Neritic Shelf Research Articles

Late Albian-Cenomanian paleoenvironmental and paleoclimatic reconstructions in northeastern Gondwana: Palynological perceptions

The late Albian-Cenomanian (100.5–93.9 Ma) is considered a greenhouse world without permanent ice caps and with high eustatic sea levels. The high sea-level led to the flooding of low-lying coastal plains and the formation of extensive shallow shelf seas. Depositional conditions and ecosystems at these marginal and shallow shelf settings were affected by changes in environmental conditions, such as climate and vegetational ecosystems in surrounding regions. This study conducts palynomorph and palynofacies and lithofacies analysis of fifty-nine cutting samples from the North Qarun-1x well in the Gindi Basin (North Western Desert, Egypt) to assess the evolution of depositional and paleoenvironmental conditions as well as paleo-vegetation pattern in response to changes in climate and sea level. The sediment succession comprises the upper Albian to Cenomanian upper Kharita and Bahariya formations and Abu Roash G Member. Biostratigraphic analysis of palynomorph composition led to define seven interval zones of upper Albian-Cenomanian. At this time, Egypt was inundated by the southern margin of the Tethys Ocean, which led to the deposition of thick siliciclastic and carbonate facies in a marginal marine depositional environment. Statistical cluster analysis of particulate organic matter (POM) allowed three palynofacies assemblages (PFA-1 to PFA-3) to be recognized, indicating oscillating depositional conditions that shifted between fluvio-deltaic to marginal shallow marine, proximal, and distal inner neritic shelf environments. PFA-1 comprises the highest proportions of total phytoclasts, indicating fluvio-deltaic to marginal shallow marine conditions. PFA-2 is dominated by moderate abundances of AOM and phytoclasts, characterizing a proximal inner neritic shelf environment. PFA-3 consists of a slightly higher concentration of AOM compared to phytoclasts, indicating a distal inner neritic shelf environment with diminished terrestrial input and increased abundances of marine microplankton. Organic palynomorphs are abundant in mid-Cretaceous strata of northern Egypt. Abundant records of humidity indicators, mainly fern spores, along with other gymnosperms (mainly Taxodiaceae and Araucariaceae) compared to low aridity indicators of gnetalean Elaterates and Ephedroids along with non-gnetalean Classopollis suggest predominantly warm and humid conditions. Two intervals in the lower part of the Bahariya Formation and in the middle part of the Abu Roash G Member showed moderate to high abundances of aridity indicators and thus, warm arid to semi-arid climate events during the early and late Cenomanian.

Seismic stratigraphy of the southern Eratosthenes High, eastern Mediterranean Sea: growth, demise and deformation of three superposed carbonate platforms (Mesozoic–Cenozoic)

Interpretation of seismic data over the southeastern flank of the Eratosthenes High shows nine principal seismic stratigraphic units overlying probable faulted basement. Among these are three superposed carbonate platforms that build a stratigraphy exceeding 3000 m. Regional comparisons suggest these range in age from Jurassic to Miocene.The Jurassic carbonate platform exhibits a layered stratigraphy and aggradational deposition style over the whole study area. A Lower Cretaceous platform subsequently developed as a linear, aggrading bank and prograded as multiple high-frequency sequences for more than 40 km into the Eratosthenes High interior, isolating an intrashelf basin that remained connected to the Levant Basin by a narrow seaway. The Jurassic platform margin was a fault-controlled, scalloped escarpment, while the mid-Cretaceous platform was strongly influenced by linear, NW–SE-orientated, fault-controlled sags.The Miocene platform, a shoaling, ‘catch-up’ neritic shelf, was established after a hiatus during which the flat top of the Cretaceous platform lay below the photic zone. The Miocene platform surface was subsequently incised by Messinian erosional channels that fed offlapping and downstepping regressive carbonate or evaporitic shorelines which tracked Messinian sea-level fall. Updoming and segmentation of the Eratosthenes High occurred during the early Messinian prior to the emplacement of Messinian salt onto its flanks.

Organic Matter Assessment and Paleoenvironmental Changes of the Middle Jurassic Main Source Rocks (Khatatba Formation) in the North Western Desert, Egypt: Palynofacies and Palynomorph Perspectives

The Middle Jurassic in the north Western Desert, Egypt, was a time of complex tectonics and increased environmental perturbations attributed to the predominant sedimentation of organic carbon-rich fine siliciclastic and carbonate deposits of the Khatatba Formation. Although some studies have addressed the hydrocarbon potential and source rock characteristics of the Khatatba Formation, a regional-scale investigation of the prevalent paleoenvironmental conditions and organic matter characteristics is still necessary. In this study, the Khatatba Formation is investigated for detailed palynofacies analysis and palynomorph composition to assess organic matter kerogen types and reconstruct the depositional paleoenvironmental patterns on a regional scale. For this purpose, 116 drill cuttings were collected from five wells in the Matruh, Shushan, and Dahab-Mireir Basins. Moderately diverse assemblages of spores, pollen, and dinoflagellate cysts are reported. Age-diagnostic dinoflagellate cysts, including Adnatosphaeridium caulleryi, Dichadogonyaulax sellwoodii, Korystocysta gochtii, Wanaea acollaris, and Pareodinia ceratophora, along with occasional records of Systematophora areolate and Systematophora penicillate, defined a Bajocian–Callovian age. Based on particulate organic matter (POM) composition, four palynofacies assemblages (PFAs) are identified. PFA-1 is the most common within the Khatatba Formation in the five studied wells. It contains high proportions of phytoclast fragments versus low contents of amorphous organic matter (AOM) and palynomorphs and is defined by a gas-prone kerogen Type III. PFA-2 is comprised of moderate abundances of AOM and phytoclast characteristics of oil-prone kerogen Type II. PFA-3 is dominated by phytoclasts and moderate to low proportions of AOM and palynomorphs of kerogen Type III, whereas PFA-4 consists of AOM and palynomorphs defining kerogen Type II. PFA-1 indicates predominant deposition in proximal active fluvio-deltaic sources to marginal marine conditions with enhanced contributions of terrestrial/riverine influx. PFA-2 and PFA-3 reveal deposition under an enhanced dysoxic to anoxic proximal inner neritic shelf due to the abundant occurrences of spores and coastal to shallow marine dinoflagellate cysts. PFA-4 suggests deposition under enhanced suboxic to anoxic distal inner neritic conditions because of enhanced AOM and abundant proximate and some chorate dinoflagellate cysts. Thus, the Middle Jurassic experienced a predominantly marginal to shallow water column in this part of the southern margin of the Tethyan Ocean where the Matruh, Shushan, and Dahab-Mireir Basins were located.

Upper Jurassic to Lower Cretaceous benthic foraminifera from South Tibet (Tethyan Himalayas): systematic, biostratigraphic, and palaeoecologic implications

The present study represents the first attempt to described foraminiferal fauna of the Gucuo Locality and is among the first work to illustrate the Upper Jurassic to Lower Cretaceous benthic foraminifers of South Tibet. The material consists in a combination of unpublished data on the Gucuo Section and published data on the Gyangze; Weimei, and Bandingsi sections.A total of 239 specimens have been currently recorded in the Upper Jurassic to Lower Cretaceous Menkadun and Gucuo formations of the Gucuo locality. To facilitate further researches on foraminifers from the Tethyan Himalayas, a total of 59 species from 38 genera have currently been repertory and described. Among the recorded specimens, a few species are significant biostratigraphic markers used to identify and constrain the age of four foraminiferal assemblages. The Textularia aff. haeusleri assemblage contains a few numbers of agglutinated species very similar to the one described in the Oxfordian to Kimmeridgian Nupra Formation of the Thakkola Region. The Tithonian Trochammina quinqueloba assemblage is a widespread biostratigraphic horizon recorded in both southern and northern parts of South Tibet, as well as in the Indian and the Proto-Atlantic oceans. The Berriasian to Valanginian Trochammina abrupta assemblage contains typical Early Cretaceous foraminifers. It confirms the Jurassic/Cretaceous boundary should be delineated at the base of the shale Unit of the Gucuo Formation, although it was previously attributed to the base of the volcaniclastic sandstone units using ammonite data. This latest being currently associated to the Valanginian to Hauterivian Ammobaculites crepinae–Textularia bettenstaedti foraminiferal assemblage.A palaeoecological reconstruction along a southwest to northeast transect of South Tibet, Himalayas, is proposed to understand the composition and dynamic of foraminiferal assemblages along with palaeoenvironmental changes. The morphogroup approach has been selected as the best methods to illustrate both sea-level and oxic changes. The northern tectonic zone of South Tibet is associated with the abundance of deep-water agglutinated foraminifers, which suggests a deep environment beyond the shelf break. The southern tectonic zone of South Tibet is rich in typical species of shallow environment and was located on the neritic shelf. The morphogroup analysis reveals localized, periodic disaerobic conditions on the sea-floor. The Jurassic to Cretaceous Boundary interval was associated with a drop of the sea-level, whereas the rise in diversity and abundance of marginal-sea taxa during the Valanginian to Hauterivian times underline a higher sea-level and a pulse in the development of epeiric seas in the southern part of South Tibet.

Near equatorial paleoclimatic evolution and control on organic matter accumulation during the Cenomanian in the Abu Gharadig Basin, southern Tethys: Insights from palynology, organic petrography, and geochemistry

The beginning of the Late Cretaceous (Cenomanian) attested to a major marine transgression interrupted by sea level falls in north Africa, which led to significant environmental perturbation and widespread accumulation of organic matter-rich sediments. During this period, northern Egypt was located at the southern edge of the Tethys Ocean and deposition of thick siliciclastic alternations between shales, siltstones, and sandstones of the Bahariya Formation took place in the BED2 gas field of the Abu Gharadig Basin, north Western Desert. An integrated approach of palynofacies analysis and palynomorph composition, organic petrography, and geochemical analysis of total organic carbon (TOC)/Rock-Eval pyrolysis, carbonate content, XRD analysis, and clay mineralogy of thirty-nine cutting samples obtained from the lower-middle Cenomanian Bahariya Formation was conducted. This approach was used to characterize organic matter and possible occurrence of source rock intervals, paleoenvironmental and paleoclimatic conditions and their role in organic matter accumulation. Moderate to high proportions of pteridophyte and water fern spores and pollen grains of Araucariaceae (Araucariacites), Cheirolepidiaceae (mainly Classopollis), and Gnetalean (Elaterate pollen) and Afropollis versus the sparsely recorded xerophilous ephedroids (Ephedripites) indicate prevalent conditions of a warm and humid climate. This is consistent with clay mineral composition that is dominated mainly by kaolinite and smectite compared to low illite and the lack of chlorite, which indicated a predominantly warm-humid climate state. Palynofacies analysis showed two assemblages; the first one is dominated by phytoclasts and indicates gas-prone Type III kerogen and deposition in a fluvio-deltaic environment. The second palynofacies assemblage is characterized by moderate abundance of phytoclasts and AOM mostly of terrestrial origin, suggesting a mixed Type III/II kerogen and deposition in a marginal to shallow neritic shelf environment. The organic geochemical characterization showed relatively low average values of TOC, S2, and HI, indicating that the Bahariya Formation is characterized by Type III gas-prone kerogen. Thermal maturity of the Bahariya Formation ranges from immature to the early stage of the oil window. Additionally, organic matter characteristics of the Bahariya Formation are qualitatively and quantitatively similar to their equivalents from nearby basins. Low TOC contents are attributed to a high sedimentation rate and an increased terrestrial input of sand- and silt-sized fractions at shallow water column settings that controlled strong dilution and decomposition versus low burial efficiency of the organic matter.

Palynomorph composition and palynofacies analysis of the Cenomanian Bahariya Formation in the north Western Desert of Egypt: Depositional palaeoenvironment and sequence stratigraphic implications

Abstract This study investigates the Bahariya Formation using 39 rock-cutting samples from the Abu Gharadig Basin in the north Western Desert, Egypt. An approach combining palynomorph composition and palynofacies analysis with lithological composition and geophysical gamma ray log values is used to assess biostratigraphic age constraints, prevalent depositional palaeoenvironments, and relative sea level changes. A moderately diverse assemblage comprised mainly of terrestrial palynomorphs versus minor content of marine palynomorphs is identified. Marker spore and pollen taxa are used to name three interval zones of early Cenomanian, and early-middle Cenomanian ages. Quantitative distribution of particulate organic matter using cluster analysis reveals two palynofacies assemblages: an older assemblage dominated by phytoclasts (mainly wood particles); and a younger assemblage characterised by moderate abundances of amorphous organic matter and phytoclasts. Palynofacies analysis suggests deposition of the Bahariya Formation in fluvio-deltaic to inner neritic shelf environments. Based on the pronounced stratigraphic variations in the particulate organic matter composition, including terrestrial:marine palynomorph ratio, lithological and gamma-ray log data, three complete transgressive-regressive sequences and one incomplete sequence and their systems tracts are defined. The transgressive-regressive trends reflect 3rd order sequences. Additionally, successive changes in relative sea level indicate a continuous rise at the topmost part of the succession.

Cenozoic Depositional Evolution and Stratal Patterns in the Western Pearl River Mouth Basin, South China Sea: Implications for Hydrocarbon Exploration

Investigating the deposition evolution and stratal stacking patterns in continental rift basins is critical not only to better understand the mechanism of basin fills but also to reveal the enrichment regularity of hydrocarbon reservoirs. The Pearl River Mouth Basin (PRMB) is a petroliferous continental rift basin located in the northern continental shelf of the South China Sea. In this study, the depositional evolution process and stacking pattern of the Zhu III Depression, western PRMB were studied through the integration of 3D seismic data, core data, and well logs. Five types of depositional systems formed from the Eocene to the Miocene, including the fan delta, meandering river delta, tidal flat, lacustrine system, and neritic shelf system. The representative depositional systems changed from the proximal fan delta and lacustrine system in the Eocene–early Oligocene, to the tidal flat and fan delta in the late Oligocene, and then the neritic shelf system in the Miocene. The statal stacking pattern varied in time and space with a total of six types of slope break belts developed. The diversity of sequence architecture results from the comprehensive effect of tectonic activities, sediment supply, sea/lake level changes, and geomorphic conditions. In addition, our results suggest that the types of traps are closely associated with stratal stacking patterns. Structural traps were developed in the regions of tectonic slope breaks, whereas lithological traps occurred within sedimentary slope breaks. This study highlights the diversity and complexity of sequence architecture in the continental rift basin, and the proposed hydrocarbon distribution patterns are applicable to reservoir prediction in the PRMB and the other continental rift basins.

Depositional setting and organic matter characterization of the Upper Devonian Antrim Shale, Michigan Basin: Implications for hydrocarbon potential

During the Late Devonian, the Michigan Basin in the Eastern Interior Seaway was characterized by phases of well-developed bottom water anoxia, triggering extensive accumulation of organic matter-rich black shales. Palynological analysis for paleoenvironmental reconstruction and biostratigraphy of the Upper Devonian rocks has been previously performed in other basins of eastern North America, but it is lacking in the Michigan Basin. Here, the Upper Devonian Antrim Shale Formation is investigated for biostratigraphic, paleoenvironmental, and sea level reconstructions, organic matter characterization, and source rock potential. 35 core samples from three drill holes across the Michigan Basin spanning the Norwood, Paxton, Lachine, and Upper members. Palynological analysis showed a moderately diverse assemblage of 31 genera represented by 53 species of relatively well-preserved prasinophyte phycomata and acritarchs, with sparse records of plant spores and freshwater algae. Based on marker prasinophytes and acritarchs, an age of late Frasnian was assigned to the Norwood and Paxton members, while an early Famennian age was proposed to the Lachine and Upper members of the Antrim Shale. Two palynofacies assemblages (PFA) were identified from the variation in the particulate organic matter (POM) composition, both of which indicate deposition in a distal inner neritic shelf environment, but the PFA-2 took place in slightly shallower conditions than the PFA-1. Geochemical screening indicated high organic matter content (up to 25 wt%, avg. 8.5 wt%) and excellent hydrocarbon generating potential of kerogen Type II for the Norwood, Lachine, and Upper members. Only the Paxton Member was dominated by poor to fair organic richness (<0.6 wt%) and low hydrocarbon potential with a mixed Type II/III kerogen. Organic petrography indicated that the dominant organic matter assemblage consists mainly of unicellular marine Tasmanites telalginite and Leiosphaeridia, followed by solid bitumen of the initial oil type. Thermal maturity, determined from VRo-eq and Tmax, indicated that all samples are in the oil window, except for some intervals in the Paxton Member.

Uppermost Jurassic to Lower Cretaceous benthic foraminiferal faunas of the Weimei and the Bandingsi localities of northern and southern parts of South Tibet – A preliminary analysis

Diverse foraminiferal assemblages from the Weimei–Gyabula, and the Menkadun–Gucuo formations have been recorded based on the study of two Upper Jurassic to Lower Cretaceous successions, respectively located in the northern and southern parts of South Tibet. Each section contains three different foraminiferal assemblages. Due to the poor state of preservation and the occurrence of long-ranged cosmopolitan species, these foraminiferal assemblages do not allow a precise age assignment for the fossil bearing strata. However, the identification of the Globulina prisca assemblage indicates a Cretaceous age for the Shale Unit of the Gucuo Formation, although the Jurassic/Cretaceous (J/K) boundary was traditionally delineated above this unit. On the other hand, the occurrence of Dorothia oxycona indicates that the uppermost Gyabula and Gucuo formations cannot be older than Aptian and middle Albian age, respectively.During the Late Jurassic to Early Cretaceous, the northern part of South Tibet was located in a deep marine environment, probably at bathyal to abyssal depths, whereas the southern part of South Tibet was part of the outer neritic shelf environment. Typical foraminiferal associations of mildly oxygen-deficient deep-water environment and increased surface productivity have been recorded during the late Early Cretaceous. The biofacies succession indicates that the late Early Cretaceous was associated with a deepening of the palaeobasin, probably associated with a global Albian transgression.

Sedimentary environments and mechanism of organic matter enrichment of dark shales with low TOC in the Mesoproterozoic Cuizhuang Formation of the Ordos Basin: Evidence from petrology, organic geochemistry, and major and trace elements

Proterozoic strata have become a new target for hydrocarbon exploration worldwide. They exhibit the potential to become a resource replacement target. To provide a reference for hydrocarbon source rock evaluation, the sedimentary environment and mechanism of organic matter enrichment of dark shales in the Mesoproterozoic Cuizhuang Fm. were investigated for the first time by combining organic and inorganic research concepts. Although the total organic carbon (TOC) content in the Cuizhuang Fm. is relatively low (mean value of 0.59%), it still can be considered as a potential source rock. Thin-section observations, main and trace element contents, and biomarker compounds revealed the following: (1) the Cuizhuang Fm. was deposited in a neritic shelf sedimentary environment at a passive continental margin, and the climate was warm and humid. The hydrocarbon-generating organisms mainly comprised prokaryotes such as cyanobacteria and other bacteria. All these characteristics meet the conditions of source rock formation. (2) The low oxygen level, felsic source provenance, and absence of submarine hydrothermal activity led to low productivity, and the low productivity and low-grade hydrocarbon-generating organisms led to low TOC content in the Cuizhuang Fm. (3) The enrichment of V in the Cuizhuang Fm. is apparent, but does not reach the hyper-enrichment threshold of 500 ppm associated with euxinic conditions, revealing anoxic ferruginous conditions. Moreover, the V content is positively correlated with TOC content, indicating that organic matter enrichment was mainly controlled by the redox conditions of the bottom water. Therefore, the sedimentary areas with high productivity were related to basic source provenance and submarine hydrothermal activity, and anoxic bottom water may have promoted the development of better source rocks. This findings may represent the main direction for identifying effective source rocks in subsequent work.

The Maastrichtian–Danian transition in the northern Farafra Oasis, Western Desert (Egypt): Implications from foraminiferal paleobathymetry and paleoenvironmental reconstructions

Two exposed sections of the Lower/Upper Maastrichtian to Danian at northern and northeastern Farafra Oasis, Western Desert (Egypt) namely, the Farafra–Ain Dalla road and the Ain Maqfi sections have been analyzed for their planktonic and benthonic foraminifera to constrain the biostratigraphy and infer the paleoenvironmental conditions. Lithostratigraphically, the successions of the present study are classified into the Khoman Chalk and the Dakhla Shale. Biostratigraphically, six planktonic foraminiferal zones; Gansserina gansseri, Racemiguembelina powelli and Plummerita reicheli of the Maastrichtian as well as the Praemurica inconstans (P1c), Praemurica uncinata (P2) and Morozovella angulata (P3a) of the Danian are recognized. A detailed quantitative benthonic foraminiferal analysis has been performed and yields a total of 172 identified benthonic foraminiferal species belonging to 62 different genera. The various foraminiferal parameters as well as cluster analysis provide good information about the paleoenvironmental conditions and sea–level fluctuations across the Maastrichtian–Danian transition. The Maastrichtian revealed outer neritic shelf with slightly shallowing conditions recorded only at the Ain Maqfi section, well oxygenated conditions with low organic matter influx coupled with sea–level rise. During the Maastrichtian–Danian transition, a paleoenvironmental shift to inner-shallow middle neritic shelf, low oxygenated conditions with moderate organic matter influx coupled with sea–level fall is interpreted. This transition is characterized by a major hiatus due to absences of Guembelitria cretacea (P0), Parvularugoglobigerina eugubina (Pα), Parasubbotina pseudobulloides (P1a) and Subbotina triloculinoides (P1b) zones and subzones. It is well marked in the field by an irregular contact and conglomeratic phosphatic dolostone bed at the Farafra-Ain Dalla road section while the disconformity surface can be detected by planktonic foraminifera in the Ain Maqfi section. The lowermost Danian is phosphatic argillaceous chalky limestone with intensive reworking. Low oxygen and/or relatively high–moderate organic matter influx within inner–middle neritic shelf paleoenvironmental conditions can be reconstructed for the lowermost Danian associated with a slight increase in sea–level during the Praemurica inconstans (P1c) zone. The uppermost Danian reflected very well oxygenated conditions and/or low organic matter influx in middle–outer neritic shelf conditions associated with sea–level rise during ages of the planktonic foraminiferal zones P2–P3a.

Strata sequence and paleochannel response to tectonic, sea-level, and Asian monsoon variability since the late Pleistocene in the South Yellow Sea

AbstractThe continental shelf strata provide information regarding sea-level fluctuation and climate changes in the Quaternary period. A 5831.47-km-long high-resolution seismic profile and borehole core (YS01) were acquired to reconstruct the evolutionary history of the strata in South Yellow Sea (SYS) during the late Pleistocene. The strata recorded three transgression events (HI, HII, and HIII) and three stages of paleochannel development (LI, LII, and LIII). Based on the distribution, thickness, and volume of the strata formed in the three transgressions, we concluded that the scale of the three transgressions during the late Pleistocene was HIII, HI, and HII, in descending order. In addition, our data show that the Yellow River extended to the Yellow Sea Trough during the last glacial maximum. The influence of the tectonic framework on sedimentation in the SYS was completely concealed by sea-level changes and sediment supply in the late Pleistocene (~Marine Isotope Stage 5). Since then, the accommodation space, a crucial prerequisite for sedimentation, has been controlled solely by sea-level changes in the SYS. Furthermore, two “source to sink” models of the neritic shelf in the marine and terrestrial environments were established, including high sea-level and shelf-exposure models.

Upper Paleocene to lower Eocene microfacies, biostratigraphy, and paleoenvironmental reconstruction in the northern Farafra Oasis, Western Desert (Egypt)

Three Paleocene-Eocene (P-E) stratigraphic transect sections namely, from the north to south, Ain Maqfi, Farafra-Ain Dalla road, and El-Quess Abu Said in the northern Farafra Oasis,Western Desert (Egypt) are described and interpreted based upon field observations, microfacies analysis, chronostratigraphy and foraminiferal paleobathymetry, to detect the effect of the Syrian Arc Fold System (SAFS) on the lateral and vertical facies changes, various stratigraphic breaks and to reconstruct the depositional paleoenvironments. Lithostratigraphically, the P-E successions are composed of the upper part of the Dakhla Formation, Tarawan Chalk and Esna Shale Formation. Vertical and lateral facies changes are noted between tectonic paleo-highs and paleo-lows in the Farafra Oasis. Eight microfacies types are recognized. The larger benthic and planktonic foraminiferal zones are here used to correlate the shallow and deeper facies. Two larger benthic (SBZ4 and SBZ6), six planktonic foraminiferal (P4–E4) and one calcareous nannofossil (NP9b) biozones are identified. The recorded basal Eocene Dababiya Quarry Member (DQM) within the Esna Shale Formation in the central Farafra Oasis is represented by units 4 and 5 of the DQMat its GSSPwith a neritic facies types. Towards the northern part of the Farafra Oasis, the P-E interval occurs within the base of the Maqfi Limestone Member that contains the larger benthic foraminiferal SBZ6 Zone and is correlated with the DQM.Amajor sea-level fall near the upper part of P5 Zone, followed by a prominent sea-level risewith a minor hiatus across the P-E interval in the Farafra Oasis reflects the complex interplay between sea level changes and tectonic signatures. Two inferred paleoenvironments, namely inner neritic and mid-outer neritic shelf have been identified.

Change in Sediment Provenance Near the Current Estuary of Yellow River Since the Holocene Transgression

Sedimentary sequence and sediment provenance are important factors when it comes to the studies on marine sedimentation. This paper studies grain size distribution, lithological characteristics, major and rare earth elemental compositions, micropaleontological features and 14C ages in order to examine sedimentary sequence and sediment provenance of the core BH6 drilled at the mouth of the Yellow River in Bohai Sea. According to the grain size and the micropaleontological compositions, 4 sedimentary units have been identified. Unit 1 (0–8.08 mbsf) is of the delta sedimentary facies, Unit 2 (8.08–12.08 mbsf) is of the neritic shelf facies, Unit 3 (12.08–23.85 mbsf) is of near-estuary beach-tidal facies, and Unit 4 (23.85 mbsf–) is of the continental lake facies. The deposits from Unit 1 to Unit 3 have been found to be marine strata formed after the Holocene transgression at about 10 ka BP, while Unit 4 is continental lacustrine deposit formed before 10 ka BP. The provenances of core BH6 sediments show properties of the continental crust and vary in different sedimentary periods. For Unit 4 sediments, the source regions are dispersed while the main provenance is not clear, although the parent rock characteristics of a few samples are similar to the Luanhe River sediments. For Unit 3, sediments at 21.1–23.85 mbsf have been mainly transported from the Liaohe River, while sediments above 21.1 mbsf are mainly from the Yellow River and partially from the Liaohe River. For Unit 2, the sediments have been mainly transported from the Yellow River, with a small amount from other rivers. For Unit 1, the provenance is mainly the Yellow River catchment. These results help in better understanding the evolution of the Yellow River Delta.

Diagenesis and Compositional Partitioning of Quaternary Cool-water Carbonate Aeolianites: Southeastern Australia

Abstract Middle to late Pleistocene stranded carbonate sea beaches, adjacent aeolian dunes, and Pleistocene aeolianites together with Holocene marginal marine calcareous sediments across southeastern Australia contain the longest record of such deposition in the modern world, stretching back to at least ∼ 900 ky (MIS 23). The grains are/were derived from a suite of cool-water, heterozoan carbonate sediments produced on the adjacent offshore shelf. Most modern shelf carbonate sediments in the region today are aragonite-rich but the aragonite is presently slowly being dissolved on or just below the seafloor before the deposits enter the rock record. Beach-dune sediments are derived mainly from the shallow < 30 meters deep adjacent seafloor. They are mollusk-rich but with most bryozoan particles having been destroyed in the surf zone. The mollusks are dominantly aragonite. These nearshore sediments are swept onshore before any significant seafloor diagenesis can take place and so are compositionally different from the eventual aragonite-poor open-shelf sediments. Aragonite skeletal fragments in the dunes progressively dissolve with time under a semi-arid climate and produce diagenetic low-magnesium calcite meteoric cements, a well-known process common in tropical, aragonitic Pleistocene aeolianites. The loss of metastable minerals with time is matched by a progressive and well-understood change in stable isotopes. Such alteration in this environment, however, seems to take longer than in tropical carbonates, with lithification herein not beginning until ∼ 200 ka and complete cementation being achieved in ∼ 450 ky Thus, neither aeolianites nor offshore marine carbonates record the composition of the original sediment; aeolianites lack the important bryozoan constituents whereas neritic deposits lack the prolific mollusks. This partitioning, revealed for the first time, answers the conundrum as to why some cool-water neritic shelf and coastal sediments in the rock record are compositionally different.

Quantative analysis and paleoecology of Middle to Upper Eocene Ostracods from Jebel Jebil, central Tunisia

One hundred and seventy collected samples from Jebil section have been carefully studied for their ostracod content and referred to 41 species belonging to 20 genera. Their vertical distribution allowed to distinguish five successive associations of ostracod assemblages; two of which are correlated with the Early Lutetian, one with the Late Lutetian, another association with the Bartonian and the last one with the Priabonian. Community structure of the collected ostracod fauna has been studied; three indices have been calculated for each sample: Shannon (diversity), Margalef (richness) and Equitability indexes. In the lower and the middle part of the Formation, they indicate a stable environment supporting high diversity ostracod communities; whereas in the upper portion the environmental conditions were unstable characterized by low diversity. The results of a multivariate statistical method, using the cluster analysis and the Detrended Correspondence Analysis of the 41 ostracod species and the 170 samples, have led to conclude that the most effective environmental factor in the study area is the paleodepth and of less importance oxygenation and salinity. Thus, it allowed to distinguish four palaeoenvironmental intervals within the Cherahil Formation: the first one represented by taxa that are known from the shallower parts of the shelf; the second interval includes the majority of the encountered species of inner neritic shelf with normal salinity; the third one, corresponding to an outer neritic domain; and the last interval refers to a circalittoral environment, is comprised mainly of Cytherella angulata and of Soudanella laciniosa triangulata.

Middle Permian Sedimentary Cycles and Facies Features in the Tieqiao Section in Laibin, Guangxi, South China

In the Late Paleozoic paleogeography, the Laibin area, South China, locates in the south-eastern margin of the Laibin-Heshan isolated carbonate platform. One global stratotype section for The Guadalupian-Lopingian (G/L) (The end of the Mid-Permian) transition is located in the Tieqiao section in Laibin area, which is regarded as appropriate one for the study. Based on the analysis, seven facies (open platform, restricted platform, neritic shelf, bioclastic shoal, reef, slope, and basin) are recognized in the Maokouan Formations, besides, three sequences have been identified in the Maokouan succession. They all represent a deeping-upward then shallowing-upward sedimentary cycle, and the microfacies alterna- tion conformable marine sequences at that time, which implies consistence of sequence number recorded in the Tieqiao section were little influenced by the role of sys-depositional fault, maybe controled by the paleoceanographic condition.

Microfacies types (MFT) and palaeoenvironment of the Umlatodoh carbonates in the Shillong Plateau of Meghalaya, NE India

One of the major marine transgressions in the Shillong Plateau is represented by the middlemost Umlatodoh Formation of the Sylhet Limestone Group during early Eocene. Based on 26 species of Larger Benthic Foraminifera (LBF) and eleven species of calcareous red algae and green algae, nine Tethyan MFT (microfacies types) viz, Miliolid Packstone-Grainstone, Algal Packstone-Grainstone, Miliolid Grainstone, Larger Benthic Foraminiferal Grainstone, Green Algae Grainstone, Alveolina oblonga Grainstone, Oolitic Packstone-Grainstone and Micrite have been identified for the Umlatodoh Limestone. These microfacies have contributed towards identification of a range of environments from low-energy, shallow neritic inner-shelf through shallow, reef shelf to more restricted environment represented by pure micrite. Petrographical characteristics indicate that these limestones are primarily microcrystalline allochemical rocks of biomicritic type and a mixture of packstone and grainstone. The foraminiferal assemblage associated with various species of coralline red algae and green algae suggests inner ramp, mud-free shallow neritic shelf carbonate sedimentation of the Umlatodoh Limestone showing depositions in calm water, low-energy conditions in the lower part changing to moderately high-energy conditions in the middle followed by low-energy restricted environment represented by micrite towards the upper part.

Principal Component and Cluster Analyses of Palynodebris from Aj-X Well, Offshore Dahomey Basin and their Environmental Significance

The palynofacies study carried out on offshore ditch cuttings from AJ-X well in Dahomey basin revealed different types of palynodebris. To assess the pattern of distribution of the palynofacies types, their abundances were submitted to Cluster analysis and Principal component analysis. The PCA diagram consists of four curves, PC1, PC2, PC3 and PC4 with a cumulative variance of 45.8% showing fluctuations in PC curves supporting the response of the palynodebris to changing environments. The cluster for the well revealed two super clusters based on the abundance and composition of the kerogen groups. These are informally designated as: super cluster X, which is subdivided into X1, X2 and X3, and super cluster Y, subdivided into Y1 and Y2. X1 is composed mostly of phytoclasts and a moderate abundance of the AOM group (Phy/AOM); X2 of high abundances of phytoclasts combined with moderate to high abundances of palynomorphs (Phy/Pal); and X3 is composed of very high abundances of phytoclasts (Phy). The high abundances of the AOM group occur in super cluster Y. Y1 is characterized by very high abundances of the AOM group. In Y2 this group occurs with phytoclasts and, in particular with the palynomorph group (AOM/Pal). The major break that occurred between clusters X and Y is strongly related to the abundance of the phytoclasts group. The size, shape and texture of all the components were integrated with sedimentological features and palynomorphs to recognize distal suboxic-anoxic shallow neritic shelf environment.

Lithostratigraphy, Palynostratigraphy and Palynofacies Indications of the Depositional Environments of Upper Cretaceous to Paleogene Sediments, Offshore Eastern Dahomey Basin, SW Nigeria

The ditch cuttings from an offshore well, 'JX' were studied for their sedimentology, palynostratigraphy, palynofacies and depositional environment history. Lithologically, poorly sorted sandstones with variable interbeds of shales and siltstones overlain by dark grey shales and interbedded limestones with occasional pyrite occurrence (Abeokuta-Afowo Formations) constituted the basal sections. Overlying is a glauconitic sequence consisting of shale with little clay and lenses of limestones (Araromi Formation). The uppermost section consists of predominantly of shales with some glauconitic materials (Oshosun Formation). Occurrences of Canningia cf. senonica, Nelsoniella; Microdinium ornatum (dinoflagellates), Crybelosporites pannuceus, Cicatricosisporites spp. (spores), and Classopollis spp., Ephedripites (pollen) assemblage suggests a late Cenomanian age for the lowermost section while the occurrences of Lejeunecysta hyaline, Selenopemphix selenoides and Hystrichokolpoma rigudae (dinoflagellates) and Peregrinipollis nigericus, Retitricolporites irregularis and Ctenolophonidites costatus (pollen) and Foveotriletes margaritae (spore) at the uppermost section indicate an Eocene age. Palynological marine index (PMI) curve and the Peridinioid to Gonyaulacoid ratio (P/G ratio) between the different palynological assemblages showed very strong fluctuations. Six broad lithofacies units with characteristic palynofacies assemblages reflecting different paleoenvironment were distinguished, which varied from estuarine through shallow inner neritic (shelf) to middle neritic shelf settings.