Shelf Lagoon Research Articles

Stratigraphy, Microfacies, Paleoenvironments and Paleoecology of Asmari Formation (Oligocene - Miocene), Zagros Basin, Western Iran

The present work involves the study of the Asmari Formation from the viewpoint of the stratigraphy, microfacies, and depositional environment. The Khorramabad area, Zagros basin, west Iran, has been investigated for the first time. For this purpose, seventy-three samples were taken from the Shurab section. Eight microfacies have been identified in the Asmari Formation: Sandy wackestone (MF1), Peloidal Mudstone (MF2), Bioclastic milina Wackestone-Packstone (MF3), Bioclastic imperforate foraminifera Wackestone-Packstone (MF4), Coral boundstone (MF5), Bioclastic packstone - grainstone (MF6), Bioclastic imperforate foraminifera grainstone (MF7), Mudstone microfacies (MF8). Two benthic foraminiferal biozones were recognized, which are Archaias asparagus- Archaias Henson Assemblage Zone, which is restricted to Chattian, and the Eulepidina- Miogypsina Assemblage Zone, which is restricted to Aquitanian. The biostratigraphic correlation with other studies in and outside Iran, executed across the studied section, enveloping the Chattian to Aquitanian, divulges a progression in the biozone thickness, indicating a migration of the depositional center towards the northwest of the basin. The Asmari Formation is deposited from an outer ramp and open shelf lagoon and is defined as a tropical environment.

Sedimentary environments in the prelude to the lagerstätten conditions of the Tlayúa Formation (Albian) in central Mexico: A microfacies approach

This contribution presents the first detailed microfacies analysis of the Lower Member of the Albian Tlayúa Formation outcropping in southern Mexico. The basis for this study consists of two measured stratigraphic sections named Tlayúa and Barranca Abuelo. The Tlayúa section is a composite section ∼48 m thick that includes the sedimentary record of the Lower Member and the first 12 m of the Middle Member. This carbonate succession directly precedes the fossil Lagerstätten deposits of the Middle Member exposed at Tlayúa Quarry. A second stratigraphic section of ∼23 m outside of Tlayúa Quarry includes the transitional contact between the Lower and Middle members and is documented herein for the first time. A detailed microfacies analysis combined with stratigraphic and sedimentologic data suggests that the Tlayúa deposits are part of a tropical shallow-water carbonate platform similar to the modern Great Bahama Banks.Microfacies analysis of the studied sections reveals the evolution of a protected carbonate setting affected by recurring storm events of different intensities. The succession records shelf lagoon sedimentation for most of the Lower Member and its gradual change to tidal flat deposition in its upper part. Shelf lagoon deposits consist of a diverse set of subtidal microfacies characterized by a carbonate fabric with low taxonomic group diversity that indicates stressed environmental conditions. Tidal flat deposits consist of shallowing upward cycles, each less than 1 m thick. Commonly, the peritidal cycles are incomplete and consist of mud-rich shallow-subtidal intertidal microfacies, which are clearly observed at the top of the Lower Member. Both microfacies are similar to those observed in the complete cycles described below. The complete cycles consist of mud-rich shallow-subtidal microfacies with low diversity of bioclasts that gradually change to intertidal microfacies characterized by the association of microbialites with desiccation features that suggest intermittent subaerial exposure. Intertidal microfacies record an important change in the composition of the carbonate factory because microbial processes generated most of the sediments in these microfacies. Supratidal deposits characterized by the presence of subaerial exposure structures and calcified pedogenic products, complete the peritidal succession and were identified at the base of the Middle Member. Thus, the presence of soil pisoid-like structures, clotted micrite, possible Microcodium structures, and alveolar textures indicate a sufficiently long subaerial exposure for the formation of pedogenic products, and therefore, a break in the marine sedimentation of the Tlayúa Formation.

Neoproterozoic extensional basins and its control on the distribution of hydrocarbon source rocks in the Yangtze Craton, South China

• Neoproterozoic source rocks in the Yangtze Craton. • Structural and sedimentary patterns of the extensional basins evaluated. • The source rocks are related to the extensional basins, climate and life evolution. • Favorable areas for the source rock distribution are predicted. Hydrocarbon exploration in the Neoproterozoic extensional basins in the Yangtze Craton of South China has made prominent progresses in the past decades. However, the spatial distribution of source rocks across the craton still remains unclear. Here, we use outcrop observations, borehole data, and reflective seismic profiles to investigate the prototype, distribution and sediment infilling of these basins in the Yangtze Craton during the periods of source rock deposition. The basin prototype is categorized based on its tectonic origin and structural-sedimentary feature as follows. (1) During the deposition of the Datangpo Formation, rift basins extensively developed, band are characterized by a series of graben or half graben structures that consist of basin-controlling normal faults. The source rocks in the Datangpo Formation were deposited as the inner shelf basin or deep shelf facies, obtaining high thickness (i.e., 200 m) in the Wuling rift basin. (2) During the deposition of the Doushantuo Formation, the intracratonic rift basin and passive continental margin basin formed. These basins were controlled by syn-depositional faults that inherited the existing rift fault structures. The source rocks of the Doushantuo Formation were deposited as shelf lagoon, slope to deep-water basin facies. (3) During the deposition of the Dengying Formation, the intraplatform rift basin and passive continental margin basin developed with influence of regional tectonic extension. Not only are these basins affected by the preexisting faults, but also by the newly developed syn-depositional faults. The argillaceous source rocks of the Dengying Formation deposited as slope and deep-water basin facies. Further in-depth analyses revealed that a range of mechanisms may have contributed to the formation and distribution of source rocks in the Yangtze Craton, such as the break-up of the Rodinia Supercontinent, paleaolatitudes, glacial-interglacial geological events, basin extension and sedimentary environments. By integrating the distribution of the source rocks and the late tectonic reworking, this study predicts several targets for the source rock distribution in the Yangtze Craton, including in the central Sichuan, northeastern Sichuan, southern Shanxi, western Hubei, northeastern Guizhou-western Hunan, and the northeastern Guangxi-central Hunan areas.

Sandstone reservoir quality in light of depositional and diagenetic processes of the Messinian Qawasim Formation, onshore Nile Delta, Egypt

• Sandstone in the upper part of the Qawasim Formation is the main reservoir in the El Tamad field. • Primarily, the sandstone is arenite in composition, well sorted with sub-rounded to rounded grains. • Diagenesis enhanced the reservoir quality of the sandstone with low-cement content, poor compaction and weak fracturing. • Clastic rocks of the Qawasim Formation were deposited in a shelf environment as stacked barrier-island bars. • Carbonate rocks of the Qawasim Formation were deposited in a shelf lagoon with open circulation. The Nile Delta region is the main gas-producing province in Egypt. Late Miocene sediments are the target of drilling in the onshore Nile Delta where it has the potentiality to store and produce hydrocarbons. Five wells of the Messinian Qawasim Formation at El Tamad Field were selected for the present study. Detailed petrographic investigations were carried out to reveal the depositional and diagenetic features of the Qawasim Formation rock units. Two main facies were distinguished in the studied formation: clastic and carbonate facies. The clastic rocks include conglomerate, sandstone and shale, while the carbonate rocks include limestone and dolostone. Depositional, diagenetic and petrophysical parameters of the sandstones of the Qawasim Formation reflect that this rock unit an excellent reservoir for hydrocarbon. These microfacies display depositional features such as well sorting, open packing and poor cementation in both coarse and fine-grained microfacies. The porosity of sandstone was enhanced by diagenetic processes such as dolomitization of calcite cement and low compaction that resulted in grain cracking, while the dissolution of feldspars had a subordinate impact on increasing porosity due to their low abundance and the resultant authigenic clay minerals.

Hrabalicrinus zitti gen. et sp. nov., and other Upper Jurassic crinoids (Echinodermata, Crinoidea) from the Brno area (Czech Republic)

Jurassic (Oxfordian) crinoids from the Brno area (Czech Republic) are described in details for the first time. A rich crinoid assemblage consisting of cups, isolated cup elements, brachial plates, columnals, pluricolumnals, and cirrals is assigned to isocrinids Isocrinus amblyscalaris (Thurmann), Balanocrinus subteres (Münster), B. pentagonalis (Goldfuss), Isocrinida indet., comatulid Hrabalicrinus zitti gen. et sp. nov., cyrtocrinids Lonchocrinus sp., Cyrtocrinus cf. nutans (Goldfuss), Pilocrinus moussoni (Desor), Tetracrinus moniliformis Münster, Cyrtocrinida indet., and millericrinids (Millericrinida indet.). Crinoids already mentioned from the Jurassic strata of the Czech Republic are subsequently revised. Apart from the isolated remains of Isocrinida, Millericrinida, and Thiolliericrinida, the presence of any other taxon reported from this area should be treated with extreme caution. A rare example of non-regenerative columnal healing (the so-called callus) in I. amblyscalaris is also described. Based on sedimentology and microfacies, the Jurassic limestones were deposited in various palaeoenvironments of upper carbonate platform and shelf lagoon (0 to >50 m palaeodepth).

Application of Fracture Barrier Analysis in Well Stimulation Planning for Upper Baturaja Limestone Formation Based on Well Log & Drill Cutting Data from OBF-01 and OBF-04 Wells, Offshore Southeast Sumatra

In general, the South Sumatra Regional Stratigraphy of the Baturaja Limestone Formation facies is deposited on the Buildup Carbonate (Reef) and the Limestone Clastic Carbonate of the Baturaja Formation which grows as a buildup reef on the platform in the Basement High (Horst) underneath is the Lemat Formation volcanic deposits. Referring to the facies model in general, the Baturaja Limestone Formation, the depositional environment starts from Shelf Lagoon Open Circulation - Winnowed Edge Sand - Organic Buildup - Fore Slope - Deep Shelf Margin - Open Sea Shelf - Basin, meaning that carbonate is formed starting from pure organic Cabonate Buildup Reef without / a little sludge / mud to the Carbonate Basin where more muddy / mud is present, this condition causes clay minerals to also more and more mix with Terigenous Clastics (Quartz, feldpar). The complexity of the Baturaja Limestone Formation requires fracture barrier analysis associated with well stimulation planning in order to increase oil productivity with the appropriate method. Fracture barrier fracture analysis is an approach method to determine the depth interval that becomes a barrier in hydraulic fracturing by correlating the results of geomechanical analysis from well log data and mineralogical analysis from drill cuttings data, so that a commonly used well stimulation method can be selected, namely hydraulic fracturing, acidizing, and acid-fracturing.From the ternary diagram plot the XRD (bulk) analysis results show that the distribution of the main minerals (Quartz, Clay, Calcite) is more dominant in the ductile zone, hard to frac category. This shows that all the depth intervals in the OBF-01 and OBF-04 wells are more ductile, and are not recommended for hydraulic fracturing. From the XRD (bulk) analysis, Calcite mineral is more dominant, so for well stimulation work it is recommended to use acidizing or acid-fracturing.

Effects of diagenesis on the geomechanical and petrophysical aspects of the Jurassic Bir El-Maghara and Masajid carbonates in Gebel El-Maghara, North Sinai, Egypt

In the present study, the Jurassic Bir El-Maghara and Masajid Formations in North Sinai are subjected to a detailed microfacies analysis to identify the effect of the diagenetic factors on their geomechanical and petrophysical properties. The Bir El-Maghara Formation consists of mudstone microfacies, whereas the Masajid Formation consists of foraminiferal and molluscan mudstone microfacies, oolitic grainstone microfacies, mudstone microfacies, and foraminiferal-algal sparitic mudstone microfacies. These microfacies were deposited in restricted platform conditions through the entire section with some oscillations into shelf lagoon conditions in the middle parts of the sequence. The porosity and bulk density of Masajid and Bir El-Maghara Formations are strongly correlated with their petrophysical and geomechanical properties. These correlation relationships can help estimate the geomechanical properties statistically in terms of the petrophysical parameters.

Giant ooids in a Neoproterozoic carbonate shelf, Simla Group, Lesser Himalaya, India: An analogue related to Neoproterozoic glacial deposits

Abstract This study is the first comprehensive documentation of giant ooids and related facies associations in the Kunihar Formation, Proterozoic Simla Group Lesser Himalaya, Himachal Pradesh, analysing the understanding of processes and controls on development of giant ooids. Based on field observations, supplemented by outcrop based facies analysis, petrography and delineation of environmental variations, four facies associations have been delineated: (i) Peritidal siliciclastic-carbonate (FA1) (ii) Shelf lagoon (FA2) (iii) Reef complex (FA3) (iv) Fore reef slope (FA4). Deposition of giant ooids and associated facies associations of Kunihar Formation occurred in a carbonate rimmed shelf with high tidal influence. Size of giant ooids from Kunihar Formation is the largest as compared to giant ooids from other geological formations. Kunihar giant ooids developed when normal ooids were washed from ooid shoals (intertidal) into slightly deeper regions (shallow subtidal) resulting in the increased dimension of ooids in suspension due to higher hydrodynamics. Scanning electron microscopy (SEM) studies support microbial origin of ooids, giant ooids and stromatolites of Kunihar Formation. Increased microbial activity in Kunihar Formation is attributed to increase in nutrients by virtue of weathering of underlying Darla volcanics. Abundant carbonate and Microbially induced sedimentary structures (MISS) deposition in lower part of Simla Group points to increased microbial activity which likely increased the volume of oxygen in Neoproterozoic atmosphere ushering in Ice House conditions during the subsequent deposition of Blaini Group. Giant ooids associated with Neoproterozoic glacial deposits throughout the world, occupy stratigraphic positions below, above or between glaciations. Simla Group is another example where giant ooids lie stratigraphically below Marinoan Blaini Tillites. Increased magmatic activity and weathering before and during the Neoproterozoic glaciations increased nutrients in marine waters which increased algal growth. Thus, giant ooids were deposited due to such phases of increased microbial activity before/after glaciation, and during interglacial periods.

Middle–Upper Jurassic marine gastropods from central Saudi Arabia

Ferrari, M., El-Hedeny, M., Zakhera, M., El-Sabbagh, A. & Al Farraj, S., May 2018. Middle‒Upper Jurassic marine gastropods from central Saudi Arabia. Alcheringa.A total of 68 gastropod specimens are reported from the Middle‒Upper Jurassic sedimentary successions exposed at central Saudi Arabia. The studied material comes from the Tuwaiq Mountain and Hanifa formations at the Khashm al Qaddiyah, Dirab, Jabal al Abakkayn and Maáshabah sections. Thirteen species are identified, described and illustrated. Among them, a new Aporrhais species (A. sauditica sp. nov.) is introduced. In addition, two further possible new Pseudomelania species from the same strata are mentioned. Other members of the assemblage include Kosmomphalus? sp. aff. K. reticulatus Fischer, Bourguetia? sp. aff. B. saemanni (Oppel), Bourguetia? sp., Ampullospira sp., Globularia? sp. cf. G. bajociana Fischer, Purpuroidea sp. aff. P. glabra Morris and Lycett, Purpuroidea sp., Cossmannea sp. aff. C. desvoidyi (d’Orbigny), Cryptoplocus sp. aff. C. depressus (Voltz) and Actaeonina? sp. Stratigraphically, seven species of this gastropod assemblage were only reported from the Middle Jurassic, whereas the other six ones are extending from the Middle to the Upper Jurassic of the studied succession. As compared with their gastropod content, the Khashm al Qaddiyah represents the richest section (33 out of 68 specimens, 48.5%), whereas the Maáshabah section showed an impoverished gastropod assemblage (only three specimens, 4.4%). The species reported here show paleobiogeographical affinities with coeval gastropod assemblages from India, east Africa, Middle East (Israel, the Sinai of Egypt) and western Tethys. The identified species confirm three depositional settings: open shelf lagoon, shoal/fore-shoal and open marine environments. The lower degree of fragmentation, poor sorting and scarcity of abrasion indicate a parautochthonous faunal assemblage.Mariel Ferrari [mferrari@cenpat-conicet.gob.ar] Instituto Patagónico de Geología y Paleontología (IPGP-CCT CONICET-CENPAT), Bvd. Brown 2915, U9120CD, Puerto Madryn‒Chubut, Argentina; Magdy El-Hedeny* [mmelhedeny@gmail.com] Department of Geology, Faculty of Science, Alexandria University, Alexandria 21568, Egypt; Mohamed Zakhera† [zakhera40@yahoo.com] Department of Geology, Faculty of Science, Aswan University, Aswan 81528, Egypt; Ahmed El-Sabbagh [ah.elsabbagh@gmail.com] Department of Geology, Faculty of Science, Alexandria University, Alexandria 21568, Egypt; Saleh Al Farraj [salfarraj@hotmail.com] College of Science, King Saud University, Riyadh 12371, Saudi Arabia. *Also affiliated with Deanship of Scientific Research, King Saud University, Riyadh 11451, Saudi Arabia. †Also affiliated with Vice Rectorate for Development and Quality, Najran University, Najran 11001, Saudi Arabia.

Microbiostratigraphy,Microfacies and Depositional Environment of the Sarvak and Ilam Formations in the Gachsaran Oilfield, southwest Iran

Microbiostratigraphy, microfacies and depositional environments of the Sarvak and Ilam formations have been studied in the Gachsaran Oilfield (well no. 335). The thickness of the Sarvak and Ilam formations are 95.5 and 66.5 meters respectively and they consist of limestone with minor shale. Biostratigraphic analysis of these formations has resulted in the recognition of two biozones in the Sarvak Formation: 1) Nezzazata–alveolinid assemblage zone, 2) Rudist debris; and one biozone in the Ilam Formation: the Rotalia skourensis–algae assemblage zone. The age of the Sarvak Formation is considered as late Cenomanian and the Ilam Formation as Santonian. The zones match with the previous biozonation scheme of Wynd (1965) in the Zagros area. The intervals are subdivided into 11 microfacies that were deposited in four major depositional environments. These include shelf lagoon, platform margin, slope, and basin depositional settings. A carbonate shelf without an effective marginal barrier is considered as a depositional model for both formations.

Lithostratigraphy and depositional environments of the upper cretaceous deposits in the central Taurides (S Turkey)

The study area is located in the Central Taurides (southern Turkey), which is bounded by the Kirkkavak fault to the west and Ecemis fault to the east. The sequences are studied in detail based on measured sections composed of the rocks deposited during the Cenomanian–Maastrichtian and located within different tectonic units previously described in the Taurides. The study materials include 217 thin section data from seven Cenomanian–Maastrichtian sequences of outcropping in different parts of the Central Taurides. The sediments deposited during the Cenomanian–Maastrichtian period in the Central Taurides are subdivided into eight units based on their lithological, paleontological, and textural properties. The lower boundaries of the upper Santonian and Campanian are unconformable contacts. The Upper Cretaceous sequence starts with the middle Cenomanian and represents a continuation of the Lower Cretaceous tidal flat and shelf lagoon sequence. Upper Turonian–Coniacian sediments are not observed due to the eustatic sea level drop. The second main transgression period of the Upper Cretaceous platform took place in the Santonian. This unit is represented by limestones composed of wackestones/packstones containing benthic foraminifera and rudist fragments, which are deposited in tidal flats and subtidal environments. The late Campanian starts with a transgression, and the environment transformed transitions into slope facies from inner platform facies, as a result of the thrust of ophiolitic rocks. In the following period, slope front and basin plain environments were dominant due to the increasing slope. Slumped pelagic limestones were deposited on the slope. Planktonic foraminiferal pelagic limestones were unconformably deposited on plaque limestone in the slope front environment depending on the increase in slope gradient and local faulting. As a result of decreasing tectonic activity, the sediments were deposited onto a stable basin plain. They were initially fed from the nearby carbonate platform and then by siliciclastic turbidites derived from the thrusted ophiolitic rocks. In this study, the lithostratigraphic properties of the Cenomanian–Maastrichtian units outcropping in various parts of the Central Taurides are described. The sedimentary deposits described here suggest different basinal conditions in the region.

Micropalaeontology and sedimentology of an inter-evaporite Arab-C carbonate exposure, Riyadh, Saudi Arabia

Alternating carbonate and evaporite successions of strata characterise the Late Jurassic, Kimmeridgian–Tithonian, Arab Formation of Saudi Arabia. Outcrops of the Arab Formation are poorly exposed and are mostly studied from subsurface samples, due to dissolution of the evaporites and the collapse of intervening carbonate strata. The type section for this formation is in the subsurface, where a more complete succession is available for study where four carbonate and evaporite members, namely D, C, B and A in ascending stratigraphic order, have been identified. The Arab has important hydrocarbon associations, due to its excellent porosity, permeability and proximity to underlying source rocks and overlying evaporite seals. This study provides new insights into the internal palaeo environment and depositional cyclicity of the Arab C, with implications to improve understanding of microfacies and porosity variations of the reservoir equivalent. The studied section provides a rare datapoint for the proximal palaeoenvironment of the Arab C carbonates that are mostly studied in the subsurface far to the east. Arab Formation carbonates are exposed along road cuts in the city of Riyadh. One recent exposure, 27 ft (8.2 m) thick, provides new micropalaeontological, sedimentological and palaeoenvironmental information for this stratigraphic unit that is otherwise elusive as an exposure. The new section is located next to the flanks of a sinkhole within gently folded strata and implies that the exposure must represent one of the supra-Arab D evaporite members, for which regional mapping indicates assignment to the C member of the Arab Formation. The Arab C consists of a succession of low diversity microfossiliferous wackestones and packstones, in which the red alga Permocalculus cf. iranae and rhaxellid sponge spicules are well represented. The exposure extends the stratigraphic range of Bramkampella arabica from the Sulaiy Formation down section into the older Arab C. Arab C microfacies are interpreted to represent deposition in a normal marine salinity, carbonate platform setting, probably within a shelf lagoon, mostly below fair-weather wavebase and within storm wavebase. Within the succession is a distinctive grainstone bed containing the benthonic foraminifera Everticyclammina kelleri, Kurnubia jurassica, Nautiloculina oolithica, Pseudocyclammina lituus and Redmondoides lugeoni and is considered to represent the top of one high frequency sequence (HFS). Immediately above the grainstone additional strata in the measured section form the base of a second HFS.

Depositional environment and micropore characteristics of the Ediacaran Doushantuo Formation black shale in Western Hubei, China

The black shale of the lower Sinian (Neoproterozoic) Doushantuo Formation, located in the Yangtze area (Hubei, China), has a great potential to contain shale gas. The main depositional environments of the region included tidal flats, shelf lagoons, shelf margins, and slopes. While black shale formed from deposits in a shelf lagoon setting, the sediments from the tidal flats were dolomitic. Several factors affect the sediments deposited in these environments, including sediment surface energy (i.e., quiescent or turbulent), salinity, and the presence of either oxic or reducing conditions. The depositional environments of segments B and D of the Doushantuo Formation in the Jiulongwan section were significantly different; specifically, segment D was anoxic to euxinic. As a result, the total organic carbon (TOC) in the segment D shale is higher than that in segment B. This study also used field emission scanning electron microscopy to show that pores in organic matter are the main gas storage space and that the organic matter pores in segment D have diameters less than 100 nm. The increasing of the organic matter content increases organic pore and porosity of the reservoir. TOC is not only the main controlling internal factors of the micropores of the shale in Doushantuo Formation but also provides the important storage space for shale gas. The quiet and hypoxia environment of the black shale in Doushantuo Formation is the main factor controlling the enrichment of organic matter, and the enrichment of organic matter is beneficial to improve the physical property of the shale reservoir. Therefore, black shale formed in the quiet and anoxic depositional environment has better reservoir physical property.

Microbiostratigraphy, Microfacies and Depositional Environment of the Sarvak Formation in Bi Bi Hakimeh Oil Field (Well No. 29), Southwest Iran

The Sarvak Formation in Bi Bi Hakimeh oil field (well No.29) with the thickness of 324 m consists of limestone in Cenomanian - Turonian period. The upper boundary of the Sarvak Formation is in the form of disconformity with the Gurpi Formation. The lower boundary of Sarvak Formation is not clear because of lack of sequential excavation. According to microbiostratigraphy studies, 4 genera and 2 species of planktonic foraminifera and 22 genera and 28 species of benthic foraminifera are identified and based on them, three biozones were introduced including Nezzazata–Alveolinids assemblage zone, Rudist debris, and Oligosteginid facies. The zones are in consistence with zones introduced by Wynd (1965). In the study area (southwest of Iran), the Sarvak Formation is subdivided into 8 microfacies that are distinguished by petrographic analysis on the basis of their depositional textures and fauna. In addition, four major depositional environments were identified in the Sarvak Formation. These include shelf lagoon, platform margin, slope and basin environmental settings, which are interpreted as a carbonate shelf without an effective barrier separating the platform from the open ocean.

Microfacies and diagenesis of the reefal limestone, Callovian Tuwaiq Mountain Limestone Formation, central Saudi Arabia

In order to document the microfacies and diagenesis of the reefal limestone in the uppermost part of the Callovian Tuwaiq Mountain Limestone Formation at Khashm Al-Qaddiyah area, central Saudi Arabia, scleractinian corals and rock samples were collected and thin sections were prepared. Coral framestone, coral floatstone, pelloidal packstone, bioclastic packstone, bioclastic wacke/packstone, algal wackestone and bioclastic foraminiferal wacke/packstone were the recorded microfacies types. Cementation, recrystallization, silicification and dolomitization are the main diagenetic alterations affected the aragonitic skeletons of scleractinian corals. All coral skeletons were recrystallized, while some ones were dolomitized and silicified. Microfacies types, as well as the fossil content of sclearctinian corals, bivalves, gastropods, brachiopods and foraminifera indicated a deposition in environments ranging from shelf lagoon with open circulation in quiet water below wave base to shallow reef flank and organic build up for the uppermost reefal part of the Tuwaiq Formation in the study area.

Biofacies, taphonomy, and paleobiogeography of the Kamar-e-Mehdi Formation of east-central Iran, a Middle to Upper Jurassic shelf lagoon deposit

The Callovian–Lower Kimmeridgian Kamar-e-Mehdi Formation of the western Tabas Block, part of the Central-East Iranian Microcontinent, represents the fill of an extensive carbonate shelf lagoon situated between a barrier carbonate platform in the east and the uplifted Yazd Block in the west. In the course of its development, the shelf lagoon experienced increasing isolation which, together with a climatic change to more arid conditions, led to deposition of evaporites and severe restriction of the benthic fauna. Sedimentation in the predominantly low-energy lagoon was controlled by an influx of fine-grained carbonate from the neighboring platform, biogenic carbonate production within the lagoon and episodic influx of siliciclastic material by high-density currents from the uplifted western margin. Much of the succession is composed of meter-scale asymmetric cycles that record decreasing terrigenous input as well as increasing shelliness and substrate firmness due to a decreasing sedimentation rate. They are interpreted as small-scale climatic cycles recording changes in humidity. Larger-scale cycles, comprising 250–400 m in thickness, document changes in terrigenous influx and may also be climate-controlled, although a tectonic origin cannot be excluded. The shelf lagoon was populated by several level-bottom communities and small patch reefs. The latter are of low diversity, composed of calcareous sponges, corals, or oysters, and are usually associated with thick microbial crusts. The level-bottom assemblages are dominated by bivalves, and rarely also by gastropods or brachiopods. Fossils occur scattered or concentrated in pavements and beds. Shell concentrations are autochthonous to allochthonous, most commonly parautochthonous. Concentrating agents were storm-induced flows and low sedimentation rates, less commonly also storm-waves. Within the small-scale cycles, infaunal assemblages prevail but are replaced by epifaunal assemblages at their top. The latter are commonly dominated by the bivalves Pseudopecten tipperi and Camptonectes (Grandinectes) teres, the two most characteristic faunal elements of the shelf lagoon. Diversity values of the benthic fauna were low to moderate. Environmental stress was mainly produced by elevated salinity values, at times possibly intensified by lowered oxygen values and soft substrate conditions. Paleobiogeographically, the benthic macrofauna exhibits a close relationship to that of European shelf seas but contains some immigrants from the southern shores of the Tethys and the northwestern Pacific. The degree of endemism was low (two taxa).

Plio–Pleistocene palaeogeography of the Llanura Costera del Caribe in eastern Hispaniola (Dominican Republic): Interplay of geomorphic evolution and sedimentation

This paper aims to reconstruct the palaeogeographic evolution of the Llanura Costera del Caribe (LCC) in eastern Hispaniola (Dominican Republic) during the Pleistocene, adding new insights to published information on Pliocene–Early Pleistocene deposits. The LCC is a generally flat region comprising the unfolded sedimentary cover of the Cordilleras Central and Oriental. Within this cover, the Pliocene–Early Pleistocene Yanigua Formation,mainly consisting of marl, changes seawards to the mainly limestone Los Haitises Formation. Both units formed in a shallow-water platform rimmed by a reef barrier at least in the latest depositional stages. The overlying Pleistocene La Isabela Formation consists of two major offlapping reef terraces, in which a reef core enclosed a lagoon, and prograded over forereef bioclastic debris. Two belts in LCC's morphostructure directly reflect its sedimentary evolution. The Inner Belt extends over the marly substrate of the Yanigua Formation and the Coastal Belt comprises three major surfaces corresponding to the depositional top of the Los Haitises Formation (Upper Surface), and to the Upper and Lower terraces of the La Isabela Formation (Intermediate and Lower Surfaces, respectively). The MIS 5e age of the 10–20 m high Lower Terrace implies a low uplift rate of 0.033–0.068 mm/yr for the Lower Surface. The Pliocene–Early Pleistocene platform was emergent in the Early–Middle Pleistocene. The Early Pleistocene reef barrier separated endorheic watersheds extending over the former shelf lagoon from the open ocean. Reefs built the Upper Terrace of the La Isabela Formation during one or several Middle Pleistocene highstands and the Lower Terrace during MIS 5e and previous highstands. Siliciclastic deposits in this terrace record the opening to the Caribbean Sea of watersheds in the eastern LCC. The Lower Terrace emersion and opening of the large drainage systems of the western and central LCC took place after MIS 5e. Relative sea level fall and emersion of a large area did not imply increased terrigenous sedimentation in the adjacent marine basin. Development of endorheic watersheds over most of the emergent surface delayed the diachronic arrival of siliciclastics into the marine basin for hundreds of thousands of years.

Stratigraphic division and correlation and sedimentary characteristics of the Cambrian in central-southern Sichuan Basin

Abstract Under the guidance of sequence stratigraphy and through analyses of large amounts of well and outcrop data, the correspondence of all formations in the Cambrian of southern and central Sichuan Basin is defined and a new scheme of stratigraphic division and correlation for the Cambrian in central-southern Sichuan Basin is proposed based on the characteristics of lithology, logging curves and fossils. The Jiulaodong Formation in southern Sichuan Basin is corresponding roughly to the Qiongzhusi Formation in central Sichuan Basin, the Yuxiansi Formation in southern Sichuan roughly to the Canglangpu, Longwangmiao and Gaotai formations in central Sichuan Basin, the Xixiangchi Group in southern Sichuan roughly to the same group in central Sichuan. The Cambrian in central-southern Sichuan Basin is divided into the Qiongzhusi Formation, Canglangpu Formation, Longwangmiao Formation, Douposi Formation and Xixiangchi Group from bottom to top. On the basis of that, sedimentary facies of the Cambrian was studied. Controlled and impacted by the paleo-uplifts formed by Caledonian Orogeny, the deposition of the Cambrian experienced three evolutionary stages: shore–continental shelf, restricted platform–evaporation flat–shelf lagoon, restricted platform. The sandy, oolitic and carst dolomites in the Longwangmiao Formation and the Xixiangchi Group are the targets of oil and gas exploration.

Relative dispersion of surface drifters in a barrier reef region

The spreading of two clusters of satellite‐tracked surface drifters was quantified by ‘relative dispersion’ and ‘relative diffusivity’ (the time derivative of relative dispersion) in two regions of the southern Great Barrier Reef (GBR): a low‐density reef matrix of complex topography and a shelf lagoon over flat bathymetry. Although based on a small data set, averaged values of these parameters over a 12 day period with concurrent measurements for the two regions indicated that relative dispersion and diffusivity of drifter pairs were 182 and 52 times greater, respectively, along the reef matrix than found in the lagoon. Relative diffusivities in the southern GBR were anisotropic and varied largely in space and time within the spatial scales of 1–100 km and temporal scales of hours to 16–20 days; they were considerably enhanced by high‐frequency tidal processes. Submesoscale processes (<20 km) were important in the southern GBR, particularly in areas with complex topography where secondary circulations around the reefs and regions of steep bathymetry caused abrupt increase in dispersion. Although the dispersion was much higher along the reef matrix, the presence of ring circulations around the islands and wake eddies in the lee of islands can act as trapping mechanisms to keep particles within the reef matrix, retarding the loss to open water areas. This provides an efficient physical mechanism for both enhancing larval connectivity between reefs and, at the same time, trapping larvae when in the vicinity of reefs.

Taphonomy and palaeoecology of high-stress benthic associations from the Upper Jurassic of Asturias, northern Spain

The late Kimmeridgian Tereñes Formation, exposed on the coast of Asturias, northern Spain, displays a complex pattern of directed changes of grain size, carbonate content, and skeletal concentrations. In its upper part, here investigated, the formation represents a protected shelf lagoon in which four major facies types are distinguished: The Nanogyra virgula mudstone (1) and the Corbulomima concentrations (2) are characterized by concentrations of small bivalves. The carbonate mudstone (3) contains pseudomorphs after gypsum crystals and thin crusts of gypsum, occasionally in connection with thin microbial layers. Finally, the silty to fine-sandy marlstone and micrite and marly silt (4) is highly bioturbated and contains a moderately diverse benthic macrofauna. These facies indicate a generally quiet environment punctuated by brief episodes of high water energy. Two low-diversity macrobenthic assemblages can be recognized, each of them strongly dominated by a single bivalve taxon. The Nanogyra virgula assemblage exhibits a higher diversity than the near-monospecific Corbulomima assemblage. The former lived in well aerated waters of slightly reduced salinity, and the latter in dysoxic waters of more strongly reduced salinity. The environmental stress responsible for the extremely low species richness and evenness is thought to be multifactorial, produced by reduced salinity, dysoxic conditions, and a soft substrate, and resulted in simple food chains. The eurytopic opportunist Corbulomima was the only element of the shelly macrobenthos that was able to thrive in the shelf lagoon under these conditions. It occurs in countless mm- to cm-thick pavements and shell beds which show evidence of winnowing, influence of weak currents, and occasionally of distal storms, as can be deduced from the orientation pattern of shells. These rhythmic Corbulomima concentrations are explained as reflecting small-scale climatic fluctuations between wetter, stormier conditions leading to mixing of the water masses and enabling colonization of the lagoonal floor by the bivalve, and drier, more tranquil conditions. The latter resulted in a stratified water mass and anoxia at the bottom. Superimposed on this rhythmic alternation are three higher orders of cycles which are partly climatic controlled, partly reflect changes in relative sea level.