Search

Grain-size analysis of the sediments in borehole M0079A, located in the Corinth Rift, was used to explore hydrodynamic conditions and provenance in the Late Pleistocene Corinth Rift. Grain-size populations that were sensitive to the sedimentary environments were characterized by frequency distribution, particle size–standard deviation and probability cumulative curves. Our results indicate the grain-size population component in the range 0.15–0.25 µm may be used as a sensitive proxy for hyperpycnal flows, which have commonly been triggered by river floods from the southern margin of the rift since c. 0.593–0.613 Ma. The high-density plumes derived from the longer rivers of the southern rift that were prevalent before c. 0.593–0.613 Ma. When sediment is supplied as hemipelagic deposition, the proportion of the total grain-size population that is in the 0.3–0.5 µm range becomes an index for suspension fall-out deposits. The core shows coarser sediments during the marine periods, and this may be linked to the current circulation related to the Ishtmia Strait opening. The study thus illustrates how the establishment of interbasinal straits can influence the details of sedimentary hydrodynamics in the deep-water axis of an adjacent depocentre.

The morphology of coastal sequences provides fundamental observations to unravel past sea level (SL) variations. For that purpose, converting morphometric observations into a SL datum requires understanding their morphogenesis. The long-lasting sequence of coral reef terraces (CRTs) at Cape Laundi (Sumba Island, Indonesia) could serve as a benchmark. Yet, it epitomizes a pitfall that challenges the ultimate goal: the overall chronology of its development remains poorly constrained. The polycyclic nature of the terraces, involving marine erosion and reoccupation of old coral colonies by more recent ones hinders any clear assignment of Marine Isotope Stages (MIS) to specific terraces, in particular the reference datum corresponding to the last Interglacial maximum (i.e., MIS 5e). Thus, to overcome these obstacles, we numerically model the genesis of the sequence, testing a range of eustatic SL reconstructions and uplift rates, as well as exploring the parameter space to address reef growth, erosion, and sedimentation. A total of 625 model runs allowed us to improve the morpho-chronological constraints of the coastal sequence and, more particularly, to explain the morphogenesis of the several CRTs associated with MIS 5e. Our results suggest that the lowermost main terrace was first constructed during the marine transgression of MIS 5e and was later reshaped during the marine regression of MIS 5e, as well as during the MIS 5c and MIS 5a highstands. Finally, we discuss the general morphology of the sequence and the implications it may have on SL reconstructions. At Cape Laundi, as elsewhere, we emphasize the necessity to address the development of CRT sequences with a dynamic approach, i.e., considering that a CRT is a landform built continuously throughout the history of SL oscillations, and not simply during a singular SL maximum.

Coastal boulders or mega-clasts, named huracanolitos in Cuba, are found along many rocky shores. They result from storms/hurricanes or tsunamis, but despite being potentially important indicators for hazard assessment, their mode of emplacement is typically unknown. We studied a coastal boulder that is probably the largest one ever described on Cuba Island. Located on a low-lying coral reef terrace on the SE Cuban shore, the reefal limestone boulder is emplaced seaward of the ruins of the Bucanero resort. The latter was built in 1989, suffered from Hurricanes Ivan (2004) and Dennis (2005), and, in October 2012, was totally destroyed by Hurricane Sandy. Despite these extreme climatic events, satellite images obtained since 1962 show no movement of the block. In order to analyse the boulder and its morphological context, we have analysed its position, shape and its morphological environment. To determine its volume, we developed a low-cost, open-source Structure from Motion photogrammetry (SfM) procedure using a simple 3D spherical target (diameter 30 ± 0.2 x 10-3 m). Using various hydrodynamic equations, we calculated the breaking wave height and associated energy responsible for the dislocation of the coral reef terrace and transport of the boulder. To determine the exposure time of the block on the terrace, we performed 36Cl analysis and U/Th dating on post-depositional secondary calcite. From these data, the climatic or tsunamigenic (by earthquake or gravity event) origin is discussed. Notably, mapping and dating of a neighboring coastal landslide were made in order to correlate or not its effect on the setting up of the boulder.Keywords: coastal boulder; extreme-wave deposit; tsunami; hurricane; Cuba

Coral reefs and mangroves are important yet vulnerable coastal ecosystems, especially given strong anthropogenic pressure such as the projected 21st century sea-level rise. Despite the symbiotic relationship of reefs and mangroves within the context of sea-level changes, few studies provided a synchronous assessment of both systems, especially on timescales beyond a few decades. We focus on the response of coral reefs and mangroves in Belitung Island (Indonesia), where previous studies have already provided initial constraints on Holocene relative sea-level history. Along 3 different coastal transects within a distance of ~40 km, we surveyed and sampled 8 sedimentary cores of 1-3 m length, containing coral fragments, mangrove mud/sands and other types of coastal sediment. The three cross-sections evidence a surprisingly variable responses to the same sea-level history: 1) at Sidjuk, a mid-Holocene (~6 ka) coral reef within a former embayment was slowly filled up with sediments providing the substrate for estuarine mangrove development, 2) at Batu Itam, ~500 m of open coast mangroves prograded on top of a mid-Holocene reef, currently occupying around one third of the area within the intertidal zone and 3) at Mendanau, ~ 1 km of open coast mangroves prograded directly on top of basement rocks, with fossil corals seemingly sparse and isolated compared to the other sites. These results demonstrate how reef and mangrove response to sea-level changes may be extremely local in nature, and directly dictated by physical parameters such as sedimentation, wave erosion and reef construction rates. The spatial heterogeneity on the scale of a few 10s of kilometers emphasizes the need for tailored solutions with regards to future sea-level rise.

The fossil record of Quaternary reef systems, as expressed in uplifted regions by sequences of stacked terraces, has been extensively used to either understand their morphodynamics, or to unravel sea level variations. Yet, because these two aspects are intimately linked, Quaternary reef analysis is often underdetermined because the analysis often focuses on single sequences, along one-dimensional profiles. Here, we take advantage of the lateral variations of coral reef sequences by documenting the morphological variations of the reef sequence on Sumba Island. Near Tambolaka, Northwest Sumba, we analyzed a reef transect, topography, and associated sedimentological record to obtain a precise coral reef stratigraphy and geomorphic patterns that can be compared to the well documented eastern counterpart. In Tambolaka, the reef sequence displays four lower layers of bedded chalky limestone units with a weakly cemented sandy matrix, which we attribute to the Middle Miocene to Pliocene Wakabukak formation based on calcareous nannofossils and planktonic foraminifers. The uppermost layer is a calcretized reefal limestone unit with a well-lithified sandy matrix, which we attribute to the Plio-Pleistocene reef sequence of the Kalianga formation. Seven marine terraces imprint the regional morphology, four of which we correlate with MIS 5e, MIS 7e, MIS 9e, MIS 11c terraces of Cape Laundi, Northeast Sumba. When scrutinized at the light of numerical models of reef development, these results indicate that the morphodynamics of reefal sequences is strongly impacted by the tectonic evolution, even at local scales. The geodynamic context sets both the extrinsic -morphology of the basement, hydrodynamics- conditions of reef development, but also the intrinsic properties - reef growth rate in particular. While the morphodynamic evolution of the sequence is at first order representative of the interplay between uplift rates and sea level oscillations, the detailed assemblage of the reef units drastically varies along the coastline.

Several crustal and lithospheric mechanisms lead to deformation and vertical motion of the upper plate during subduction, but their relative contribution is often enigmatic. Multiple areas of the Hellenic Forearc have been uplifting since Plio-Quaternary times, yet spatiotemporal characteristics and sources of this uplift are poorly resolved. The remarkable geology and geomorphology of Kythira Island, in the southwestern Hellenic forearc, allow for a detailed tectonic reconstruction since the Late Miocene. We present a morphotectonic map of the island, together with new biostratigraphic dating and detailed analyses of active fault strikes and marine terraces. We find that the Tortonian-Pliocene stratigraphy in Kythira records ~100 m of subsidence, and a wide coastal rasa marks the ~2.8-2.4 Ma maximum transgression. Subsequent marine regression of ~300-400 m and minor E-W tilt are recorded in ~12 marine terrace levels for which we estimate uplift rates of ~0.2-0.4 mm/yr. Guided by simple landscape evolution models, we interpret the coastal morphology as the result of initial stability or of slow, gradual sea-level drop since ~2.8-2.4 Ma, followed by faster uplift since ~1.5-0.7 Ma. Our findings on- and offshore suggest that E-W extension is the dominant mode of regional active upper crustal deformation, and N-S normal faults accommodate most, if not all of the uplift on Kythira. We interpret the initiation of E-W extension as the result of a change in plate boundary conditions, in response to either propagation of the North Anatolian Fault, incipient collision with the African plate, mantle dynamics or a combination thereof.

International audience; The Java Back-arc Thrust scars the entire back-arc area of Java Island, but the faults' nature, timing, and activity remain partly elusive. Characterizing the structure and activity of the seismogenic Java Back-arc Thrust (historical earthquakes up to 7 Mw) is a cornerstone to evaluate associated geohazards. We focus on the western part of Java Back-arc Thrust that reaches the megalopolis of Jakarta. We combine morphotectonic data, seismic reflection, electric resistivity profiles, kinematic, structural field measurements, paleoseismological trenching, and sediment dating (optically stimulated luminescence and 14C). Our results suggest that the interplay between the faults, volcanoes, and sedimentary basin modulates the propagation of the fault system across and along-strike. The West Java Back-arc Thrust has been active from Pliocene to Recent, but with a laterally variable tempo and tectonic regime. While tectonic activity was sustained for longer times in the eastern part, the West Java Back-arc Thrust broke through the Jakarta Basin in the west, possibly only since the Late Pleistocene, and partitions into a network of immature transpressive structures. We conclude that the West Java Back-arc Thrust has a high seismic hazard that requires a careful risk evaluation along its trace, as it threatens the numerous infrastructures of the densely populated West Java.

Quantifying paleo sea-level changes is an important challenge given its intricate relation with paleo-climate, -ice-sheets and geodynamics, but pre-Holocene uncertainties currently span several tens of meters. The world’s coastlines provide an enormous geomorphologic dataset, and recent modelling studies have showed their potential in constraining paleo sea-level through forward landscape evolution modeling. We take a next step, by applying a Bayesian approach to invert the geometry of marine terrace sequences to paleo sea-level. Using a Markov chain Monte Carlo sampling method, we test our model on synthetic profiles and two observed marine terrace sequences. The synthetic profiles – with known input parameters – show that there are optimal values for uplift rate, erosion rate, initial slope and wave base depth to obtain a well-constrained inversion. Both the inversion of synthetic profiles and a terrace profile from Santa Cruz (Ca, US) show how sea-level peaks are easier to constrain than sea-level troughs, but that also solutions for peaks tend to be non-unique. Synthetic profiles and profiles from the Corinth Rift (Greece) both show how inverting multiple profiles from a sequence can lead to a narrower range of possible paleo sea-level, especially for sea-level troughs. This last result emphasizes the potential of inverting coastal morphology: joint inversion of globally distributed marine terrace profiles may eventually reveal not only local relative sea-level histories, but catalyse a better understanding of both global paleo sea-level and glacio-isostatic adjustments.