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1.
A thick sequence of late Dinantian (Asbian–Brigantian) carbonates crop out in the Buttevant area, North Co. Cork, Ireland. A mud-mound unit of early Asbian age (the Hazelwood Formation) is the oldest unit described in this work. This formation is partly laterally equivalent to, and is overlain by, over 500 m of bedded platform carbonates which belong to the Ballyclogh and Liscarroll Limestone Formations. Four new lithostratigraphic units are described within the platform carbonates: (i) the early Asbian Cecilstown Member and (ii) the late Asbian Dromdowney Member in the Ballyclogh Limestone Formation; (iii) the Brigantian Templemary Member and (iv) the Coolbane Member in the Liscarroll Limestone Formation. The Cecilstown Member consists of cherty packstones and wackestones that are inferred to have been deposited below fair-weather wavebase. This unit overlies and is laterally equivalent to the mud-mound build-up facies of the Hazelwood Formation. The Dromdowney Member is typified by cyclic-bedded kamaenid-rich limestones possessing shell bands, capped by palaeokarst surfaces, with alveolar textures below and shales above these surfaces. The carbonates of this unit were deposited at or just below fair-weather wavebase, the top of each cycle culminated in subaerial emergence. The Templemary Member consists of cyclic alternations of subtidal crinoidal limestones capped by subtidal lagoonal crinoid-poor, peloidal limestones possessing coral thickets. Intraclastic cherty packstones and wackestones characterize the Coolbane Member, which is inferred to have been deposited below fair-weather wavebase but above storm wavebase. The early Asbian Cecilstown Member has a relatively sparse micro- and macrofauna, typified by scattered Siphonodendron thickets, archaediscids at angulatus stage and common Vissariotaxis. Conversely, macro- and microfauna is abundant in the late Asbian Dromdowney Member. Typical late Asbian macrofossils include the coral Dibunophyllum bipartitum and the brachiopod Davidsonina septosa. The base of the late Asbian (Cf6γ Subzone) is recognized by the first appearance of the foraminifers Cribrostomum lecompteii, Koskinobigenerina and the alga Ungdarella. The Cf6γ Subzone can be subdivided into two biostratigraphic divisions, Cf6γ1 and Cf6γ2, that can be correlated throughout Ireland. Relatively common gigantoproductid brachiopods and the coral Lonsdaleia duplicata occur in the Brigantian units. The base of the Brigantian stage (Cf6δ Subzone) is marked by an increase in the abundance of stellate archaediscids, the presence of Saccamminopsis-rich horizons, Loeblichia paraammonoides, Howchinia bradyana and the rarity of Koninckopora species. Changes in facies at the Cecilstown/Dromdowney Member and the Ballyclogh/Liscarroll Formation boundaries coincide closely with the changes in fossil assemblages that correspond to the early/late Asbian and the Asbian/Brigantian boundaries. These facies changes are believed to reflect major changes in relative sea-level on the Irish platforms. The sea-level variations that are inferred to have caused the facies changes at lithostratigraphic boundaries also brought in the new taxa that define biostratigraphic boundaries. Moreover, many of the Dinantian stage boundaries that are defined biostratigraphically in Great Britain, Belgium and the Russian Platform also coincide with major facies boundaries caused by regressive and transgressive episodes. The integration of detailed biostratigraphic analyses with facies studies will lead to better stratigraphic correlations of Dinantian rocks in northwest Europe. © 1997 John Wiley & Sons, Ltd.  相似文献   

2.
Three Upper Viséan to Serpukhovian limestone formations from the Adarouch region (central Morocco), North Africa, have been dated precisely using foraminiferans and calcareous algae. The lower and middle part of the oldest formation, the Tizra Formation (Fm), is assigned to the latest Asbian (upper Cf6γ Subzone), and its upper part to the Early Brigantian (lower Cf6δ Subzone). The topmost beds of this formation are assigned to the Late Brigantian (upper Cf6δ Subzone). The lower part of the succeeding Mouarhaz Fm is also assigned to the Late Brigantian (upper Cf6δ Subzone). The Akerchi Fm is younger than the other formations within the region, ranging from the latest Brigantian (uppermost Cf6δ Subzone) up to the Serpukhovian (E1–E2). The base of the Serpukhovian (Pendleian Substage, E1) is repositioned, to coincide with the appearance of a suite of foraminiferans including Archaediscus at tenuis stage, Endothyranopsis plana, Eostaffella pseudostruvei, Loeblichia ukrainica, Loeblichia aff. minima and Biseriella? sp. 1. The upper Serpukhovian (Arnsbergian Substage, E2) is marked by the first appearance of Eostaffellina ex. gr. paraprotvae and Globoomphalotis aff. pseudosamarica. The biostratigraphical scheme used for the reassessment of the foraminiferal zones and subzones in the Adarouch area closely compares with that for the British succession in northern England (Pennine Region), where the stratotypes of the Upper Viséan (Asbian and Brigantian) and Early Serpukhovian (Pendleian) substages are located. Thus, a succession equivalent to an interval from the Melmerby Scar Limestone to the Great (or Little) Limestone is recognized. These assemblages are also compared to other foraminiferal zones proposed in other regions of Morocco. Several foraminiferans have been identified that are proposed as potential Serpukhovian markers for other basins in Western Europe, and compared to sequences in Russia and the Donets Basin, Ukraine. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

3.
The stratigraphy of the upper Viséan (Asbian to Brigantian) carbonate succession in southeast Ireland is revised on the basis of seven quarry and two borehole sections. Six lithological units have been distinguished, two units (units 1 and 2) in the upper Asbian Ballyadams Formation, and four units (units 4 to 6) in the Brigantian Clogrenan Formation (both formations are dated precisely using foraminiferans, calcareous algae and rugose corals). The boundary between the Ballyadams and Clogrenan formations is redefined 19 m below the horizon proposed by the Geological Survey of Ireland, and thus, lithological characteristics of both formations are redescribed. The upper part of the Ballyadams Formation is characterized by well‐developed large‐scale cyclicity, with common subaerial exposure surfaces. Fine‐ to medium‐grained thin‐bedded limestones with thin shales occur in the lower part of cycles, passing up into medium‐grained pale grey massive limestones in the upper part. The Clogrenan Formation is composed mainly of medium‐ to coarse‐grained thick limestone beds with variable presence of shales; but no large‐scale cyclicity. There is a decrease in the number of subaerial exposure surfaces towards the top of the formation and common chert nodules; macrofauna occurs mostly concentrated in bands. The six units recognized in the Carlow area are comparable with other units described for the same time interval (Asbian–Brigantian) from south and southwest Ireland, demonstrating the existence of a stable platform for most parts of southern Ireland, controlled principally by glacioeustatics. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

4.
The microbiota of the upper Viséan (Asbian–Brigantian) rocks in the Lough Allen Basin in northwest Ireland is analysed. The Middle Mississippian sequence studied extends from the upper part of the Dartry Limestone/Bricklieve Limestone formations of the Tyrone Group to the Carraun Shale Formation of the Leitrim Group. The rocks have been traditionally dated by ammonoid faunas representing the B2a to P2c subzones. The Meenymore Formation (base of the Leitrim Group) also contains conodont faunas of the informal partial‐range Mestognathus bipluti zone. The upper Brigantian Lochriea nodosa Conodont Zone was recognized by previous authors in the middle of the Carraun Shale Formation (Ardvarney Limestone Member), where it coincides with upper Brigantian ammonoids of the Lusitanoceras granosus Subzone (P2a). Foraminifera and algae in the top of the Dartry Limestone Formation are assigned to the upper Cf6γ Foraminifera Subzone (highest Asbian), whereas those in the Meenymore Formation belong to the lower Cf6δ Foraminifera Subzone (lower Brigantian). The Dartry Limestone Formation–Meenymore Formation boundary is thus correlated with the Asbian–Brigantian boundary in northwest Ireland. For the first time, based on new data, a correlation between the ammonoid, miospore, foraminiferan and conodont zonal schemes is demonstrated. The foraminiferans and algae, conodonts and ammonoids are compared with those from other basins in Ireland, northern England, and the German Rhenish Massif. Historically, the Asbian–Brigantian boundary has been correlated with several levels within the P1a Ammonoid Subzone. However, the new integrated biostratigraphical data indicate that the Asbian–Brigantian boundary in northwest Ireland is probably located within the B2a Ammonoid Subzone and the NM Miospore Zone, but the scarcity of ammonoids in the Tyrone Group precludes an accurate placement of that boundary within this subzone. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

5.
The upper Viséan–Serpukhovian strata in the type region for the Serpukhovian Stage is an epeiric‐sea succession ca. 90 m in thickness. The predominantly Viséan Oka Group (comprising the Aleksin, Mikhailov, and Venev formations) is dominated by photozoan packstones with fluvial siliciclastic wedges developed from the west. The Lower Serpukhovian Zaborie Group is composed of the Tarusa and Gurovo formations. The latter is a new name for the shale‐dominated unit of Steshevian Substage age in the studied area. The Zaborie Group is composed of limestones and marls in its lower (Tarusa and basal Gurovo) part and black smectitic to grey palygorskitic shales in the main part of the Gurovo Formation. The Gurovo Formation is capped by a thin limestone with oncoids and a palygorskitic–calcretic palaeosol. The Upper Serpukhovian is composed of a thin (3–12 m) Protva Limestone heavily karstified during a mid‐Carboniferous lowstand. The succession shows a number of unusual sedimentary features, such as a lack of high‐energy facies, shallow‐subtidal marine sediments penetrated by Stigmaria, the inferred atidal to microtidal regime, and palustrine beds composed of saponitic marls. The succession contains many subaerial disconformities characterized by profiles ranging from undercoal solution horizons to palaeokarsts. Incised fluvial channels are reported at two stratigraphic levels to the west of the study area. The deepest incisions developed from the Kholm Disconformity (top of the Mikhailov Formation). This disconformity also exhibits the deepest palaeokarst profile and represents the major hiatus in the Oka–Zaborie succession. The new sea‐level curve presented herein shows two major cycles separated by the Kholm Unconformity at the Mikhailov/Venev boundary. The Lower Serpukhovian transgression moved the base‐level away from falling below the seafloor so that the section becomes conformable above the Forino Disconformity (lower Tarusa). The maximum deepening is interpreted to occur in the lower dark‐shale part of the Gurovo Formation. The base of the Serpukhovian Stage is defined by FADs of the conodont Lochriea ziegleri and the foraminifer Janischewskina delicata in the middle of the sequence VN2. The Aleksinian–Mikhailovian interval is provisionally correlated with the Asbian (Lower–Middle Warnantian) in Western Europe. Based on FODs of Janischewskina typica and first representatives of Climacammina, the Venevian is correlated with the Brigantian in Western Europe. The Tarusian–Protvian interval contains diverse fusulinid and conodont assemblages, but few forms suitable for international correlation. FADs of the zonal conodont species Adetognathus unicornis and Gnathodus bollandensis at several metres above the Protvian base suggest correlation of the entire Zaborie Group and may be the basal Protvian to the Pendleian. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

6.
Despite the importance of south Cumbrian sections for the Arundian–Holkerian (mid Viséan) boundary, beyond the stratotype proposed at Barker Scar in 1976, little else is known regionally about this boundary and its relationship to adjacent formations. We re-evaluate the Dalton Formation, making its upper and lower boundaries regionally more consistent and precisely-defined, in good quality outcrops with associated biostratigraphy. The Dalton Formation is formally divided into the Blackstone Member and overlying Raven's Member. Rich foraminiferal assemblages of the Cf4δ subzone are recognized in the Blackstone Member and the lower to mid Raven's Member in nine sections. The Cf4γ–Cf4δ boundary is recognized in the underlying Red Hill Limestone Formation in 3 sections. The upper part of the Raven's Member shows the first appearance of taxa assigned to the Cf5α and Cf5β subzones of the Holkerian in three of the sections. Bentonitic shales in the mid and upper-most part of the Raven's Member were evaluated for zircon and apatite geochronology, although only 4 out of 504 analyses yielded Carboniferous ages, indicating an almost entirely detrital source. This detritus was northerly or northeasterly-derived and predominantly from the Southern Uplands Terrane with subsidiary input from the Lewisian Complex or eastern Greenland sources. Petrographic analysis identified 13 microfacies indicating that the Dalton Formation represents the inner to outer part of a southward inclined shelf, in which east–west changes in microfacies were generated by synsedimentary faulting inherited from northwest–southeast aligned basement structures.  相似文献   

7.
在西藏自治区北部区域地质调查过程中 ,在班戈地层分区的下白垩统郎山组之上发现一套厚约 180 0 m的滨海—浅海相碳酸盐岩、陆源细碎屑岩岩石组合 ,该套组合不属于郎山组定义之内 ,且出露良好、层序稳定、标志清楚、厚度巨大、化石丰富 ,并具有很强的可填图性。因此 ,我们将整合于郎山组之上 ,岩性以杂色微—薄层状钙质泥岩、钙质 (粉 )砂岩与泥质生物灰岩的韵律产出为特征 ,产圆笠虫、腹足类、双壳类等化石的地层命名为康曲组 ;将整合于康曲组之上 ,下部以灰—深灰色中薄层状泥质生物灰岩、圆笠虫泥晶灰岩、钙质泥岩等韵律出现为特征 ,上部以灰色中—厚层状含硅质结核圆笠虫泥晶灰岩、圆笠虫泥晶灰岩夹灰绿色薄层状钙质砂岩为特征 ,富含圆笠虫、腕足类、腹足类、双壳类及海胆等化石的地层定名为余穷组。时代均置于早白垩世晚期  相似文献   

8.
Abstract In mid‐Middle Cambrian time, shallow‐water sedimentation along the Cordilleran passive margin was abruptly interrupted by the development of the deep‐water House Range embayment across Nevada and Utah. The Marjum Formation (330 m) in the central House Range represents deposition in the deepest part of the embayment and is composed of five deep‐water facies: limestone–argillaceous limestone rhythmites; shale; thin carbonate mud mounds; bioturbated limestone; and cross‐bedded limestone. These facies are cyclically arranged into 1·5 to 30 m thick parasequences that include rhythmite–mound, rhythmite–shale, rhythmite–bioturbated limestone and rhythmite–cross‐bedded limestone parasequences. Using biostratigraphically constrained sediment accumulation rates, the parasequences range in duration from ≈14 to 270 kyr. The mud mounds are thin (<2 m), closely spaced, laterally linked, symmetrical domes composed of massive, fenestral, peloidal to clotted microspar with sparse unoriented, poorly sorted skeletal material, calcitized bacterial(?) filaments/tubes and abundant fenestrae and stroma‐ tactoid structures. These petrographic and sedimentological features suggest that the microspar, peloids/clots and syndepositional micritic cement were precipitated in situ from the activity of benthic microbial communities. Concentrated growth of the microbial communities occurred during periods of decreased input of fine detrital carbonate transported offshore from the adjacent shallow‐water carbonate platform. In the neighbouring Wah Wah Range and throughout the southern Great Basin, coeval mid‐Middle Cambrian shallow‐water carbonates are composed of abundant metre‐scale, upward‐shallowing parasequences that record high‐frequency (104?105 years) eustatic sea‐level changes. Given this regional stratigraphic relationship, the Marjum Formation parasequences probably formed in response to high‐frequency sea‐level fluctuations that controlled the amount of detrital carbonate input into the deeper water embayment. During high‐frequency sea‐level rise and early highstand, detrital carbonate input into the embayment decreased as a result of carbonate factory retrogradation, resulting in the deposition of shale (base of rhythmite–shale parasequences) or thin nodular rhythmites, followed by in situ precipitated mud mounds (lower portion of rhythmite–mound parasequences). During the ensuing high‐frequency sea‐level fall/lowstand, detrital carbonate influx into the embayment increased on account of carbonate factory pro‐ gradation towards the embayment, resulting in deposition of rhythmites (upper part of rhythmite–mound parasequences), reworking of rhythmites by a lowered storm wave base (cross‐bedded limestone deposition) or bioturbation of rhythmites by a weakened/lowered O2‐minimum zone (bioturbated lime‐ stone deposition). This interpreted sea‐level control on offshore carbonate sedimentation patterns is unique to Palaeozoic and earliest Mesozoic deep‐water sediments. After the evolution of calcareous plankton in the Jurassic, the presence or absence of deeper water carbonates was influenced by a variety of chemical and physical oceanographic factors, rather than just physical transport of carbonate muds.  相似文献   

9.
A Lower Carboniferous platform sedimentary sequence (the Paprotnia Beds) in the Bardo Unit of the central Sudetes (NE part of the Bohemian Massif, SW Poland) is biostratigraphically well dated, based on rich macro- and micro-fossil evidence, as Late Viséan (late Asbian, crenistria, Go III α zone). The beds contain several bentonite layers, one of which was dated using the U–Pb SHRIMP method on volcanic zircons and yielded an age of 334 ± 3 Ma. This date fits well to the recently established chronostratigraphic limits of the Viséan, and is consistent with the newest isotopic age constraints of 336.5–332 Ma for the Asbian boundaries.  相似文献   

10.
The Bowland Basin (northern England) contains a series of carbonates and terrigenous mudstones deposited during the Ivorian to early Brigantian. Two regional depositional environments are indicated by facies and facies associations. Wackestone/packstone and calcarenite facies indicate deposition in a carbonate ramp environment, while lime mudstone/wackestone, calcarenite and limestone breccia/conglomerate facies, often extensively slumped, represent a carbonate slope environment. Stratigraphic relations suggest that the depositional environment evolved from a ramp into a slope through the Dinantian. Two main sediment sources are indicated by the sequence; an extra-basinal terrigenous mud source and a supply of carbonate from the margins of the basin. Deposition from suspension and from sediment gravity flows, in situ production and remobilization of sediment during sedimentary sliding were important processes operating within the basin. Periods of enhanced tectonic activity in the late Chadian to early Arundian and late Asbian to early Brigantian are indicated by basin-wide horizons of sedimentary slide and mass flow deposits. Both intervals were marked by a decline in carbonate production resulting from inundation and uplift/emergence. The first of these intervals separates deposition on a seafloor with gentle topography (carbonate ramp) from a situation where major lateral thickness and facies variations were present and deposition took place in a carbonate slope environment. The second interval marks the end of major carbonate deposition within the Bowland Basin and the onset of regional terrigenous sedimentation.  相似文献   

11.
Analysis of a 275 m‐thick section in the Milford Borehole, GSI‐91‐25, from County Carlow, Ireland, has revealed an unusual sequence of shallow subtidal, peritidal and sabkha facies in rocks of mid?‐late Chadian to late Holkerian (Viséan, Lower Carboniferous) age. Sedimentation occurred on an inner ramp setting, adjacent to the Leinster Massif. The lower part of the sequence (late Chadian age) above the basal subtidal bioclastic unit is dominated by oolite sand facies associations. These include a lower regressive dolomitized, oolitic peloidal mobile shoal, and an upper, probably transgressive, backshoal oolite sand. A 68 m‐thick, well‐developed peritidal sequence is present between the oolitic intervals. These rocks consist of alternating stromatolitic fenestral mudstone, dolomite and organic shale, with evaporite pseudomorphs and subaerial exposure horizons containing pedogenic features. In the succeeding Arundian–Holkerian strata, transgressive–regressive carbonate units are recognized. These comprise high‐energy, backshoal subtidal cycles of argillaceous skeletal packstones, bioclastic grainstones with minor oolites and algal wackestones to grainstones and infrequent algal stromatolite horizons. The study recognizes for the first time the peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in the eastern Irish Midlands. These strata appear to be coeval with similar evaporite‐bearing rocks in County Wexford that are developed on the southern margin of this landmass, and similar depositional facies exist further to the east in the South Wales Platform, south of St. George's Land, and in Belgium, south of the Brabant Massif. The presence of evaporites in the peritidal facies suggests that dense brines may have formed adjacent to the Leinster Massif. These fluids may have been involved in regional dolomitization of Chadian and possibly underlying Courceyan strata. They may also have been a source of high salinity fluids associated with nearby base‐metal sulphide deposits. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

12.
The Mobarak Formation is near the town of Kiyasar in the south-east of Sari city, northern Iran. This formation conformably overlies the Geirud Formation (Upper Devonian). The lower part of the Mobarak Formation consisting of shales and thin- to medium-bedded limestone toward the top of these sequences changes into alternations of dark limestone and interbedded gray to black shales. Weathered yellow thick-bedded shales are observed at the top of the section. This formation is covered unconformably by sandstones attributed to the Dorud Formation (Lower Permian). The thickness of the formation in this region is 250 m. Four rock units have been recognized in this section. Foraminiferal biostratigraphy shows that the age of the Mobarak Formation in the Kiaysar region ranges from Lower Tournaisian to Early Middle Visean. The foraminifer Zones FAZ1 and FAZ2 are correlated with the Lower Tournaisian and Upper Tournaisian, whereas Zones FAZ3 and FAZ4 correlate with the Visean. Affinities exist between specimens recorded in the Kiyasar section with species known from other regions in eastern and Central Alborz, but there are important differences in their appearance.  相似文献   

13.
Various early Paleozoic (Cambrian Series 3–Middle Ordovician) reefs are found in the Taebaek Group, eastern Korea, located in the eastern margin of the Sino-Korean Block. They occur in every carbonate-dominant lithostratigraphic unit of the group, but their morphology and composition differ markedly. The Daegi Formation (middle Cambrian: Cambrian Series 3) contains siliceous sponge-Epiphyton reefs formed in a shallow subtidal environment, which is one of the earliest metazoan-bearing microbial reefs after the archaeocyath extinction. The Hwajeol Formation (upper Cambrian: Furongian) encloses sporadic dendrolites consisting of Angulocellularia, which developed in a relatively deep subtidal environment, representing a rare deeper water example. The onset of the Ordovician radiation resulted in the formation of microbialite–Archaeoscyphia–calathiid patch reefs in shallow subtidal deposits of the Lower Ordovician Dumugol Formation. Subsequent late Early Ordovician relative sea-level fall established extensive peritidal environments, forming microbial mats and stromatolites of the Lower–Middle Ordovician Makgol Formation. Ensuing Ordovician radiation resulted in one of the earliest metazoan skeletal reefs of the Middle Ordovician Duwibong Formation, constructed by stromatoporoid Cystostroma and bryozoan Nicholsonella, and developed around shallow shoals. These reefs reflect ongoing evolution and sea-level change during the early Paleozoic, and exemplify a rare glimpse of peri-Gondwanan records of reef evolution, which warrant detailed investigations and comparison with their counterparts in other regions.  相似文献   

14.
周志澄  张瑛 《沉积学报》1991,9(4):120-127
安徽巢县凤凰山剖面和州组上段生物扰动灰岩含丰富的遗迹化石Thalassinoides sp,属于Glossifungites遗迹相,它发育在没有完全石化的间断面上,反映了一种潮间带潮坪的沉积环境。Glossifungites遗迹相的发现,在地层学上证实了巢县以北和州组与黄龙组之间为间断接触,缺失或只存在一部分相当于巢县以南普遍存在的老虎洞组白云岩时代的地层。  相似文献   

15.
The ‘Calcaires à Productus’ of the Montagne Noire are microbial build-ups. Two formations are defined and dated respectively as Uppermost Visean (Upper Warnantian–Brigantian) and Serpukhovian on the basis on corals. That makes these limestones out to be younger than previously stated (Lower and base of Upper Warnantian–Asbian and base of Brigantian) and indicates that the development of the olistoliths and thrusts including them, due to the Variscan orogeny, was at least as young as the Upper Serpukhovian. The Serpukhovian limestones of the Montagne Noire are correlated with the Lanet Limestone (Mouthoumet Massif, Corbières) and Ardengost Limestone (central Pyrenees). To cite this article: É. Poty et al., C. R. Geoscience 334 (2002) 843–848.  相似文献   

16.
The Eyam Limestone Formation of Steeplehouse Quarry, Wirksworth, Derbyshire, UK yields a diverse assemblage of Lower Carboniferous vertebrate remains. The assemblage is dominated by dermal denticles of the enigmatic selachian Petrodus patelliformis M’Coy, 1848, but also contains teeth of petalodonts, hybodonts and neoselachians. Actinopterygian remains also occur. The assemblage has yielded the earliest Neoselachian, Cooleyella fordi (Duffin and Ward, 1983) and the earliest British lonchidiid, Reesodus wirksworthensis (Duffin 1985). The first occurrence of the enigmatic spiny shark Acanthorhachis (Listracanthidae) is reported from the Viséan, extending its range back some 10 million years. Associated invertebrate remains and sedimentological data indicates a thriving fore-reef environment, deposited in a low energy off-reef setting. The vertebrate remains are well preserved with little abrasion, indicating short transport distances. Conodont elements indicating a late Brigantian age (Early Carboniferous, Viséan) have unusual and extensive euhedral apatite overgrowths.  相似文献   

17.
Bulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ13C and δ18O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ13C and δ18O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ13C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ13C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies.  相似文献   

18.
Tidal-flat and shoal deposits of carbonate fades in the Qiziqiao Formation are widely distributed over the vast areas of Guangxi, Guangdong and Hunan provinces, constituting an important stratigraphic unit where strata-bound and stratiform ore deposits (galena-sphalerite-pyrite) are found. These types of ore deposit seem to have close relations to the tidal deposits. Recognition of tidal deposits is based upon rock fabric, texture, structure, fossil assemblage, and particular sedimentary cycle. The typical sequence of tidal-flat deposits consists mainly of three units:Amphipora limestone, laminated limestone and dolostone (from the bottom to the top). This sequence represents a complete process of sedimentation from low-tideflat through intertidal to high-tideflat or supratidal. The sequence of shoal deposits of carbonate facies consists chiefly of grainstone and algal oösparite. Two major types of tidal deposit (open and restricted marine facies) can be distinguished, with eleven microfacies as follows: 1) calcarenite (grainstone) with sparite (MF-1); 2) algal oösparite (MF-2); 3) oncolite (alga-, or stromatoporoid-encrusted grains) (MF-3); 4) bioclastic grainstone (biosparite) or rudstone with sparite (MF-4);5) Amphipora limestone (MF-5); 6) dark fossil-poor micrite (MF-6); 7) pelsparite or peloidal grainstone with sparite (MF-7); 8) laminated pellet mudstone-wackstone (MF-8); 9) micrite with onkoids (MF-9); 10) rudstone or floatstone (MF-10; and 11) bedded dolomite-gypsum-dolomite (MF-11).  相似文献   

19.
云南兰坪中排李子坪铅锌矿矿床成因   总被引：1，自引：0，他引：1  
孔云丽  戚林坤 《云南地质》2009,28(3):275-279
铅锌矿产于下白垩统景星组下段泥质粉砂岩、钙质砂岩和中侏罗统花开左组上段砂岩夹薄层泥灰岩介壳灰岩中,脉状、透镜状、层状、似层状等产出。NNE向断裂的交叉夹持部位的旁侧派生次级断裂及层间破碎带是主要容矿构造,矿床成因为沉积-中低温热液改造构造控矿型。  相似文献   

20.
西藏西北部晚二叠世地层新资料   总被引：1，自引：0，他引：1  
<正> 藏北地区的晚二叠世地层,文世宣等(1979)报导了羌塘东部双湖热觉茶卡一带的上二叠统,命名为热觉茶卡组。梁定益等(1983)在羌塘西部多玛附近发现位于下二叠统之上和下三叠统之下的一套含Waagenophyllum和腕足类的碎屑岩和碳酸盐地层,称吉普日阿群。诺林(1946)把塔什立克湖至窝尔巴错一带的二叠系称塔什立克湖群,该群的顶  相似文献   

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