Lagoonal Facies Research Articles

Palaeogene marine stratigraphy in China

Palaeogene deposits are widespread in China and are potential sequences for locating stage boundaries. Most strata are non-marine origin, but marine sediments are well exposed in Tibet, the Tarim Basin of Xinjiang, and the continental margin of East China Sea. Among them, the Tibetan Tethys can be recognized as a dominant marine area, including the Indian-margin strata of the northern Tethys Himalaya and Asian-margin strata of the Gangdese forearc basin. Continuous sequences are preserved in the Gamba–Tingri Basin of the north margin of the Indian Plate, where the Palaeogene sequence is divided into the Jidula, Zongpu, Zhepure and Zongpubei formations. Here, the marine sequence ranges from Danian to middle Priabonian (66–35 ma), and the stage boundaries are identified mostly by larger foraminiferal assemblages. The Paleocene/Eocene boundary is found between the Zongpu and Zhepure formations. The uppermost marine beds are from the top of the Zongpubei Formation (~35 ma), marking the end of Indian and Asian collision. In addition, the marine beds crop out along both sides of the Yarlong Zangbo Suture, where they show a deeper marine facies, yielding rich radiolarian fossils of Paleocene and Eocene. The Tarim Basin of Xinjiang is another important area of marine deposition. Here, marine Palaeogene strata are well exposed in the Southwest Tarim Depression and Kuqa Depression. They comprise mostly neritic and coastal lagoon facies of the Tethyan realm. Palaeontological evidence suggests that the Paleocene/Eocene boundary here is in middle of the Qimugen Formation. The Tarim Basin was largely drained by Late Oligocene. To the east, the marine offshore Palaeogene strata are widespread in the North Taiwan and East Zhejiang depressions of the continental shelf basin of East China Sea. Abundant fossils including foraminifera, calcareous nannofossils, ostracods, pollen and bivalves occur in the marine environment. Biostratigraphically, the sequence is well correlated with the international planktonic foraminiferal and nannofossil zonations.

Stratigraphy, facies, and depositional environments of the Paleogene sediments in Cairo–Suez district, Egypt

The study of eight stratigraphic sections along Cairo–Suez district, between Gebel Ataqa and Gebel Mokattam, reveals a thick Lower Eocene Ypresian carbonate section, approximately 210 m thick, at Gebel Abu Treifiya just to the west of Gebel Ataqa. It is represented by the Minia Formation rich in Nummulites praecursor, Orbitolites pharaonum Schwager, Alveolina frumentiformis Schwager, and one of the precursors of Nummulites gizehensis group. The Minia Formation is a fairly clear, warm, and shallow marine facies. The Middle Eocene Lutetian sediments are totally missing in all studied sections probably reflecting instability in deposition echoed in the active block movements the area witnessed since the Paleozoic. The Bartonian sea transgressed over the area depositing Gebel Hof Formation at the base, Observatory Formation in the middle, and Qurn Formation at the top. The Gebel Hof Formation was deposited in an open-marine environment, passed upward into shallow marine, neritic to reefal facies; for the Observatory and Qurn formations, the former changed laterally into a sheltered lagoon facies, Sannor Formation, rich in Somalina stefaninii Silvestri, Dictyoconus aegyptiensis Chapman, and Idalina cuvillieri Bignot and Strougo. The Upper Eocene sediments, Maadi Formation, are mainly represented by a carbonate–clastic section showing shallowing-upward cycles resulted by the retreating of the sea shoreline northward during the Late Eocene, with high supply of terrigenous sediments. The lower carbonate cycle of the Maadi Formation was deposited in a restricted platform and tidal flats, whereas the upper siliciclastic cycle containing Carolia placunoides banks was probably deposited in a winnowed platform edge. These depositional environments were developed in grabens, ramps, and footslopes of the down-faulted Middle Eocene blocks, i.e., syn-tectonic deposition. The fluviatile Oligocene sediments were highly controlled by the structural and topographic lows, where a substantial thickness was deposited, Gebel Ahmer Formation, occupying several grabens and gently sloping areas between many synthetic faults.

Time‐lapse seismic modelling of CO2 fluid substitution in the Devonian Redwater Reef, Alberta, Canada

ABSTRACTCommon shot ray tracing and finite difference seismic modelling experiments were undertaken to evaluate variations in the seismic response of the Devonian Redwater reef in the Alberta Basin, Canada after replacement of native pore waters in the upper rim of the reef with CO2. This part of the reef is being evaluated for a CO2 storage project. The input geological model was based on well data and the interpretation of depth‐converted, reprocessed 2D seismic data in the area. Pre‐stack depth migration of the ray traced and finite difference synthetic data demonstrate similar seismic attributes for the Mannville, Nisku, Ireton, Cooking Lake, and Beaverhill Lake formations and clear terminations of the Upper Leduc and Middle Leduc events at the reef margin. Higher amplitudes at the base of Upper‐Leduc member are evident near the reef margin due to the higher porosity of the foreslope facies in the reef rim compared to the tidal flat lagoonal facies within the central region of the reef. Time‐lapse seismic analysis exhibits an amplitude difference of about 14% for Leduc reflections before and after CO2 saturation and a travel‐time delay through the reservoir of 1.6 ms. Both the ray tracing and finite difference approaches yielded similar results but, for this particular model, the latter provided more precise imaging of the reef margin. From the numerical study we conclude that time‐lapse surface seismic surveys should be effective in monitoring the location of the CO2 plume in the Upper Leduc Formation of the Redwater reef, although the differences in the results between the two modelling approaches are of similar order to the effects of the CO2 fluid replacement itself.

Microfacies, Diagenetic and Depositional Environment of Kazhdumi Formation (Aptian-Albian), Dezful Embayment, Zagros, NW Iran

Facies determination of the Kazhdumi Formation in Solabdar and Chilingar oil wells which are the oldest Iranian oil wells is very important since the Kazhdumi was known as a source rock in these fields. Lithological composition of this formation is limestone, shale and glauconite bearing sandstone in the base which is an obvious sign of disconformity between Kazhdumi and Dariyan formations. The Diagenetic effective processes in this area include: dissolution, substitution (hematitization, glauconitization), micritization and bioturbation, cementation, compaction, stylolitization and porosity (interapartical, moldic, intercrystaline, channel). These processes are in relation to diagenetic environments such as meteoric, marine phreatic and deep burial. Kazhdumi Formation in the studied fields is composed of seven carbonate facies and two clastic facies (shale and sandstone) as follows: bioclast wackestone/ packstone- bioclast packstone– bioclast wackestone/packstone–sandstone (lagoon facies) and bioclast wackestoneglobigerin, wackestone (planktonic wackestone)/crinoidal globigerina wackestone –oligosteginid wackestone (open marine facies). By considering these facies, it is suggested lagoon and open marine sedimentary environments for Kazhdumi Formation in the studied fields.

Carbon oxygen isotope analysis and its significance of carbonate in the Zhaogezhuang Section of Early Ordovician in Tangshan, North China

In the Early Ordovician Zhaogezhuang (赵各庄) Section of Tangshan (唐山), North China, the Yeli (冶里) Formation is composed of an entire third-order sequence, with facies ranging from the inner ramp restricted platform and open marine to the middle and even outer ramps. The Liangjiashan (亮甲山) Formation is dominated by highstand system tracts (HST) with predominantly inner ramp grain-shoal and lagoon facies. Analyzing the carbon and oxygen isotope during the whole-rock carbonate reveals the δ13C values in the Yeli Formation range from −7.11‰−0.76‰ (PDB), with the mean value at −2.98‰, while the δ18O values range of −9.09‰–−4.65‰ with the mean value at −6.12‰. The δ13C values in the Liangjiashan Formation range of −1.15‰−0.3‰, and the mean value of −0.57‰; the δ18O values are −8.76‰–−7.48‰, and the mean value is −8.06‰. The δ13C values in the Yeli Formation decrease, but at the bottom of the Liangjiashan Formation the values increase steadily. In the middle-upper formation, there is an extended fluctuation between 0–−1.00‰. The δ13C trend in the studied section is similar to that of the contemporary sections, except that it has much lower δ13C values and a more negative excursion. The correlation between the δ13C changes and the eustatic events, as well as the sedimentary facies, indicates that in the Tangshan area, the carbon isotope evolution can be attributabled to the processes of the eutrophic sea/oligotrophic sea, the seafloor organism-mediated oxidation in shallow water and the organic reduction after maximum flooding. The changes in the carbon isotope contents were primarily affected by the regional relative sea level changes. Compared to the other coeval data, the Early Ordovician of the Tang shan area is also severely depleted in 18O, with all of the δ18O sample values being below −5‰, except for one sample with a value of -4.65‰. With the ancient seawater having a δ18O value of −5.5‰ (SMOW), it is reasonable to delineate a temperature of less than 37 °C.

Carbonate platform development in a Paleoproterozoic extensional basin, Vempalle Formation, Cuddapah Basin, India

Sedimentological investigation of the late Paleoproterozoic (Orosirian) Vempalle Formation of the Cuddapah Basin, Dharwar craton, India, reveals three facies association that range from supratidal to deep subtidal. Sedimentary rocks of this succession are dominated by heterolithic carbonate mudstone, intraformational carbonate conglomerate, and a variety of columnar, domal, and stratiform microbialite facies. Deposition occurred in an extensional regime during development of a low-gradient ramp, where the distribution of microbialite facies is distinctly depth-partitioned. A gradual increase in synoptic relief of columnar stromatolites through the section, and the upward transition from stratiform to columnar microbialites, record a prolonged marine transgression with little or no influx of terrigenous detritus. Siliciclastic influx along the northeastern side of the shelf reflects the redistribution of topographic highs concomitant with large scale volcanic activity. Redistribution of topographic highs eventually led to progradation of peritidal facies and shutting down of the carbonate factory. Earthquake-induced ground shaking and voluminous volcanism experienced by this platform point to the reactivation of a deep-seated mantle-plume that resulted in thermal doming of the Dharwar crust prior to the onset of Cuddapah deposition. Isotopic and elemental chemistry of a selection of Vempalle Formation carbonate rocks record elevated Mn2+ and Fe2+ concentrations and depleted carbon isotope values in inner ramp lagoonal facies, relative to more open marine stromatolitic facies. Patterns of isotopic and elemental variation suggest the presence of geochemically distinct water masses—either within the water column or within substrate pore fluids—that resulted from a combination of globally low marine oxygenation and restricted oceanographic circulation in inner ramp environments. These data suggest that, even in the aftermath of Early Paleoproterozoic oxygenation, that ocean chemistry was heterogeneous and strongly affected by local basin conditions.

Facies analysis and sequence stratigraphy of the Asmari Formation in the northern area of Dezful Embayment, south-west Iran

This paper concerns the sequence stratigraphy of the Oligocene (Rupelian–Chattian)–Early Miocene (Burdigalian) Asmari Formation based on microfacies analysis of the sediments in the Ghale Nar Oilfield, central area of Zagros fold-thrust belt. Fourteen facies types typical for upward shallowing trend from open marine (MF 1–3), to shoal (MF 4-5), semi-restricted and restricted lagoon (MF 6–12) and finally to near-shore lagoon (MF 13-14) depositional environments were identified. Based on the environmental interpretations, we reconstructed a homoclinal ramp mainly represented by its inner and middle sectors. Facies types MF 4–14 are characterized by the occurrence of large and small porcelaneous benthic foraminifera representing a shallow-water setting of an inner ramp influenced by wave and tide processes. MF1 (Pabdeh Formation), and 2 -3 with planktonic, large and small hyaline benthic foraminifera represent between FWWB and SWB. Four third-order depositional sequences were recognized. In this study, in order to better correlate the identified sequences with pre-defined sequences of the Asmari Formation in the Dezful Embayment and the Izeh zone that are chronologically well-defined, we use their number instead of their name. Sequence 2 mainly consists of an open marine environment in the base, followed by semi-restricted to restricted lagoonal facies. Sequences 36 are characterized by semi-restricted to restricted lagoonal facies. Moreover, the regional relative sea-level change curves correlate well with the global sea-level change curves.

Ostracods and rock facies across the Givetian/Frasnian boundary interval in the Sourd d’Ave section at Ave-et-Auffe (Dinant Synclinorium, Belgium)

Ostracods from the Sourd d'Ave section have been collected in the Moulin Boreux and Fort Hulobiet Members (Fromelennes Fm., Givet Group) and in the Pont d'Avignon Member (Nismes Fm., Frasnes Group). Ostracods collected in the Fromelennes Fm. by Milhau (1983a) and in the Nismes Fm. by Casier (1987a) have been also reviewed. Forty-four ostracod species are identified in the Fromelennes Fm. and 25 in the Nismes Fm. They belong exclusively to the Eifelian Mega-Assemblage, and several assemblages indicative of restricted and shallow marine, sometimes agitated, environments are recognized in the Fromelennes Fm. The great rarity of ostracods in the upper part of this formation provides evidence for less favourable lagoonal conditions probably related to increasing aridity at the end of the Givetian. In the Frasnes Group, assemblages are exclusively open marine and indicative of increasing water depth. The majority of ostracod species recognized in the Givet Group are missing in the base of the Frasnes Group as a consequence of the Frasnes Event. A systematic list of ostracods identified in the Fromelennes Fm. at Sourd d'Ave is published as an annex. Systematic sampling has been carried out in order to establish the sedimentological evolution of the environments and to detail the Givetian-Frasnian (G/F) transition. This allowed recognition of 13 microfacies types that replicate the standard sequence of Preat & Mamet (1989) from open marine shallow subtidal to restricted supratidal near emersion. The Boreux Member and the Fort Hulobiet Member display restricted facies (Amphipora, spongiostromid and algal bafflestones and bindstones, loferites with desiccation lumps) with poorly fossiliferous beds interbedded with higher energy peloidal and sometimes oolitic grainstone facies. Laminite horizons, sometimes with small-sized lateral linked hemispheroid stromatolites are uncommon, and are associated with dolomicrites showing pseudomorphs of evaporite minerals. These evaporitic facies become common in the upper part of the Fort Hulobiet Member suggesting the palaeoclimate became more arid at the G/F transition. Metre-scale cyclicity is pervasive throughout the Givetian part of the section. The boundary between the Givet Group and the Frasnes Group is very distinctive in the field, and is characterized by a transition from restricted evaporative lagoonal facies to open marine interbedded marly shales and nodular limestones. The upper part of the Fort Hulobiet Member consists of interbedded biostromes (semi-restricted stromatoporoid boundstones) followed by Amphipora floatstones, then fossil-poor units and restricted supratidal laminites with well-developed fenestral fabrics. The Frasnian Pont d'Avignon Member contains a rich faunal assemblage (bryozoans brachiopods, molluscs, nautiloids, tentaculitids) suggesting an abrupt drowning from the marginal Givetian carbonate platform into a Frasnian distal ramp or deep basinal environment below or near storm wave base.

Neoproterozoic sedimentary facies and glacial periods in the southwest of Tarim Block

Based on the analysis of sedimentary facies and chemical index of alteration (CIA) in Nanhua and Ediacaran Systems in the southwest of Tarim Block, some features of glacial records in Neoproterozoic become more clear. Six sedimentary facies have been divided in the study area, including alluvial fan facies, lacustrine facies, glacial facies, littoral facies, neritic facies, and lagoonal facies, showing that this area underwent a process from continent to marine, with mainly littoral and neritic sedimentation. Two cold events have been recognized by analysis of CIA values in the study area, called Bolong and Yutang glaciation, respectively. They present as thick-layer tillite deposition in the Bolong Formation and thin-layer tillite deposition in the Yutang Formation, respectively. The Bolong glacial period in the study area can be correlated to the Yulmeinak glacial period in Aksu area, Tereeken glacial period in Qurugtagh area, and the Nantuo glacial period in South China, which is equivalent to the universally acknowledged Marinoan glacial period. The Yutang glacial period can be correlated to the Hankalchough glacial period in Qurugtagh, which is equivalent to Gaskers glaciation in Newfoundland.

Dolomite formation in the shallow seas of the Alpine Triassic

AbstractThe Alpine Triassic units of Switzerland, Northern Italy and Western Austria offer an extensive geological archive, in which the enigmatic process of dolomite formation can be studied in a palaeoenvironmental context. Recent studies clearly demonstrate that large amounts of the Alpine Triassic dolomites are late diagenetic or hydrothermal. Nevertheless, as part of multiple generations of diagenetic overprint, some generations of fine‐crystalline, Ca‐rich dolomite appear strictly confined to their depositional facies and show signs of very early formation at surface temperatures in specific ancient depositional environments. In this review, three cases of Alpine Triassic dolomites are discussed, where dolomite rocks may have formed during or soon after sedimentation. The sedimentary facies indicate contrasting palaeoenvironmental conditions and, hence, document three different possible processes of dolomite formation: (i) In the Dolomite Mountains (Northern Italy), dolomite beds of the partly isolated Middle Triassic (Anisian/Ladinian) Latemar Platform are confined to the very top of shallowing‐upward lagoonal facies cycles. (ii) Dolomite beds of the San Giorgio Basin (Southern Switzerland), an intra‐platform basin that opened during the Anisian/Ladinian transition, are associated with organic carbon‐rich shales, which were deposited in a deeper water environment under anoxic conditions. (iii) In the entirely dolomitized platform facies of the Dolomia Principale (Hauptdolomit Formation), a very early generation of fine‐crystalline dolomite occurs in the shallowest part of evaporative peritidal cycles. This platform extended over thousands of square kilometres along the Tethys margin during the Late Triassic (Carnian and Norian) and large amounts of carbonate were deposited under hypersaline sabkha‐like conditions. Representing three distinct depositional environments, these three different Triassic systems show features in common with several dolomitization models developed from the study of modern dolomite‐forming environments; for example, the sabkha model, the evaporative lagoon/lake model, the organogenic model and the microbial model. Although these actualistic models may be applicable to reconstruct the palaeoenvironmental conditions during dolomite formation, dolomite‐forming processes during the Triassic were apparently quite different from the modern world in terms of distribution and scale. Recent developments in stable‐isotope geochemistry and high‐resolution geochemical probing offer the possibility to make better reconstructions of Triassic palaeoceanographic conditions and suggest a non‐actualistic approach to better understand dolomite formation during the Triassic.

Depth transect of an Upper Triassic (Rhaetian) reef from Gosau, Austria: Microfacies and community ecology

In the Late Triassic (~235–201.3Ma) scleractinian corals and hypercalcified sponges built large, diverse reef ecosystems, the most famous of which are the Dachstein reefs. This study presents a depth transect along an Upper Triassic (lower Rhaetian) Dachstein reef from the Gosausee margin of the Dachsteingebirge (Gosau, Austria). The Gosausee microbial-sponge-coral reef is a complete barrier reef with an almost continuous fore reef to lagoon transect preserved (a very rare occurrence for the Triassic), and thus provides a window into depth zonation of Dachstein-type reef facies and biotic succession. The Gosausee reef facies exhibit strong depth control and five classic reef facies or zones are identified: the fore reef, reef front, reef crest, back reef, and lagoon facies. Thin, rare microbial fabrics and a high abundance of fine-grained, mud-rich skeletal wackestones (transported reef debris) characterize the deepest fore reef. As the reef shallows, muddy sediments decrease in abundance and are replaced by microbial fabrics, corals, and cements. Abundant sponges, microbial crusts, and thick, marine cements typify the reef crest, whereas microbialite-coated phaceloid corals are dominant in the back reef facies. Heavily cemented oncoids or microbial-sponge bindstones are characteristic of the lagoon. Based on their compositional and biotic similarities, the Gosausee reef was likely part of the same barrier reef systems as the source reef for the Gosaukamm reef breccia (one of the classic Norian–Rhaetian Dachstein reefs). The reef zones of the Gosausee margin can be used to interpret the depth or reef zone of less well preserved reef fragments, can inform models of community ecology and niche utilization in the Late Triassic, and highlight the need for additional research into the environmental factors that controlled biotic distribution in Upper Triassic reefs.

A Late Carboniferous algal mound from the Tarim Basin, NW China: internal structure and palaeoecology

A Beresella‐dominated algal mound and its internal construction from Paojianggou, southwestern Tarim Basin, NW China are studied for the first time. This algal mound was constructed during the early–middle late Moscovian and its age is constrained by fusulinids of the Fusulina kamensis and F. quasicylindrica zones of middle Pennsylvanian age. The mound substrate beds are composed of crinoid and brachiopod grainstones. Crinoid bank and brachiopod‐dominated skeletal shoals indicate a high‐energy environment and produced a local topographic high on which the mound grew. Komia bafflestone of the foremound facies further increased the relief of the crinoid bank on the seafloor. The lower part of the mound framework is dominated by Shamovella–Archaeolithoporella boundstone, while the main (upper) mound framework consists predominantly of Beresella boundstone. Grainstone–packstone characterizes the windward mound flank facies. The mound capping facies include a lower skeletal packstone–wackestone and an upper lagoon facies marlstone and black shale. Various algal colonies performed different roles in the formation of the mound. Komia colonies were the mound constructors that baffled higher energy currents and entrapped fine sediment. Archaeolithoporella, Shamovella and possibly microbes (cyanobacteria) acted as the binders to stabilize mud and fine‐grained sediment. Beresella colonies were the major framework constructors whose network entrapped the mud to build the mound.The growth of the Paojianggou mound in the Tarim Basin has undergone three major algal colonization stages: Komia baffling, laminar Archaeolithoporella and Shamovella encrusting and binding, and Beresella binding and stabilizing. The Paojianggou mound developed in a protected lagoonal position behind the shoals and grew during a short rise in sea level followed by a long‐period sea‐level fall. Its demise was caused by terrigenous siliciclastic input due to regional tectonic uplifting in the Tarim Basin during the latest Moscovian time. Copyright © 2012 John Wiley & Sons, Ltd.

Magnetic susceptibility and its relationship with paleoenvironments, diagenesis and remagnetization: examples from the Devonian carbonates of Belgium

To better understand the origin of the initial magnetic susceptibility (χin) signal in carbonate sequences, a rock magnetic investigation that includes analysis of acquisition curves of the isothermal remanent magnetization (IRM) and hysteresis parameters, was undertaken on Devonian carbonates from the Villers and Tailfer sections, Belgium. Both sections are divided into a lower unit, dominated by biostromal and external ramp facies (biostromal unit) and an upper unit, only consisting of lagoonal facies (lagoonal unit). The variations in χin signal are mainly driven by magnetite variation, mostly pseudo-single-domain (PSD) magnetite. Clay minerals, pyrite, hematite and obviously calcite and dolomite are also present but their contribution to the χin pattern is not significant. There is a correlation between detrital proxies (Zr, Rb, Al2O3, TiO2) and χin for the Tailfer biostromal unit and the entire Villers section. The pervasive presence of fine-grained magnetite is interpreted as related to remagnetization. In absence of external fluids, the iron released during the smectite to illite transition remains in situ. In those situations χin may reflect an inherited primary synsedimentary signal. In the lagoonal unit of the Tailfer section, remagnetization appears to have obscured the original detrital information prompting the need for an evaluation of the composition of the susceptibility signal for individual case studies.

Study on Sedimentary Facies of Carboniferous Kalashayi Formation in the South of Tahe Oilfield

Tahe Oilfield is an significant petroleum exploitation and development area in Tarim Basin for SINOPIC. Carboniferous Kalashayi Formation has become an important oil-bearing series for increasing and maintaining production, and for finding backup resource in this Oilfield. Based on the theories and methods of sedimentology, by using the sesmic, log and core data, the sedimentary facies of Carboniferous Kalashayi Formation in the south of Tahe Oilfield are analyzed deeply. Through the study of characteristics of the rock type and texture,sedimentary structure, sedimentary sequence, stratic structure, paleontology, geochemistry and geophysics of Kalashayi Formation, the Sand-shale Section is attributed to tidal flat deposits which has a nature of estuary. The study area are mainly dominated by intertidal zone, including mud flat, sand-mud flat, sand flat, tidal channel and mouth bar microfacies. The Up-mudstone Section developed lagoonal facies. This study provides favorable support for the further petroleum exploration in study area.

A new karren feature: hummocky karren

Karren are small-scale landforms on karst surfaces and many types have been described so far. Here we present an apparently new feature which was found on the Hochschwab karst massive in the Northern Calcareous Alps of Austria. So far only few outcrops each having less than 1 m² within a very restricted area have been found. Morphometric analysis reveals that the karren consist of a randomly distributed, dispersed assemblage of small hummocks and depressions in between. The mean distance between neighbouring hummocks is 4 to 5 cm and the mean height is 0.85 cm. Longitudinal sections are gently sinuous. The occurrences are delimited by thin soil cover with grassy vegetation. The karst features continue below that vegetation cover. Therefore, it is clear that the karren have formed subcutaneously. Corroded fissures where water could infiltrate into the epikarst are absent. The bedrock lithology is Middle Triassic limestone of the Wetterstein Formation in lagoonal facies. Geological structures do not govern the feature. The surface is not a bedding plane and small joints and fractures do not govern the arrangement of the hummocks. Thin section analysis regarding rock texture and dolomite components show that there is no compositional difference between hummocks and depressions. Geochemical analyses show that the limestone is very pure with a very low content of Magnesia. Slightly higher Magnesia contents at the hummock surfaces are significant. The data obtained so far only indicate that some dissolution mechanism but not any rock property governs the irregular array. As there exist no descriptions of comparable features in literature, the name “hummocky karren” is suggested for that type of karren landform.

Biotically constrained palaeoenvironmental conditions of a mid-Holocene intertidal lagoon on the southern shore of the Arabian Gulf: evidence associated with a whale skeleton at Musaffah, Abu Dhabi, UAE

Whale remains (a left and right mandible, scapula, humerus and fragmentary radius and ulna as well as parts of the cranium and rostrum) belonging to a probable humpback whale ( Megaptera cf. novaeangliae) were found in the well-described sabkha sequence exposed in the Musaffah Industrial Channel, Abu Dhabi, United Arab Emirates. More precisely, the whale remains were found in a series of sediments representing a range of lagoonal facies. The sediments surrounding the whale bones were age-dated at approximately 5200 14C yrs BP and are therefore interpreted to correspond to the previously documented late Flandrian sea-level peak, preceding a fall in sea-level which culminated in the supratidal sabkha overprint of the carbonates. Associated with the whale remains is an assemblage of molluscs, foraminifera and ostracods. Together with the inferred presence of sea grass and algae, these facies are interpreted to indicate a very shallow subtidal to intertidal lagoonal environment. Cirripede remains found associated with the skeleton were identified as those of the whale barnacle Coronula diadema and hence had their origins with the whale. Significantly, the low species diversity of microfossils suggests that higher salinities existed in the mid-Holocene lagoon than are present in modern counterparts. This is here inferred to be related to the onset of continental aridity in Arabia during the mid-Holocene.

Mesozoic deltaic system along the western margin of the Indian plate: lithofacies and depositional setting of Datta Formation, North Pakistan

Over 1 km thick Mesozoic sedimentary sequence is exposed over a wide area in the Upper Indus basin of north Pakistan along the western margin of the Indian Plate. The Mesozoic sequence is comprised of clastic facies in the lower part, while carbonate facies are dominant in the upper part. About 200 m thick mixed sequence of interbedded sandstone, siltstone, clay, and carbonaceous shale represents the lower Jurassic Datta Formation in the Salt and Trans Indus Ranges in North Pakistan. The Datta Formation constitutes important reservoir horizons in a number of oil fields in the western Himalayan foreland basins where it is encountered at a depth of about 4 km in various wells. The Datta Formation is described from different parts of the range front to understand the internal architecture of various sedimentary facies and their depositional system. The thickness and lithofacies assemblages of the Datta Formation change in different parts of the range front as well as in subsurface of the Upper Indus basin. The Datta Formation represents a coarsening upward deltaic sequence in most parts of the basin. On the basis of lithological variations and sedimentary structures, a number of depositional facies have been recognized which include channel belt facies, floodplain/abandoned channel facies, swamp facies, and lagoonal facies. Further north, in the Kalachitta and Hazara regions, the siliciclastic facies change to more complex assemblages of interbedded bauxite, silcrete, marl, and some limestone. These sediments represent deposition in a delta-plain setting of a fluvial-dominated delta with northwestward flowing channels.

Early dolomitization of a Lower Cretaceous shallow water carbonate platform: was microbial activity a major controlling factor?

Early dolomitization of carbonate rocks has long been problematic as precipitation experiments at room temperature have shown that dolomite will not be precipitated chemically under Earth surface conditions. To explain the widespread occurrence of early dolomite in the geological record, bacterial mediation of dolomitization has been proposed. The strongest evidence comes from modern hypersaline environments where dolomitization was shown to take place under anoxic conditions. In this study, part of the Qatar Carbonates and Carbon Storage Research Center (QCCSRC), we investigate whether microbial mediation of early dolomitization took place on the Barremian-Berriasian carbonate platform of the Jurf Formation of Southern Oman. The sediments are characterized by a finely crystalline, poorly ordered dolomite present in various proportions. The amount of dolomite seems to be controlled by facies with tidal flats and algal/microbial facies being totally dolomitized and restricted lagoonal facies being only partially dolomitized. Results indicate that the dolomite is penecontemporaneous to sedimentation and that the dolomitizing fluids are low temperature seawater formed in an evaporative lagoon. Thus, reflux mechanisms played a role in the process of dolomitization. One of the major questions of our study is what controls the heterogeneity of the distribution of dolomite on this platform. We hypothesize that bacteria present in tidal flats and algal/microbial mats facies played an important role in controlling dolomitization. Our goal is to highlight the role of biomineralization in controlling the early diagenesis of the Jurf Formation. To test this we will use a multi-proxy approach using mineralogy, trace elements, stable and clumped isotopes, and SEM analysis, in an integrated field, petrographic and geochemical study of a Barremian-Berriasian carbonate platform exposed in the Haushi-Huqf area. A bed-by-bed sampling was done for two 15-meter thick outcrop sections separated by one kilometer. Initial results highlight the importance of considering the presence of microbial activity to build more accurate dolomitization models and in assessing the lateral heterogeneity of dolomite bodies.

Latitudinally different responses of Tethyan shoal-water carbonate systems to the Early Aptian oceanic anoxic event (OAE 1a)

Ocean-wide anoxic events represent intensively investigated anomalies in the global carbon cycle. Most previous research has focussed on hemipelagic and pelagic settings and on the relationship between black-shale deposition and carbon-isotope excursions. The study of ocean-wide anoxic events and coeval shallow-water settings is now increasingly seen as an interesting complementary approach, but one that is not without problems. Whereas platform drowning characterizes the Early Aptian of the northern Tethyan margin, Lithocodium–Bacinella-rich facies and ongoing shoal-water sedimentation at the southern Tethyan margin (Oman) bears important information on potential causes of carbon-cycle perturbations. The present paper seeks to test the supra-regional relevance of the Oman data by investigating coeval central Tethyan limestones. Three Lower Aptian shoal-water sections in Istria (Croatia), deposited on the isolated Adriatic Carbonate Platform, are investigated applying chemostratigraphy (carbon and strontium) and detailed sedimentological analysis. The focus is on peritidal to lagoonal facies characterized by mass occurrences of Lithocodium–Bacinella, an enigmatic microencruster community. Lithocodium–Bacinella facies occurs predominantly in layers ranging from one to several centimetres in stratigraphic thickness, with several layers merging to metre-thick packages. Growth fabrics within the layers include oncoidal morphotypes, lumps, interconnected patches and columns, layers and rare nodular to massive bindstone facies. These growth patterns show a remarkable regional extent and consistency over study sites distributed several kilometres apart. This widespread distribution suggests that specific Lithocodium–Bacinella morphotypes might serve as regional stratigraphic markers. The high-resolution carbon-isotope chemostratigraphy presented here is based on pristine rudist shells and matrix micrite samples and calibrated against strontium-isotope data obtained from screened rudist low-Mg calcite. The chemostratigraphic data are consistent with existing biostratigraphic data and place the studied strata at the onset of Early Aptian oceanic anoxic event 1a. Moreover, results indicate the near-coeval nature of Lithocodium–Bacinella bloom facies in Istria and Oman. The outcomes of this study point to latitudinally different responses of Tethyan shoal-water carbonate systems (platform drowning versus Lithocodium–Bacinella blooms) to the ocean-wide anoxic event 1a.

The sedimentary structure of Upper Pleistocene–Holocene deposits in Venice and its effects on the stability of the historic centre

The results of a recent study (Geologia e Progettazione nel centro storico di Venezia”, Secondo Convegno Nazionale: La riqualificazione delle città e dei territori, Venezia 7 dicembre 2007, Quad. IUAV 54, Il Poligrafo ed. 2008) focused on Venice’s historic centre have led to the identification of a peculiar sedimentary structure (multistorey sandbody). This structure was created during the Upper Pleistocene by the multiple overlapping of sandbodies corresponding to stream channels, all of which crossed the area where the city now lies. The erosion and deposition events responsible for these stratigraphic conditions are the effects of both climatic variations during the Wurmian glaciation and changes in the fluvial regime. The sedimentary structure was completed during the Holocene by the deposition of sand in fluvial and tidal channels on top of the previous Pleistocene stream channel sediments. The deposits of this structure are heteropic with those of the rhythmic succession (cyclothemic organization) belonging to the alluvial plain of the Upper Pleistocene and the Holocene lagoon facies of tidal plain found in the surrounding area. The geological model of the city is able to explain the mechanical behaviour of the soil in the western and eastern areas of the historic centre where the losses in elevation are considered at present as a consequence of the urban development in the last centuries, since the vertical movements due to groundwater exploitation of the deep aquifers have more than likely ended. According to a methodology known as operational facies concept, which first determines the facies relationship, then discriminates the intervals with lithological variability and finally fits together the information coming from the terrain properties, it can be inferred that the losses in elevation are related to the evolution of the deformation processes acting nowadays in the urban settlement. The results of the aforementioned study reveal, in fact, that these processes are provoked by the residual component of a long-term secondary consolidation and by the geochemical subsidence which occurs between salt water intrusion and clay containing organic matter and peat.