Testing glacial isostatic adjustment as a cause of Early Pleistocene river network reorganization in the area of Lithuania (NE Europe)

Glacial isostatic adjustment (GIA) induces lithospheric bending that can strongly influence depositional systems located within ice-sheet forebulges. While previous studies have documented the role of GIA in river-network reorganization during the last glacial cycle, its impact on sedimentary systems earlier in the Quaternary remains poorly constrained.Here, we investigate the pre-glacial Daumantai Formation, exposed in several outcrops beneath the oldest tills in the Baltija Highlands of eastern Lithuania. This fluvial succession was dated using combined 10Be–26Al exposure–burial dating to ~0.95 ± 0.1 Ma, while the same approach indicates that the overlying tills were deposited only ~30–50 kyr later. Importantly, the succession records a distinct change in palaeocurrent directions, initially toward the southeast and subsequently toward the northwest, occurring prior to the first documented advance of the Fennoscandian Ice Sheet (FIS) into the region. This shift is interpreted as a reorganization of the river network rather than merely a modification of river planform geometry, as the palaeocurrent reorientation is consistently documented at several sites across distances exceeding 10 km.GIA was simulated using the ICEAGE normal-mode modelling framework to assess the potential role of lithospheric bending and associated slope changes in river-network reorganization. Four ice-sheet configurations were tested: (1) a late Gauss and (2) an early Matuyama extent after Batchelor et al. (2019, https://doi.org/10.1038/s41467-019-11601-2), (3) an additional, larger ice sheet extending ~150 km northwest of the study area, and (4) a MIS 20–24 ice-sheet extent from Batchelor et al., which directly overlies the analysed succession. A 380 kyr modelling scenario included five glacial cycles comprising ice growth, deglaciation, and ice-free periods, with 40 kyr and 100 kyr periodicities and increasing amplitudes. The modelling results indicate that the study area was affected by forebulge development associated with all tested ice-sheet extents. The two smaller ice sheets induced southeastward surface tilting, whereas the larger configuration produced northwestward tilting, with maximum slope changes reaching ~0.002°.The resulting time-dependent uplift and subsidence fields were subsequently used as inputs for landscape evolution modelling to investigate the impact of episodic glacial loading and unloading on surface processes. Erosion and sedimentation were simulated using a stream-power–law, finite-difference approach under imposed time-varying three-dimensional deformation. Preliminary results suggest that repeated north–south tilting associated with glacial cycles exerts a strong control on fluvial dynamics and can locally lead to drainage reversals.The postdoctoral project CosmoLith was caried out under the “New Generation Lithuania” plan (Nr. 10-036-T-0008) financed under the European Union economic recovery and resilience facility instrument NextGenerationEU. The research was supported by the Slovak Research and Development Agency under the contract No. APVV-21-0281.