This study isolates the controls on the alluvial architecture of two interfingering late Lutetian fluvial systems preserved at the northern margin of the Tremp-Graus Basin, in the south central Pyrenees. One of these, a northerly, mountain-belt-derived, high-gradient, bedload-dominated gravelly system, displays marked variations in its alluvial architecture, with systematic changes in the width, depth, internal facies arrangement and stacking pattern of its channel bodies. The other, an easterly, basin-axis-derived, lower gradient, poorly drained alluvial system, displays no discernible systematic variations in alluvial architecture.
A common control that would similarly affect both of the interfingering fluvial systems cannot be responsible for their contrasting architectural patterns. Variations in base level, despite the contemporaneous shoreline being only 18 km downstream, climate, or the tectonic rotation of the study area are therefore discounted. Fluctuations in sediment supply to the northerly derived system resulting from the episodic uplift of its drainage basin are instead proposed, although intrinsic controls cannot be totally discounted. Similar sediment flux variations in the headwaters of the easterly derived system are envisaged, but its longer, lower-gradient transport path was effective in dampening out these fluctuations upstream of the study area.