Coarse-Grained, Jurassic Fluvial-Tidal Deltas in a Tectonically Active Basin: Lajas Formation in Southern Neuquen Basin, Argentina

Abstract

A southern Neuquen Basin outcrop analogs of the hydrocarbon-bearing Mid-Jurassic Lajas Formation, Argentina is examined. The spatial and temporal changes occur during repeated regressive-transgressive phases of Lajas deltas building across the shelf and the accumulated hundreds of meters thick deposits as a topset succession of shelf-margin clinoforms. About 1,000 m of sedimentary logs with observations on grain size, sedimentary structures, and paleocurrent direction have been collected. The log measurements in multiple locations combined with kilometers of walking of outcrop belt, photomosaic, and drone photos allowed facies description and interpretation. The dominant facies of the area is medium-coarse sandstone with uni- or bi-directional cross-strata and parallel laminations. Sandstones showing an upward coarsening and thickening tendency are interpreted as tidal dominated delta front, and sharp based and channelized coarse-grained sandstone beds contain abundant wood logs and cut down into parallel laminated or bi-directional cross-bedded sandstones. Dm to meter thick mudstone intervals which are highly bioturbated with marine trace fossils alternate with the sandstones. Evidence of significant wave influence is very rare. There are also strike-continuous, transgressive cross-stratified sandstone bodies near the shelf break that appear to have migrated along shoreline, and are important for understanding off-shelf sediment transportation. The sand-rich deltaic system is unusual as it contains relatively little mud and this character is interpreted to derive from a combination of source proximity, syn-depositional tectonic activity, and strong tidal reworking of the river-driven sediment dispersal. The strong and extensive tidal-current reworking within the Lajas delta-estuarine system was due to the occurrence of syn-depositional tectonic movements, creating narrow and convergent geometry in Mid-Jurassic, and coastlines that were irregular and protected from oceanic waves. In addition, measured paleocurrents suggest that there may have been an anti-clockwise tidal gyre within this narrow southern end of the basin that may have tidally swept the shelf-edge coastlines. This outcrop analog study that shows the shoreline was dominated by fluvial-tidal processes can be directly applied to adjacent hydrocarbon fields in Neuquen Basin, and as a wider importance, can improve understanding of fluvial-tidal depositional systems in tectonic active settings.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018