Short, sharp shelves: unravelling palaeoenvironmental controls on sediment supply across active rift-margins to deep-water lakes

Deep-water systems in active rift margins are complex, highly dynamic, and often characterised by short, steep source-to-sink configurations promoting high and sustained sediment supply to substantially underfilled, deep-water depocentres. The timing and character of coarse siliciclastic sediment delivery to deep-water environments in active rift basins is governed by complicated interactions of tectonics, climate, eustasy, and shelf process regime. However, existing sediment-supply models are largely geared towards longer term changes (~10⁶ yrs) in tectonics, or eustatic variability, with relatively limited consideration for higher order (10⁴-10⁵ yrs) changes in catchment sediment flux due to the difficulty in deconvolving controls at these timescales. In the case of deep-lacustrine, syn-rift systems, the excess accommodation resulting from substantial subsidence, limited storage in short or absent shelves, and suppressed water-level variability in lacustrine settings, means climate-controlled catchment sediment flux may become a dominant allogenic control on mesoscale stratigraphic architecture. However, well-constrained conceptual models and examples of the stratigraphic record of sediment supply variability to deep-water syn-rift lakes are rare due to the paucity of exhumed deep-water syn-rift systems.

The West Xylokastro Fault Block, on the southern margin of the Gulf of Corinth, Greece presents an exhumed Early-Mid Pleistocene deep-lacustrine syn-rift fan system of the Rethi-Dendro Formation. The stratigraphic correlation possible in the West Xylokastro Fault Block permits the tying of stratigraphic architecture recorded in the up-dip Ilias fan-delta, supplying sediment to the deep-water realm, and their stratigraphy and palaeoenvironmental record down-dip. Through combining fieldwork, digital-outcrop investigation with palynological and palaeomagnetic sampling of a behind-outcrop research borehole, a chronostratigraphically-constrained palaeoenvironmental record reveals important changes in sediment supply variability to the deep-water consistent with obliquity-paced oscillations of forest coverage and glacial-interglacial climate variability. Integration of the borehole data, with outcrop observations, highlights that thick, laterally pervasive mudstone intervals can be tied to transgressions of the fan delta, interpreted to be related to sediment supply shutdowns. Sediment supply shutdowns occur during the warming phase of the most severe interglacials of the Early Pleistocene, where the palynological record shows the substantial expansion of warm-temperate, forested vegetation biomes. The role of vegetation in governing sediment supply, through hindering sediment erosion and transport from a catchment, has commonly been linked to reduced sediment yields throughout interglacials. However, the observations in the West Xylokastro stratigraphy highlight that such a model may not appropriate for very severe interglacials where highstand conditions may become highly seasonal, with semi-arid or dry summers, but with very large storms in winters and/or too warm to sustain lowland forests. In these conditions more open, scrubland vegetation permits the erosion and transport of coarse-grained material to the deep-lacustrine fan during the interglacial highstand.

The study informs new conceptual models for climate- and vegetation-controlled sediment supply to deep-water in active margins, which highlight the substantial potential complexity of palaeoenvironmental controls on sediment flux. Binary interglacial vs glacial sediment supply models are likely insufficient for characterising the complexity presented by deep-water syn-rift stratigraphy and highlights a greater need for deep-water stratigraphers to identify suitable proxies (such as palynology) for constraining palaeoenvironmental change.