Sedimentology of a transgressive mixed‐energy (wave/tide‐dominated) estuary, Upper Devonian Geirud Formation (Alborz Basin, northern Iran)

The Upper Devonian (Frasnian to Famennian) Geirud Formation exposed in the northeastern margin of the Albortz Basin, Iran, records the development and the evolution of a mixed‐energy (wave‐tide dominated) estuary adjacent to an open coast. Four vertically‐ stacked groups of facies associations were detected in supporting this interpretation: (i) a river‐dominated, bay‐head delta zone, including basal fluvial channel belts, flood plains and tidally‐influenced channels; (ii) a mixed‐energy, central basin zone, consisting of sand‐rich and mud‐rich tidal flats, associated with tidal channels and bars; (iii) a wave‐dominated, coastal and open‐marine shelf zone, comprising siliciclastic marine deposits; and (iv) an open‐marine shelf zone representing the uppermost stratigraphic interval of the Geirud Formation and consisting of carbonate offshore and bioclastic‐rich deposits. The large‐scale depositional architectures of the Geirud strata are exposed among three main investigated areas. The correlation of the three sections suggests that the Geirud Formation includes two major stratigraphic intervals: (i) a lowermost compound incised‐valley fill; and (ii) an overlying transgressiveregressive interval. The latter also comprises shoreface and open‐shelf sediments over the estuarine deposits, instead of coastal barrier or estuarine mouth sand deposits, which are usually considered as important depositional elements in the classical estuary facies models. The stratigraphic evolution reconstructed in this paper documents the infill of incised valleys during an important episode of relative sea‐level rise and the ensuing overstepping of these valleys by an estuarine system which, in turn, became transgressed by siliciclastic shoreface and carbonate open marine facies.