Sedimentological Criteria Research Articles

A Tide-Dominated Beach from the Cambro-Ordovician Cabos Formation of Northwestern Spain

Abstract: Beaches on which tidal processes play a significant role, as opposed to purely wave-dominated beaches, are little known from the rock record due to a lack of suitable sedimentological criteria and facies models. Cambrian strata of the Cabos Formation in Asturias, Spain, with a wealth of tide-generated sedimentary structures and a close similarity to modern tide-dominated beaches, provide an opportunity to advance our understanding of ancient tidal beaches. Our field-based analysis of the Bres and Taramundi members identifies three lithofacies: Facies 1—simple bedsets of fine sandstone, divided into (1A) sandstone with tabular and wedge-shape geometries and horizontal to low-angle laminae, (1B) sandstone with tabular, wedge-shape, and wave-form geometries and decimeter-scale planar cross-bedding, and (1C) sandstone with a channel form and erosional base; Facies 2—composite bedsets of mudstone and fine sandstone; and Facies 3—intraformational breccias composed of (3A) rip-up clasts, (3B) sandstone clasts, and (3C) skeletal clasts. The numerous tidal structures testify to bed-load transport by unidirectional and bidirectional tidal currents, alternating deposition from bed load and suspended load, emergence during the ebb stage of tidal cycles, tidal scour, and regular fluctuations of tidal-current velocity. In contrast, structures indicative of swash–backwash, oscillating waves, and storms are much less conspicuous. This study argues that the Cabos sandstone and mudstone accumulated on a tide-dominated, wave-influenced beach distinguished by four morphologic zones: a flat sandy supratidal beach (Subfacies 1A), a sandy intertidal beach characterized by small 2-D dunes of a ridge-and-runnel system (Subfacies 1B), cross-shore channels (Subfacies 1C), and the subtidal nearshore with a mix of mud and rippled sand (Facies 2). Rare storm processes on the tidal beach are suggested by the breccias (Facies 3). As increasingly more local examples like the Cabos are recognized in the rock record, sedimentologists can begin to construct a generalized facies model for this largely unexplored sedimentary environment.

Extraordinary flood events and the response to monsoonal climatic change during the last 3000 years along the middle Yangtze River valley, China

Well-dated long-term flood data series is very important in studying of hydrological response to past climatic change along the mainstream of the Yangtze River. Five palaeoflood events recorded by slackwater deposits were identified by the sedimentological criteria and analytical results. A detailed chronological framework of palaeoflood events during the late Holocene was established by optically stimulated luminescence dating in combination with anthropogenic remains, and with pedo-stratigraphic correlations with well dated Holocene loess-soil profiles within the middle Yangtze River basin. The five palaeoflood events were dated to 2.43±0.18ka, 1.82±0.17ka, 0.94±0.07ka, 0.72±0.06ka and 0.59±0.05ka, respectively. These results are consistent with geological records at other palaeoflood sites and historical flood records along the middle Yangtze River valley. A regional synthesis of the palaeoflood chronology over the last 3000years was compiled along the middle Yangtze River. Extraordinary flood events generally seem to be correlated with late Holocene climatic variability (i.e. Neoglacial cooling, Roman Warm Period, Dark Ages Cold Period, Medieval Climate Anomaly, and Little Ice Age). The high-resolution climatic proxies from stalagmites of the Dongge cave and Heshang cave, ice-cores of the GRIP, the total solar irradiance variations and sunspot number, and ENSO activities suggest that these hydroclimatic events are possibly related to the weaker Asia summer monsoon and the cooling climatic events, as well as the stronger ENSO activities during the late Holocene in the Yangtze River valley. These results provide insights into the response of hydroclimatic system to global change in the large rivers of Asia.

Evidence for seagrass meadows and their response to paleoenvironmental changes in the early Eocene (Jafnayn Formation, Wadi Bani Khalid, N Oman)

The recognition and understanding of vegetated habitats in the fossil record are of crucial importance in order to investigate paleoecological responses and indirectly infer climate and sea-level changes. However, the low preservation potential of plants and macroalgae hampers a direct identification of these environments in the geological past. Here we present sedimentological and paleontological evidences as tool to identify the presence of different seagrass-vegetated environments in the shallow marine settings of the lower Eocene Jafnayn platform of Oman and their responses to paleoenvironmental changes. The studied lower Eocene deposits consist of well bedded, nodular packstones dominated by encrusting acervulinid and alveolinid foraminifera passing upward to an alternance of packstones with echinoids and quartz grains and grainstones rich in Orbitolites, smaller miliolid foraminifera and quartz grains. The presence of seagrass is inferred by the occurrence of encrusting acervulinids and soritid Orbitolites, as well as by their test morphologies together with further sedimentological criteria. The clear shift observed in the faunal assemblages and sedimentary features may be related to a major reorganization of the carbonate system passing from a carbonate platform to a ramp-like platform with increased terrigenous sedimentation. Heterotroph tubular acervulinids and oligotroph alveolinids of the carbonate platform were replaced upward by more heterotroph organisms such as large, discoidal Orbitolites and smaller miliolids, most likely due to enhanced nutrient levels which would have led to a change of phytal substrate, from cylindrical-leaf dominated grasses into flat-leafed ones.

A depositional model for seismo-turbidites in confined basins based on Ionian Sea deposits

This study investigates Ionian Sea seismo-turbidite (ST) deposits that we interpret to be triggered by major historic earthquakes and tsunamis in the Calabrian Arc. ST beds can be correlated with the AD 1908 Mw 7.24 Messina, AD 1693 Mw 7.41 Catania, and AD 1169 Mw 6.6 Eastern Sicily earthquakes while two previously unknown turbidites might have been generated by the AD 1818 Mw 6.23 Catania and AD 1542 Mw 6.77 Siracusa earthquakes.Textural, micropaleontological, geochemical and mineralogical signatures of STs reveal cyclic patterns of STa, STb, STc and STd sedimentary units for each earthquake with an associated tsunami. The STa unit contains multiple ST stacks with different mineralogy, geochemistry foraminiferal assemblages and sedimentary structures that are deposited from synchronous multiple slope failures and turbidity currents. The STb homogenite graded mud unit overlying the STa unit is deposited by the waning flows of the multiple turbidity currents that are trapped in the Ionian Sea confined basin. The STc laminated and marine-sourced unit results from seiching of the confined water mass that appears to be generated by earthquake ruptures combined with tsunami waves. The STd unit is a tsunamite cap deposited by the slow settling suspension cloud created by tsunami wave backwash erosion of the shoreline and continental shelf. This tsunami process interpretation is based on the textural gradation of the upper unit and a more continental source of the tsunamite cap which includes C/N>10 and the presence of inner shelf foraminifera with a lack of abyssal species. This interpretation is in agreement with the lack of a tsunamite cap for the turbidite likely linked to the AD 1542 historic earthquake that is not associated with a tsunami. The new sedimentologic criteria identifies the final seiche and tsunamite cap deposits of STs and provides a model that can now be tested in other locations to better understand the different depositional processes of seismo-turbidites in confined basins.

Investigating extreme flood response to Holocene palaeoclimate in the Chinese monsoonal zone: A palaeoflood case study from the Hanjiang River

Palaeoflood events recorded by slackwater deposits (SWDs) were investigated extensively by sedimentological criteria of palaeohydrology along the upper Hanjiang River valley. Modern flood SWDs were collected for comparison with palaeoflood SWD in the same reaches. Three typical palaeoflood SWDs were observed within Holocene loess-soil blanket on the first river terrace land. The grain size distributions of palaeoflood SWDs are similar to modern flood SWDs, whereas they are different from eolian loess and soil. Palaeoflood SWD lies in three major pedo-stratigraphic boundaries (TS/L0, L0/S0, and S0/Lt) in the Holocene loess-soil profiles. The chronology of three palaeoflood episodes was established by OSL dating and pedo-stratigraphic correlation with the well-dated Holocene loess-soil profiles in the upper Hanjiang River basin. Holocene palaeoflood events were dated to 9500–8500, 3200–2800, and 1800–1700 a B.P., respectively. Palaeoflood discharges were estimated by the palaeoflood model (i.e., slope–area method and step-backwater method). The highest discharges are 51,680–53,950m3s−1 at the 11,500-time scale in the Xunyang reach of the upper Hanjiang River valley. Holocene extraordinary hydroclimatic events in the Hanjiang River often result from abnormal atmospheric circulations from Southwest monsoons in the Chinese monsoonal zone. These results provide a regional expression of extreme flood response to Holocene palaeoclimate to understand the effects of global climatic variations on the river system dynamics.

Onshore tsunami sediment transport mechanisms inferred from heavy mineral assemblages

The aim of this study is to discuss and to extend the characterization of (palaeo)tsunami deposits, and their source materials, based upon the detailed study of their heavy mineral assemblages. Results obtained from three distinct locations (Portugal, Scotland and Indonesia), different coastal contexts and chronologies (the tsunami events studied took place at 1500 cal. yr BP, AD 1755 and 2004) are summarized and discussed in order to contribute to the sedimentological study of onshore (palaeo)tsunami deposits. Results indicate that heavy mineral assemblages primarily reflect local specific conditions. For example, in the Portuguese sites, ca. 90% of the heavy mineral population consists of tourmaline+andalusite+staurolite, whereas in the Scottish samples garnet+amphiboles can be dominant in 90% of the assemblage, where at the Indonesian study site amphiboles+andalusite were the most frequent minerals. The application of Principal Component Analysis for each site reveals that the first two components explain at least 55% of the total variance. In the three studied areas, hydraulic sorting by density was observed and a higher presence of the denser heavy minerals of the assemblages was detected. However, it is important to stress that the sediment source plays a key role in the establishment of the heavy mineral assemblages of the (palaeo)tsunami deposits. In this study, relationships between the likely source sediments and the (palaeo)tsunami deposits were described and whenever possible sediment sources were clearly identified. Furthermore, it was also possible to detect the backwash signal using the analysis of the heavy minerals (e.g. higher frequency of denser minerals or variation in the presence of rounded or euhedral zircon). The work presented here, in contributing to the enhancement of sedimentological criteria presently available to recognize and differentiate extreme marine inundation deposits, also highlights new areas for future research.

Constraints on Water Depth of Massive Sulfide Formation: Evidence from Modern Seafloor Hydrothermal Systems in Arc-Related Settings

The results of three decades of seafloor research provide the most reliable information on the importance of water depth in massive sulfide formation. Available data from over 130 occurrences show that water depths of seafloor vent sites vary with plate tectonic setting and the regional magmatic and volcanic environment. The shallowest hydrothermal systems in subduction-related settings are hosted by arc volcanoes. These shallow vent sites have a number of features in common with subaerial epithermal systems. Massive sulfide occurrences in arc-related rifts, the most likely setting for many ancient analogs, are generally restricted to water depths from ~700 to 2,000 m, with rifts developing within old arc crust at the deeper end of this range. Back-arc spreading centers proximal to arcs host massive sulfide deposits at depths of ~1,500 to 2,000 m. The deepest hydrothermal systems occur along mature back-arc spreading centers distal to volcanic arcs where water depths range from ~2,000 to 3,700 m. These deeper vent sites probably represent the best modern analogues of ophiolite-hosted massive sulfide deposits. Boiling of the hydrothermal fluids is common at volcanic arcs and in arc-related rifts. In these environments, elevated magmatic gas contents of the hydrothermal fluids can contribute to the widespread occurrence of phase separation and associated gas loss. By contrast, the high ambient pressures in deep marine hydrothermal systems along mature back-arc spreading centers prevent fluids from boiling during their ascent to the seafloor. Boiling controls the maximum temperature at which hydrothermal fluids discharge at the seafloor and, therefore, influences the metal content of seafloor sulfide deposits. Copper-rich massive sulfides typically occur at water depths exceeding ~1,000 m, whereas Zn- and Pb-rich occurrences may form at any water depth. Boiling can be an important control on Ag and Au grades but is not the only factor controlling precious metal enrichment in massive sulfides. Shallow marine hot spring deposits can be highly enriched in trace metals such as As, Hg, and Sb. Submarine volcanic arc and back-arc settings are geologically complex and significant variations in water depth can occur over short distances. Paleoenvironmental reconstruction of these environments in ancient volcanic terranes is hampered by the lack of unequivocal volcanological or sedimentological criteria that indicate water depth. The relationships established here using modern seafloor observations provide important constraints on the paleoenvironmental setting of ancient volcanic-hosted massive sulfide deposits.

Younger Dryas flood events: A case study from the middle Warta River valley (Central Poland)

The Weichselian Late Glacial sequence composed of Younger Dryas overbank deposits and the Alleröd/Younger Dryas organic-rich series from the Warta River valley, Central Poland, were studied within a project, including sedimentological criteria, analysis of aquatic invertebrates (cladocerans and chironomids) remains and diatoms. The well-dated sequence of events on the floodplain, mainly due to the determination of the age of in situ riparian forest, provided a basis for a reconstruction of local conditions of palaeofloods in a wide context of global climatic changes at the Weichselian decline. The flash floods were largely affected by catchment topography, forest destruction in response to the Younger Dryas cooling, and permafrost reappearance. The floods have been detected as the causative factors in a river style change.

Optical dating and sedimentary record from the terrace depositional profile of the Warta River (Central Poland)

Abstract Results of OSL dating and sedimentary studies from the profile of the low alluvial terrace of the middle Warta River are presented. The samples were dated using the single-aliquot regenera-tive method. Dating was used to establish a timing of the Weichselian Late Glacial events in the river valley environment. Stable conditions on the floodplain are expressed by the deposition of organic-rich series radiocarbon dated at 12 900-12 600 cal BP and 11 600-10 770 cal BP. Samples for OSL dating were collected from the mineral material deposited during the intensification of flood events during the Weichselian decline. The results obtained for the alluvia range from 12.78 ± 0.62 ka b2k to 14.33 ± 0.74 ka b2k. Sedimentological criteria allowed to distinguish between particular flood events. Overestimation of OSL ages is probably a result of rapidity of environmental changes in that time.

Of sand and mud: Sedimentological criteria for identifying the turbidity maximum zone in a tidally influenced river

AbstractThe thickness and lateral distribution of sand and mud beds and bedsets on channel bars from the tidally influenced Fraser River, British Columbia, Canada, are quantitatively assessed. Fifty‐six vibracores totalling ca 114 m of vertical section are used to tabulate bed thicknesses. Statistical calculations are undertaken for nine channel bars ranging from the freshwater and tidal zone, to the sustained brackish water and tidal zone. The data reveal that thickness trends can be organized into three groups that broadly correspond to time‐averaged hydrodynamic and salinity conditions in the various distributary channels. Thick sand beds (up to 30 cm) and thin mud beds (up to 5 cm) characterize the freshwater tidal zone. The tidal and freshwater to brackish‐water transition zone comprises thin sands (up to 10 cm) and thicker muds (up to 19 cm), and the sustained brackish water tidal zone consists of thin muds (up to 6 cm) with relatively thicker sands (up to 25 cm). The results suggest that the locus of mud deposition occurs in the tidal freshwater to brackish‐water zone, probably reflecting mud flocculation and deposition at the turbidity maximum. Landward of the turbidity maximum, mud deposition is linked to tidal influence (tidal backwater effect and reverse eddy currents on channel margins) as mud beds thin in the landward direction. These results support the hypothesis that mud deposition is greatest at the turbidity maximum and decreases in both the seaward and landward direction. This study also showcases that mud‐bed thicknesses are greatest towards the turbidity maximum and thin in both the landward and seaward direction. In the rock record, the apex of mud deposition probably marks the position of the palaeo‐turbidity maximum.

Toward a standard stratigraphical classification practice for the Baltic Sea sediments: the CUAL approach

The Late Pleistocene and Holocene glacial and postglacial sediments of the Baltic Sea basin are conventionally classified into units according to the so‐called Baltic Sea stages: Baltic Ice Lake, Yoldia Sea, Ancylus Lake and Litorina Sea. The Baltic Sea stages have been identified in offshore sediment cores by fundamentally different criteria, precluding detailed comparisons of the sediment units amongst different sea areas and studies. Here, long sediment cores and reflection seismic and pinger sub‐bottom profiles were studied from an offshore area in the Gulf of Finland, northern Baltic Sea. The strata are divided on the basis of sedimentological criteria into three allostratigraphical formations with subordinate allostratigraphical members and lithostratigraphical formations, following the combined allostratigraphical and lithostratigraphical (CUAL) approach. Sedimentological features are recommended as the primary stratigraphical classification criteria because they do not require the palaeoenvironmental inferences of salinity and water level that are inherent in the conventional classification practice. The presented stratigraphical division is proposed as a flexible template for future stratigraphical work on the Baltic Sea basin, whereby lower‐rank allounits and lithounits can be included and removed locally, while the alloformations will remain at the highest hierarchical level and guarantee regional correlatability. The stratigraphical division is compatible with international guidelines, facilitating communication to the wider scientific community and comparison with other similar basins.

Standardised recording of sediments in the excavation of the Sabaean town of Sirwah, Yemen

The application of sediment description in excavations helps to interpret sedimentary archives on archaeological sites. Although there are a few standardised approaches for describing deposits in excavations, stratigraphic records carried out by archaeologists can differ significantly from those carried out by geoscientists. In order to compare the results of both types of records, anthropogenic deposits of the Sabaean town of Sirwah, Yemen were investigated with regard to sedimentological criteria. Apart from field description and radiocarbon dating, the texture, structure and chemical data were studied in order to demonstrate whether or not they are suitable for determining post-sedimentary modifications and for refining stratigraphy as developed by archaeologists. The comparison of sediment units and cultural layers of a selected part of an archaeological section demonstrates that a stratigraphic record carried out according to a soil manual substantiates depositional history. In addition, the paper gives some new insights into the occupation and building history of the unique Sabaean culture at the Ramlat As-Sab'atayn desert margin of Yemen in the first millennium BC.

Comment on “Internal waves, an under-explored source of turbulence events in the sedimentary record” by L. Pomar, M. Morsilli, P. Hallock, and B. Bádenas [Earth-Science Reviews, 111 (2012), 56–81

This discussion is an attempt to alert the sedimentologic community that there is an immediate need to carry out fundamental research on process sedimentology of internal waves and internal tides in modern oceans. Despite the exhaustive theoretical, experimental, observational, and numerical analyses of modern internal waves and internal tides by oceanographers, with their focus on ocean mixing and modeling, there are absolutely no core-based studies by sedimentologists on the origin of primary sedimentary structures formed by baroclinic currents associated with internal waves and internal tides in modern marine environments. As a consequence, there are no objective sedimentological criteria for distinguishing internal-wave deposits. Amid this knowledge vacuum, the temptation to interpret ancient rock record as internal-wave deposits must be tempered.

Background levels of potentially toxic metals from soils of the Pisa coastal plain (Tuscany, Italy) as identified from sedimentological criteria

Identification of reliable background values of potentially toxic metals in sediments requires detailed integration of geochemical data with accurate sedimentological studies. Through analysis of 60 soil samples from the Pisa coastal plain, this study shows to what extent sediment provenance and facies characteristics may influence the natural distribution of potentially toxic metals (Cr, Ni, Cu, Zn, Pb) within alluvial and coastal sediments. Metals supplied to the alluvial plain are mostly concentrated within the finest sediment fraction (floodplain clays), while coarser crevasse and overbank deposits exhibit invariably lower metal contents. Beach-ridge sands display the lowest metal concentrations. Transport of ophiolitic detritus by the longshore drift may account for locally high Cr concentrations within beach deposits. Geochemical fingerprinting of individual facies associations in terms of natural metal contents results in the construction of a geologically-based geochemical map. This map offers a more reliable depiction of spatial distribution of background levels than interpolation techniques based uniquely upon statistical methods. Matching background values against metal concentrations from topsoil samples leads to the reliable assessment of the pollution status of Pisa coastal plain. Metal contents exceeding the threshold values designated for contaminated areas (Cr) simply reflect catchment geology, and are not the product of artificial contamination. On the other hand, anthropogenic disturbance may be detected even where metal contents (Pb, Cu) lie below the threshold values. The use of sedimentological criteria is presented here as a pragmatic tool to enhance predictability of natural metal contents in sediments, with obvious positive feedbacks for legislative purposes and environmental protection.

Microtextural characteristics of quartz grains transported and deposited by tsunamis and storms

The complex transport and depositional processes associated with tsunamis and storms and the peculiarities of local inundation present questions associated with the recognition and differentiation of the sedimentary signature of these events. This work presents a study of quartz grains transported and deposited by tsunami and storm waves with the objective of identifying specific microtextural signatures caused by high-energy marine inundations and to correlate them with their principal sedimentary sources. In this empirical study, 1150 quartz grains (78 samples) and their microtextural signatures were observed, analyzed and classified using scanning electron microscope photomicrographs. The results suggest that although no specific microtextural signature is associated with high energy inundations, there are strong increases in the percentage of fresh surfaces and percussion marks when compared with the potential source material. Moreover, tsunami and storm grains present the greatest microtextural variance among all the grains analyzed. Nevertheless, specific local conditions and sediment concentrations constrain the microtextural implications on tsunami or storm grains. One laboratory experiment was designed to test microtextural implications in grains subjected to variable velocities, sediment concentration and time. The surface microscopic signature in quartz grains of high-energy events further contributes to the development of more efficient sedimentological criteria to identify deposits associated with tsunami and storm events.

Holocene palaeoflood events recorded by slackwater deposits along the lower Jinghe River valley, middle Yellow River basin, China

AbstractPalaeoflood slackwater deposits (SWDs), are the natural record of overbank flooding and are often found within aeolian loess‐soil profiles along the river valleys of the middle Yellow River basin. These pedo‐stratigraphic sequences are studied using a multi‐disciplinarily approach to reconstruct Holocene hydro‐climatic variations. Our field investigations along the lower Jinghe River valley have identified palaeoflood SWDs at several sites along the riverbanks based on sedimentological criteria. Analytical results, including magnetic susceptibility, particle‐size distribution and concentrations of chemical elements, indicate that these well‐sorted palaeoflood SWD beds were deposited from the suspended sediment load in floodwaters. We identify two episodes of extraordinary palaeoflood events along the Jinghe River valley. These hydro‐climatic events were dated to 4200–4000 and 3200–2800 a BP, by using the optically stimulated luminescence method in combination with archaeological dating of retrieved anthropogenic remains, and with pedo‐stratigraphic correlations with the previously studied Holocene pedo‐stratigraphy in the Jinghe River drainage basin. The flooding events are therefore considered to be a regional expression of known climatic events in the northern hemisphere and demonstrate Holocene climate was far from stable. This study provides important data in understanding the interactions between regional hydro‐climatic systems and global change in semi‐arid and sub‐humid regions. Copyright © 2012 John Wiley & Sons, Ltd.

A new palaeoenvironmental model for the evolution of the Byzovaya Palaeolithic site, northern Russia

Recently, the sediment stratigraphy and geochronology of the well‐known Palaeolithic site Byzovaya in northern Russia were investigated. New technological analyses of the artefacts suggest a Middle Palaeolithic Mousterian culture, and occupation by Neanderthals, not Modern humans as previously thought. We present here a new and detailed documentation of the stratigraphy, including the geological context of the artefacts and faunal remains. From sedimentological criteria we confidently interpret the find‐bearing strata as debris‐flow deposits, covered by aeolian sediments. The chronology is based on radiocarbon and luminescence (OSL) dates from the find‐bearing and overlying strata. The results are utilized to reconstruct the geological history at the excavation area. The stratigraphy varies considerably across the excavation area. The most intact and undisturbed part of the sequence was found inside the most recent Excavation II. In this part the artefacts and bones appear to have been permanently sealed and protected by aeolian sand. The older Excavation I shows a more complicated stratigraphy, as the finds may have been temporarily exposed during the early Holocene owing to ravine incision and slumping activity. The individual radiocarbon dates that were collected from different parts of the site and from various stratigraphic positions are re‐investigated in this study. By using Bayesian statistics the conclusion is that the site was occupied during a restricted period around 30.6–34.7 ka. A series of partly unpublished OSL dates of coversand from different sites demonstrates a regional aeolian signal during the Lateglacial in northern Russia, 15–14 ka.

Process-sedimentological challenges in distinguishing paleo-tsunami deposits

There has been a lively debate since the 1980s on distinguishing between paleo-tsunami deposits and paleo-cyclone deposits using sedimentological criteria. Tsunami waves not only cause erosion and deposition during inundation of coastlines in subaerial environments, but also trigger backwash flows in submarine environments. These incoming waves and outgoing flows emplace sediment in a wide range of environments, which include coastal lake, beach, marsh, lagoon, bay, open shelf, slope and basin. Holocene deposits of tsunami-related processes from these environments exhibit a multitude of physical, biological and geochemical features. These features include basal erosional surfaces, anomalously coarse sand layers, imbricated boulders, chaotic bedding, rip-up mud clasts, normal grading, inverse grading, landward-fining trend, horizontal planar laminae, cross-stratification, hummocky cross-stratification, massive sand rich in marine fossils, sand with high K, Mg and Na elemental concentrations and sand injections. These sedimentological features imply extreme variability in processes that include erosion, bed load (traction), lower flow regime currents, upper-flow regime currents, oscillatory flows, combined flows, bidirectional currents, mass emplacement, freezing en masse, settling from suspension and sand injection. The notion that a ‘tsunami’ event represents a single (unique) depositional process is a myth. Although many sedimentary features are considered to be reliable criteria for recognizing potential paleo-tsunami deposits, similar features are also common in cyclone-induced deposits. At present, paleo-tsunami deposits cannot be distinguished from paleo-cyclone deposits using sedimentological features alone, without historical information. The future success of distinguishing paleo-tsunami deposits depends on the development of criteria based on systematic synthesis of copious modern examples worldwide and on the precise application of basic principles of process sedimentology.

Contrasted textural and taphonomic properties of high-energy wave deposits cemented in beachrocks (St. Bartholomew Island, French West Indies)

This study investigates the preservation potential of high-energy wave deposits by beachrocks located along shores contrastingly exposed to storms. In order to discriminate between clast populations originating in different formational and depositional conditions, namely fair-weather vs. extreme wave conditions, the applicability of quantitative textural and taphonomic analyses of sand- to boulder-sized sediments was evaluated using samples from beachrocks and non-cemented coarse-grained coastal deposits. The size, shape and roundness of coarse clasts were characterized by measuring axial dimensions (L, I and S), from which shape descriptors were derived. Taphonomic scores were established at macroscopic and microscopic (thin-section) scales by rating the degree of physical and biologically-mediated alteration of skeletal material. Combined with the examination of depositional morphologies and facies architectures, the procedure enables characterization of fair-weather and extreme wave-generated deposits, and provides criteria to distinguish between single-event deposits and reworked material resulting from the coalescence of sediments produced during multiple events. The distinction of single short-lived event deposits from those reflecting long-term reworking is crucial for correctly interpreting similar deposits in the geological record and evaluating the historical hazard along coastlines. Site-dependent controls such as the nature of available sediments, the geomorphology of coastal depositional settings, and the tectono-climatic context proved to be important for interpreting the origin of the high-energy wave deposits studied here. In the ongoing debate on the signatures of tsunamigenic and storm sediments in fringing coastal settings, the present study demonstrates that one promising research direction is the use of multiple sedimentological criteria to characterize event materials, including their morphology and taphonomy, combined with the precise characterization of site-specific controls.

Geomorphic and Sedimentologic Evidence of Extreme Wave Events Recorded by Beachrocks: A Case Study from the Island of St. Bartholomew (Lesser Antilles)

Abstract Caron, V., 2012. Geomorphic and sedimentologic evidence of extreme wave events recorded by beachrocks: a case study from the Island of St. Bartholomew (Lesser Antilles). In spite of their preferential location inside tropical belts where extreme wave events occur, and in spite of their potential for rapidly lithifying coastal sediments and by doing so promoting their preservation, beachrocks have so far been poorly investigated for the record of tsunamis or storms. This contribution focuses on beachrock occurrences on the island of St. Bartholomew (French West Indies), which is located in a region at risk from such natural hazards. Geomorphic and sedimentologic criteria were used to characterize beachrocks from shores frequently exposed to extreme wave events. Besides known effects of waves on beachrocks, including exhumation and landward transport of broken slabs, this article reports less well-documented perturbations such as the immersion of beachrock layers and cementation of event deposits. ...