Stochastic alluvial fan and terrace formation triggered by a high-magnitude Holocene landslide in the Klados Gorge, Crete

Abstract

Abstract. Alluvial fan and terrace formation is traditionally interpreted as a fluvial system response to Quaternary climate oscillations under the backdrop of slow and steady tectonic activity. However, several recent studies challenge this conventional wisdom, showing that such landforms can evolve rapidly as a geomorphic system responds to catastrophic and stochastic events, like large-magnitude mass wasting. Here, we contribute to this topic through a detailed field, geochronological, and numerical modelling investigation of thick (>50 m) alluvial sequences in the Klados catchment in southwestern Crete, Greece. The Klados River catchment lies in a Mediterranean climate, is largely floored by carbonate bedrock, and is characterised by well-preserved alluvial terraces and inset fans at the river mouth that exceed the volumes of alluvial deposits in neighbouring catchments of similar size. Previous studies interpreted the genesis and evolution of these deposits to result from a combination of Pleistocene sea-level variation and the region's long-term tectonic activity. We show that the >20 m thick lower fan unit, previously thought to be late Pleistocene in age, unconformably buries a paleoshoreline uplifted in the first centuries CE, placing the depositional age of this unit firmly in the late Holocene. The depositional timing is supported by seven new radiocarbon dates that indicate middle to late Holocene ages for the entire fan and terrace sequence. Furthermore, we report new evidence of a previously unidentified valley-filling landslide deposit that is locally 100 m above the modern stream elevation, and based on cross-cutting relationships, it predates the alluvial sequence. Observations indicate the highly erodible landslide deposit as the source of the alluvial fill sediment. We identify the likely landslide detachment area as a large rockfall scar at the steepened head of the catchment. A landslide volume of 9.08×107 m3 is estimated based on volume reconstructions of the mapped landslide deposit and the inferred scar location. We utilise landslide runout modelling to validate the hypothesis that a high-magnitude rockfall would pulverise and send material downstream, filling the valley up to ∼100 m. This partial liquefaction is required for the rockfall to form a landslide body of the extent observed in the valley and is consistent with the sedimentological characteristics of the landslide deposit. Based on the new age control and the identification of the landslide deposit, we hypothesise that the rapid post-landslide aggradation and incision cycles of the alluvial deposits are not linked to long-term tectonic uplift or climate variations but rather stochastic events such as mobilisation of sediment in large earthquakes, storm events, or ephemeral blockage in the valley's narrow reaches. The Klados case study represents a model environment for how stochastically driven events can mimic climate-induced sedimentary archives and lead to deposition of thick alluvial sequences within hundreds to thousands of years, and it illustrates the ultrasensitivity of mountainous catchments to external perturbations after catastrophic events.

Paleo‐denudation rates suggest variations in runoff drove aggradation during last glacial cycle, Crete, Greece

“…The Chrissoskalitissa catchment contains significant gypsum bedrock, which could promote slope failures. Aggradation due to landsliding has been shown to occur in a similar carbonate catchment on Crete (Bruni et al, 2021). However, CRN denudation rates measured elsewhere have been demonstrated to be accurate and consistent, despite significant landsliding within catchments (Roda‐Boluda et al, 2019), suggesting that mass wasting may not necessarily bias denudation‐rate measurements.…”

“…Ott et al (2019b) documented average uplift rates of ~ 1.2 mm/a since 71 ka at Elafonisi and < 0.1 mm/a since 125 ka for the hanging wall of the Sfakia fault on which the fan is deposited. The Krios paleoshoreline, which can be traced along the western coast of Crete, documents Late Holocene uplift during either a single (Shaw et al, 2008) or a sequence of earthquakes (Ott et al, 2021) by about 8 m at Elafonisi and 2.9 m at Sfakia (Angelier, 1979;Ott et al, 2021). However, on a Late-Pleistocene timescale, rock uplift rates along the coast of Crete are relatively steady (Strasser et al, 2011;Gallen et al, 2014;Ott et al, 2019b;Robertson et al, 2019).…”

How have Cretan rivers responded to late Holocene uplift? A multi‐millennial, multi‐catchment field experiment to evaluate the applicability of Schumm and Parker's (1973) complex response model

“…Along the south central coast alone, over 40 rivers drain predominantly limestone gorges that commonly preserve morphological and sedimentary evidence for Quaternary changes in sediment supply and flow regime that have resulted from variations in tectonic activity (including uplift, faulting, local subsidence, and seismic shaking), climate, and human land use (e.g., Booth, 2010;Bruni et al, 2021;Maas, 1998;Maas & Macklin, 2002;Maas et al, 1998;Macklin et al, 2010;Noble, 2004;Pope et al, 2008).…”

“…This is especially the case in those parts of Cretan catchments underlain by more easily erodible Neogene sediments such as the Messara Plain, where major agricultural intensification led to accelerated fine-grained sedimentation in the downstream Anapodaris Gorge during the mid-12th century CE (Macklin et al, 2010). However, in the steepland catchments of south-western Crete that tend to be underlain by more mechanically resistant lithologies (e.g., limestones, phylitte quartzites), tectonic activity, climatic fluctuations, and/or stochastic events (e.g., mass movements) have been shown to be the primary controls of major valley floor incision and aggradation phases (Bruni et al, 2021;Macklin et al, 2010;Maas & Macklin, 2002;Maas et al, 1998;Pope et al, 2008Pope et al, , 2016. Valley-floor vegetation (e.g., maquis, phrygana, Cretan pine [Pinus brutia] and oleander [Nerium oleander]) tends to be patchy and has had little influence on these incision and aggradation phases (Maas & Macklin, 2002;Macklin et al, 2010).…”

Tracing past extreme floods on an alluvial fan using geophysical surveying

“…The sedimentary architecture of alluvial fan systems results from different cycles of erosion and sedimentation (e.g., de Haas et al., 2019; Harvey, 2012; Mather et al., 2017). Although the sediments stored in an alluvial fan are a fraction of those transported from its feeder catchment, they can document the legacy of floods, sediment‐laden flows, and debris flows (Bruni et al., 2021; Davies & Korup, 2007; Korup, 2004). Careful investigations and analyses of such sediments may help to understand how and under which conditions sediments were transported, deposited, and preserved (e.g., Bardou & Jaboyedoff, 2008; Crosta & Frattini, 2004).…”