fm. bypass the shelf and steep slope to build the initial conglomerate base of the fan. In summary, the earliest Los Molles conglomeratic fans are linked with high relief of the basin margin that later decreased and formed sandstone dominated fans.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015