--> Abstract: Tidal Dunes of the Eocene Baronia Sandstone, Ager Basin, Spain: Distinguishing Tidal Dunes from Tidal Bars; Why Bother?, by Cornel Olariu, Ron J. Steel, Robert W. Dalrymple, Murray K. Gingras, and Jean-Loup Rubino; #90078 (2008)

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Tidal Dunes of the Eocene Baronia Sandstone, Ager Basin, Spain: Distinguishing Tidal Dunes from Tidal Bars; Why Bother?

Cornel Olariu1, Ron J. Steel1, Robert W. Dalrymple2, Murray K. Gingras3, and Jean-Loup Rubino4
1Department of Geological Sciences, The University of Texas at Austin, Austin, TX
2Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON, Canada
3Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada
4TOTAL, Pau, France

Ager Basin is a piggy-back basin located in the South Central Pyrenees, Spain, and was active during the Eocene. The >200 m-thick Baronia Sandstone crops out in the Ager syncline for more than 10 km in an east-west direction. Paleocurrent directions are east-west, parallel to the axis of the basin. The lower part of the Baronia succession is composed of cross-stratified sandstone bodies alternating with meters to tens of meters of bioturbated sandy mudstones. The fundamental building blocks in the cross-bedded sandstones are 2m to 4m thick and have horizontal extensions of 200-400 m before they thin and pass down-current into bioturbated silty sandstones and muddy sandstones. The sandstone bodies are formed by decimeter-scale bidirectional cross strata that thin in a basinward direction and pass into alternations of rippled siltstones and sandstones that is more bioturbated. The cross-bedded units have a coarsening-up motif and stack to form larger-scale lithosomes that also thin down-current. The sandstones are composed of siliciclastic material or of intrabasinal bioclastic debris. The Cruziana ichnofacies is the dominant ichnological expression in the bioturbated muddy sandstone, suggesting that the tidal bars accumulated below fair-weather wavebase.

The lower Baronia deposits are interpreted to have been formed by subtidal compound dunes within a strait or a seaway rather than as elongate tidal bars. This interpretation is based on the observations that: (1) the orientation of the cross-bedded sets is similar to that of the dipping master surfaces; (2) there are no channels as would be expected with tidal bars, and (3) the fully marine, distal ichnofossil assemblage.

For reservoir modeling it is important to distinguish between tidal bars (flow-parallel bodies) and tidal dunes (flow-transverse bodies).

 

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