Quantitative reconstruction of landscape dynamics and tectonics from sediment calibre and architecture: an example from the Kerinitis megadelta, Gulf of Corinth, Greece

In tectonically active landscapes, fault movement drives both the creation of accommodation space (i.e., basins), and the production of topography on which geomorphic processes act (i.e., mountains). The action of fluvial processes on those mountains will route eroded sediment into the basins; in many extensional mountain belts, this leads to the deposition of coarse alluvial fans or Gilbert deltas in the hanging-walls of normal faults as they slip and create accommodation space. The stratigraphic architecture and sedimentary characteristics of such deposits clearly respond to and thus in principle can record the tectono-climatic environment controlling the system. This implies that key stratigraphic variables, such as grain size and unit thicknesses, can be quantitatively inverted to recover a tectono-climatic history. However, confounding variables also active in erosional-depositional systems (e.g., far-field base level control, stochastic processes, signal degradation during transport) may complicate attempts to decode this archive and may buffer or shred tectono-climatic signals before they are preserved.

The well-exposed early to middle Pleistocene deltaic stratigraphy of the Corinth Rift, central Greece, provides a rare opportunity to test these ideas quantitatively. Here, we present a preliminary data set attempting to decode the geomorphic and hence tectono-climatic history of a key section of the rift directly from the grain size and architecture of a very large (~500 m thick), fault controlled, and now uplifted Gilbert delta in the Kerinitis valley, located on the southern margin of the Gulf of Corinth. We used a series of high-resolution drone surveys to obtain 27 vertical transects through the incised delta, from which detailed grain size and sediment thickness data were obtained from photogrammetric analyses (~10,000 images). Our data enabled us to produce a highly detailed correlation of stratal horizons within the deltaic package, from which we were able to describe the evolution of grain size trends both downstream and through the ca. 800 ky lifespan of the delta. We are able to resolve a marked acceleration of the driving fault from the delta stratigraphy itself, which is recorded in a sudden increase in downstream fining rate, driven by more rapid extraction of sediment from the river supplying material to the delta. The timing of this increase correlates with independent constraints from stratigraphic form on the onset of “rift climax” in this delta. Post fault acceleration, we demonstrate that the fining rates begin to fall back, consistent with transient response to tectonic perturbation in the upstream catchment and with a wave of incision sweeping up through the terrestrial system. Our results demonstrate that sophisticated insights into fault evolution can be drawn from deltaic stratigraphy, and emphasise the importance of transient landscape response in creating rift zone sedimentary archives.