Bioclastic Sands Research Articles

Trace elements in Holocene sediments of the southern Doñana National Park (SW Spain): historical pollution and applications

The multidisciplinary analysis of core sediments from Donana National Park, south Spain, permits to delimitate both the paleoenvironmental changes and the geochemical evolution of this area during the Holocene. In a first phase (10–6.5 kyr cal), this area was occupied by freshwater/brackish marshes with periodical alternation of dry periods and humid intervals. In a second phase, these marshes were inundated during the Flandrian transgression (~6.5 kyr cal), with the deposition of bioclastic sands. The third phase (6.5–3.6 kyr cal) is characterized by the transition to an old lagoon, with unpolluted, bottom sediments. The geochemical concentrations of these clayey sediments can be used as a geochemical background for present-day and future environmental evaluations of this area. In the following 600 years approximately, two tsunamis caused the partial infilling of this area with bioclastic, marine sediments. In the interval comprised between these two tsunamis, this lagoon was polluted with heavy metals derived from historical mining activities. This environmental contamination represents one of the oldest evidences of mining pollution in the world (>3 kyr cal BP).

Molluscan assemblages in littoral soft bottoms of the Alboran Sea (Western Mediterranean Sea)

The structure of the molluscan assemblages inhabiting the subtidal bottoms off the west coast of Malaga province (southern Spain) and their relation to sediment characteristics were analysed with both univariate and multivariate parameters. Five significantly different molluscan assemblages were identified and assigned to ‘sables fins bien calibrés’ (SFBC, well sorted fine sands); ‘détritique côtier’ (DC, coastal bioclastic sands); ‘détritique envasé’ (DE, muddy bioclastic sands); and ‘coralligène’ (CO, coralligenous) biocoenoses of Pérès and Picard classification. A total of 234 molluscan species were identified, with gastropods as the dominant group (135 species). An increase of diversity and evenness with depth has been observed, with the highest values for both indexes on a rocky outcrop, and the lowest in the shallower fine sand assemblage dominated by few species. The different molluscan assemblages inhabiting these sublittoral bottoms were conditioned by depth, percentage of gravel and percentage of clay. A large proportion of tropical West African species is found in the area, some of them reaching their distributional limit towards the Mediterranean Sea. The southern Iberian coasts, in the confluence of Atlantic and Mediterranean waters and between Africa and Europe, are therefore highlighted as one of the areas with highest molluscan species richness in Europe, and require a conservation policy in order to preserve this unique European biodiversity heritage.

Experimental mechanical and chemical compaction of carbonate sand

Uniaxial compression tests were conducted on bioclastic sand and crushed calcite crystals. Mechanical and chemical processes were investigated to better quantify petrophysical properties of carbonates and their evolution with burial or during fault zone processes. The grain size was in the range 63–500 μm, and the samples were saturated with water in equilibrium with carbonate, glycol, decane, or air. During loading, effective stress was increased to 32 MPa. Mechanical compaction processes (i.e., grain rearrangement, crushing) could be separated from chemical processes (i.e., pressure solution, subcritical crack growth). P and S waves monitored during the tests showed low velocity in samples saturated with reactive fluids. This suggested that chemical reactions at grain contacts reduced the grain framework stiffness. Creep tests were also carried out on bioclastic sand at effective stress of 10, 20, and 30 MPa. No creep was observed in samples saturated with nonreactive fluids. For all the samples saturated with reactive fluids, strain as a function of time was described by a power law of time with a single exponent close to 0.23. Parameters controlling creep rate were, in order of importance, grain size, effective stress, and water saturation. Microstructural observations showed that compaction of bioclastic carbonate sand occurred both mechanically and chemically. Crack propagation probably contributed to mechanical compaction and enhanced chemical compaction during creep. Experimental compaction showed that compaction of carbonates should be modeled as a function of both mechanical and chemical processes, also at relatively shallow depth and low temperature.

Late Cretaceous sedimentary evolution of a northern sector of the Adriatic Carbonate Platform (Matarsko Podolje, SW Slovenia)

Cretaceous shallow-marine carbonate rocks of SW Slovenia were deposited in the northern part of the Adriatic Carbonate Platform. A 560-m-thick continuous Upper Cenomanian to Santonian carbonate succession has been studied near Hrusica Village in Matarsko Podolje. With regard to lithological, sedimentological, and stratigraphical characteristics, the succession has been divided into nine lithostratigraphic units, mainly reflecting regressive and transgressive intervals of larger scale. During the latest Cenomanian and Early Turonian, hemipelagic limestones were deposited on top of shallow-marine lagoon and peritidal Upper Cenomanian deposits indicating relative sea-level rise. Subsequently, the deeper marine depositional setting was gradually filled by clinoform bioclastic sand bodies overlain by peritidal and shallow-marine low-energy mainly lagoonal lithofacies. Similar lithofacies of predominately inner ramp/shelf depositional settings prevail over the upper part (i.e., Coniacian to Santonian) of the succession. In the area, the Upper Cetaceous carbonate rocks are separated from the overlying Lower Eocene (Upper Paleocene?) carbonate sequence by regional unconformity denoted by distinct paleokarstic features. On the Adriatic Carbonate Platform the deeper marine carbonate setting, developed at the Cenomanian/Turonian boundary, is usually correlated with OAE2 and related eustatic sea-level rise. Similarly, subsequent reestablished shallow-marine conditions are related to Late Turonian long- and short-term sea-level fall. However, we are suggesting that deeper marine deposits were deposited in a tectonically induced intraplatform basin formed simultaneously with the uplift of the northern and northeastern marginal parts of the Adriatic Carbonate Platform.

Geomorphic and stratigraphic evidence for an unusual tsunami or storm a few centuries ago at Anegada, British Virgin Islands

Waters from the Atlantic Ocean washed southward across parts of Anegada, east-northeast of Puerto Rico, during a singular event a few centuries ago. The overwash, after crossing a fringing coral reef and 1.5 km of shallow subtidal flats, cut dozens of breaches through sandy beach ridges, deposited a sheet of sand and shell capped with lime mud, and created inland fields of cobbles and boulders. Most of the breaches extend tens to hundreds of meters perpendicular to a 2-km stretch of Anegada’s windward shore. Remnants of the breached ridges stand 3 m above modern sea level, and ridges seaward of the breaches rise 2.2–3.0 m high. The overwash probably exceeded those heights when cutting the breaches by overtopping and incision of the beach ridges. Much of the sand-and-shell sheet contains pink bioclastic sand that resembles, in grain size and composition, the sand of the breached ridges. This sand extends as much as 1.5 km to the south of the breached ridges. It tapers southward from a maximum thickness of 40 cm, decreases in estimated mean grain size from medium sand to very fine sand, and contains mud laminae in the south. The sand-and-shell sheet also contains mollusks—cerithid gastropods and the bivalve Anomalocardia—and angular limestone granules and pebbles. The mollusk shells and the lime-mud cap were probably derived from a marine pond that occupied much of Anegada’s interior at the time of overwash. The boulders and cobbles, nearly all composed of limestone, form fields that extend many tens of meters generally southward from limestone outcrops as much as 0.8 km from the nearest shore. Soon after the inferred overwash, the marine pond was replaced by hypersaline ponds that produce microbial mats and evaporite crusts. This environmental change, which has yet to be reversed, required restriction of a former inlet or inlets, the location of which was probably on the island’s south (lee) side. The inferred overwash may have caused restriction directly by washing sand into former inlets, or indirectly by reducing the tidal prism or supplying sand to post-overwash currents and waves. The overwash happened after A.D. 1650 if coeval with radiocarbon-dated leaves in the mud cap, and it probably happened before human settlement in the last decades of the 1700s. A prior overwash event is implied by an inland set of breaches. Hypothetically, the overwash in 1650–1800 resulted from the Antilles tsunami of 1690, the transatlantic Lisbon tsunami of 1755, a local tsunami not previously documented, or a storm whose effects exceeded those of Hurricane Donna, which was probably at category 3 as its eye passed 15 km to Anegada’s south in 1960.

Hydrodynamic behaviour of coarse bioclastic sand from shelly cheniers

AbstractCheniers from Mont‐Saint‐Michel bay (France) are coarse shelly sand ridges migrating on the mudflat up to the salt marshes where they accumulate and merge in a littoral barrier. In this macrotidal setting and low wave forcing, the cheniers are rarely submerged. However, they are found to move up to several metres during coincidence of spring tide and wave activity. Their processes of migration, morphology and internal structure (composition of the beddings, grain size, sorting and grain arrangement) are thought to be closely related to the hydrodynamic behaviour of the coarse and shelly sediment.This paper focuses on the hydrodynamic behaviour of bioclastic sand sampled from the cheniers: settling velocities of the shell fragments were measured using a 2 m long sedimentation tube. Thresholds of motion under unidirectional current, velocity and turbulence vertical profiles were characterized in a small recirculating flume using Laser Doppler Anemometry (LDA).The flat‐shaped bioclastic particles feature low settling velocities and reveal a good resistance to the re‐suspension effect of the flow when imbricated in a sediment bed. The shear stress in the bottom boundary layer has been measured in the viscous and log sub‐layers. Nikuradse roughness heights (ks) for shell debris beds of different sizes have been quantified. It is found that ks ≈ 2·56d50. This value is close to the ones used for classic rounded sand grains despite their major differences of shape.The dual behaviour of the shell fragments (low settling velocity, good resistance to unidirectional flow) should be considered as a key to understanding how this coarse material is transported across the tidal flat, and finally accumulated as cheniers. Further flume experiments including wave activity and tidal fluctuations are necessary to better quantify these complex processes. Copyright © 2010 John Wiley & Sons, Ltd.

Distribution of the archaeocyath-calcimicrobial bioconstructions on the Early Cambrian shelves

The differences and variety of structural, depositional, and compositional features observed in the Early Cambrian microbial-archaeocyath buildups preserved in the present-day continents, suggest a direct correlation between the physico-chemical factors of deposition and the structural architecture of the buildups. This can be explained in terms of their palaeogeographic collocation on the shelves (depth, energy), and hence of the areal distribution of epeiric basins and perioceanic/pericontinental platforms. Data on the analysis of biohermal communities and their architectures indicate that the relative development of the main reef-building components, as well as their evolution within the reef communities, reflects the dominant physico-chemical factors, mainly temperature and nutrient availability, and the physiography of the primary depositional setting. The bioaccumulations show different reef building styles, defined by the types of associated facies and by the early diagenetic features. They are represented by (i) mud-supported simple ( Kalyptrae) to compound mounds locally with stromatactis-like cavities; (ii) dendrolitic bioherms and crustose buildups with large shelter cavities and low synoptic relief; (iii) cement-supported skeletal reefs with wave resistant frameworks often associated with oolitic shoals, and (iv) bioclastic sands, developed at photic and shallow sub-photic depths on low-angle/low-energy ramps (i–ii) or on high-energy conditions on platforms distally rimmed (iii) or occasionally swept by storm currents (iv). The results of the analysis provide information on the spatial conditions of the primary depositional settings of the first metazoan involved in reef building in the history of the Earth, and suggest that the architecture of the bioconstructions was controlled not only by the physiography of the depositional setting and global/astronomic climate but also by local climatic conditions constrained by the latitudinal distribution of the Early Cambrian continental blocks.

The record of tidal cycles in mixed silici–bioclastic deposits: examples from small Plio–Pleistocene peripheral basins of the microtidal Central Mediterranean Sea

AbstractThe Pliocene–Pleistocene peripheral marine basins of the Mediterranean Sea in southern Italy, from Basilicata and western Calabria to northern and eastern Sicily, represent tectonically formed coastal embayments and narrow straits. Here, units of cross‐stratified, mixed silici–bioclastic sand, 25 to 80 m thick, record strong tidal currents. The Central Mediterranean Sea has had a microtidal range of ca 35 cm, and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out‐of‐phase reversal of the tidal prism in narrow straits linking the Tyrrhenian and Ionian basins. The siliciclastic sediment was generated by local bedrock erosion, whereas the bioclastic sediment was derived from the contemporaneous, foramol‐type cool‐water carbonate factories. The cross‐strata sets represent small to medium‐sized (10 to 60 cm thick) two‐dimensional dunes with mainly unidirectional foreset dip directions. These tidalites differ from the classical tidal rhythmites deposited in mud‐bearing siliciclastic environments. Firstly, the foreset strata lack mud drapes and, instead, show segregation of siliciclastic and bioclastic sand into alternating strata. Secondly, the thickness variation of the successive silici–bioclastic strata couplets, measured over accretion intervals of 2 to 3 m and analysed statistically, reveal only the shortest‐term, diurnal and semi‐diurnal tidal cycles. Thirdly, the record of diurnal and semi‐diurnal tidal cycles is included within the pattern of neap‐spring cycles. Differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea, with the tidal currents locally enhanced in peripheral basins. It is suggested that this particular facies of mud‐free, silici–bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub‐tidal environment of straits and embayments and the shortest‐term tidal cycles.

Geologic setting of the Abdur Archaeological Site on the Red Sea coast of Eritrea, Africa

The Abdur Archaeological Site was defined initially by the discovery and dating (125 +/− ka by Ur/Th) of in situ stone tools within uplifted marine terrace deposits located along the southern Red Sea coast of Eritrea, near the small village of Abdur. These tools represent some of the earliest well-dated evidence for human occupation of coastal marine environments. The site is located on the Buri Peninsula along the eastern shoreline of the Gulf of Zula and covers an area of approximately 7 km by 1 km. Three main stratigraphic units are defined and discussed from oldest to youngest: (1) The Buri Formation is defined herein as a sequence of brackish–freshwater (estuarine) and fluvial–deltaic sediments consisting of mudstones, siltstones, sandstones, conglomerates and limestones with ash and pumice beds. Ar–Ar dating of pumice and tephras puts the time of deposition of this unit from about 0.91 to 0.72 Ma (Early–Middle Pleistocene). Several stone tools were discovered in the Buri Formation, indicating early human occupation of a coastal environment during the Early to Middle Pleistocene. (2) The Abdur Volcanic Complex (AVC) is a small basaltic shield complex that forms the highlands along the eastern part of the Abdur Site and extends to the north and south of the area. Basalts from this center were dated from 2.12 to 0.17 Ma, indicating that the volcanic complex has been tectonically and magmatically active prior to, during and after deposition of the Buri Formation. (3) The Abdur Reef Limestone (ARL) is the remnant of a shallow marine reef system deposited approximately 125 ka (last glacial highstand, isotope stage 5e) along the margins of the Abdur volcanic highlands. The ARL consists of a basal transgressive lag deposit overlain by extensive buildups of mollusks, echinoderms, bioclastic sands and corals up to 11 m thick. Numerous stone tools in the ARL fall into two tool kits, bifacial hand axes of the Acheulian industry and Middle Stone Age-type (MSA) obsidian flakes and blades. Their distribution suggests that foraging activities of early humans varied with environmental setting. The Buri Formation has been folded and faulted prior to deposition of the ARL, with dips as high as 36° and vertical locally along fault zones. The ARL has been uplifted up to 8–15 m and tilted 1°–5° in a seaward direction. The area is cut by numerous faults, part of a regional NNW-trending fault system. Occurrences of Buri Formation equivalents to the north along the Buri Peninsula as well as in the area southwest of the Alid volcano suggest the area west of the escarpment was a broad lowland characterized by rivers, coastal lakes, and estuaries along the Red Sea coast during the Early to Middle Pleistocene. Reefs equivalent to the ARL covered large parts of the adjacent Buri Peninsula, the north end of Mt. Ghedem across the Gulf of Zula, and the Dahlak Archipelago to the north.

Geochemistry of marine sediments of the Brazilian Northeastern continental shelf

The marine sediment samples collected from the northeastern Brazilian continental shelf, at water depths between 20 and 80 m, consisted mainly of sands with an almost total absence of gravel and granules. Medium, coarse and very coarse sand grains are mostly composed of halimeda, lithothamnium, rodoliths and bioclastic sands with a carbonate content varying between 77 and 96 %. The chemistry in general shows a decreasing content of Ca (86.1 %) >Si (6 %) > Cl (3.6 %) > Sr (0.8 %) > K (0.66 %) > S (0.62 %) > Al (0.6 %) > Na (0.55%) > Mg (0.43 %) > Fe (0.4 %) > P (0.2 %) > Br (0.04 %) in the samples. There was no correlation between CaCO3 and chemical contents and grain size with depth and bio-components. With the exception of Sr of marine origin, all other elements (P, S, Br, Cl, Fe) are of continental origin. The lithothamnium of some offshore samples shows higher CaCO3 content, while Mg and Na are present only in halimedas. Bioclastic sands contain no Br, and silt and clay fractions are rare and characterize samples closer to the coast. These marine bioclastic granulates are of very pure biogenic calcium carbonates and are thus highly to be recommended for economic purposes.

Wind transport of foraminiferal tests into subaerial dunes: an example from western Ireland

Abstract. INTRODUCTIONThe empty tests of dead foraminifera behave as sedimentary particles and are subject to transport, although their different shapes and effectively low density means that their hydraulic equivalence is greater than that of spherical quartz grains (see Haake, 1962). Their estimated traction velocities range from c. 4 cm s−1 to c. 13 cm s−1 (Snyder et al., 1999). The presence of calcareous foraminiferal tests in a fossil sedimentary deposit would normally be taken as an indication of deposition in a marine environment. However, it has long been known that wind can transport tests from a carbonate beach into adjacent carbonate dunes as in Dogs Bay, Connemara, Eire (Murray, 1973) and Abu Dhabi, Persian Gulf (Murray, 1970). The purpose of this Notebook is to provide some details of the Dogs Bay occurrence and to comment on how such deposits might be recognized in the rock record.In western Ireland the coastal geology consists of hard rocks, yet the beaches are commonly composed primarily of calcareous bioclastic sands (Guilcher & King, 1961; Keary, 1967). Dogs Bay (Lat. 53° 24′ N Long. 9° 58′ W) lies on the west-facing side of a tombola which is approximately 200 m wide and 400 m long. The tombola is made up of sand dunes that are mainly vegetated except along the margins adjacent to the beaches. Dogs Bay is exposed to Atlantic storms and the surface layer of the beach is reworked on each tidal cycle. Both the beach and the dunes are composed of . . .

Giant sandwaves in the Hola glacial trough off Vesterålen, North Norway

Seabed data acquired from the Hola glacial trough off Vesterålen, North Norway, reveal the morphology of spectacular sandwave fields and large sandwaves in great detail. The sandwave fields are among the deepest known, occurring in water depths of 200–260 m. Multibeam bathymetry and backscatter, shallow seismic, sediment samples and video data collected by the MAREANO programme and bottom current modelling results have been used to interpret morphology, transport and distribution of the seabed sediments. Sand supply to the Hola trough was initially from wave and current reworking of glacial and glaciomarine deposits during the eustatic sea level lowstand and sea level rise after the last glaciation. More recently deposited bioclastic sand dominates the upper part of the sand unit, which locally reaches 10 m thickness. Sand predominantly accumulates behind ridges, where current strength decreases, and in topographic lows. Straight-crested and barchan-type sandwaves up to 3 km long and 7 m high are interpreted to form by tidal and geostrophic currents with bottom current speeds reaching 0.7–0.8 m s − 1 . A large cluster of coral reefs occur immediately east of the barchan-type sandwaves, where modelling shows bottom current velocities up to c. 1 m s − 1 . Sandwaves occur in four major sandwave fields. Strong variation in the nature of sandwaves over short distances, e.g., migration direction, between neighbouring sandwave fields point to important local variations in current regime and sedimentary processes. The present work demonstrates how local topography and interaction of several current systems may produce complex depositional patterns on a local scale. Detailed multibeam mapping and video inspection of open shelf troughs in other narrow shelf settings worldwide may reveal similar sandwave fields.

The significant role of sediment bio-retexturing within a contemporary carbonate platform system: Implications for carbonate microfacies development

Assessments of carbonate platform reef–lagoon sediments and benthic habitats around Rodrigues Island (south-west Indian Ocean) have been undertaken in order to examine carbonate sediment textural properties and the controls on texturally-defined sediment fabrics. Reef–lagoon sediments, sampled from across the expansive (~ 8 km wide) carbonate-dominated windward platform, principally comprise poorly sorted medium- to coarse-grained bioclastic sands, composed of a low diversity of grain constituents — predominantly non-geniculate coralline algal bioclasts. Despite a marked homogeneity in sediment compositional and grain size properties, eight distinct sediment textural groups can be identified that form a heterogeneous mosaic across the contemporary reef–lagoon system. Only along the narrow outer platform margins (reef crest, sand apron and outermost lagoon environments) do we observe consistent (predictable) transitions in sediment textural groups, where physical processes are the primary drivers of selective sediment transport and sorting. In contrast, across the main expanse of the lagoon, the sediment substrates are characterised by an irregular mosaic of texturally-defined sediment groups — formed primarily as a function of sediment bio-retexturing. The burrowing activities of alpheid and callianassid shrimps are particularly important in this respect and impart a distinctly unique textural fabric to the upper sediment horizons in the environments in which the respective organisms occur. The consequence of this is that, at the platform system scale, individual, texturally-defined sediment groups are relatively poor indicators of prevailing hydrodynamic regimes or of local sediment production, reflecting more the biological action of organisms inhabiting the substrate. This has important implications for understanding the development of carbonate sediment fabrics in the context of palaeoenvironmental reconstructions and for interpreting a key diagnostic criteria of carbonate microfacies.

Taphonomy and taphofacies models of the Pliocene deposits of Vejer de la Frontera (Cádiz, SW Spain)

A quantitative taphonomic analysis of the fossil assemblages, together with a sedimentary study of the Pliocene deposits of the Vejer Basin (Cádiz, Spain), has been carried out. This multidisciplinary analysis has allowed us to establish with great precision the palaeoenvironmental conditions under which these materials were formed. The Pliocene deposits can be divided into three units, named from bottom to top: Unit 1, Unit 2 and Unit 3. Unit 1 is formed of medium-to-coarse-grained sands at the base, corresponding to subtidal bar deposits. At the top, the sands become coarser, and beach and littoral dunes were formed. Within these sedimentological contexts, the fossil remains exhibit a strong degree of taphonomic destruction due to high hydrodynamic energy. Unit 2, consisting of clays, corresponds to deposits formed in a protected coastal bay. A low-energy environment allowed the fossils to maintain a good state of conservation and even thin-shelled bivalves with a delicate ornamentation are preserved. Unit 3 comprises highly bioclastic sands and calcarenites-calcirudites. This unit displays a shallowing upwards trend, passing from deposits generated under shallow subtidal conditions in the lower part to materials deposited on beaches and coastal dunes in the upper part. Under these conditions, the fossil remains show a high degree of taphonomic destruction. In Units 1 and 3, the fossils are mainly accumulated in horizontal beds, with extensive lateral continuity and with erosive bottoms and graded tops. These accumulations of fossils are interpreted as tempestites. As compared with the fossils preserved in surrounding sediments, the taphonomic attributes of these bioclasts suggest a superposition of the background and event processes. During normal conditions, the high energy of the littoral environments produced a strong degree of taphonomic destruction. During storm events, a mixture of taphonomic attributes was produced: high fragmentation and disarticulation and little abrasion and edge rounding of the fragments. This mixture was a consequence of sudden burial after storms, which prevented long periods of exposure of the bioclasts in the taphonomically active zone. Three taphofacies models are proposed, based on taphonomic attributes of fossils contained in the different units: inner-shelf taphofacies, protected shallow-lagoon taphofacies and storm bed taphofacies.

A Mediterranean Holocene restricted coastal lagoon under arid climate: Case of the sedimentary record of Sabkha Boujmel (SE Tunisia)

The Holocene sedimentary record of Sabkha Boujmel (SE Tunisia) is expressed by a shallowing-upward carbonate lagoon–tidal flat cycle (2.3 m thick) unconformably overlying continental silt–sandy sediment, Late Würmian in age. The sedimentary package of this cycle starts with transgressive marginal shallow marine (intertidal to subtidal) bioclastic sands grading upwards to black mudstone, rich in organic matter (T.O.C. up to 1.3%) deposited within a lagoon protected from the sea by Upper Pleistocene lithified sand spits. The uppermost part of the cycle is represented by oobioclastic carbonate sands covered with dead biodegraded microbial mats and/or reddish sands of aeolian origin deposited in intertidal to supratidal environments. The facies arrangement, particularly the spatial distribution of the ancient and the more recent microbial mats, records the progressive infilling of the lagoon as well as the progradation of the shoreline during the last 2000 years. The organic-rich facies which provide an age varying between 4130 and 6800 yr B.P. were deposited when the Boujmel lagoon started to be progressively separated from the Mediterranean Sea. The main factors controlling the facies and the thickness variation are the local topographic sea-floor irregularities most likely controlled by the inheritance morphology resulting from an important fluviatile digging that occurred during the last glacial maximum, the relative sea-level fluctuations, the hydro-isostatic rebound and the climate.

Closure of a seaway: stratigraphic record and facies (Guadix basin, Southern Spain)

During the late Tortonian (upper Miocene), the Guadix Basin in S Spain formed one of the Betic corridors that connected the Mediterranean Sea with the Atlantic Ocean. The closure of this connection occurred in a series of steps, documented by three sedimentary units. A lower unit, consisting of basinal marls, shallow-water calcarenites and sands records the formation of a wide seaway. During deposition of the following unit this narrowed to a strait no more than 2 km in wide, triggering an intensification of currents that caused migration of submarine dunes preserved as giant cross-beds in bioclastic sands and conglomerates. Current flowed from the Mediterranean to the Atlantic. The third unit constitutes the youngest marine episode of the filling of the Guadix Basin. At this stage, the connection between the Mediterranean Sea and the Atlantic Ocean was broken, and a system of coastal coral reefs was established in the northern part of the Basin.

Pleistocene coralline algal build-ups ( coralligène de plateau ) and associated bioclastic deposits in the sedimentary cover of Cutro marine terrace (Calabria, southern Italy)

Abstract Carbonate build-ups mainly constructed by encrusting coralline red algae are currently developing on Mediterranean soft bottoms, at depths ranging from 20 m to 160 m. They are usually referred to as ‘ coralligène de plateau ’. Few fossil examples of these bioconstructions have been described in the literature and their evolution in the context of a stratigraphic cycle has never been modelled in detail. Cutro marine terrace (Calabria, southern Italy) preserves mid-Pleistocene deposits assigned to Marine Isotope Stage (MIS) 7 or MIS 9. Extensive algal build-ups representing the deepest unit of the succession occur in the outer and central portion of the terrace, interpreted as the most distal setting from the palaeo-shoreline. Two studied sections, Vrica and Telegrafo, showed that the solid biogenic framework grew over a basal rhodolithic layer, which was stabilized by the binding activity and overgrowth of non-geniculate Corallinales (calcareous red algae). Therefore, these bioconstructions represent a rare fossil example of coralligène de plateau . At the Telegrafo section, Titanoderma pustulatum has been identified as the major rhodolith component. The build-ups are dominated by T. pustulatum , associated mainly with Mesophyllum spp. and locally with Lithophyllum stictaeforme . Rhodalgal bioclastic deposits are found in lateral contact with the build-ups. The two facies developed together under a hydrodynamic regime where phases of sedimentation from storm-driven currents alternated with phases of calm conditions. They were deposited during a single stratigraphic cycle beginning with the generation of a ravinement surface during the transgressive systems tract (TST) and ending with the burial of the coralligène by well-sorted shoreface bioclastic sands. Optimal and extensive growth of the coralligène took place during the highstand systems tract (HST).

Sedimentary features of tsunami backwash deposits in a shallow marine Miocene setting, Mejillones Peninsula, northern Chile

Miocene shoreface sandstones in the Caleta Herradura half-graben, northern Chile, contain an exceptionally coarse deposit that, based on sedimentologic and stratigraphic features, is regarded as having been laid down during a tsunami event by non-cohesive and sediment-laden subaqueous density flows. Interpretations of the principal sediment-depositing mechanisms effective in the tsunami surges rely largely on field observations of deposit geometry and internal sedimentary characteristics. This example comprises two erosively based sedimentation units that were probably deposited by successive waves in the tsunami wave train. The Lower Unit consists of a clast-supported, polymodal, boulder-bearing breccia composed mostly of angular clasts and fewer well-rounded clasts. Framework components are mostly chaotic but may also exhibit either inverse-to-normal grading or crude normal grading. Laterally, changes in characters of depositional facies are common and abrupt. The sand-sized, bioclastic-rich matrix is poorly sorted and very similar to the underlying lower shoreface bioclastic sandstone, implying that soft sediments eroded at the lower erosional surface contributed to the tsunami deposit. The bulk of the Upper Unit is a poorly sorted, breccia-bearing sandstone. Pebbles and cobbles are scattered, massive or normally graded. Sporadic outsized boulders, emplaced as debris fall deposits, may occur along the erosional base. An array of signatures, such as unusually coarse grain size in comparison to the surrounding deposits, erosional bases, the mixed sources of sediments, multiple erosional and depositional events, normal size grading or massive texture, are all considered distinctive features of tsunamigenic deposits. Backwash deposition is indicated by the incorporation within the tsunami deposits of sediments derived from mixed sources, such as angular clasts from nearby subaerial settings, rounded clasts reworked from beach gravels, and bioclastic sand eroded from older, and unconsolidated, shoreface deposits. Notwithstanding the absence of soft-sediment deformation features, the marked facies change from a lower shoreface into an upper shoreface environment through the tsunamiites provides directional information about the origin of the tsunami wave. It was most likely produced by a sea floor fault displacement associated with an episode of sudden, probably coseismic coastal uplift.

Textura, Composición y Foraminíferos Bentónicos de los Sedimentos Superficiales en los Bancos de Salmedina, Caribe Colombiano

Salmedina reef banks are mud diapirs that include three main (and smaller) shoals conforming true submarine mesas. A surface sediment study was performed with detailed sampling of the mesas and more spaced sampling of the mesas and more spaced sampling of the pre-reefal zone. Granulometry of sediments, calcium carbonate content, organic matter, composition of particles > 150 micrometers, and relative abundance of benthonic foraminifera were measured. Distribution maps and cluster analysis enable us to differentiate four different sedimentary facies: (1) Bioclastic sands of reef mesas (2) Lithoclastic sand gravels of diapiric origin from the Burbujas shoal (3) Lithobioclastic perirreefal silts (4) Biolithoclastic peri-reefal very fine sands and silts. Burbujas shoal is an active mud volcano that present a zones of lithoclastic sandy gravels caused by the degradation of the volcano cone stucture, while westward sediments grades toward typical sediments of reef mesas. Salmedina shoal, by contrast, presents two kinds of reef mesa supply of calcareous algae. Differences in peri-reefal mud sediments are possibly ti the northern influence of the magdalena river, and the southern reefal mesas calcareous supply. Amp istegina gibbosa is the dominant specie of benthonic foraminifera and it is associated to the reef production. In contrast, Anomalina globulosa is associated to areas the terrogenous continental influence.

The stratigraphic architecture and evolution of the Burdigalian carbonate—siliciclastic sedimentary systems of the Mut Basin, Turkey

This study describes the coeval development of the depositional environments in three areas across the Mut Basin (Southern Turkey) throughout the Late Burdigalian (early Miocene). Antecedent topography and rapid high-amplitude sea-level change are the main controlling factors on stratigraphic architecture and sediment type. Stratigraphic evidence is observed for two high-amplitude (100–150 m) sea-level cycles in the Late Burdigalian to Langhian. These cycles are interpreted to be eustatic in nature and driven by the long-term 400-Ka orbital eccentricity-cycle-changing ice volumes in the nascent Antarctic icecap. We propose that the Mut Basin is an exemplary case study area for guiding lithostratigraphic predictions in early Miocene shallow-marine carbonate and mixed environments elsewhere in the world. The Late Burdigalian in the Mut Basin was a time of relative tectonic quiescence, during which a complex relict basin topography was flooded by a rapid marine transgression. This area was chosen for study because it presents extraordinary large-scale 3D outcrops and a large diversity of depositional environments throughout the basin. Three study transects were constructed by combining stratal geometries and facies observations into a high-resolution sequence stratigraphic framework. 3346 m of section were logged, 400 thin sections were studied, and 145 biostratigraphic samples were analysed for nannoplankton dates (Bassant, P., 1999. The high-resolution stratigraphic architecture and evolution of the Burdigalian carbonate-siliciclastic sedimentary systems of the Mut Basin, Turkey. PhD Thesis. GeoFocus 3. University of Fribourg, 277 p.). The first transect (Alahan) is on the northwestern basin margin. Here, the siliciclastic input is high due to the presence of a river system. The siliciclastic depocentre migrates landwards during transgressions, creating an ecological window allowing carbonates to develop in the distal part of the delta. Carbonate production shuts down during the regression when siliciclastics return. The second transect (Pirinç) is also situated on the northern basin margin 12 km to the east of the Alahan section. It shows a complete platform-to-basin transition. An isolated carbonate platform complex develops during the initial flooding, which is drowned during a time of rapid sea-level rise and environmental stress, associated with prograding siliciclastics. The shelf margin then retrogrades forming large-scale clinoform geometries and progrades before a major sea-level fall provokes slumping collapse, followed by rebuilding of the shelf margin as sea level rises again. The third transect (Silifke) has a steep asymmetric Pre-Miocene valley-topography, forming a narrow strait, linking the Mut Basin to the Mediterranean. Strong tidal currents are generated in this strait area. Siliciclastic input is low and localised. Eighty metres of cross-bedded bioclastic sands are deposited in a tidal regime at the base. Subsequently, carbonate platforms backstep against the shallow-dipping northern flank, while platforms only develop on the steep southern flank when a firm wide shallow-marine substrate is provided by a bench on the footwall block. The energy of the environment decreases with increased flooding of the strait area. Third-order sequences and higher-order parasequences have been identified in each transect and correlated between transects. Correlations were made using biostratigraphic data and high-resolution sequence stratigraphy in combination with the construction of the relative sea-level curve for each site. The third-order highstands are stacked in a proximal position and separated by exposure surfaces, while the lowstands, deposited in a distal setting, are separated by deep-marine (offshore or subphotic) deposits. The parasequences produce dominantly aggradational and progradational geometries with transgressive ravinement surfaces and exposure surfaces developing at times. Reconstruction of the depositional profile shows that the third-order sequences are driven by relative sea-level oscillations of 100–150 m, and that these may be attributed to 400-Ka orbital eccentricity cycles. The parasequences are driven by eustatic 20–30 m sea-level oscillations, which may be attributed to the 100-Ka orbital eccentricity cycles. The isolated carbonate build-ups in the Pirinç and Alahan transects develop at the same time as bioclastic tidal deposits in the Silifke area during the transgression of sequence 1. This is caused by a difference in hydrodynamic regime: a direct result of basin morphology funneling tidal currents in the Silifke area. We also demonstrate how during the highstands a siliciclastic delta system progrades in the Alahan area, while only 12 km to the east, a fringing carbonate platform develops, showing how siliciclastic input can have a very localised effect on carbonate environments. The exceptional quality of the outcrops with its variety of environments and its location at the Tethyan margin make this site a good candidate for a reference model for Burdigalian reef and platform architectures.