Intra-shoreface erosion in response to rapid sea-level fall: depositional record of a tectonically uplifted strand plain, Pacific coast of Japan

This study documents lowering of the surf zone (i.e. the upper shoreface) leading to intra-shoreface erosion, following two rapid relative sea-level falls along a tectonically uplifted coast during the Holocene, and the characteristics of the resultant prograding shoreface deposits. These findings are based on high-resolution analysis and radiocarbon dating of three new drill cores obtained from the Kujukuri strand plain, Pacific coast of eastern Japan, combined with previously published borehole data and information on modern shoreline profile adjustments. A shallowing-upward sandy succession composed of lower and upper shoreface facies, foreshore and backshore facies was recognized in the drill cores. Two rapid falls in relative sea-level at 2·3 to 2·6 and 1·8 to 2·0 ka are recorded by downstepping of the base of the foreshore facies, and farther seawards by the lowering of an erosional boundary between the upper and lower shoreface facies. Superimposed bed profiles of an adjacent modern beach define an envelope, the base of which reflects shore-normal migration of longshore bars and troughs. The base of the envelope represents an erosional surface that divides the surface mobile layer above from preserved deposits beneath. The surface is concave upwards and steeper than the mean beach profile, and exhibits a flat platform approximately at the lower limit of the upper shoreface equating to the storm surf zone. The seaward transition of this surface, rather than the mean equilibrium profile, controls the metre-scale to decimetre-scale internal structure of the Kujukuri shoreface deposits. Depositional models for sea-level fall based on an exponential equilibrium profile do not adequately account for the presence and migration of longshore bars and troughs.     

Deltaic sedimentation in the Devonian of Western Libya

Middle and Upper Devonian deposits from the Aouinet Ouenine Formation in the southern Ghadames Basin of western Libya provide a well exposed example of a deltaic complex containing both progradational and transgressive facies. Progradational facies comprise both laterally accreting and incised distributary channels overlying prodelta deposits. Also present is a progradational beach environment showing build-up from an offshore shelf through nearshore shelf to shoreface and foreshore sub-environments. Over-lying these progradational facies are transgressive tidal-flat, washover-fan, foreshore and nearshore deposits.The characteristics and interrelationships of the different facies are explained by two sedimentation models: progradational facies existed contemporaneously during phases of active sediment supply whereas the transgressive facies existed contemporaneously during periods of diminished or absent detrital influx.     

Characteristics of an open coast tidal flat: Example from Daman,west coast of India

This study highlights lithofacies and biofacies characteristics of the open coast tidal flat near Daman on the eastern flank of Gulf of Khambhat. Sedimentological and biological observation record six facies within the tidal flat area including older beach, beach face, sand flat, mud flat/mixed flat, sand bar and beach rock. Distinct sedimentary structures, foraminiferal assemblage and bioturbation intensity characterize each facies. A wide variety of wave and current generated sedimentary features characterize the sand flat facies. Semiconsolidated sands of older beach running parallel the coastline at a level higher than the present beach face possibly records the latest sea level highstand. The beach rock reflects early cementation of sands in tropical environments. Foraminifera are widely distributed in sand flats, mixed flats and mud flats and grouped into two biofacies — Ammonia-Elphidium-Quinqueloculina biofacies (sand flat and mixed flat) and Trochammina-Miliammina biofacies (mud flats). The beach face and sand bar facies contain forminifera reworked from sand flat and mud/mixed flat. Seasonal variation in depositional style is marked by deposition of fresh mud deposited over large areas of the intertidal flat during monsoon time, most of which is washed away by waves and current actions well before the onset of the next monsoon.     

Changes in beach water table elevation during neap and spring tides on a sandy estuarine beach, Delaware Bay, New Jersey

A field investigation of temporal and spatial changes in wind and wave characteristics, runup and beach water table elevation was conducted on the foreshore of an estuarine beach in Delaware Bay during neap (April 9, 1995) and spring (April 16, 1995) tides under low wave-energy conditions. The beach has a relatively steep, sandy foreshore and semi-diurnal tides with a mean range of 1.6 m and a mean spring range of 1.9 m. Data from a pressure transducer placed on the low tide terrace reveal a rate of rise and fall of the water level on April 16 of 0.09 mm s⁻¹ resulting in a steeper tidal curve than the neap tide on April 9. Data from three pressure transducers placed in wells in the intertidal foreshore reveal that the landward slope of the water table during the rising neap tide was lower than the slope during spring tide, and there was a slower rate of fall of the beach water table relative to the fall of the tide. Wave heights were lower on April 9 (significant height from 17.1 min records <0.16 m). The water table elevation was 0.08 m higher than the water in the bay at the time of high water, when maximum runup elevation was 0.29 m above high water and maximum runup width was 2.0 m. The elevation of the water table was 0.13 m higher than the maximum elevation of water level in the bay 74 min after high water, when wave height was 0.12 m and wave period was 2.7 s. The use of mean bay water level at high tide will underpredict the elevation of the water table in the beach, and demarcation of biological sampling stations across the intertidal profile based on mean tide conditions will not accurately reflect the water content of the sandy beach matrix.     

Evolution and stratigraphy of a regressive barrier/backbarrier complex: Kiawah Island, South Carolina

Analysis of 75 vibracores from the backbarrier region of Kiawah Island, South Carolina reveals a complex association of three distinct stratigraphic sequences. Beach ridge progradation and orientation-controlled backbarrier development during the evolution of Kiawah Island, and resulted in deposition of: (1) a mud-rich central backbarrier sequence consisting of low marsh overlying fine-grained, tidal flat/lagoonal mud; (2) a sandy beach-ridge swale sequence consisting of high and low marsh overlying tidal creek channel and point bar sand, and foreshore/shoreface; and (3) a regressive sequence of sandy, mixed, and muddy tidal flats capped by salt marsh that occurs on the updrift end of the island. Central backbarrier deposits formed as a result of the development of the initial beach ridge on Kiawah Island. Formation of this beach ridge created a backbarrier lagoon in which fine-grained estuarine and tidal flat mud accumulated. Washovers, oyster mounds, and tidal creek deposits form isolated sand and/or shell-rich lenses in the lagoon. Spartina alterniflora low marsh prograded into the lagoon as the tidal flats aggraded. Barrier progradation and sediment bar-bypassing at Stono Inlet created digitate beach ridges on the northeast end of Kiawah Island. Within the beach-ridge swales, tidal flats were disconformably deposited on shoreface and foreshore sand of the older beach ridges. Tidal creek drainage systems evolved to drain the swales. These rapidly migrating creeks reworked the tidal flat, foreshore, and shoreface sediments while redepositing a fining-upward sequence of channel lag and point bar deposits, which served as a substrate for salt marsh colonization. This resultant regressive sedimentary package marks the culmination of barrier island development and estuary infilling. Given enough time and sedimentation, the backbarrier sequence will ultimately prograde over the barrier island, reworking dune, beach, and foreshore sediments to form the upper sand-rich bounding surface of the barrier lithosome. Preservation of the regressive sequence is dependent upon sediment supply and the relative rate of sea-level rise, but the reworking of barrier islands by tidal inlets and migrating tidal creeks greatly alter and complicate the stratigraphic sequence.     

Distribution of heavy metals in the Piscinas beach sediments (SW Sardinia, Italy)

We have studied sediments of the Piscinas beach (SW Sardinia, Italy), which is supplied by two streams that wash mine dumps of abandoned lead and zinc mines at Montevecchio and Ingurtosu, situated inland from the supply basin of the beach itself. A study of the texture, mineralogy and geochemistry of the sediments was conducted for the purpose of assessing the possible influence of the mine waste on the composition of the sediments, looking for any anomalous enrichments in heavy metals. Furthermore, to evaluate and quantify metal release into the sea, samples of Posidonia oceanica, a bioaccumulator marine plant, were also examined. The results indicate that the distribution of heavy metals in the foreshore sediments is particularly affected by the contribution of the streams, while in the shoreface the distribution is affected by the currents that disperse the sediments both out to sea and southwards. The metal contents of the Posidonia oceanica are correlated with the different stages of activity of the mines. Received: 28 January 1998 · Accepted: 22 April 1998     

Facies associations of the Bathonian Hamam Formation from Northwestern Jordan

Two stratigraphic sections of the Hamam Formation (Bathonian Stage, Middle Jurassic) exposed in the western part of Wadi Zarqa region, northwestern Jordan, are described and interpreted on the basis of palynoflora and facies analysis in order to reconstruct their depositional environments and sequence stratigraphic framework, which not discussed before. Five facies associations have been identified in the Hamam Formation characterized by a mixed carbonate–siliciclastic ramp setting, ranging from incised fluvial valley fill facies, beach foreshore restricted inner ramp to high-energy shoals and mid-ramp settings. The palynoflora includes well-preserved miospore assemblages which are recorded only from the incised fluvial valley fill facies for the first time and yielded 64 miospore species belonging to 40 genera. Most of these taxa are long-ranging and have been reported from Jurassic and Cretaceous rocks worldwide, except Callialasporites dampieri, Murospora florida, Granulatisporites jurassicus, Piceites expositus, Pityosporites parvisaccatus, Leptolepidites verrucatus, and Protopinus scanicus which have short ranges in the Middle Jurassic. Furthermore, these rocks are rich in shallow-marine Neo-Tethys macro-invertebrates supporting a Bathonian age. Two third-order depositional sequences bounded by three regional unconformities at the Bajocian–Bathonian and Bathonian–Callovian stage boundaries as well as within the Bathonian are defined based upon facies characteristics and stratal geometries. A regional correlation of sequence boundaries of similar age indicates that they are eustatic in origin.     

Taphonomic differentiation of Acropora palmata facies in cores from Campeche Bank Reefs, Gulf of México

A common assumption in the geological analysis of modern reefs is that coral community zonation seen on the surface should also be found in cores from the reef interior. Such assumptions not only underestimate the impact of tropical storms on reef facies development, but have been difficult to test because of restrictions imposed by narrow‐diameter cores and poor recovery. That assumption is tested here using large‐diameter cores recovered from a range of common zones across three Campeche Bank reefs. It is found that cores from the reef‐front, crest, flat and rubble‐cay zones are similar in texture and coral composition, making it impossible to recognize coral assemblages that reflect the surface zonation. Taphonomic signatures imparted by variations in encrustation, bioerosion and cementation, however, produce distinct facies and delineate a clear depth zonation. Cores from the reef‐front zone (2–10 m depth) are characterized by sections of Acropora palmata cobble gravel interspersed with sections of in‐place (but truncated) A. palmata stumps. Upper surfaces of truncated colonies are intensely bioeroded by traces of Entobia isp. and Gastrochaenolites isp. and encrusted by mm‐thick crustose corallines before colony regeneration and, therefore, indicate punctuated growth resulting from a hurricane‐induced cycle of destruction and regeneration. Cores from the reef crest/flat (0–2 m depth) are also characterized by sections of hurricane‐derived A. palmata cobble‐gravels as well as in‐place A. palmata colonies. In contrast to the reef front, however, these cobble gravels are encrusted by cm‐thick crusts of intergrown coralline algae, low‐relief Homotrema and vermetids, bored by traces of Entobia isp. and Trypanites isp. and coated by a dense, peloidal, micrite cement. Cores from the inter‐ to supratidal rubble‐cay zone (+0–5 m) are only composed of A. palmata cobble gravels and, although clasts show evidence of subtidal encrustation and bioerosion, these always represent processes that occurred before deposition on the cay. Instead, these gravels are distinguished on the basis of their limited bioerosion and marine cements, which exhibit fabrics formed in the intertidal zone. These results confirm that hurricanes have a major influence on facies development in Campeche Bank reefs. Instead of reflecting the surface coral zonation, each facies records a distinctive, depth‐related set of taphonomic processes, which reflect colonization, alteration and stabilization following the production of new substrates by hurricanes.     

Kyanite-eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks,Adula nappe,Central Alps

In the southern Adula nappe (Central Alps), two stages of regional metamorphism have affected mafic and pelitic rocks. Earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite-hornblende eclogites was followed by a Tertiary overprint which varied from greenschist to high-grade amphibolite facies. Despite a common metamorphic history, contrasting equilibration conditions are often recorded by high-pressure mafic eclogite and adjacent predominantly lower-pressure pelite assemblages. This pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading. The rates are controlled by a mechanism in which dehydrating metapelites provide the H₂O required for simultaneous overprinting of enclosed mafic eclogites by hydration.Quantitative mass balance modelling based on corona textures is used to show that overprinting of metapelites during unloading involved dehydration reactions. The relatively rapid rate of dehydration reactions led to nearly complete reequilibration of metapelites to amphibolite facies assemblages.After the formation during high-pressure metamorphism of mafic eclogites, later lower-pressure reequilibration by hydration to amphibolites was slow, and therefore incomplete, because it depended on large scale transport of H₂O from adjacent, dehydrating metapelites.The facies contrast observed between rocks of different bulk composition is thus a consequence of the general tendency of metamorphic rocks to retain the most dehydrated assemblage as the final recorded state.     

Depositional environments of coastal sediments of Calangute,Goa

The grain-size statistics and environmental conditions of deposition of the beach and dune sediments from the Calangute region, Goa, have been studied. The results of Student's t test and bivariate plots reveal that there exist distinct differences in grain-size parameters of the sediments from different environments and that these differences are highly significant. The study shows that the variations in mean grain size (Mz), graphic standard deviation (O₁), graphic skewness (Sk₁), simple sorting measure (So_s) and simple skewness measure (O_s) are significant in identifying the sediments from different environments. Further, the study reveals the usefulness of grain-size parameters not only in differentiating beach and dune sediments, but also in delineating the beach into foreshore and backshore. The differences in grain-size characteristics of the sediments between the environments reflect the transport, erosional and depositional mechanisms (active hydrodynamic processes in the beach foreshore and aerodynamic processes in the backshore and dune environments) prevailing in the area of study.     

Wavy lamination in a mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, Aichi, central Japan

Although sandy foreshore facies are generally characterized by parallel lamination, wavy lamination is predominant in the mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, which crops out along the Pacific coast of the Atsumi Peninsula, Aichi, central Japan. The foreshore facies consists of three sedimentary subfacies; interbeds of gravel and parallel laminated sand of the lower foreshore facies, parallel laminated fine to medium sand beds containing scattered pebbles and cobbles of the middle foreshore facies, and wavy laminated fine to medium sand beds containing scattered pebbles and cobbles of the upper foreshore facies. A lack of erosional surfaces in the middle foreshore facies indicates the continuous accumulation of sand in flat beds under upper plane bed flow. The wavy laminated sands of the upper foreshore facies exhibit erosional surfaces indicative of repeated deposition and erosion. The erosional surfaces are undulatory, with depressions (10 cm wide and 3 cm deep) that contain scattered pebbles and cobbles. These depressions reflect backwash erosion of sand around and below the pebbles and cobbles. Sand draping over the undulating erosional surfaces forms the wavy lamination. The wavy laminated sand with scattered pebbles and cobbles is a key facies of an upper foreshore or swash zone, and is a good sea-level marker.     

Discrimination of particle zonation on a pebble beach

The extent to which pebble shapes are zonally sorted on a pebble beach is examined. On the basis of comprehensive point samples from a pebble beach in west Wales, and using factor analysis, a pebble shape index for each point sample is derived. It is proposed that the sum of squares (%RSS) explained by a linear trend surface of shape index values, approximates to the degree to which the second derivative of pebble shape with respect to distance downbeach, tends towards zero. If the second derivative is zero there is no zonation of sample shape values but, rather, a uniform trend. Strength of zonation on one beach is calculated for eight beach samples during 1971. A heterogeneity index (Ht) is introduced to show the degree of difference between clustered point samples on the basis of shape and size. On the basis of %RSS and Ht value, discrimination between pebble facies type on the sample beach over time can be made.     

An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates

The feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south-west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (R_w), maximum projection sphericity (δ_p), oblate-prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δ_p and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade-shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and R_w is observed, hence the abundance of disc-shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates.     

Discrimination between coastal subenvironments using textural characteristics

The objective of this study was to discriminate between modern beach subenvironments based on textural characteristics obtained using the graphical (percentile) method, the moment method, and the log-hyperbolic distribution (LHD). A total of 126 surface sedimentation units were sampled at the nodes of a 21 x 6 rectangular grid (1000 m²) on a carbonate sand beach, Oahu, Hawaii. Sampling was conducted at low energy conditions from the lower foreshore to the backshore. Non-parametric discriminant analysis was used as an objective tool in defining distinct subenvironments. Confidence bands around the canonical variates derived from the graphic mean, sorting, skewness and kurtosis indicated four separate subenvironments (lower foreshore, mid-foreshore, upper foreshore and backshore). Three distinct subenvironments were identified using the mean, sorting (standard deviation) and skewness measures derived by the method of moments. A similar subenvironment distinction was obtained using five statistics of the LHD (gamma, γ; nu ν; delta, δ; tau, τ; and xi, ξ). No significant difference was noted in textural characteristics between the upper foreshore and backshore zones, and these zones were grouped into one subenvironment. These results indicate that different process scenarios would be needed to explain different subenvironment partitioning based simply on the approach adopted. Discriminant analysis indicated that fewer subenvironment samples were misclassified and separation distances between subenvironments in bivariate canonical plots were greater for the standard moment measures compared with the statistics derived from fitting the computationally intensive LHD model. Examination of the mass frequency grain size distributions indicated that the LHD was generally the most appropriate model. These observations were confirmed by the hyperbolic shape triangle which indicated that the LHD rather than the more commonly used log-normal distribution was generally optimal in describing sediments. These results support the use of the LHD statistical measures in subenvironment discrimination over the graphic-inclusive measures.     

雪峰古陆边缘上石炭统陆源碎屑和碳酸盐混合沉积

湖南沅陵雪峰古陆边缘上石炭统主要包括下列岩相类型 :冲积扇砂砾岩相、前滨海滩砂岩相、浅滩碳酸盐相及混积岩相、潮下碳酸盐岩相及滩后局限碳酸盐岩相。通过沉积相分析和古岩溶面的确定 ,在该区上石炭统中共划分出 13个海侵海退旋回及 5个三级层序。对上石炭统中发育的混积岩也进行了讨论。     

雪峰古陆边缘上石炭统陆源碎屑和碳酸盐混合沉积

湖南沅陵雪峰古陆边缘上石炭统主要包括下列岩相类型 :冲积扇砂砾岩相、前滨海滩砂岩相、浅滩碳酸盐相及混积岩相、潮下碳酸盐岩相及滩后局限碳酸盐岩相。通过沉积相分析和古岩溶面的确定 ,在该区上石炭统中共划分出 13个海侵海退旋回及 5个三级层序。对上石炭统中发育的混积岩也进行了讨论。     

Millennial/submillennial-scale sea-level fluctuations in western Mediterranean during the second highstand of MIS 5e

This paper investigates a series of small-scale, short-lived fluctuations of sea level registered in a prograding barrier spit that grew during the MIS 5e. This interglacial includes three highstands (Zazo et al., 2003) and we focus on the second highstand, of assumed duration ～10 ± 2 ka, given that U–Th ages do not provide more accurate data. Geometry and 3D architecture of beach facies, and thin-section petrography were used to investigate eight exposed offlapping subunits separated by seven conspicuous erosion surfaces, all interpreted as the result of repeated small-scale fluctuations of sea level.Each subunit records a relatively rapid rise of sea level that generated a gravelly shoreface with algal bioherms and a sandy uppermost shoreface and foreshore where most sand accumulated. A second range of still smaller-scaled oscillations of sea level has been deduced in this phase of sea-level fluctuation from lateral and vertical shifts of the foreshore-plunge-step-uppermost shoreface facies.Eventually, progradation with gently falling sea level took place and foreshore deposits underwent successive vadose cementation and subaerial dissolution, owing to relatively prolonged exposure. Later recovery of sea level re-established the highstand with sea level at approximately the same elevation, and there began deposition of a new subunit. The minimum sea-level variation (fall and subsequent rise) required to generate the observed features is 4 m. The time span available for the whole succession of events, and comparison with the Holocene prograding beach ridge complex in the nearby Roquetas (Almería) were used to calculate the periodicity of events. A millennial-suborbital time scale is suggested for fluctuations separating subunits and a decadal scale for the minor oscillations inside each subunit.     

Coarse‐sand beach ridges at Cowley Beach,north‐eastern Australia: Their formative processes and potential as records of tropical cyclone history

Storm surges generated by tropical cyclones have been considered a primary process for building coarse‐sand beach ridges along the north‐eastern Queensland coast, Australia. This interpretation has led to the development of palaeotempestology based on the beach ridges. To better identify the sedimentary processes responsible for these ridges, a high‐resolution chronostratigraphic analysis of a series of ridges was carried out at Cowley Beach, Queensland, a meso‐tidal beach system with a >3 m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years. The ridge crests sit +3·5 to 5·1 m above Australian Height Datum (ca mean sea‐level). A ground‐penetrating radar profile shows two distinct radar facies, both of which are dissected by truncation surfaces. Hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at +2·5 m Australian Height Datum, equivalent to the fair‐weather swash limit during high tide. The lower facies comprises a sequence of seaward‐dipping reflections. Beach progradation thus occurs via fair‐weather‐wave accretion of sand, with erosion by storm waves resulting in a sporadic sedimentary record. The ridge deposits above the fair‐weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events. Inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide. The ridges may also include an aeolian component as cyclonic winds can transport beach sand inland, especially during low tide, and some layers above +2·5 m Australian Height Datum are finer than aeolian ripples found on the backshore. Coarse‐sand ridges at Cowley Beach are thus products of fair‐weather swash and cyclone inundation modulated by tides. Knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges.     

Depositional facies,environments and sequence stratigraphic interpretation of the Middle Triassic–Lower Cretaceous (pre-Late Albian) succession in Arif El-Naga anticline,northeast Sinai,Egypt

The Middle Triassic–Lower Cretaceous (pre-Late Albian) succession of Arif El-Naga anticline comprises various distinctive facies and environments that are connected with eustatic relative sea-level changes, local/regional tectonism, variable sediment influx and base-level changes. It displays six unconformity-bounded depositional sequences. The Triassic deposits are divided into a lower clastic facies (early Middle Triassic sequence) and an upper carbonate unit (late Middle- and latest Middle/early Late Triassic sequences). The early Middle Triassic sequence consists of sandstone with shale/mudstone interbeds that formed under variable regimes, ranging from braided fluvial, lower shoreface to beach foreshore. The marine part of this sequence marks retrogradational and progradational parasequences of transgressive- and highstand systems tract deposits respectively. Deposition has taken place under warm semi-arid climate and a steady supply of clastics. The late Middle- and latest Middle/early Late Triassic sequences are carbonate facies developed on an extensive shallow marine shelf under dry-warm climate. The late Middle Triassic sequence includes retrogradational shallow subtidal oyster rudstone and progradational lower intertidal lime-mudstone parasequences that define the transgressive- and highstand systems tracts respectively. It terminates with upper intertidal oncolitic packstone with bored upper surface. The next latest Middle/early Late Triassic sequence is marked by lime-mudstone, packstone/grainstone and algal stromatolitic bindstone with minor shale/mudstone. These lower intertidal/shallow subtidal deposits of a transgressive-systems tract are followed upward by progradational highstand lower intertidal lime-mudstone deposits. The overlying Jurassic deposits encompass two different sequences. The Lower Jurassic sequence is made up of intercalating lower intertidal lime-mudstone and wave-dominated beach foreshore sandstone which formed during a short period of rising sea-level with a relative increase in clastic supply. The Middle-Upper Jurassic sequence is represented by cycles of cross-bedded sandstone topped with thin mudstone that accumulated by northerly flowing braided-streams accompanying regional uplift of the Arabo–Nubian shield. It is succeeded by another regressive fluvial sequence of Early Cretaceous age due to a major eustatic sea-level fall. The Lower Cretaceous sequence is dominated by sandy braided-river deposits with minor overbank fines and basal debris flow conglomerate.     

Grenats des andésites et des rhyolites de Slovaquie,origine des grenats dans les séries andésitiques

The investigated garnets from the andesitic series (andesites — rhyolites) occured as megacrysts. It was shown they were not formed in a late magmatic period and they are not xenocrysts coming from the metamorphic basement. Better, they indicate high pressure crystallisation phases in the eclogitic facies, during which the almandine content of the garnets increases with the rising degree of acidity of the basalt-eclogitic melt.The electron microprobe analysis showed that an isobaric evolution could manifest by a normal Mg-Fe zonation, whilst a polybaric crystallisation would result in an inverse Fe-Mg zonation. Two types of magmatic garnets are shown: one that has a low Ca content and is formed under medium pressure in the presence of H₂O, and the other with higher Ca content, formed under high pressure in the absence of H₂O.