Climate and tectonic modulation of sand delivery to deep water lakes in rift basins. Late Pliocene, Corinth Rift, Greece

The sedimentary record of deep-water lakes is often used to investigate past climate and environmental change. Correct identification however, of the main controls driving the production and transport of sediment to the deepest parts of tectonically active basins is often challenging, especially when trying to differentiate autogenic from allogenic factors. This study focuses on the changes observed in a deep-water sedimentary system that evolved from mudstone-dominated to the development of a lacustrine sand-dominated channel-lobe distributary fan and back to mudstone deposition during the Pliocene climatic optimum in the Corinth Rift, Greece. This is a multidisciplinary study that involves the integration of sedimentology and structural geology with digital outcrop modelling, palynology, palaeomagnetology and geochronology.

The studied sedimentary system consisted of a coarse-grained delta (Mavro delta) that fed the deep-water Rethi Dendro Formation (RDF) in the Amphithea fault block during the Pliocene and Early Pleistocene. These syn-rift deposits were sourced from a major hinterland catchment, the Olvios catchment, draining the southern, fault-controlled margin of the rift. The depocentre was located at the exit of a structurally controlled sediment fairway, ~15 km from the source of sediment and ~12 km basinwards from the basin margin coastline. The stratigraphy of the RDF in the study area is well constrained due to the combination of detailed surface mapping and logging with LiDAR, photogrammetry and UAV surveys of large-scale exposures in the Sythas river valley. These were integrated with the information obtained from the analysis of rock cores obtained through wells drilled immediately behind the cliffs where the RDF is exposed. The Amphithea fault block has a half-graben configuration and tilting of the hangingwall was one of the main tectonic controls on the evolution of the depocenter fill. The stratigraphic interval considered in this study is ~130 m thick. It was deposited above a ~6° angular unconformity and it is composed predominantly of fine-grained hemipelagic dominated units, interrupted by the development of an ~30 m thick sandstone-dominated channelized lobe unit. Detailed palynological analysis of this interval shows significant changes in pollen and spore assemblages that are used to interpret the palaeoflora developed in the drainage catchments. The palynoflora in fine-grained hemipelagic intervals is dominated by temperate forests mixed with subtropical elements, whereas the channelized lobe unit is dominated by palynofloras typical of open herbaceous vegetation including steppic taxa, suggesting a dryer and cooler climate. The shift from forest- to herbaceous-dominated palynological assemblages is gradual, recorded from 5 m below the facies change marking the base of the channelized lobe unit.  In contrast, the top of the channelized lobe unit coincides with the abrupt change back to the forest-dominated pollen assemblage. The correlation between the establishment of a sand-dominated channel-lobe distributary fan in the basin floor with the predominance of open herbaceous vegetation is interpreted to reflect the highly erodible condition of land covered in this type of vegetation. In contrast, during periods when forests are dominant, erodibility decreases.