Ichnology of Deltaic Mouth-Bar Systems of the Lajas Formation (Middle Jurassic) in the Sierra de la Vaca Muerta, Neuquén Basin, Argentina

Ichnologic and sedimentologic studies of the Lajas Formation (Middle Jurassic) in Sierra de la Vaca Muerta allowed the recognition of two different types of deltaic mouth bars, each of them showing trace fossil suites with different characteristics. Type I deltaic mouth bars consist of fine to coarse sandstones and fine conglomerates completely reworked by fair-weather and storm wave action, revealing a predominance of basinal hydraulic processes (e.g., waves) during bar construction and progradation. Trace fossil assemblages are composed of Ophiomorpha and Haentzschelinia in the foreset beds, and Polykladichnus, Skolithos, and Arenicolites in the topset beds. Type II deltaic mouth bars comprise sandstones that are fine to coarse and massive or present high angle cross-stratification and current ripples migrating in the opposite direction to the inclination of the foresets. These bars are interpreted to have been deposited during intervals of extraordinary fluvial discharge when wave action was restricted to the topset part of the bars. Whereas equilibrium trace fossils occur in the bottomset beds, escape trace fossils and Ophiomorpha are recorded in the distal foreset beds. In the topset beds, Skolithos and Polykladichnus specimens are very abundant. In general, the two types of mouth bars show low diversity, low abundance of trace fossils and a simple tiering structure. Such traits reflect environmental stresses mainly produced by fluctuating hydraulic energy, salinity, sediment input and high mobility of the substrate.