Facies and depositional settings of the Middle Eocene-Oligocene carbonates in Kutch

Middle Eocene Fulra Limestone and Oligocene Maniyara Fort Formation represent platform carbonate deposits of Kutch at the north-western margin of India. These carbonates contain larger benthic foraminifera, including Alveolina, Assilina, Discocyclina, Lepidocyclina, Miogypsina, Nummulites and Spiroclypeus. This study presents paleodepositional and paleobathymetric interpretations for both formations using benthic foraminifera in combination with lithological association, sedimentary structures and early diagenetic features. The six carbonate facies comprising the Fulra Limestone indicate a depositional spectrum ranging from bar-lagoon to mid-ramp depositional conditions. It records several shallowing upward cycles, leading to emergence and formation of paleokarst. The four carbonate facies of the Maniyara Fort Formation represents deposition within the inner ramp setting in bar-lagoon and patch-reef environment, while intervening fine siliciclastics correspond to episodes of relative sea level fall. Nummulitic accumulations form low-relief bars within the fair weather wave base in both the formations. The depositional setting of the Paleogene carbonate in Kutch broadly resembles Eocene platformal deposits in the circum-Tethys belt.     

The role of oceanographic conditions on Cenozoic carbonate platform drowning: Insights from Alpine and Apennine foreland basins

The drowning of carbonate platforms is a fascinating topic because it may be induced by several concomitant factors. A key setting to investigate carbonate platform drowning is a foreland basin. The Nummulitic Limestone ramp developed in the foreland of the western Alps during the Bartonian, while the Lithothamnion and Bryozoan Limestone ramp deposited in the foreland of the central Apennines between the Burdigalian and Serravallian. The drowning of Nummulitic Limestone is related to the reduction in light for the photo‐dependent biota according to the progressively increasing depth and poor efficiency of aphotic carbonate factory. The drowning of the Lithothamnion and Bryozoan Limestone ramp is attributed to the deterioration of the environmental conditions that predate tectonic subsidence related to the Apennine orogenesis. Eutrophication triggered by upwelling events induced a crisis in the shallow‐water carbonate production, while during the following tectonic subsidence the shallow‐water carbonate factory was definitively inactive.     

MECO and Alpine orogenesis: Constraints for facies evolution of the Bartonian nummulitic and Solenomeris limestone in the Argentina Valley (Ligurian Alps)

The Eocene represents the last greenhouse interval before Present. The maximum warming during the early Eocene was followed by a long-term cooling trend culminating in the Antarctica glaciation at the base of the Oligocene. Superimposed on this long-term cooling trend there is a prominent transient warming event known as the middle Eocene climatic optimum (MECO) occurring during the early Bartonian. The carbonate ramp succession cropping out in the Argentina Valley (Maritime Alps) offers new insights on the evolution of shallow water realms during this time interval. This ramp displays two main facies belts, middle and outer ramp. The middle ramp is recorded by larger benthic foraminifer floatstone to rudstone, passing to Solenomeris branches and nodule floatstone to rudstone evolving to branching coralline algal floatstone. The outer ramp is dominated by bioturbated marly wackestone to packstone alternating with larger benthic foraminifer floatstone with a silty matrix. The investigated ramp was affected by continuous dispersion and reworking of the skeletal components as other Tethyan Eocene ramps. During the Eocene, the Alpine foreland was influenced by fine terrigenous input controlling the trophic conditions and promoting seawater stratification and the development of a strong pycnocline, for which many perturbations could propagate as internal waves. The reworking of skeletal components of the ramp has been ascribed to the action of internal waves. The switch of carbonate production from a carbonate factory dominated by larger benthic foraminifera to a factory in which the encrusting foraminifer Solenomeris was the main carbonate producer biota, is indicative of a radical change in palaeoenvironmental conditions affecting the early Bartonian. The acme of Solenomeris often coincided with the crisis of carbonate producers during intervals of an evident deterioration of environmental conditions. In this case the acme is probably related to the adverse conditions linked to the (MECO) warming event. Finally, the drowning of the nummulitic ramp has been caused by light reduction for the photo-dependent biota due to progressively increasing depth linked to flexural subsidence of the foreland plate, and minor efficiency of the aphotic carbonate factory.     

A New Hiatus within the Lutetian of the El Basatin Section,Gebel Mokattam,Egypt:Field and Sedimentological Observations,with Special Emphasis on Nummulites

The Eocene succession of the El Basatin Section in Gebel Mokattam, east of Cairo, consists, from base to top, of two main units; the Mokattam and Maadi Formations. The Mokattam Formation consists of two Members, the Building Stone Member and the Giushi Member. The Upper Building Stone Member yielded six species of Nummulites belonging to the Upper Lutetian. These species are: Nummulites farisi Hussein et al., 2004; Nummulites cf. praegizehensis Boukhary and Hussein-Kamel, 1993; Nummulites cf. gizehensis(Forsk?l, 1775); Nummulites discorbinus(Schlotheim 1820) and Arxina schwageri(Silvestri, 1928) emended by Boukhary et al. 2012 and Nummulites crassichordatus Boukhary et al., 2010. The Giushi Member yielded three species that indicate a Bartonian age. These species, which continued from their first appearance in the Upper Building Stone Members, are N. discorbinus, A. schwageri and N. crassichordatus. The Maadi Formation, which has been previously considered to be of Bartonian–Priabonian age, is devoid of fossils in the study section. The two members of the Mokattam Formation represent a carbonate platform facies. The deposition of the Upper Building Stone Member was disturbed during the Lutetian by slumping and a convolute-bedding interval, indicating a short hiatus. The subsequent regression resulted in a very shallow marine to near-shore facies in the above Maadi Formation.     

Facies analysis and palaeoenvironmental interpretation of the Late Oligocene Attard Member (Lower Coralline Limestone Formation), Malta

The Oligocene represents a key interval during which coralline algae became dominant on carbonate ramps and luxuriant coral reefs emerged on a global scale. So far, few studies have considered the impact that these early reefs had on ramp development. Consequently, this study aimed at presenting a high‐resolution analysis of the Attard Member of the Lower Coralline Limestone Formation (Late Oligocene, Malta) in order to decipher the internal and external factors controlling the architecture of a typical Late Oligocene platform. Excellent exposures of the Lower Coralline Limestone Formation occurring along continuous outcrops adjacent to the Victoria Lines Fault reveal in detail the three‐dimensional distribution of the reef‐associated facies. A total of four sedimentary facies have been recognized and are grouped into two depositional environments that correspond to the inner and middle carbonate ramp. The inner ramp was characterized by a very high‐energy, shallow‐water setting, influenced by tide and wave processes. This setting passed downslope into an inner‐ramp depositional environment which was colonized by seagrass and interfingered with adjacent areas containing scattered corals. The middle ramp lithofacies were deposited in the oligophotic zone, the sediments being generated from combined in situ production and sediments swept from the shallower inner ramp by currents. Compositional characteristics and facies distributions of the Attard ramp are more similar to the Miocene ramps than to those of the Eocene. An important factor controlling this similarity may be the expansion of the seagrass colonization within the euphotic zone. This expansion may have commenced in the Late Oligocene and was associated with a concomitant reduction in the aerial extent of the larger benthonic foraminifera facies. Stacking‐pattern analysis shows that the depositional units (parasequences) at the study section are arranged into transgressive–regressive facies cycles. This cyclicity is superimposed on the overall regressive phase recorded by the Attard succession. Furthermore, a minor highstand (correlated with the Ru4/Ch1 sequence) and subsequent minor lowstand (Ch2 sequence) have been recognized. The biota assemblages of the Attard Member suggest that carbonate sedimentation took place in subtropical waters and oligotrophic to slightly mesotrophic conditions. The apparent low capacity of corals to form wave‐resistant reef structures is considered to have been a significant factor affecting substrate stability at this time. The resulting lack of resistant mid‐ramp reef frameworks left this zone exposed to wave and storm activity, thereby encouraging the widespread development of coralline algal associations dominated by rhodoliths.     

Benthic foraminiferal biostratigraphy of the tertiary sediments from the Elazig and Malatya Basins,Eastern Turkey

Tertiary sequences in the Elazig and Malatya Basins, eastern part of Taurus Orogenic Belt, are investigated with the aim of defining the benthic foraminiferal biozones. Tertiary geological units from bottom to top are as follows: Basement rocks, Zorban Formation, Yildiztepe Formation, Suludere Formation, Gedik Formation (Malatya Basin); Elazig Magmatics, Keban Metamorphics, Harami Formation, Kuscular Formation, Seske Formation, Kirkgecit Formation (Elazig Basin). Middle-Upper Eocene Yildiztepe, Suludere and Gedik Formations; Upper Paleocene-Lower Eocene Seske Formation and Middle-Upper Eocene Kirkgecit Formation are all characterized by interbedded clastics and carbonate rocks. Six stratigraphic sections are studied in detail for foraminiferal biostratigraphy. Eight benthic foraminiferal biozones are reported. These are; Coskinolina rajkae biozone in the Late Paleocene (Thanetian), Assilina yvettae, Idalina sinjarica biozones in the Late Paleocene; Asterocyclina alticostata gallica biozone in the Early Eocene (Late Cuisian), Nummulites millecaput biozone in the Middle Eocene (Middle Lutetian), Nummulites aturicus biozone in the Middle Eocene (Late Lutetian), Nummulites perforatus biozone in the Middle Eocene (Bartonian), Nummulites fabianii biozone in the Late Eocene (Priabonian). Some key taxa are illustrated.     

Early Bartonian orthophragminids (Foraminiferida) from Reineche Limestone,north African platform,Tunisia: taxonomy and paleobiogeographic implications

The orthophragminids in lower Bartonian Reineche Limestone member, a fossiliferous shallow-marine unit exposed in Cap Bon peninsula in Tunisia, are represented by 17 species assigned to Discocyclinidae Galloway 1928 and Orbitoclypeidae Brönnimann 1946. These taxa, associated with nummulitids and alveolinids, belong to the lineages of Discocyclina Gümbel 1870, Nemkovella, 1987, Orbitoclypeus Silvestri 1907, and Asterocyclina Gümbel 1870, described for the first time from north Africa lying at the southern margin of Tethyan ocean during Paleogene. We identified Nemkovella evae, previously not recorded in upper Lutetian/lower Bartonian and younger Eocene deposits of northern Tethyan platforms, and erected a new subspecies, N. evae reinechensis n. ssp. A comparison of Reineche orthophraminids, assigned to orthophragmines zone (OZ) 12 and shallow benthic zone (SBZ 17), to the well-described coeval assemblages at northern Tethyan platforms in Italy, Hungary, Turkey, and to those in Kutch Basin in the Indian subcontinent suggests that some species are confined to certain paleogeographic domains. Orbitoclypeus haynesi, the only orbitoclypeid and the most abundant orthophragminid in lower Bartonian deposits in Kutch, appears to be the most common orbitoclypeid in Reineche Limestone. In Europe, this species is not known and is replaced by Orbitoclypeus varians, the most common orbitoclypeid in middle Eocene of central Europe. Both species occur in varying proportions in marine successions in Turkey. Asterocyclina sireli, identified so far only in Turkey, occurs in Reineche Limestone and in lower Bartonian deposits in Kutch. This species is recorded for the first time in the Indian subcontinent. Relying on present study, as well as our recent studies in Kutch Basin, we conclude that the generic and specific diversity of orthophragminids decreases eastward from the peri-Mediterranean region to Indian subcontinent and to the western Pacific.     

Paleogene stratigraphy of Kutch,India: an update about progress in foraminiferal biostratigraphy

The Paleogene sections of Kutch are the reference for the regional chronostratigraphic units of India. The ages of these dominantly shallow marine carbonates are mainly based on larger benthic foraminifera (LBF). The taxonomic revisions of the LBF and the progressively refined shallow benthic zonations (SBZ) have necessitated the present study on updating the stratigraphy of the area. The sedimentation in Kutch commenced with the deposition of volcaniclastics in terrestrial environments in the Paleocene. The marine transgression in SBZ 5/6 deposited finer clastics and carbonates, designated as Naredi Formation, in early Eocene. There is no evidence of marine Paleocene in Kutch. A major hiatus spanning SBZ 12 to SBZ 16 was followed by the development of a carbonate platform and deposition of Harudi Formation – Fulra Limestone during the Bartonian, SBZ 17. The hiatus corresponds to a widespread stratigraphic break in Pakistan and India to Australia, referred as the ‘Lutetian Gap.’ The Maniyara Fort Formation is assigned to SBZ 22 B and SBZ 23, and its age is revised to Chattian. Climate played a major role in building up of the Paleogene stratigraphic succession of Kutch, the carbonates formed during the warming intervals and the stratigraphic gaps were in the intervening cooling periods.     

Bartonian orthophragminids from the Fulra Limestone (Kutch,W India) and coeval units in Sulaiman Range,Pakistan: a synthesis of shallow benthic zone (SBZ) 17 for the Indian Subcontinent

Orthophragminids from the Bartonian Fulra Limestone in Kutch, India and the coeval units in Sulaiman Range in Pakistan suggest the establishment of a significant number of endemic species in the Indian subcontinent (Eastern Tethys). Among a total of fifteen species of Discocyclina, Orbitoclypeus and Asterocyclina, six of them appear to be confined to Indian subcontinent while seven species are common both to the peri-Mediterranean/Europe region (Western Tethys) and Indian subcontinent. Two species, Asterocyclina sireli, a four-ribbed species of possibly Indo-Pacific origin, and Orbitoclypeus haynesi that form large populations in Fulra Limestone, appear to have spread into North Africa and Turkey but not into European platforms as a response to Middle Eocene Climatic Optimum (MECO). The lack of Lutetian and Priabonian fauna in the studied sections, either due to a hiatus or unsuitable depositional environments, hampers the establishment of the actual stratigraphic ranges of the identified taxa. Our record provides us to characterize the orthophragminids in shallow benthic zone (SBZ) 17 for Eastern Tethys in detail by comparing the data from the above localities with those from the North Africa, Europe and Turkey, showing the change in diversity.     

Depositional framework and controls on mixed carbonate-siliciclastic gravity flows: Pennsylvanian-Permian shelf to basin transect, south-western Great Basin, USA

Mixed carbonate-siliciclastic sediment gravity flow deposits of Late Pennsylvanian to Early Permian age are exposed in the Death Valley - Owens Valley region of east-central California. The Mexican Spring unit constitutes the upper part of the Keeler Canyon Formation and is characterized by turbidites, debris flow deposits and megabreccias, all of mixed carbonate-siliciclastic composition. The mixed composition of the Keeler Canyon Formation provides an opportunity to link facies architecture to controls on depositional system development. Depositional relationships indicate that the deposits represent a non-channellized base of slope carbonate apron system with inner, outer and basinal facies associations. These gravity flow deposits are characterized by repeated stacked, small scale (<15 m) coarsening and thickening upward cycles with superimposed medium scale (>100 m) coarsening and thickening upward cycles. Contemporaneous outer shelf and upper slope deposits of the Tippipah Limestone are exposed at Syncline Ridge on the Nevada Test Site. The deposits consist of carbonate buildups directly overlain by cross bedded, quartz-rich sandstone and conglomerate which filled channels that traversed across the previously existing carbonate shelf. Detritus was transported to the west, down the upper slope by gully systems that fed the temporally persistent base of slope apron of the upper part of the Keeler Canyon Formation. This style of deposition differs from point-sourced siliciclastic submarine fan depositional systems. However, the Keeler Canyon system has lithofacies similar to some sandy siliciclastic turbidite systems, such as the delta-fed submarine ramp facies model, which is a line-sourced, shelf-fed system that is not supply limited. The mixed clastic apron systems of the Keeler Canyon Formation differ from classical carbonate aprons in that the former is characterized by an abundance of sedimentary cycles. Controls on the development of these cycles and of the facies distribution may have resulted from changes in type and rate of sediment supply, relative sea level changes and/or tectonic events. Interpretation of the data is focused on relative changes in sea level as the most significant control on development of the depositional system. Relative sea level changes serve two important functions: (1) they provide a mechanism for bringing coarse siliciclastic and bioclastic grains together on the outer shelf, and (2) shelf margin collapse may be initiated during relative lowstands allowing for transport of the sediment to the deep basin and development of deep basinal cycles. Therefore, an abundance of mixed clastic gravity flow deposits such as these in the rock record may be an indicator of periods of high frequency changes in relative sea level, which is a characteristic of Late Palaeozoic sea level history.     

Early Eocene calcareous algae and benthic foraminifera from Meghalaya,NE India: A new record of microfacies and palaeoenvironment

Early Eocene carbonate sediments of the Umlatdoh Limestone (Meghalaya, N-E India) represent a shallow marine shelf environment. The major biotic components characterizing these carbonates are calcareous green algae and small to larger benthic foraminifera. Based on the biogenic associations and general sedimentological features, five major facies types (MFTs) are distinguished. They are dominated by poor to moderately sorted grainstones followed by packstones, rudstones and wackestones. Considerable abundance of Halimeda, scarcity of z-corals and poor to moderate occurrence of filter-feeding organisms imply mesotrophic to a slightly oligotrophic nutrient regime. Rare occurrence of geniculate coralline algae is probably due to the lack of suitable substrate and environmental conditions. High incidence of grainstones and packstones, fairly preserved microfossils and few reworked specimens indicate a parautochthonous mode of deposition. Preponderance of Alveolina and Nummulites indicate the possible advent of larger foraminiferal turnover (LFT) in the east Tethys during or even before early Eocene. A conceptual palaeoenvironmental model for the studied succession is provided to showcase various facies gradients, bathymetry levels and shelf zones pertinent to the Umlatdoh Limestone.     

From Cycles to Sequences:Sequence Stratigraphy and Relative Sea Level Change for the Late Cambrian of the North China Platform

In the Late Cambrian, the North China Platform was a typical carbonate ramp platform. The Upper Cambrian of the northern part of the North China Platform is famous for the development of bioherm limestones and storm calcirudites and can be divided from bottom to top into the Gushan, Changshan and Fengshan formations. In this set of strata, the deep-ramp mudstone and marls and the shallow-ramp packstones and grainstones constitute many carbonate meter-scale cycles of subtidal type. More tidal-flat dolomites axe developed in the Upper Cambrian of the southern margin of the North China platform, in which limestone and dolomite beds also constitute many carbonate meter-scale cycles of the peritidal type. These cycles are marked by a variety of litho-facies successions. There are regularly vertical stacking patterns of meter-scale cycles in long-term third-order sequences, which is the key to discerning such sequences. Third-order sequence is marked by a particular sedimentary-facies succession that is the result of the environment-changing process of deepening and shoaling, which is genetically related to third-order sea level changes. Furthermore, four third-order sequences can be grouped in the Upper Cambrian of the North China Platform. The main features of these four third-order sequences in the northern part of the platform can be summarized as follows: firstly, sequence-boundaries are characterized by drowning unconformities; secondly, the sedimentary-facies succession is generally constituted by one from deep-ramp facies to shallow-ramp facies; thirdly, a succession of “CS (?) HST” (i.e., “condensed section and highstand system”) forms these four third-order sequences. The chief features for the third-order sequences in the southern part of the North China Platform comprises: more dolomites are developed in the HSTs of third-order sequences and also developed more carbonate meter-scale cycles of peritidal types; the sedimentary-facies succession of the third-order sequences is marked by “shallow ramp-tidal flat”; the sequence boundaries are characterized by exposure punctuated surfaces. According to the changes for the third-order sequences from the north to the south, a regular sequence-stratigraphic framework can be established. From cycles to sequences, the study of sequence stratigraphy from litho-facies successions to sedimentary-facies successions exposes that as follows: meter-scale cycles that are used as the basic working unit actually are litho-facies successions formed by the mechanism of a punctuated aggradational cycle, and third-order sequences that are constituted by regularly vertical stacking patterns of meter-scale cycles are marked by sedimentary-facies successions. On the basis of the changing curve of water depth at each section, the curve of the relative third-order sea level changes in the late Cambrian of the North China Platform can be integrated qualitatively from changing curve of water depth. The correlation of Late Cambrian long-term sea level changes between North China and North America demonstrates that there are not only similarities but also differences, reflecting control of long-term sea level changes both by global eustacy and by regional factors.     

High‐resolution sequence stratigraphy of the Maastrichtian‐Ypresian succession along the eastern scarp face of Kharga Oasis,southern Western Desert,Egypt

A thick Maastrichtian‐Ypresian succession, dominated by marine siliciclastic and carbonate deposits of the regionally recognized Nile Valley and Garra El‐Arbain facies associations, is exposed along the eastern escarpment face of Kharga Oasis, located in the Western Desert of Egypt. The main objectives of the present study are: (i) to establish a detailed biostratigraphic framework; (ii) to interpret the depositional environments; and (iii) to propose a sequence stratigraphic framework in order to constrain the palaeogeographic evolution of the Kharga sub‐basin during the Maastrichtian‐Ypresian time interval. The biostratigraphic analysis suggests the occurrence of 10 planktonic zones; two in the Early Maastrichtian (CF8b and CF7), four in the Palaeocene (P2, P3, P4c and P5) and four in the Early Eocene (E1, E2, E3 and E4). Recorded zonal boundaries and biostratigraphic zones generally match with those proposed elsewhere in the region. The stratigraphic succession comprises seven third‐order depositional sequences which are bounded by unconformities and their correlative conformities which can be correlated within and outside Egypt. These depositional sequences are interpreted as the result of eustatic sea‐level changes coupled with local tectonic activities. Each sequence contains a lower retrogradational parasequence set bounded above by a marine‐flooding surface and an upper progradational parasequence set bounded above by a sequence boundary. Parasequences within parasequence sets are stacked in landward‐stepping and seaward‐stepping patterns indicative of transgressive and highstand systems tracts, respectively. Lowstand systems tracts were not developed in the studied sections, presumably due to the low‐relief ramp setting. The irregular palaeotopography of the Dakhla Basin, which was caused by north‐east to south‐west trending submerged palaeo‐highs and lows, together with the eustatic sea‐level fluctuations, controlled the development and location of the two facies associations in the Kharga Oasis, the Nile Valley (open marine) and Garra El‐Arbain (marginal marine).     

Sequence stratigraphy of a Mesozoic carbonate platform-to-basin system in western Sicily

Sequence stratigraphic studies of the Triassic through Paleogene carbonate successions of platform, slope and basin in western Sicily (Palermo and Termini Imerese Mountains) have identified a sedimentary cyclicity mostly caused by relative oscillations of sea level. The stratigraphic successions of the Imerese and Panormide palaeogeographic domains of the southern Tethyan continental margin were studied with physical-stratigraphy and facies analysis to reconstruct the sedimentary evolution of this platform-to-basin system. The Imerese Basin is characterized by a carbonate and siliceous-calcareous succession, 1200–1400m thick, Late Triassic to Eocene in age. The strata display a typical example of a carbonate platform margin, characterized by resedimented facies with progradational stacking patterns. The Panormide Carbonate Platform is characterized by a carbonate succession, 1000–1200 m thick, Late Triassic to Late Eocene, mostly consisting of shallow-water facies with periodic subaerial exposure. The cyclic arrangement has been obtained by the study of the stratigraphic signatures (unconformities, facies sequences, erosional surfaces and stratal geometries) found in the slope successions. The recognized pattern has been compared with coeval facies of the shelf. This correlation provided evidence of sedimentary evolution, influenced by progradation and backstepping of the shelf deposits. The stratigraphic architecture of the platform-to-basin system is characterized by four major transgressive/regressive cycles during the late Triassic to late Eocene. These cycles, framed in a chronostratigraphic chart, allows the correlation of the investigated shelf-to-basin system with the geological evolution of the African continental margin during the Mesozoic, showing tectono-eustatic cycles. The first cycle, encompassing the late Triassic to early Jurassic, appears to be related to the late syn-rift stage of the continental margin evolution. The following three cycles, spanning from the Jurassic to Eocene, can be related to the post-rift evolution and to thermal subsidence changes.     

Facies, dissolution seams and stable isotope compositions of the Rohtas Limestone (Vindhyan Supergroup) in the Son valley area, central India

The early Mesoproterozoic Rohtas Limestone in the Son valley area of central India represents an overall shallowing-upward carbonate succession. Detailed facies analysis of the limestone reveals outer- to inner-shelf deposition in an open marine setting. Wave-ripples, hummocky cross stratifications and edgewise conglomerates argue against a deep marine depositional model for the Rohtas Limestone proposed earlier. Stable isotope analysis of the limestone shows that δ¹³C and δ¹⁸O values are compatible with the early Mesoproterozoic open seawater composition. The ribbon limestone facies in the Rohtas Limestone is characterized by micritic beds, each decoupled in a lower band enriched and an upper band depleted in dissolution seams. Band-wise isotopic analysis reveals systematic short-term variations. Comparative enrichment of the heavier isotopes in the upper bands is attributed to early cementation from sea water and water derived from the lower band undergoing dissolution because of lowering of pH at depth. The short-term positive shifts in isotopic compositions in almost every upward gradational transition from a seamed band to a non-seamed band support the contention that dissolution seams here are of early diagenetic origin, although their formation was accentuated under overburden pressure.     

Isotopic and sedimentological clues to productivity change in Late Riphean Sea: A case study from two intracratonic basins of India

Enriched¹³C/¹²C ratios with δ¹³C ∼3%0 (w.r.t PDB) of two Late Riphean (∼ 700-610 Ma) intracratonic carbonate successions viz., Bhander Limestone of Vindhyan Basin and Raipur Limestone of Chattisgarh Basin suggest higher organic productivity during this period. This view is supported by sedimentological evidence of higher biohermal growth and consequent increase in depositional relief in the low gradient ramp settings inferred for these basins. Oxygen isotope analysis of these carbonates show distinct segregation between enriched deeper water carbonate mudstone and depleted shallow water stromatolite facies that received fresh water influx. This shows that facies-specific analyses can be useful in understanding the depositional setting of these sediments.     

Platform and off-platform carbonates of the Upper Cambrian of western Maryland, U.S.A.

Upper Cambrian carbonates in western Maryland are comprised of platform facies (Conococheague Limestone) west of South Mountain and basin facies (Frederick Limestone) east of South Mountain. Conocheague platform carbonates contain interbedded non-cyclic and cyclic facies. Non-cyclic facies consist of cross-stratified grainstones, thrombolitic bioherms, and graded, thin-bedded dolostones. These were deposited in shallow, subtidal shelf lagoons. Cyclic facies are composed of repeated sequences of cross-stratified grainstone; ribbon-rock; wavy, prism-cracked laminite; and planar laminated dolostone. The cyclic facies are shallowing-upward cycles produced by lateral progradation of tidal flats over shallow, nearshore subtidal environments. Cyclic and non-cyclic facies are interbedded in the Conococheague in a layer cake fashion, but no higher-order cyclicity can be found. The Frederick Limestone is dominated by monotonously thick sequences of graded, thin-bedded limestones, interbedded with massive peloidal grainstones and beds of breccia up to 10 m thick in the lower Frederick. The breccias contain transported megaclasts of Epiphyton-Girvanella boundstones. The basal Frederick was deposited in a slope-to-basinal setting east of a rimmed shelf. An Epiphyton-Girvanella marginal reef along the shelf edge was the source of the blocks in the breccias. The upper Frederick Limestone formed on a carbonate ramp.     

晚三叠世龙门山前陆盆地早期(卡尼期)碳酸盐缓坡和海绵礁的淹没过程与动力机制

晚三叠世龙门山前陆盆地分布于扬子克拉通西缘,属于印支期造山楔构造负载驱动的挠曲型前渊凹陷.其中卡尼期马鞍塘组是分布于底部不整合面之上的第一套地层单元,记录了前缘隆起边缘碳酸盐缓坡和海绵礁的构建和淹没过程.据钻孔揭示马鞍塘组的最大厚度超过250m,显示为西北厚东南薄的楔形结构,从北西向南东依次分布了深水盆地、碳酸盐缓坡和海绵礁和浅水滨岸带等沉积物类型.其中碳酸盐缓坡和海绵礁分布于前陆盆地的远端,呈面向西的条带状展布,其走向线与龙门山冲断带的走向大致平行.碳酸盐缓坡和海绵礁的厚度介于30～100m之间,由北西向南东变薄.在垂向上,马鞍塘组由3部分构成,下部为鲕粒滩和生物碎屑滩,中部为海绵礁,上部为黑色页岩,显示为向上变细、变深的沉积序列.在Li et al.(2003)盆地模拟的基础上,本次对卡尼期前陆盆地的沉降速率、沉积速率、海绵礁生长速率、相对海平面上升速率进行了定量计算,其中沉降速率为0.10mm·a-1、沉积速率为0.04mm·a-1、海绵礁生长速率为0.03mm·a-1、相对海平面上升速率介于0.01mm·a-1～0.05mm · a-1之间.研究结果表明:在卡尼期早期,相对海平面处于初始上升阶段,相对海平面上升速率较小,盆地处于欠补偿状态,沉积了碳酸盐缓坡型鲕粒滩和生物碎屑滩;在卡尼期中期,相对海平面上升速率等于海绵礁生长速率,海绵礁持续保持垂直向上的生长状态,形成了高度达100余米的塔礁;在卡尼期晚期,相对海平面上升速率大于海绵礁生长速率,礁顶的水深逐步变大,导致礁体被淹溺致死,从而在卡尼期形成了鲕粒灰岩滩-生物碎屑滩-海绵礁灰岩-页岩的向上变细、变深的沉积序列,显示了前陆盆地早期碳酸盐缓坡和海绵礁生长并被淹没的特有模式.本次研究成果表明龙门山前陆盆地的底部不整合面和碳酸盐缓坡、海绵礁的淹没过程是扬子板块西缘印支期造山楔逆冲构造负载的挠曲变形的产物,显示了在卡尼期松潘-甘孜残留洋盆的迅速闭合和造山楔构造负载向扬子板块的推进过程.     

Global geoheritage significance of Ordovician stratigraphy and sedimentology in the Cliefden Caves area,central western New South Wales

Globally significant geoheritage features of the Cliefden Caves area, in the Belubula River Valley between Orange and Cowra in central western New South Wales, comprise a richly fossiliferous shallow-water limestone succession of Late Ordovician age (the Cliefden Caves Limestone Subgroup) overlain by deep-water laminites and allochthonous limestones of the Upper Ordovician Malongulli Formation. Key features of the Ordovician geology of the Cliefden Caves area that have been identified using the Geoheritage Toolkit as being of international significance are the abundance of unique and exceptionally diverse fossils in the Fossil Hill Limestone (forming the lower part of the Cliefden Caves Limestone Subgroup), which supplement detailed interpretation of carbonate-dominated deposition within an Ordovician volcanic island setting. The fossiliferous limestones preserve biostromes and local small bioherms of stromatoporoids and corals, and recurrent in situ and disarticulated/imbricated Eodinobolus shell beds formed in shallow, quiet-water, dominantly muddy carbonate sediments that passed up-sequence to clay-free carbonate environments. These mud-dominated carbonate sediments are interspersed with higher-energy conditions, represented by skeletal, lithoclastic and calcrete-ooid grainstones overlying disconformities, leading to the identification of subaerial disconformities and associated diagenesis in the Fossil Hill Limestone. The Fossil Hill Limestone is succeeded by massive limestones in the middle part of the Cliefden Caves Limestone Subgroup and then, in turn by the Vandon Limestone and the deeper-water graptolitic laminites of the Malongulli Formation—this completes a succession that is rarely preserved in the geological record, further enhancing the geoheritage significance of the Cliefden Caves area.     

Facies Associations and Discontinuous Surfaces in the Syringothyris Limestone Formation of Tethyan Margin of Gondwana,Kashmir

The early Carboniferous sedimentation of the Tethyan Margin of Gondwana in the Kashmir Himalaya represents alternating siliciclastic - carbonate succession consisting of distinct stratigraphic sequences which are bounded by discontinuities. The discontinuities in the sedimentation are related to environmental changes in the form of subaerial exposure, subaqueous erosion, subaqueous omission or changes in texture and facies. These distinct surface zones or time significant boundaries can be correlated across the depositional platform. Low stand, high stand and transgressive sedimentation units in the lower and middle parts of early Carboniferous Syringothyris Limestone Formation in Banihal area have been recognised. This is explained by superposition of high frequency and low amplitude sea level fluctuations on a large-scale trend under greenhouse conditions during the early Carboniferous period. The facies associations present in the early Carboniferous succession of the Himalaya broadly represent intertidal (peritidal), shallow subtidal, deeper subtidal, off-shore-slope and deeper environments. Discontinuities that are interpreted as progradational, retrogradational and aggradational phases of sedimentation bound these facies associations. This formation represents continental margin depositional setting which is authenticated by deposition of siliciclastic sediments. This marginal depositional setting is greatly affected by numerous dynamic processes including tectonic and other active sea as well as continental processes. The records of all those processes in this formation reflect the eustatic changes in sea level. These periodic eustatic changes have generated the various discontinuities, stratigraphic sequences or systems tracts. Overall it appears that interplay of many processes such as sediment supply, thermal and tectonic activity, eustatic and climatic changes in the Kashmir Tethyan depositional basin generated these distinct depositional sequences during the early Carboniferous period.