Anatomy of a laterally migrating tidal bar in front of a delta system: Esdolomada Member, Roda Formation, Tremp-Graus Basin, Spain



The first sandstone unit of the Esdolomada Member of the Roda Formation (hereafter referred to as ‘Esdolomada 1’) was formed by a laterally-migrating, shelf tidal bar. This interpretation is based on detailed mapping of the bedding surfaces on the digital terrain model of the outcrop built from light detection and ranging data and outcrop photomosaics combined with vertical measured sections. The Esdolomada 1 sandbody migrated laterally (i.e. transverse to the tidal currents) towards the south-west along slightly inclined (1.6° to 4.6°) master bedding surfaces. The locally dominant tidal current flowed to the north-west. This current direction is indicated by the presence of stacked sets of high-angle (average 21°) cross-stratification formed by dunes that migrated in this direction, apparently in an approximately coast-parallel direction. The tidal bar contains sets and cosets of medium-grained cross-stratified sandstone that stack to reach a thickness of about 5·5 m. Individual cross-bed sets average about 50 cm thick (with a range of 10 to 70 cm) and have lengths of ca 130 to 250 m in a direction perpendicular to the palaeocurrent. Set thickness decreases in the direction of migration, towards the south-west, and the degree of bioturbation increases, so that the cross-bedded sandstones gradually change into highly bioturbated finer-grained and thinner-bedded sandstones lacking any cross-stratification. The rate of thinning of individual dune sets as they are traced down any obliquely-accreting master surface is some 40 cm per 100 m (0·004) for the older, thicker sandstones, whereas the younger, thinner beds thin at a rate of 15 cm over 100 m (0·0015). The tidal bar has a sharp base and top and is encased in finer-grained bioturbated, marine sandstones. The Esdolomada bar crest was oriented north-west to south-east, parallel to the tidal palaeocurrents and to the nearby palaeoshoreline, but built by lateral accretion towards the south-west. Lateral outbuilding generated a flat-topped bar with a measured width of about 1700 m, and a preserved height of 5·5 m. The bar, disconnected from a genetically related south-westward prograding delta some 2 km to the north-east, developed during the transgressive phase of a sedimentary cycle. The tidal bar was most probably initiated as a delta-attached bar at the toesets of the delta front and during transgression evolved into a detached tidal bar.