Ichnofabric Analysis and Classification in the Deltaic Depositional Environment of the Eocene Roda Sandstone Formation, Spain

Geoffray Musial^1,2
1Interprétation Structurale et Sédimentaire, TOTAL S.A, Pau, France
²Dynamique de la lithosphère, Université de Montpellier2, Montpellier, France

In some cases, the analysis of ichnofabric distribution can be critical for the determination of the depositional environment, notably through diversity of ichnogenera, substrate and bioturbation degree.

The Roda Sandstone Formation in Spain is a lowstand depositional systems tract controlled by fluvial and tidal currents. The detailed descriptions of seven cored wells along with field observations allow to identify nine ichnofabrics.

As the sea level falls, strong fluvial currents contribute to the development of prograding steep foresets in sandy lobes where three ichnofabrics are distinguished. They are dominated by Thalassinoides in topsets of sandy bars, Planolites in the foresets and Conichnus associated with large Ophiomorpha in more tidally influenced foresets.

Two ichnofabrics appear when tidal currents become dominant at the onset of transgression. The first is characterized by Macaronichnus in transgressive beaches and the second ichnofabric by Skolithos and small Ophiomorpha common in proximal sandy subtidal bars. In the distal silty facies corresponding to the bottomsets of subtidal bars and the lower shoreface, both diversity and degree of bioturbation increase and three ichnofabrics are identified by several associations of the following ichnotaxa: Paleophycus, Planolites, Ophiomorpha, Teichichnus, Scolicia and Phycosiphon.

When the sea level rises, the transgressive sandstones are rich in reworked bioclasts, for instance Alveolinidae (Foraminifera). The transgressive surfaces occurring above the prograding delta lobes are characterized by Glossifungites ichnofacies where Diplocraterion is recognized.

Finally, during high sea level conditions, Nummulitidae shells accumulate on the sea floor and then are trapped in Thalassinoides burrows.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas