Depositional patterns of crevasse deltas in a Jurassic fluvial-dominated tide-influenced lower delta plain (Neuquén Basin, Argentina)

GUGLIOTTA, M.; FLINT, S.S.; HODGSON, D.; VEIGA, G.D.

Congreso; 30th IAS Meeting of Sedimentology; 2013

A typical delta usually shows three main depositional areas: a delta plain, a delta front and a prodelta. Many studies have addressed modern and ancient delta systems and in particular sedimentary facies and architecture of the delta front area, but the range of processes and depositional elements recorded in delta plain settings is still poorly understood. In particular, the role of tides and the interplay of tide and fluvial processes in this area and specifically in non-channelized settings have received limited attention. This study addresses the facies model of the lower delta plain distributary channels and interdistributary areas. The Middle Jurassic Lajas Formation study section represents a fluvial-dominated deltaic system influenced by tidal processes throughout most of the 500 m thick succession. Facies associations include distributary channels, side bars, crevasse channels that consists of erosive-based, moderate- to well- sorted sandstones with mud clasts, sparse pebbles, pieces of silicified wood and plant/carbonaceous debris and fining upward trends. Distributary and crevasse channels are usually structureless, but they may show trough cross-stratification, planar cross-stratification and current ripples. The depositional model proposed consists of small deltaic bodies referred to as crevasse deltas, which are active for relatively short periods (e.g. seasonal) and result in the filling of low-energy, non-channelized areas between distributary channels (i.e. interdistributary areas or bays), and may cause the avulsion of the distributary channel. The initiation phase of is usually via single crevasse-splays (pioneers), which are recorded by structureless, moderate-sorted sandstones. The splay evolves into an organized crevasse delta that comprises sandstones and heterolithic sediments showing coarsening/thickening upward trends and containing mud clasts, mud drapes, sparse to abundant trace fossils and rare current and wave ripples. The crevasse prodelta facies consists of lateral extensive, bioturbated, structureless mudstones with rare oyster fossils. The role of tides in this system is minor, but distinct. Tidal reworking occurs in particular in the crevasse mouth bars facies and in particular in its medial part. Channels seem to be not affected by tidal processes, which is interpreted to be due to the strength of the fluvial currents although the lack of tidal indicators could also be ascribed to low preservation compared with interdistributary areas. The correct identification of the sedimentological features of this zone and its depositional model as recorded in stratigraphic successions will improve knowledge of sedimentation processes in deltas and their understanding in terms of hydrocarbon reservoir/seal systems.