Print this pageArchitecture of Obliquely Migrating Tidal Bars: Esdolomada Member, Roda Formation, Graus-Tremp Basin, Spain
The Esdolomada Member of the Roda Sandstone Formation crops out in the Tremp-Graus Basin of the south-central Spanish Pyrenees. This study describes the architecture of the first sandstone unit of the Esdolomada Member.
Detailed mapping of the bedding surfaces on the digital terrain model of the outcrop built from LIDAR data and outcrop photomosaics combined with vertical measured sections show that the sand unit is an obliquely-migrating tidal bar. The core of the bar contains sets of medium-grained sandstone that stack up to reach a thickness of about 5.5 m. The sandstone typically contains sets of high-angle (average 21 degrees) cross-stratification oriented toward NW indicating oblique migration of 2-D dunes on slightly inclined (1.6-2.8 degrees) accretion surfaces which dip toward SW. The bar has its crest oriented NW-SE, parallel to the tidal paleocurrents and to the paleo-shoreline, but builds by lateral accretion toward SW. Individual cross-bedded sandstone sets that represent flow units are thinning at a rate of 0.004 (or 4 m per km) and pass laterally and distally into highly bioturbated muddy sandstone.
Vertical aggradation and lateral outbuilding generated a flat-topped bar with a measured width of at least 1732 m, an estimated length of at least 20 km and a preserved height of about 5.5 m. The bar, disconnected from a genetically related southward prograding delta some 2 km to the NE, may have developed during the transgressive phase of a sedimentary cycle.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009