Palynofacies and TOC analysis of marine and non-marine sediments across the Middle–Upper Jurassic boundary in the central-northern Lusitanian Basin (Portugal)

The Middle–Upper Jurassic transition is a geodynamic benchmark in the evolutionary history of several peri-Atlantic basins. Contrary to the vast Tethyan and peri-Tethyan areas, in the Lusitanian Basin (Portugal), this interval corresponds to a major basin-wide disconformity preceded by a complex forced regression that induced sharp facies variations across the depositional systems. The Middle Jurassic units below are fully marine, whereas the Upper Jurassic sediments (Lower?-Middle Oxfordian), correspond to freshwater/brackish lacustrine, grading into punctuated brackish-restricted lagoonal and shallow-marine paleoenvironments. This study presents total organic carbon and palynofacies data of 34 samples (total analyzed thickness about 85 m) collected from three key sections encompassing the Middle–Upper Jurassic transition in the central-northern sector of the Lusitanian Basin. The palynofacies of the analyzed part of Middle Jurassic units (Cabo Mondego Formation) are characteristic of marine environments at their base, evidencing upwards a regressive trend of the depositional systems. Total organic carbon content is generally low, reaching up to 1.94 wt%. The Upper Jurassic Cabaços Formation presents kerogen assemblages mostly of continental origin, although punctuated by minor intervals of marine influence. Total organic carbon content is more variable, reaching up to 30.56 wt%. Intraclasts of re-deposited fragments of microbial mats were found incorporated in the kerogen assemblages, which point to highly dynamic erosional and depositional processes. Diversity of Botryococcus sp. occurrences was confirmed as an indicator of the degree of paleoenvironmental stability. The vertical distribution and comparison of the kerogen assemblages of the different sections indicate major changes of these parameters among relatively close settings and along narrow vertical intervals, attesting to the high sedimentary dynamics observed in the Lusitanian Basin.     

Relationships between foraminiferal assemblages and depositional sequences in Jahrum Formation,Ardal area (Zagros Basin,SW Iran)

The Jahrum Formation was deposited in the foreland basin in southwest Iran (Zagros Basin). The Zagros mountain belt of Iran, a part of the Alpine–Himalayan system, extends from the NW Iranian border through to SW Iran, up to the strait of Hormuz. The various facies of the Jahrum Formation were deposited in four main genetically related depositional environments, including: tidal flat, lagoon, shoal and open marine. These are represented by 14 microfacies. The Jahrum Formation represents sedimentation on a carbonate ramp. Tidal flat facies are represented by fenestral fabric, stromatolitic boundstone and thin-bedded planes. Carbonate deposition in a shallow marine lagoon was characterised by wacke–packstone, dominated by various taxa of imperforate foraminifer. The shoals are made up of medium- to coarse-grained skeletal and peloidal grainstone. This facies was deposited predominantly in an active high energy wave and current regime, and grades basinward into middle ramps facies are represented by wackestones–packstones with a diverse assemblage of echinoderm and large benthic foraminifers with perforate wall. Outer ramp facies consist of alternating marl and limestones rich in pelagic foraminifera. There is no evidence for resedimentation processes in this facies belt. The sequence stratigraphy study has led to recognition of three third-order depositional sequences.     

Sequence stratigraphic analysis and sea-level history of the Upper Jurassic deposits (Mozduran Formation), south of Aghdarband,NE Iran

Abstract

The Kopet-Dagh Basin is a large sedimentary basin in northeastern Iran that host the giant Khangiran and Gonbadli gas fields. The Mozduran Formation with its various sedimentary facies is an important reservoir widely distributed in the basin. A sedimentological analysis of Upper Jurassic Mozduran Formation resulted in an accurate reconstruction of the sedimentary environments and the sequence stratigraphic framework south of Aghdarband. The strata consist of six different facies associations including 12 carbonate, one evaporate and two siliciclastic subfacies. On the basis of their various components, structural and textural characteristics, these facies were deposited on a homoclinal ramp in tidal flat to open marine environments ranging from supratidal to subtidal settings. Facies A1 and A2 represent open marine, B1-B4 Shoal, C1-C4 lagoonal and D1, D2, E, T1 and T2 tidal flat and Salina environments. In addition, based on detailed field and laboratory studies on the facies architecture, several large-scale (long-term) depositional sequences could be distinguished in the stratigraphic sections of the study area. These sequences are composed of LST, TST and HST that are separated by a SB1 and SB2 sequence boundaries. The paleogeography of the study area during the Late Jurassic time is reconstructed in five block diagrams.     

Palynofacies Analysis of Inner Continental Shelf and Middle Slope Sediments offshore Egypt, South-eastern Mediterranean

Marine palynological studies of Quaternary sediments usually focus on dinoflagellate and pollen assemblages for paleoceanographic and paleohydrographic interpretations of past events. This paper focuses on the use of palynofacies analysis for paleohydrological reconstructions of deltaic and deep sea environments to evaluate transport of organic matter to the ocean. These palynodebris data are used to interpret palaeoenvironmental and paleohydrographic changes in two marine cores from the continental shelf (core-1, 27 m water depth) and middle slope (core-2, 1030 m) offshore Egypt, south-eastern Mediterranean Sea, during the latest Pleistocene and the Holocene. The relative abundances of various types of sedimentary organic matter such as phytoclasts, zooclasts, amorphous organic matter and palynomorphs are related to paleohydrographic changes of the overlying water column. Based on the total palynodebris and organic carbon content, sediments of the inner continental shelf core are characteristic of a prodelta environment proximal to a fluvio-deltaic source and moderately distal oxic environments with enhanced structured organic matter preservation potential. In contrast, the palynodebris of the middle slope core show that the basal sediments (105-140 cm depth) indicate suboxic to dysoxic bottom water conditions, followed by anoxic-suboxic bottom water conditions for the interval from 30-85 cm, which represents the S1 Sapropel. The top sediments of core-2 (0-25 cm) were deposited under oxic bottom water conditions, suggesting good ventilation in the water column. A quantitative approach was also used for interpreting the Holocene sea-level changes, based on the correlation between phytoclast and organic matter abundances. Sedimentation rate in the continental shelf is varied, being relatively very low (6.7 cm/kyr) in the basal part and increased upward to be 20 cm/kyr (depth 115-120 cm). In the upper Gray clayey silt layer the sedimentation rate was high (about 40-45 cm/kyr) due to the high discharge from El Manzala Lagoon and Damietta Nile branch. In the middle slope depth the sedimentation rate was relatively low and uniform, around 14 cm/kyr.     

Maastrichtian facies succession and sea-level history of the Hossein-Abad,Neyriz area,Zagros Basin

The Maastrichtian shallow-water carbonate platform (Tarbur Formation) is described from outcrop in southwest Iran. It is characterised by eight microfacies types, which are dominated by larger foraminifera, rudist debris and dasycladacean algae. They are grouped into four distinct depositional settings: tidal flat, lagoon, barrier and open marine. The depositional settings include stromatolitic boundstone of tidal flat, peloidal dasycladacean miliolids wackestone and peloid bioclastic imperforate foraminifera wackestone of restricted lagoon, Omphalocyclus miliolids bioclast packstone–grainstone and miliolids intraclast bioclast packstone–grainstone of open lagoon, rudist bioclast grainstone of inner-platform shoals and rudist bioclast floatstone–rudstone and bioclastic wackestone of open-marine environments.

The facies and faunal characters are typical of a ramp-like open shelf. The lack of reef-constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. On the basis of facies analysis, three depositional sequences (third order) are defined.     

Cyclic sedimentation across a Middle Ordovician carbonate ramp (Duwibong Formation), Korea

Summary The Middle Ordovician Duwibong Formation (about 100 m thick), Korea, comprises various lithotypes deposited across a carbonate ramp. Their stacking patterns constitute several kinds of meter-scale, shallowing-upward carbonate cycles. Lithofacies associations are grouped into four depositional facies: deep- to mid-ramp, shoal-complex, lagoonal, and tidal-flat facies. These facies are composed of distinctive depositional cycles: deep subtidal, shallow subtidal, restricted marine, and peritidal cycles, respectively. The subtidal cycles are capped by subtidal lithofacies and indicate incomplete shallowing to the peritidal zone. The restricted marine and peritidal cycles are capped by tidal flat lithofacies and show evidence of subaerial exposure. These cycles were formed by higher frequency sea-level fluctuations with durations of 120 ky (fifth order), which were superimposed on the longer term sea-level events, and by sediment redistribution by storm-induced currents and waves. The stratigraphic succession of the Duwibong Formation represents a general regressive trend. The vertical facies change records the transition from a deep- to mid-ramp to shoal, to lagoon, into a peritidal zone. The depositional system of the Duwibong Formation was influenced by frequent storms, especially on the deep ramp to mid-ramp seaward of ooid shoals. The storm deposits comprise about 20% of the Duwibong sequence.     

Évolution du trait de côte à l’Holocène supérieur dans la Sebkha El-Guettiate de Skhira (Golfe de Gabès, Tunisie) à travers sa faune d’ostracodes et de foraminifères

A 122 cm-long core was taken in the El-Guettiate Sebkha of Skhira (southeastern part of Tunisia) in order to investigate the recent palaeoenvironmental evolution of this region. The quantitative and qualitative analysis of ostracod and benthic foraminifera assemblages coupled with a correspondence analysis allows the reconstruction of palaeoenvironmental changes during the Holocene in this area. Four typical associations of ostracods (open marine, coastal marine, lagoonal and estuarine brackish) and two associations of benthic foraminifera (coastal and lagoonal) were distinguished. The onset of restricted lagoonal environments linked to the building-up of sand spit led to the onset of restricted lagoon and brackish environment at cal. 5408 years BP. The Shannon and equitability index of diversity were used to decipher the structural variations of the populations of ostracods and benthic foraminifera along the sampled core. We note a reduction in the Shannon index from the bottom to the top, which indicates a progressive isolation of the biotope. The open lagoonal episode is characterized by high values of diversity. During restricted lagoonal episodes the Shannon and equitability index are reduced. The correspondence analysis reveals an environmental gradient related to the marine influence. It shows an antagonism between the widely opened estuarian lagoonal species and those of restricted lagoon. The less opened estuarian lagoonal taxa occupy an intermediate position. Based on these evidences, microfauna carried out in the El-Guettiate Sebkha allows us to recognize four phases beginning with a widely opened estuarian lagoon (ca. cal. 7460 years PB), followed by a restricted lagoon (ca. cal. 5408 years BP) and finally a brackish lagoon evolving towards the present-day sebkha environment. The opened estuarine lagoon is characterized by high values of species richness and diversity indices.     

Palaeoenvironmental research on diatoms in early and middle Holocene deposits in central North Holland (The Netherlands)

In the geological research of the Holocene coastal deposits of The Netherlands, diatoms are used to reconstruct the sedimentary facies, palaeo-tide levels and salinity gradients during the deposition of the sediments. In this paper, the results of diatom research from 4 borings, taken from the early and middle Holocene deposits of central North Holland are presented. The oldest marine influenced sediments in the area are the deposits of the ‘Velsen layer’, a clay layer rich in organic matter and deposited about 8000–7000¹⁴C years before present (BP) at a depth of 20–12 m below present mean sea-level. This clay layer was formed in a shallow, permanently submerged environment with a limited tidal influence (lagoonal or pond-like conditions). The salinity changed from brackish/freshwater to marine/brackish. The younger sandy and clayey sediments, formed about 7000–4500 BP at a depth of 16–3 m below present mean sea-level, are classified as ‘tidal channel’ and ‘interchannel’ deposits. It is argued, on the base of both diatom and non-diatom criteria, that the lower and middle parts of the interchannel deposits in the central area of the palaeo-tidal basin of North Holland were formed in a subtidal environment. The upper part of the interchannel deposits and the deposits at the fringe of the North Holland tidal basin were formed in the intertidal zone or, at the fringe of the basin, even in the supratidal zone. The salinity in the North Holland tidal basin during the sedimentation of the tidal channel and interchannel deposits was marine/brackish to marine. This study indicates that diatoms, besides their palaeoecological applications, have chronostratigraphical significance (on a regional scale). The diatomsCymatosira belgica andActinoptychus splendens appear to be a useful (eco)stratigraphical marker in the Holocene coastal deposits of The Netherlands and Belgium.     

La transgression thanétienne (Paléocène supérieur) dans l’Aurès occidental (Algérie), d’après les associations de Foraminifères de la coupe d’El Kantara

In the W-Aures (Algeria), the El Kantara pass displays about 50 m of Red Marls overlying the Maastrichtian limestones with Laffitteina. The Red Marls are, in their turn, overlain by the Thanetian marly limestones of the river El Haï. The micropaleontological inventory of these Red Marls establishes the co-occurrence of subaerian (Microcodium = Paronipora), fresh-water (Charophyta) and marine microfossils (Foraminifers). The stratigraphical significance of these microfossils is discussed. The upper part of the Red Marls, yielding abundant Valvulina and scarce Glomalveolina, is of Thanetian age. The age of the lower part is less established, but the occurrence of Microcodium suggests a Thanetian age too. Located at the margin of the northern opening of the trans-Saharan epeiric sea, the El Kantara section establishes a Thanetian age for the beginning of the Paleocene transgression. Danian deposits are missing, in spite of the absence of an obvious hiatus on the field. After general emersion during the early Paleocene, the Thanetian transgression starts with the set up of lakes at the depositional area of the Red Marls. Marine influences, limited at first, become gradually prevailing, and end with the deposition of neritic marly limestones of river El Haï.     

Paleoenvironmental reconstruction based on palynofacies analysis of the Aptian–Albian succession of the Sergipe Basin,Northeastern Brazil

Palynofacies analyses were carried out on 272 core samples from two wells (GTP-17-SE and GTP-24-SE) in the Sergipe Basin, with the aim of reconstructing the paleoenvironments of the upper Aptian–middle Albian. For the palynofacies analysis the kerogen categories were counted and subjected to cluster analyses. For detailed environmental analysis, kerogen distribution trends and parameters were applied, based on percentages of kerogen categories. Total organic carbon analysis was also used. The stratigraphic distribution of palynofacies associations that defined eight palynofacies units in well GTP-17-SE and ten in well GTP-24- SE reflects a continuous terrestrial influx throughout the succession. Amorphous organic matter (AOM) and palynomorph groups show moderate to high percentages, especially in well GTP-24-SE. The increase in relative abundance of these groups suggests that a long-term a transgression and/or a decreasing terrestrial influx occurred in the area. From the bottom to top six depositional environments were identified in wells GTP-17-SE and GTP-24, respectively, and they reflect the progressive increase in marine influence as the basin evolved. Brackish lagoon conditions changed to more open marine conditions as sea level changed during the separation of South America from Africa.     

Les Ostracodes et le Quaternaire d’Aigion (golfe de Corinthe, Grèce)

The Upper Pleistocene and the Holocene of Aigion have delivered an abundant microfauna with 35 ostracode species. The Pleistocene from Aigion borehole generally provides Ostracodes from oligohaline environment with Cyprideis torosa, Candona angulata and Tyrrhenocythere amnicola while, in the Holocene, marine infralittoral species dominate with Cytheridea neapolitana, Carinocythereis whitei, Loxoconcha ovulata and Cytherois frequens. The marine sedimentation occurred at depth from some meters to some tens of meters. In the Aghios Constantinos section, the lagoonal marls are characterised by Euxinocythere schuldtae and a dwarf species of Xestoleberis. Then, a drastic environmental change occurs around the Pleistocene-Holocene boundary and presumably affected the whole Corinth gulf.     

Palynofacies associated to hyperpycnite deposits of the Miocene,Cabo Viamonte Beds,Austral Basin,Argentina

The Miocene deposits in the Punta Basílica locality, southernmost Argentina, are included within the Cabo Viamonte Beds, Cabo Domingo Group, in the Austral foreland basin of Tierra del Fuego province. The prograding clinoform systems were accumulated during a weak compressional tectonic regime that allowed the development of a narrow shelf. Paleoenvironmental reconstructions suggest that these clinoforms comprise two dominant architectural elements, channel-levee and lobe complexes, formed mainly by density hyperpycnal currents in outer shelf to depositional slope environments. The transitional and recurrent (vertical and lateral) alternation between sedimentary structures without rheologic boundaries associated with the co-occurrence of plant remains (Nothofagus) are diagnostic criteria for the recognition of hyperpycnites. This type of density flow typically transports large volumes of sediment and organic matter from proximal to deep-marine settings. Four palynofacies types were recognized in a cluster analysis. In general, the palynofacies show predominance of spores and pollen grains, tissues, cuticles, and spongy to fibrous amorphous organic matter (plant and/or freshwater to brackish algae derived), which reflect different positions within the depositional system (e.g., levee-channel and lobe deposits). The co-occurrence of inshore (Batiacasphaera spp., Lingulodinium sp.) with relatively more oceanic (Operculodinium centrocarpum, Spiniferites spp.) dinoflagellates is a strong indication that shallow-water assemblages have been displaced into deep-water settings. Due to the presence of Lingulodinium hemicystum (first appearance data: 23.0 Ma.) and Pentadinium laticinctum (last appearance data: 11.6 Ma.) an age not older than Miocene and not younger than the Serravallian/Tortorian boundary for the Punta Basílica section is proposed.     

Sedimentary facies and paleoenvironmental interpretation of the Oligocene larger-benthic-foraminifera-dominated Qom Formation in the northeastern margin of the Tethyan Seaway

The well-exposed outcrops of the Bujan, northern Abadeh, and Varkan stratigraphic sections of the Qom Formation in the Iranian part of the “northeastern margin” of the Tethyan Seaway were characterized by abundant biogenic components dominated by foraminifers, coralline red algae, and corals. The Qom Formation is Rupelian–Chattian in age in the study areas. Based on the field investigations, depositional textures, and dominant biogenic components, fifteen (carbonate and terrigenous) facies were identified. These facies can be grouped into four depositional environments: open marine, open lagoon, restricted lagoon, and continental braided streams. The marine facies were deposited on a ramp-type platform. The euphotic inner ramp was characterized mainly by imperforate foraminifera, with co-occurrence of some perforate taxa. These facies passed basinward into a mesophotic (middle) ramp with Neorotalia packstone (F5), coral, coralline algae, perforate foraminiferal packstone (F4), and coral patch reefs (F7). The deeper, oligophotic ramp facies were marly packstones with planktonic and hyaline benthic foraminifera, including large lepidocyclinids and nummulitids. The abundance of perforate foraminifera and the absence of facies indicating restricted lagoonal or intertidal settings suggest that the Varkan section was deposited mainly in open marine settings with normal salinity. The prevalence of larger benthic foraminiferal and red algal assemblages, together with the coral facies, indicates that carbonate production took place in tropical–subtropical waters.     

Facies of a Lower Ordovician carbonate shelf (Mungok Formation: Taebaeksan Basin,Korea)

Summary The lithologic associations within the Lower Ordovician Mungok Formation in Korea define four depositional facies that formed across a continental margin fringing the Sino-Korean block: these facies represent lagoonal/restricted marine, shoal, inner shelf, and outer shelf environments. The stacking pattern of these facies reveals two systems tracts composed of five depositional sequences. The lower highstand systems tract consists of the lagoonal/restricted marine and shoal facies, whereas the upper lowstand systems tract comprises, in ascending order, inner shelf, outer shelf, and inner shelf facies. Three trilobite biofacies are recognized in the Mungok Formation: i.e.,Yosimuraspis, Kainella, andShumardia biofacies in ascending order. TheYosimuraspis Biofacies is dominated byYosimuraspis but also containsJujuyaspis andElkanaspis. The predominance of the endemic eponymous taxon suggests a lagoonal/restricted marine environment. The nearly monotaxicKainella Biofacies, which comprises pandemic genera such asKainella and occasionallyLeiostegium, may represent a less restricted environment than theYosimuraspis Biofacies. TheShumardia Biofacies occurs in the marlstone/shale lithofacies through relatively thick stratigraphic interval and is dominated by cosmopolitan trilobite taxa with some endemic species. The lithofacies and cosmopolitan trilobite assemblage of theShumardia Biofacies indicate that it occupied an outer shelf environment. The vertical succession of lithofacies and trilobite biofacies in the Mungok Formation records in general a shift from a restricted, shallow water environment to deeper-water environment.     

New eocene oysters and the final regression at the southern Rim of the central High Atlas (Morocco)

The oysters Ostrea (Ostrea) todraensis n. sp. and Ostrea (Ostrea) sidialiensis n. sp. are described from the uppermost marine beds south of the central High Atlas in two sections west of Tinerhir. The oysters and the codiacean alga Ovulites margaritula (Lamarck, 1801) Lamarck, 1816 indicate a final regression at the end of Middle Eocene times, i.e. earliest at the end of the Lutetian and latest at the end of the Bartonian. The oysters lived in muddy lagoonal environments sealed off from the sea by oolitic-bioclastic carbonate sand shoals and bordered in landward direction by sebkha-like redbed deposits. Taxonomy and systematic affinities of the oysters are discussed. Their shell structures and depositional history are described in respect to sedimentary infillings. Some selected aspects of biometry explain differences in shell morphometry both between different valves of the same species and equivalent valves from the different populations, illustrating the importance of size and location of the adductor muscle scar.     

Revision of the palynofacies model of Tyson (1993) based on recent high-latitude sediments from the North Atlantic

This study presents data on the distribution of the palynofacies assemblages during the last 1500 years in sediments from Greenland and Faroe Islands fjords and North Atlantic deep-water sites. The sediments studied consist of olive grey to brown muddy silt occasionally mottled and faintly laminated. The chronology is based on ²¹⁰Pb and ¹³⁷Cs measurements and Accelerator Mass Spectrometry ¹⁴C dating of shells and plant remains. As a result, the distribution of particulate organic matter at the high latitudes is controlled by the distance from the shore and water depth. Changes in water depth do not affect the assemblages near the shore. In order to provide a tool using palynofacies analysis for assessing high-latitude depositional settings, we propose to revise the palynological model of Tyson (1993). The revised model is a ternary plot of the relative amount of (1) amorphous organic material, (2) phytoclasts +sporomorphs, and (3) foraminifera + dinoflagellates +acritarchs +other marine algae in the sediment. Based on the integration of quantitative palynofacies analysis and environmental data, the revised model has the potential for discriminating spatial and redox status differences and providing information about terrestrial/fresh water influx at the high latitudes.     

Précisions sur le patron d’empilement de dépôts du Cénomanien-Turonien de Somolinos (Chaîne Ibérique, Espagne) d’après l’étude palynologique

The Somolinos outcrop was widely studied in detailed correlations, stratigraphical architecture and high frequency sequence analysis of Central System and Iberian range successions. A rich assemblage of echinoids, ammonites, bivalves and bryozoans was described; conversely, the palynological study of the Late Cretaceous levels from Somolinos has never been performed up to now. A palynological study of two intervals within a monotonous alternation of grey marls and nodular limestone succession in the Somolinos section was carried out. The two intervals sampled are located in two correlative sequences of Late Cenomanian-Early Turonian and mid Turonian ages, respectively. Both sequences are separated by a sequence boundary, but its location and age in the stratigraphic record differs depending on the authors and the Basin considered. A rich palynofacies with abundant dinoflagellate cysts, pteridophyte spores, conifer pollens and breviary angiosperm grains of the normapolles group is described; besides, inner linings of acid resistant microforaminiferal tests were also found. The analysis of the palynological diagrams clearly indicates that the upper interval corresponds to open sea neritic sediments, deposited in more distal sedimentary environments than those of the lower interval, which were deposited in more proximal environments. This evolution of sedimentary environments represents a major sedimentary change in the general shallowing upwards trend of the late Albian-mid Turonian 2nd order succession, revealing a major facies belt retrogradation of the sequence containing the upper interval. This retrogradational event allows to accurately place the sequence boundary and to improve the arrangement of the low frequency (3rd order) depositional stacking pattern of the Early-mid Turonian in the Iberian Basin and other European and Tethyan basins.     

Late Mississippian to Early Pennsylvanian conodont biofacies in central Montana

Later Mississippian (upper Chesterian) to Early Pennsylvanian (lower Morrowan) conodont faunas were recovered from limestones and calcareous mudstones of the Alaska Bench Formation in central Montana. These faunas define four biofacies, each recognized by the dominance of a particular single element genus for which the biofacies is named. The Adetognathus biofacies represents lagoonal, tidal flat, and barrier sediments. The Declinognathodus/Idiognathoides biofacies represents a deposition in shallow, open marine, offshore conditions. The Rhachistognathus biofacies represents a transitional environment between restricted, intertidal environments and normal marine, offshore environments. The Neognathodus biofacies represents shallow-water bay or lagoonal deposition, where environmental controls have effectively excluded other conodont faunas. The investigation of conodont biofacies provides improved biostratigraphic and paleoenvironmental uses of conodonts.     

Palynostratigraphic,palynofacies, organic geochemical and palaeoenvironmental analysis of the Silurian Tanezzuft Formation in the Ghadames Basin of north-west Libya

Two palynofacies associations are documented from the Silurian Tanezzuft Formation in the Ghadames Basin. These are characteristic of the basal ‘Hot Shale’ and the overlying deposits, referred to here as the Cold Shale. The former reflects deposition in distal suboxic anoxic conditions and is dominated by highly oil-prone amorphous organic matter (AOM) typical of deposition in generally anoxic, restricted marine basins. Only a few acritarchs, prasinophyte algae and chitinozoans occur in association: virtually no spores or cryptospores were recorded. Thick-walled prasinophytes are most numerous in this part of the Tanezzuft Formation in both the Ghadames and Murzuq basins, suggesting enhanced surface water productivity. Deposition took place after the melting of the Late Ordovician ice sheets, which led to a major marine transgression. The palynofacies recorded from the overlying ‘Cold Shale’ deposits indicate deposition in distal shelf and basin conditions that were also relatively anoxic. They contain more palynomorphs, especially acritarchs, and generally less AOM. The phytoplankton assemblages are dominated by simple and thin-walled prasinophyte algae (leiospheres), suggesting dysoxic–anoxic conditions. Overall the middle and the upper parts of the Tanezzuft Formation are regarded as being deposited in distal dysoxic–anoxic shelf, distal dysoxic–oxic shelf and distal suboxic–anoxic basin respectively. Because chitinozoans are very rare, age determinations of the samples investigated are based mainly on acritarchs. The Hot Shale is dated as early-mid Rhuddanian (early Llandovery) whereas the rest of the formation is considered to late Rhuddanian–Telychian in age. As documented previously from other samples of the Tanezzuft Formation in both the Ghadames and Murzuq basins, the Hot Shale has a very high TOC content and excellent source potential for liquid hydrocarbons, whereas rest of the formation is less rich in organic matter with larger terrestrial and oxidized components and hence reduced potential for sourcing hydrocarbons (both oil and gas).     

Facies analysis and sequence stratigraphy of an Upper Jurassic carbonate ramp in the Eastern Alborz range and Binalud Mountains, NE Iran

Upper Jurassic (Oxfordian-Kimmeridgian-Tithonian?) strata of NE Iran (Lar Formation) are composed of medium- to thick-bedded, mostly grainy limestones with various skeletal (bivalves, foraminifera, algae, corals, echinoderms, brachiopods, and radiolaria) and nonskeletal (peloids, ooids, intraclasts, and oncoids) components. Facies analysis documents low- to high-energy environments, including tidal-flat, lagoonal, barrier, and open-marine facies. Because of the wide lateral distribution of facies and the apparent absence of distinct paleobathymetric changes, the depositional system likely represents a westward-deepening homoclinal ramp. Four third-order depositional sequences can be distinguished in each of five stratigraphic measured sections. Transgressive system tracts (TST) show deepening-upward trends, in which shallow-water (tidal flat and lagoonal) facies are overlain by deeper-water (barrier and open-marine) facies. Highstand systems tracts (HST) show shallowing-upward trends in which deep-water facies are overlain by shallow-water facies. All sequence boundaries in the study area (except at the top of the stratigraphic column) are of the nonerosional (SB2) type. Correlation of depositional sequences in the studied sections show that relatively shallow marine (tidal-flat, lagoonal, barrier, and shallow open-marine) conditions dominated in the area. These alternated with deep-water open-marine wackestone and mudstones representing zones of maximum flooding (MFZ).