Autogenic architecture of a river-dominated delta in a high-accommodation foreland depocenter (Neogene, Paratethys, Romania)

ABSTRACT Sedimentary architecture studies of river-dominated deltas are important for understanding the role of autogenic vs allogenic dynamics, but examples in high-accommodation settings are rare. In this study, we analyze the kilometer-thick stratigraphic record of a river-dominated delta that accumulated during the Miocene–Pliocene (Carpathian foreland of Romania) in two sections, 13 km apart. The sedimentary successions show a high average (1.45 m/kyr) accumulation rate and are correlated along the dominant paleocurrent direction, substantiating a sequence stratigraphic model of the delta. Facies analysis reveals eight facies associations in three major depositional environments: delta top, subaqueous delta, and lake-dominated coast. The reconstructed deltaic system is characterized by crevasse splays and mires on the delta top, friction- to inertia-dominated river-mouth bars in the subaqueous delta, minor wave influence, absence of tides, minor transgressive reworking, and occasional debris flows. The delta morphology may have varied from highly bifurcating and radial to more fixed and elongated. In the record, we recognize a multitude of facies cycles that have a different expression depending on whether they formed in the proximal, medial, or distal domain. These cycles allow the recognition of hundreds of high-frequency sequences (HFSs), representing 1–12-m-thick alternations between delta (lobe) progradation and abandonment. These sequences become indistinguishable downstream at variable distances, extending for up to 25–40 km from the delta top. The reconstructed model of HFS is comparable with other river-dominated deltas but shows a more gradual lateral transition between delta-top and shallow-water deposits. The autogenic mechanisms of delta-lobe switching as well as sediment storage and release may have been responsible for the generation of the HFS. These sequences are stacked progradationally into larger sequences. The progradational nature of the observed HFS is mainly characteristic for outflowing lakes, even though the hydrology of the Dacian Basin changed over the studied time period. The perceived lake-level stability at 1000–10,000 year time scales may occur because of a relatively wet climate as well as the high resolution of the record. The whole studied record reveals net aggradation, indicating a peculiar long-term balance of sediment supply and accommodation. The record includes sequences at several scales and invites further research to distinguish potential cyclic orbital controls.