Shrimps and leaves: Phytodetrital pulses and bioturbation in deposits of a river-dominated delta (Middle Jurassic Lajas Formation, Neuquén Basin, Argentina)

Abstract

The Middle Jurassic Lajas Formation of the Neuquén Basin (Argentina) records mainly deltaic sedimentation. However, the precise role of the depositional mechanisms involved is still controversial. Well core YPF Nq. Laj-8 records the overall vertical transition from delta-front mouth bars to delta plain. Discrete mouth-bar successions typically comprise a lower unbioturbated sandstone unit that passes upwards into a more intensely bioturbated sandstone unit. Phytodetrital material is abundant through the mouth-bar succession forming distinct layers in the lower units and becoming part of the lining of the burrow walls of Ophiomorpha irregulaire and Gyrolithes isp. in the upper units, representing two successive developmental stages of deltaic sandy mouth bars. Layers rich in phytodetritus exhibiting the same characteristics have been traditionally interpreted in outcrop as mudstone drapes generated by tidal action during slack-water periods. However, more recent research suggests that these layers can be best regarded as produced by river-fed flows, which is consistent with a river-dominated delta model.