Parallel Lamination Research Articles

Sedimentation on high‐energy sand flats in the Bay of Fundy: The record of tidal‐bore activity and deposition from high‐concentration suspensions of sand

AbstractThe Cobequid Bay – Salmon River estuary, Bay of Fundy, does not possess particularly large tidal bores (maximum ca 30 cm). Nevertheless, detailed logging of 13 peels made from cores spread throughout the entire area influenced by tidal bores indicates that inferred tidal‐bore deposits comprise approximately 15% of the deposits in the upper‐flow‐regime sand flats in the inner part of the estuary, averaging more than 10 tidal‐bore layers per metre of core. A landward increase in tidal‐bore height is accompanied by an increase in the number and thickness of the tidal‐bore layers. The deposit of a single tidal bore begins with structureless sand (from <1 cm to >10 cm thick) that can be gradationally overlain by sand with indistinct parallel lamination, which in turn passes upward into the well‐defined parallel lamination that is the dominant sedimentary structure in these sand‐flat deposits. The bases of the structureless layers range from prominently erosional to passively draping over the ripples formed during the preceding ebb tide. The structureless and indistinctly laminated layers are interpreted to be the deposit of bore‐generated hyperconcentrated suspensions of sand that have similarities with turbidity currents in which near‐bed turbulence is extinguished or damped. The tidal‐bore Froude number (Frtb), which determines the nature of the tidal bore, together with the height of the tidal bore, influences the amount of sand in suspension, and presumably also controls the nature of the deposits. Tidal bores with high Froude numbers (Frtb = 2 to 3) that are breaking create deposits that are analogous to the deposits of high‐concentration turbidity currents; most bore deposits documented to date from other ancient and modern settings are of this high‐energy type. Tidal bores that are closer to the lower limit of bore occurrence (Frtb = 1) generate deposits that will be difficult to distinguish from the background sedimentation.

The effects of lithology and facies types on the anisotropy parameters and upscaling factor of the sand reservoirs in the deep-water Sadewa field, Kutei Basin, East Kalimantan, Indonesia

Abstract The anisotropy and upscaling factors adjustment are very important in the proper integration of the core ultrasonic and log sonic data for hydrocarbon reservoir characterization. Related to this issue, this paper evaluates the effects of the lithology and facies types on the anisotropy parameters and upscaling factor. The data are taken from the deep-water oil-gas Sadewa field located in the Kutei Basin, East Kalimantan, Indonesia at water depths of 500–750 m. The main reservoirs in this field are the Upper Miocene sand reservoirs deposited in the upper slope fan and channel facies. Fifty core plugs sampled at depths around 3000–4000 m were collected for thin-section petrography analysis and ultrasonic measurements. The thin-section petrography analysis shows that both facies areas are dominated by greywackes with parallel lamination structure and/or intergranular porosity. The 1 MHz ultrasonic velocities were measured in the 50 core plugs. Meanwhile, 10–40 KHz dipole sonic log data sampled at the same plug’s depth positions were used to calculate the elastic (Vp, Vs, Poisson ratio, etc) and Thomsen’s e, γ and δ anisotropy parameters. The results show that for the parallel lamination and intergranular porosity greywacke samples, e parameter is the best for compensating the anisotropy effect in the integrated core and log data reservoir quality determination (sand shale ratio, percentage of quartz contents, the effective and the total porosities). However, when the samples are non-intergranular porosity greywacke, the anisotropy effect in the core-log data correlation, is too irregular to be compensated by any elastic and anisotropy parameter. The core-log data upscaling factor in the channel facies is much smaller than the upper slope fan facies, and it is in line with the gamma-ray log data which indicates that the sandy channel facies is more homogeneous than the intercalated shale-sand upper slope fan facies. The overall results suggest that the lithology and facies types significantly affect the anisotropy parameters and upscaling factor. In addition, the best elastic or anisotropic parameters) should be determined to minimize the effects in the core and log data integration for hydrocarbon reservoir characterization.

Paleoarchean terrestrial to shallow marine sedimentation on Singhbhum Craton, eastern India (the Western Iron Ore Group)

Distinguishing shallow-marine and terrestrial (fluvial) sedimentary deposits is often difficult in the absence of fossils and challenging in deformed and metamorphosed early Precambrian terrains. Herein we report for the first time a > 3.4 Ga shallow marine to terrestrial deposit from the Western Iron Ore Group (WIOG), Singhbhum Craton, India. The WIOG sedimentary succession, lying below a ~ 3.4 Ga banded iron formation, has been divided into three facies associations: a lower alluvial fan-fluvial association, a middle shallow-marine association, and an upper fluvial association. The bottommost conglomerate–very coarse- to coarse-grained sandstone constitutes the alluvial fan-braided fluvial association. Sandstones of the facies association two bears mud drapes and herringbone cross-stratification and thus indicates tidal influence. These shallow-marine sandstones bear wave ripples, ladderback ripples, and adhesion structures and indicate an intertidal depositional environment. The overlying fluvial facies association is characterized by several fining-upward cycles of conglomerate–coarse- to medium-grained sandstones with large-scale trough cross beds, parallel lamination and penecontemporaneous deformation structures and minor shale, indicating the braided nature of the postulated river system. The WIOG succession indicates an overall shallowing-up trend because of sea level fall. The sedimentary structures and texture of the WIOG fluvial deposit are very similar to recent fluvial deposits and are likely to have formed by similar sediment transportation and depositional processes in a tectonically stable sedimentary basin.

Age, depositional environment, and tectonic significance of an Early Neoproterozoic volcano‐sedimentary sequence in the eastern Beishan orogenic belt, southern Central Asian Orogenic Belt

The Beishan orogenic belt, located in the southern Central Asian Orogenic Belt, includes several discrete arc terranes. Here, we report on a newly discovered Early Neoproterozoic volcano‐sedimentary sequence in the eastern Beishan orogenic belt in order to constrain the composition and origin of the Shuangyingshan‐Huaniushan Terrane. The ~766‐m‐thick volcano‐sedimentary sequence is very low‐grade metamorphosed, and characterized by muddy and silty slate with the well‐developed parallel laminations and minor thinly‐bedded limestone in its lower and upper sections, and thick packages of sandstone‐conglomerate and basaltic rocks in the middle section. Lava flow units in the basalt are very common, and the sandstone‐conglomerate exhibits sedimentary structures typical of gravity currents, including erosional and slump features, and Bouma sequences. These features suggest that syn‐sedimentary volcanism occurred and the sequence formed in an active, deep marine environment. The basalt has a zircon U–Pb age of 901 ± 10 Ma, indicating that this sequence formed during the Early Neoproterozoic. Basalt samples have moderate SiO2 (45.33–49.18%), and high TiO2 (1.27–4.04%) contents, Mg# values of 25–48, and plot in the alkali basalt field in the Nb/Y versus Zr/TiO2 diagram, indicative of an evolved alkali basalt magma. They show inclined chondrite‐normalized REE patterns with (La/Yb)N ratios of 4.74–6.53, no pronounced negative Eu anomalies (Eu/Eu* = 0.87–1.22), no obvious negative Zr, Hf, Nb, and Ta anomalies in the primitive‐mantle‐normalized trace‐element diagram, consistent with oceanic island basalts. Considered together, these lithologies resemble those of an oceanic seamount. Our results, and the presence of the previously reported Gubaoquan granitic and mafic rocks (905–865 Ma) that formed in an active continental margin setting and underwent an amphibolite‐facies metamorphism, suggest that the Shuangyingshan‐Huaniushan Terrane may be an accreted terrane that consists of different lithologies with various origins that formed during the Neoproterozoic.

An insight to the initiation of Cretaceous sedimentation in Northeast Indian Craton

The time transgressive Cretaceous sedimentation in northeast Indian craton is represented by the deposition of the Upper Cretaceous Mahadek sediments in Meghalaya. The Lower unit of these sediments is comprised of alternate sandstone and conglomerate horizons, and the Upper unit is composed of arkosic to sub-arkosic sandstones. These sediments are overlain by the calcareous litho-units of Langpar Formation of Cretaceous–Paleocene age. Lithofacies identified from various exposures and studied lithologs reflect a variation from marginal marine to marine depositional set-up in the Mahadek sediments. The Lower Mahadek facies is dominated by large-scale trough beds of medium to coarse-grained sediments which initiates with a conglomerate horizon succeeded by thin beds of shale and fine-grained sandstone with parallel laminations, which are followed by successive deposition of gritty sand and pebbly beds with coarsening upward sequence, suggesting product of delta plain in a marginal marine setting. Moreover, the bioturbated coaly fine-grained sandstone is further representative of tidal flat-tidal channel swamp formed in reducing condition and the trace fossil assemblages depict the channel fills. The Upper Mahadek facies is dominated by glauconitic-gritty fossiliferous sandstone showing fining upward sequence and hummocks, typical of marine environment. This fining upward sequence along with the vertical and lateral facies variation implies superimposition of retrograding shorelines on an earlier progradational alluvial fan sand complex. These different-scale cycles are interpreted as the sedimentary response to movements of the basin margin faults at variable frequencies, related to the contemporary faults. This depositional model of the Cretaceous sediments may further help in paleogeographic reconstruction of the northeast Indian craton during the Cretaceous period.

Dactyloidites ottoi en depósitos marinos someros de la Formación Coris, Mioceno, Costa Rica

Dactyloidites ottoi traces associated with medium grained, mature, siliceous sandstone facies, with planar and curved parallel lamination and HCS cross laminations were developed in shallow marine environments (upper shoreface) of the Coris Formation (Miocene). High-energy environmental conditions were caused by waves, storms, tides and currents. Episodic sedimentation with high-energy erosive events caused by storms alternating with low-energy deposition with organic matter input favored sediment rework by organisms. These ichnofossils are infrequent in the sedimentary record and they had not previously been reported in Costa Rica. They are constituted by elongated elements which radiate from a central shaft and create a series of rosetted or fan-shaped tiered galleries. The organism that created these structures is yet unknown, however, several studies coincide that it must have been a feeding fodichnia trace produced by a worm-like organism which is part of the Cruziana Icnofacies.

Regressive Shoreface Depositional System of the Cretaceous Yolde Formation of the Gongola Sub-Basin Northern Benue trough N. E. Nigeria

The characterization of lithofacies of the Yolde Formation at Briyel town in the Gongola Sub-basin of the Northern Benue Trough indicated coarsening upwards cycles composed transitional succession of six lithofacies that comprises of trough cross-beds, planar cross-beds, massive beds, ripple laminations, parallel laminations and mudstone. This coarsening and thickening upward symmetry is defined by a densely bioturbated mud dominated lower part transitionally grading into heterolithic interval composed thinly bedded sandstone and mudstones both of which developed below fair-weather wave base. This is capped by a succession of moderately bioturbated trough-planar crossbedded sandstone with poly-directional paleocurrent system. These assemblages reflect shoreface deposits and devoid of hummocky cross-stratification therein may account for a moderate wave oceanographic system. Evolution of this setting in the Gongola Sub-basin is indicative of a scenario of a broaden coastline in the coastal palaeogeography of the Yolde Formation typically suppressing due to frictional damping.

Identification characteristics of volcanic deposits in Cimarga, Southeast Banten

Volcanic eruption(s) in the past occurred around rawa danau volcanic complex is marked by the presence of a caldera and widespread volcanic deposits in the Banten Province as Tuff Banten. This study aims to determine the possible mechanism of the eruption based on volcanic deposits. Moreover, this study will also present the prolonged research process to understand the dynamics of volcanic deposits in Banten. Field study and sampling of volcanic deposits were completed followed by thorough petrographic analysis. There are three outcrops all with different characteristics. There are 6 layers in outcrop 1. Out of six layers, layer 1 (P1) until layer 5 (P5) appear similar, consisting of pumice rich (as fragment) and lapilli layer, while layer 6 (P6) consists of massive ash. In outcrop 2, there are 5 layers, which show cross and parallel laminations in each layer. Outcrop 3 contains 3 layers with the top and bottom layers consisting of lithic and pumice lapilli that are medium in size. Based on grain size from each outcrop, grain size from outcrop 1 until outcrop 2 progress into finer grains. From these characteristics outcrops 1 indicated flow deposits, outcrop 2 indicated surge deposits, and outcrop 3 indicated flow deposits. Therefore the facies from all three outcrops lean towards medial to distal with outcrop 3 as a distal facies.

Paleocurrent analysis, petrographic, geochemical and statistical appraisal of Neoproterozoic siliciclastic sediments, NE Voltaian Basin, Ghana: a multidisciplinary approach to paleogeographic reconstruction

This study used sedimentary structures, facies analysis, petrographic studies, statistical techniques and geochemical data of the Bombouaka/Gambaga Group of the NE Voltaian Basin, Ghana to decipher the source of the sediments. Field measurements covered the Tossiegou, Poubogou, and Panabako Formations of the Bombouaka/Gambaga Group. The influence of hydraulic sorting was interpreted from the effects of highly resistant heavy minerals (identified in the petrographic studies) from polycyclic sources. Principal component analysis (PCA) was applied to the geochemical data to understand element associations. Two broad facies types were identified; lithofacies and ichnofacies. The lithofacies were observed as (a) asymmetric ripple marks on sandstones, (b) parallel lamination on silty/argillaceous sediments, (c) straight-crested and bifurcated ripples on shales, (d) flute casts and climbing ripples on sandy shales, (e) bipolar herringbone cross-bedding on quartz-rich sandstones, and (f) wavy lamination and cross-bedding on feldspathic sandstones. However, the ichnofacies include only Skolithos on quartzitic sandstones. The structural analyses suggest that about 85% of the paleocurrents were from NE to SW direction with subordinate directions from WNW to ESE. Based on the predominant paleocurrent directions, the sediments were probably derived from the basement Birimian rocks (metavolcanic and metasedimentary rocks with associated granitoids), previously paleogeographically located in the Amazonian Craton but now eroded, transported and transformed into the Birimian Supergroup from which the Voltaian sediments were derived. The PCA points to similar sources of granitoids and metasedimentary rocks. Therefore, the geochemical results and the PCA interpretations support the paleocurrent structures-inferred sources of the sediments in the NE Voltaian Basin.

Depositional Facies along the Banks of Guadalquivir Estuary (SW Spain)

Morales, R.; Borrego, J.; Carro, B.M., and Morales, J.A, 2020. Depositional facies along the banks of Guadalquivir Estuary (SW Spain). In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 573–577. Coconut Creek (Florida), ISSN 0749-0208.The Guadalquivir Estuary is located at center of the Cadiz Gulf. It is an estuary supplied by one of the longest rivers of Spain. The Guadalquivir estuarine channel extends from the town of Sevilla to its mouth in front of Sanlúcar de Barrameda (Cádiz), bordering Doñana National Park along its southeastern margin. Guadalquivir River has an average discharge of 185 m3/s, with a strong seasonal character. This estuarine zone is characterized by a mesotidal range, with a mean tidal amplitude of two meters. Water usually has a high concentration of suspended matter, being one of the most turbid estuaries in the world. The channel margins mainly develop muddy facies with a clearly visible parallel lamination that has been described and interpreted as a result of the alternation between wet and dry seasons and longer climatic cycles. A zonation of the estuary is suggested as a result of the tidal facies distribution.

Field Geological Characteristic of Mesozoic Paremba Sandstone, Bantimala Complex, South Sulawesi

We conducted preliminary survey to study field geological characteristic of Mesozoic Paremba Sandstone, Bantimala Complex, South Sulawesi in order to investigate the field occurrence and the petrographic characteristic of some samples from the sandstone unit. The samples were collected from measuring section campaign in three rivers, namely Mallapie, Ceppaga and Bontorio. The field investigation shows that The Paremba sandstone occurs as a tectonic slice within mélange complex which is sandwiched by schist and melanges. The sandstones range from fine to coarse-grained sandstone forming a layer of 0.5 up to 50 cm in thickness interlayered with siltstone and shale, forming a sequence set. The sequences are intercalated with other lithologies such as limestone, mudstone, conglomerate, breccia and chert, forming a distinctive unit. In certain place, the units are intruded by basalt dike. Generally, the sandstones are grey to blackish in colour, sometimes show brown to yellowish colour if weathered. Some sandstone is calcareous as shown by strong reaction with acid and the occurrence of carbonate mineral but other samples relatively show less carbonate minerals. In all three rivers, the sandstone units have been strongly tectonized as seen by irregular fold and fault from the strike and dip of the unit layer. Petrographic analyses show that the studied sandstones consist of arkosic sandstone, siltstone and calcareous sandstone which are composed of quartz, orthoclase, plagioclase and calcite in the groundmass with opaque minerals occur as main accessory. Some samples contain muscovite, especially calcareous sample. The sandstone unit shows a typical of coarsening- and thickening updward trend with flaser, lenticular and parallel lamination sedimentary structure. It is interpreted that the sandstone were deposited in shallow marine or shelf region.

Characterization of carbonate rocks through X‐ray microfluorescence and X‐ray computed microtomography

Carbonate rocks play an important role in petroleum geology by acting as reservoir rocks, generators, and even hydrocarbon sealants, accounting for about half of the oil and gas reserves known in the world. The study of these carbonate rocks have become very important in the hydrocarbon exploration scene in Brazil because of they consist in analogous for reservoir rocks of the presalt interval. Thus, the objective of this research was to use X‐ray microfluorescence (micro‐XRF) and X‐ray microtomography analysis, as complementary techniques, in order to characterize samples of carbonate rocks in respect to their structures, textures, mineralogy, and pores. The microtomographic analyses allowed the identification of the horizontal structures as parallel lamination, horizontal, and vertical fractures filled by calcite and biotic constituents (gastropods bioclasts). Different composition of minerals were also identified, as calcite, quartz, feldspars, iron sulfides, and oxides. The porosity (ranges <1 to17%), and the high‐density elements could also be quantified, as well as their distribution in each sample. The micro‐XRF analysis present a direct relationship with the distribution of minerals that compound carbonate rocks, highlighting some structures, as well as helping to identify trace and minor elements in the carbonates (Mn, Sr, and Mg).

Depositional sub-environment of Gombe Sandstone in Kumo and its environs, part of Gongola Basin, Northern Benue Trough, Northeastern Nigeria

Evaluation of Gombe Formation in the area around Kumo part of Gongola arm was carried out with a view to determine its depositional sub environment in the area through detailed geologic mapping, litholog construction and petrographic as well as sieve analyses of the sandstone samples. Detailed geologic mapping in the study was carried out hence the production of the geologic map of the area. Other pertinent geologic information such as color, texture, sedimentary structures, thicknesses of different beds and their vertical and lateral changes were determined. Four samples each were collected and subjected to laboratory analyses (i.e sieve analysis and petrography). Three stream sections were identified and logged in the field and plotted using sed log computer program. The sections include Kandahar stream section where a sequence of sandstones, mudstone and ironstone of Gombe Formation along a stream channel with a total thickness of 5.2 m is recognized; the GRA stream section where strata of Gombe Formation with a thickness of 2.06 m comprising sandstone, mudstone and ferriginized sandstone caping were logged and the Jauro Bose stream section where strata with a total thickness of 2.0 m comprising of sandstones and ferriginized reddish brown sandstone and mudstone with ferriginized sandstone interbeds. The Gombe Formation in the study area is characterized by yellow to brown sandstones with interbedded mudstones, ferriginized sandstones and ironstones. Grain size analysis indicate that the sandstones of Gombe Formation in the study area is fine to very fine grained, moderately well sorted, strongly coarse skewed and leptokurtic to very leptokurtic. Petrographic results indicate that all the analyzed samples contain large percentages of quartz in excess of 65 – 71 % with greater than 10% matrix and are characterized by angular to subrounded grains of quartz, feldspar and rock fragment and fall within the domain of feldspathic wacke and lithic wacke in the QFL diagram for sandstone classification scheme. Field evidences like interbeds of mudstones within the sandstone beds, flaser beddings, parallel laminations and bioturbations suggest that the sandstones were deposited in a transitional environment influenced by tides. Additional evidence is observed from laboratory analyses results especially sieve analysis that indicate fine to very fine grained sands and moderately well sorting, coupled with the characteristic shapes and the level of maturity obtained from the petrographic results like the large percentage of quartz and the subangular to subrounded grains. Integrating the field evidences and the laboratory results, the depositional sub environment for the Gombe Formation in the study area is interpreted to be intertidal environment of tidal flat of an estuary.

Microfacies and diagenetic evolution of the limestones of the upper part of the Crato Formation, Araripe Basin, northeastern Brazil

Abstract This paper presents the results of a petrographic and diagenetic study of the laminated limestones of the upper part of the Aptian to Albian Crato Formation, northeast of Brazil. The applied techniques were optical microscopy, cathodoluminescence and scanning electron microscopy (SEM) coupled to a wavelength-dispersive spectrometer (WDS). Petrographic analysis has revealed that most of the laminated limestones are calcilutites with a dominance of a micritic matrix, indicating a low-energy depositional environment. Microstructures such as microfaults, microfractures, microslumps, and loop bedding were observed. Based on textural, structural and paleontological features, seven microfacies were recognized: massive limestone, limestone with parallel laminations, limestone with undulated laminations, limestone with slumps, limestone with loop bedding, limestone with ostracods and limestone with peloids. In addition, the processes of cementation, dissolution, replacement, recrystallization and compaction, which are related to different diagenetic stages, were also recognized. The diagenetic constituents found in the sections include calcite, pyrite, silica and sulfates. We can conclude that a large part of the microstructures (microfaults, microfractures, microslumps and loop bedding) can be related to local seismicity, probably due to the reactivation of the Patos Shear Zone. The diagenetic constituents indicate an early to late diagenesis (eogenetic, mesogenetic and telogenetic stage).

The Neostratotype of Itapecuru Formation (Lower-Middle Albian) and Its Impact for Mesozoic Stratigraphy of Parnaíba Basin.

This study defines the neostratotype of the Lower-Middle Albian Itapecuru Formation in the Parnaíba Basin, Northeast Brazil. In this sedimentary succession along the right bank of the Itapecuru River near the Itapecuru-Mirim City, Maranhão State, three lithofacies associations are recognized. The first is silty claystone with some very fine sand, micaceous, reddish color, thin laminated, showing a tabular aspect, interpreted as floodplain fines in a distal position. The second is clayey siltstone, reddish color, with ripple cross-lamination, very thin cross-laminated wacke lenses, grading to thin parallel lamination, in apparently tabular layers, interpreted as crevasse splay and distributive channels deposits in successive avulsion events. The third is fine quartz wacke, reddish to grayish color, showing grouped trough cross bedding, interpreted as channel filling deposits. These facies associations correspond to part of a fluvial depositional system, with predominantly fine-grained rocks, and small lakes in the floodplain, in an oxidizing environment and marked by seasonality. These field data, reinforced by grain size and petrographic analyses showed that, contrary to previous descriptions, the Itapecuru Formation consists mainly of mudrocks. This research also shows that the medium to coarse sandstones and conglomerates, previously included in the Itapecuru Formation, belong to the underlying Grajaú Formation.

Sedimentological and geochemical evidence for seismoturbidite generation in the Kumburgaz Basin, Sea of Marmara: Implications for earthquake recurrence along the Central High Segment of the North Anatolian Fault

Holocene earthquake history of the Central High Segment of the North Anatolian Fault is examined here for the first time based on analysis of seismoturbidites within a 21-m-long piston core recovered from the Kumburgaz Basin in the Sea of Marmara. The visual lithological description combined with detailed grainsize analyses indicate that the deep basin hemipelagic sediments are interrupted by 28 turbidite units during the last 6.1 cal kyrs BP. The turbidites show strong segregation and a sharp boundary between a coarse basal part and overlying homogenite as inferred from detailed sedimentological and geochemical data. Several amalgamated turbidites are recognized by repeated fining upward sequences with no intervening homogenite indicating multiple episodes of traction and deposition as a result of various slope failures and turbidity currents. Each amalgamated unit was possibly triggered by the same earthquake event rupturing in the Sea of Marmara. The most common sedimentary feature is the continuous parallel lamination that was presumably introduced by long lasting water oscillations on suspended sediments due to the seiche effect. The establishment of geochemical criteria and exclusive sedimentary processes distinguish earthquake triggered turbidites (seismoturbidites) from other trigger factors. Moreover, such distinction allows us to evaluate hydrodynamic sedimentary conditions and processes in the Kumburgaz Basin. The base of most seismoturbidites are associated with a sharp increase in Mn concentration that can be explained by a diagenetic enrichment of Mn at the oxic/anoxic interface of the sediments near the seafloor prior to the deposition of the turbidite. An age-depth model of the studied core based on seven AMS 14C ages allows correlation between historical earthquakes and seismoturbidites in the Kumburgaz Basin. At least the latest nine of them fit well with the previously recorded major earthquake events between ca. ~500 cal yrs BP and 2.5 cal kyrs BP.

Morpho-sedimentary characteristics and generated primary sedimentary structures on the modern microtidal sandy coast of eastern Nile Delta, Egypt

Spatial distribution and implication of primary sedimentary structures are combined with morpho-sedimentary features to suggest a sedimentologic–neoichnological model for the low microtidal sandy coast of eastern Nile Delta. Ten linear transects normal to the shoreline were investigated across four to five parallel geomorphic landforms. Intertidal sub-environment is characterized by abundant marine-affinity surface sedimentary structures (such as beach cusps, swash marks, air holes/volcanoes, foam impressions, raindrop imprints, rill marks and tool marks) in addition to the parallel lamination and oscillation ripples. Lower supratidal sub-environment is characterized by parallel lamination, current and wind ripples. Nebkas and wind ripples dominate the nebka/inter-nebka landforms, whereas microbial-induced sedimentary structures which including cyanobacterial mats, efflorescent salt crusts, polygonal cracks and tepee structures prevail in salinas and sabkhatized landform. Marine life-trace assemblage in the intertidal zone, belonging to infaunal and epifaunal bivalves and gastropods, resembles the poor expressions of shallow-marine Skolithos ichnofacies. Predation holes in bivalves and clavate borings in drifted tree trunks are analogous to Oichnus simplex and Teredolites isp., respectively. Transitional marine life-trace assemblage of abundant ghost crab burrows and trackways represents a modern example of seaward suites of the paleoichnological backshore Psilonichnus ichnofacies. Terrestrial life-trace assemblage of beetles, ants, vertebrate tracks and rhizoliths show similarity to the landward expression of Psilonichnus ichnofacies. Despite low to moderate preservation potentiality of modern microtidal sedimentary structures, the preserved structures are invaluable in revealing depositional processes and paleoenvironments of ancient coastal systems.

Shell concentrations analysis in the Lower Permian carbonate rocks (Khan Formation) in Central Iran (Kalmard area)

ABSTRACT Shell beds are sedimentary features that can potentially provide important palaeoenvironmental information. Stratigraphic, taphonomic, and paleontologic features frequency along with their occurrence in the shell beds in carbonate rocks of Khan Formation in Central Iran vary across lithofacies. In Khan Formation, four taphonomic and sedimentologic types of shell beds have been distinguished, which are fair weather wave concentrations (bryozoan dominated shells), storm wave concentrations (articulated brachiopod dominated shells), primary biogenic concentrations (fusulinid dominated shells), and proximal temoestite (including echinoderm dominated, gastropod dominated, bivalve dominated and disarticulated brachiopod dominated shells). Bryozoan shells were formed under strong hydrodynamic energy above fair weather base. The large articulated brachiopod shells are developed below the fair-weather wave-base ; Fusulinid shells formed in lagoon environment of homoclinal carbonate ramp which approve in situ concentration with no transportation. In echinoderm, gastropod and disarticulated bivalve and brachiopod shell beds, the occurrence of disarticulated and convex-up shells in packstone, sharp and erosional base, normal grading, parallel and hummocky lamination in the uppermost part, all indicate proximal tempestites in medium hydrodynamic force between the storm wave-base and fair-weather wave-base. These four types of shell concentrations are found in distinct positions within the depositional sequence of Khan Formation.

Hepaticites(Marchantiophyta) in the Lower Permian Arroyo Totoral Formation (La Rioja Province, Argentina)

In this contribution we present the first record of a liverwort from the Arroyo Totoral Formation in the Paganzo Basin. The sample is preserved as a compression, founded in a greenish-gray shale with parallel lamination, associated with gymnosperms. The delicate thallus differentiated into wings and midrib with elongate cells but lacking evidence of air-pores like those developed in complex thalloid liverworts, suggests that the fossil is a simple thalloid liverwort. The lack of reproductive traits restricted the assignation to Hepaticites sp. This record supports the interpretation of local humid conditions and improves the Gondwanan Paleozoic record of Bryophytes.

Heavy mineral beach placers of the Ordovician Amdeh Formation (Member 4, Wadi Qahza, Saih Hatat, eastern Oman Mountains): Where is the main source area?

We studied an excellently exposed section of the slightly metamorphic siliciclastic Amdeh Formation. The section is near the base of the Amdeh Member 4 (Am4). Am4 is of Early to Mid-Ordovician age and 1677 m thick. Two lensoid placers were examined for their mineral composition (X-ray diffraction, transmitted and reflected light microscopy), geochemistry (X-ray fluorescence), in-situ gamma-ray emissions and the sedimentary structures of the host rocks. The placers were deposited in a beach environment as indicated by sedimentary structures in the associated quartzites, such as parallel lamination, parting lineations and sand volcanoes and as indicated by the compositional and textural maturity of the host rock and the placers. Placer deposition may have been favored by rising sea-levels. Overall, the dark placers contain the following heavy minerals: anatase (up to 35–40%) > ilmenite > zircon > hematite (a few %) > and apatite (<1%, may be present in trace amounts as indicated by low phosphorous contents in XRF analysis). The presence of monazite is indicated in our study by LREEs plus yttrium. Due to the high amounts of ilmenite and anatase, the placers' titanium contents are high (up to 20.5 wt%). In both placers, the concentrations of the light rare earth elements (LREEs) lanthanum, cerium, praseodymium and neodymium plus yttrium (geochemically affine to HREEs) are noteworthy (between ∼3000 and 7000 ppm). The source rocks can be expected in metamorphic and acidic plutonic rocks in the basement of Oman, which may have shed detritus from exposed basin flanks, but other potential source areas lay to the north and south of the basin. A source area with pre-enriched quartz in sandstone to explain the massive quartz influx would also be possible. The coastal/shallow shelf facies of Am4 shows very little bathymetric variability despite its thickness, which requires rapid and continuous subsidence. We attribute basin subsidence to rifting, withdrawal (and subrosion?) of salt in the subsurface (Ara Formation) due to sediment loading and possibly to gravity-driven subsidence due to the presence of dense ophiolite units below the Ara Salt. The absence of the Amdeh Formation in the west (Jabal Akhdar Dome) may be explained by rift shoulder uplift. While the Amdeh Formation accumulated in the rift basin, the Jabal Akhdar region occupied parts of the northwestern rift shoulder.