Lower Middle Ordovician Research Articles

An integrated trilobite and conodont biostratigraphy across the base of the Laurentian Whiterockian Series (lower Middle Ordovician) at its stratotype, Whiterock Canyon Narrows, Nevada

AbstractThe outcrop at Whiterock Canyon Narrows, Nevada, is the stratotype for the Middle Ordovician Whiterockian Series in Laurentia. Contrasts in the distribution of trilobites and conodonts between two parallel sections at the stratotype demonstrate the presence of an unconformity separating Ibexian faunas (the historic trilobite Zone J, the more recent restricted “Pseudocybele nasuta” trilobite Zone, theReutterodus andinusconodont Zone) from overlying Whiterockian faunas (Zone L, thePsephosthenaspis pseudobathyurustrilobite Zone, theOrthidiellabrachiopod Zone, theTripodus combsiconodont Zone). The unconformity represents the erosional loss of a minimum of 10 m of shale and bedded limestone within the upper Ninemile Formation. In comparison to more continuous section in the Ibex region, Utah, the equivalent to the thinPsephosthenaspis microspinosatrilobite Zone and, possibly, the uppermost interval of the underlying “P.nasuta” Zone are missing.Illaenus welchinew species is described from the Whiterockian faunas of the upper Ninemile and Antelope Valley formations.

Geochemistry and charge history of oils from the Yuqi area of Tarim Basin, NW China

The geochemistry, origin and charge history of oils from the Yuqi area of Tarim Basin have been investigated, through GC, GC-MS and fluid inclusion microthermometry analysis. The Yuqi oils accumulated mainly in three intervals: (1) in the Lower-Middle Ordovician Yingshan Formation (O1-2y) carbonate reservoirs; (2) in the overlying Upper Triassic Halahatang Formation (T3h); and (3) in the Lower Cretaceous Yageliemu Formation (K1y) sandstones. Oils from different reservoirs have distinct physical properties, varying from extra-heavy (O1-2y), heavy (T3h), to light oils (T3h and K1y). However, their geochemical compositions show a high degree of similarity, which indicates that they derive from the same source rock. Abundant tricyclic terpanes, gammacerane, dibenzothiophene and C21C22steranes, together with a low level of diasteranes, indicate an anoxic marine source rock for oils in the Yuqi area. Oil-oil correlation shows that Yuqi oils derive from the same source bed as Tahe oils. The co-occurrence of intact n-alkanes and 25-norhopanes in all the samples supports the proposition that there is a mixture of an early filled severely biodegraded oil and a late filled fresh oil.In this study, charge history is examined on the basis of integration of fluid inclusion homogenization temperature data with 1D burial-thermal history models. Two episodes of oil charging are identified in the O1-2y reservoir (well YQX1-1) at around 436-420 Ma (Middle-Late Silurian) and 10-3 Ma (Miocene to Pliocene), respectively. For the samples from the T3h and K1y intervals, only one episode of oil charge is indicated by the homogenization temperatures of coexisting aqueous inclusions with an inferred timing around 10-3 Ma. The T3h heavy oil reservoir is assumed to be a secondary hydrocarbon pool, which accumulated by re-migration and re-distribution of hydrocarbons from O1-2y hydrocarbon pools. The few early biodegraded oils in the K1y light oils were probably picked up along the migration pathway during the late fresh oil charging.

The ages and tectonic setting of the Faja Eruptiva de la Puna Oriental, Ordovician, NW Argentina

The Ordovician Faja Eruptiva de la Puna Oriental is a magmatic, predominantly intrusive belt in the Puna of northwestern Argentina with a N-S extension of ca. 400km. Scarce isotope geochemical ages and biostratigraphic data on some of the folded Faja Eruptiva country rocks assign the magmatism either to the Lower and lower Middle Ordovician, or to the latest Ordovician. Interpretations of origin and tectonic framework of the Faja Eruptiva are controversial and vary between arc, back-arc and collisional-orogenic settings.We present high-resolution La-ICP-MS U–Pb age and Hf isotope data on zircons from 10 plutonic samples covering the magmatic belt along a length of 200km in the northern Argentinian Puna. The xenocrystic and magmatic zircon age data have a wide spread between 2700Ma and 440Ma. Concordia and weighted mean age data document protracted magmatism in two phases between 480 and 460Ma, and between 453 and 444Ma, and constrain the time of the last intrusions at 444±3Ma and at 445±2Ma thus defining this last and main phase of intrusion at 444Ma.εHf(t) values define a main vertical trend centered at 500Ma with εHf(t) values between +3 and −16 indicating significant mixing of juvenile early Paleozoic melts with Paleoproterozoic crustal components. A second trend is formed by zircons with ages between 1.1Ga and c. 500Ma and predominantly positive εHf values between +8 and −3 and originates in juvenile mantle compositions between 1.6 and 1.1Ga. The spread of the zircon and Hf data document that the Faja Eruptiva intrusives have experienced large-scale contamination by the hosting crustal basement. It follows that the basement of the Puna is formed either by the upper Proterozoic-lower Cambrian Puncoviscana Formation as an erosional product of the Proterozoic orogenic belts of SW Amazonia or that the Puna including its Puncoviscana basement is underlain by a crust shaped by these orogenies.The main intrusive event at 444Ma has been linked to the Oclóyic orogeny in the Late Ordovician. The plutons intruded very likely in a sinistral strike-slip regime after the main folding phase of the Oclóyic orogeny had deformed the Ordovician sedimentary country rocks.

Integrated conodont biostratigraphy and carbon isotope chemostratigraphy in the Lower–Middle Ordovician of southern Sweden reveals a complete record of the MDICE

AbstractOne of the few and most complete records of the MDICE (Middle Darriwilian Isotope Carbon Excursion) is herein documented from Baltoscandia. Based on a core section penetrating the condensed Lower–Middle Ordovician succession (~46 m) on the island of Öland, southeastern Sweden, we provide an integrated scheme for carbon isotope chemostratigraphy (313 samples) and conodont biostratigraphy (29 samples) for this period. The carbonate succession in the Tingskullen core records 12 conodont zones and 6 subzones, including theOepikodus evae, Trapezognathus diprion, Baltoniodus triangularis, B. navis, B. norrlandicus, Lenodus antivariabilis, L. variabilis, Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus(Microzarkodina hagetianaandMicrozarkodina ozarkodellasubzones),E. suecicus, Pygodus serra(E. foliaceus, E. reclinatus, E. robustusandE. lindstroemisubzones) andPygodus anserinuszones in ascending order. The δ13Ccarbrecord reveals an apparently complete record of the MDICE, including a rising limb, a well-defined peak and a falling limb. The anomaly covers a thickness ofc. 27 m in the core and spans theEoplacognathus pseudoplanus, E. suecicus, Pygodus serraandP. anserinusconodont zones. Combined with the new, detailed conodont biostratigraphy, the MDICE in the Tingskullen core can be used for detailed correlation with successions from Baltica, North America, the Argentine Precordillera, South China and North China.

Reefs in the Early Paleozoic Taebaek Group, Korea: A Review

AbstractVarious 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.

Dolomitisation of the Lower-Middle Ordovician Yingshan Formation in the Tazhong area, Tarim Basin, western China

The carbonate unit in the Lower-Middle Ordovician Yingshan Formation in the Tazhong region of the Tarim Basin in western China is partially to completely dolomitised. Three types of dolomite were identified in this unit: microcrystalline (<20 μm) to finely crystalline (20–50 μm) subhedral dolomite rhombs (Rd1), finely crystalline (50–250 μm) anhedral to subhedral mosaic dolomites (Rd2), and medium to finely crystalline (200–500 μm) euhedral to subhedral sucrosic dolomites (Rd3). Three types of calcite cement were also identified: coarsely crystalline mosaic calcite cement in mouldic pores (Cc1), coarsely crystalline intergrown calcite cement (Cc2), and coarsely crystalline sparry and veined calcite cement in the fractures (based on hand sample observation) (Cc3). The δ13C values and 87Sr/86Sr ratios of the Rd1 dolomite in the Yingshan Formation are consistent with those of Lower to Middle Ordovician marine limestone, suggesting that the isotopic ratios were inherited from the precursor limestone. Stratigraphic, petrographic and geochemical data constrain the formation of the Rd1 dolomite to a shallowly buried environment involving seawater with elevated salinity. The Rd2 dolomite yields lower δ18O values (−7.5 to −4.7 ‰) and contains less Sr and Na and more Fe and Mn than the Rd1 dolomite (−5.3 to −3.2 ‰). Therefore, the dolomitising fluids of the Rd2 dolomite were likely derived from the infiltration and diffusion of residual evaporitic water resulting from intense compaction at elevated temperatures. The Rd3 dolomite generally features lower δ18O values (−9.0 to −7.0 ‰) than the Rd2 dolomite, but the δ18O values of portions of the Rd2 and Rd3 dolomites overlap. Thus, the Rd3 dolomite may represent recrystallisation of the Rd2 dolomite. The Rd3 dolomite contains significantly lower Sr and Na contents and significantly higher Fe and Mn contents than the Rd1 and Rd2 dolomites, suggesting that the dolomitic fluids precipitated under reducing conditions during burial. The late-stage, medium-coarse to very coarse calcite postdates all the dolomites and is present as cement in fractures and pores. The δ18O values of the Rd3 dolomite and Cc2 calcite cements (−12.6 to −7.9 ‰) are similar, suggesting that the calcite cement in the pores may represent calcite supersaturation associated with burial dolomitisation. The Cc3 calcite cements feature slightly higher δ13C values (+0.2 to +0.4 ‰) and higher radioactive 87Sr/86Sr ratios (0.709280 to 0.709483) than those of the limestone (0.707955 to 0.708231) and Rd3 dolomite (0.707723 to 0.708345) and, in places, lower δ18O values (−11.6 to −6.3 ‰) than the latter. Consequently, the Cc3 calcite cement either formed from the upward migration of basinal fluids in a burial environment or was impacted by hydrothermal activity.

Paired carbon isotopic analysis of Ordovician bulk carbonate (δ13Ccarb) and organic matter (δ13Corg) spanning the Great Ordovician Biodiversification Event

Paired measurements of bulk carbonate (δ13Ccarb), organic matter (δ13Corg), and their difference (Δ13C) can be used to estimate changes in isotopic fractionation through time as a function of O2/CO2 in the atmosphere. However, because local scale processes can also affect Δ13C, it is essential to compare sections from widely separated water masses. Here we present new δ13Corg data from Ordovician carbonate rocks from the Great Basin, Oklahoma, and Appalachian Basin and compare with published δ13Ccarb records from these sections and paired δ13C values from other carbonate successions around North America. These new data complement previous studies that focused on Upper Ordovician δ13Ccarb excursions and now provide a composite Ordovician δ13Corg record. New Lower Ordovician (Tremadocian Stage) δ13Corg data range from ca. −26 to −28‰, decreasing throughout the Lower–Middle Ordovician (Floian–Dapingian Stages) to ca. −29 to −31‰. δ13Corg values remain at their lowest throughout the Sandbian and are similar to other published Upper Ordovician (Sandbian–Katian) δ13Corg data from North America. Δ13C values from well-preserved intervals generally vary between +26 to +28‰ throughout the Lower to Middle Ordovician (Tremadocian to early Darriwilian), but increase to +31‰ during the mid–late Darriwilian and mid Sandbian, similar to published data from younger Late Ordovician positive δ13C excursions known as the Guttenberg (GICE) and Hirnantian (HICE) events. The overall Δ13C trend shows a ~3‰ increase throughout the Early–Middle Ordovician and coincides with a previously interpreted period of ocean cooling and some of the earliest pulses of global biodiversity of marine invertebrates and planktonic organisms. Modeling studies predict that pCO2 decreased during this time, suggesting that the effect of pCO2 on Δ13C may have been overwhelmed by other controls, such as an in increase in pO2 or a higher O2/CO2 ratio during this biodiversification event.

Deep Endichnial Cruziana from the Lower-Middle Ordovician of Spain — A Unique Trace Fossil Record of Trilobitomorph Deep Burrowing Behavior

Full reliefs of Cruziana furcifera from the Lower-Middle Ordovician quartzite sandstone beds (Pochico Formation, southern Spain) points to deep, infaunal burrowing of trilobites. Some specimens show an unusual vertical extension with a wider lower part and a narrower upper part in cross section. They are referred to trilobites, which burrowed deeply in the sediment and were oriented obliquely head down and tail up. Deep burrowing seems to be common for other members of the Cruziana rugosa group, foremost C. rugosa and C. furcifera, less for C. goldfussi. The deep burrowing recorded in the discussed trace fossils can be referred to the earliest common infaunalization caused by trilobites and other arthropods during the Ordovician, probably in a response to a food competition on the sea floor, which promoted a behavioral plasticity within the same taxon or closely related taxa of trilobites.

Ordovician carbonate rock matrix fractured-porous reservoirs in Tahe Oilfield, Tarim Basin, NW China

Abstract Features of porphyritic limestone matrix fracture-vug reservoirs in Tahe Oilfield, Tarim Basin, NW China was examined by core observation, thin section identification, cathodoluminescence, microscopic fluorescence, scanning electron microscope and energy spectrum analysis. Thick and widespread in the Lower-Middle Ordovician Yijianfang and Yingshan Formations, sand-clastic masses in the porphyritic limestone are rich in oil, quite a proportion of oil and gas produced currently is from their matrix rather than completely from the fracture cave system in the past understanding. In irregular spot or ribbon shape on cores, dolomitic sand-clastic masses usually account for about 40% of the total surface of core in many layers, arenite section has a surface porosity of around 39%. The arenite section is well-crystallized dolomite, the dolomite crystals are mainly 100−350 μm in diameter, equivalent to medium-fine sand; high-pressure mercury injection experiment results show that the reservoir physical property of sand-clastic masses is much better than that of micrite limestone, with a porosity of 12.57%−36.39%. There developed abundant stylolites and microcracks around the sand-clastic masses, which connect the oil bearing units of dolomitc sand-clastic masses, making micropores and microcracks communicate and become effective reservoir space.

Carbon isotope (δ13Ccarb) stratigraphy of the Lower–Middle Ordovician (Tremadocian–Darriwilian) in the Great Basin, western United States: Implications for global correlation

New stable carbon isotope data (δ13Ccarb) from Lower–Middle Ordovician (Tremadocian to Darriwilian) carbonate mudstone and wackestone rocks of the Pogonip Group are presented from two sections in the Great Basin region (USA) — Shingle Pass (east-central Nevada) and the Ibex area (western Utah). The Pogonip Group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the Late Cambrian (Furongian) to Middle Ordovician (Darriwilian). The Shingle Pass and Ibex area sections have been previously studied for their conodont biostratigraphy and contain a North American Midcontinent conodont fauna that range from the Cordylodus intermedius Zone (uppermost Cambrian) to the Phragmodus polonicus Zone (Darriwilian). The δ13C trend has four distinct characteristics recognized in both Great Basin sections: 1) a drop in δ13C from +1‰ at the base of the Ordovician (Tremadocian) to −0.7‰, 2) a 1 to 2‰ positive δ13C shift in the uppermost Rossodus manitouensis Zone during the late Tremadocian, 3) a gradual δ13C increase from −2‰ to ca. 0‰ during the end of the Early Ordovician (Floian), and 4) a steady δ13C decrease from 0‰ to −4 to −5‰ during Middle Ordovician (Dapingian–Darriwilian).In the Lower Ordovician, δ13C trends reported here from the Great Basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies. Instead, these Lower Ordovician isotope data most likely reflect primary seawater chemistry and changes in δ13C on a global scale. This interpretation is supported by the excellent correlation of δ13C in the Lower Ordovician to other δ13C trends reported from the sections in the Argentine Precordillera (La Silla and San Juan formations) and in western Newfoundland (St. George and Table Head groups). These correlations using δ13C are consistent with published biostratigraphic data and provide an integrated and high-resolution chemo-biostratigraphic framework for the Lower Ordovician sedimentary record of the Laurentian margin. The Middle Ordovician portion of the δ13C curves in the Great Basin represented by the Kanosh and Lehman formations shows significant isotopic depletion relative to the section in Argentina. Thus, although there is some indication that minima and maxima in the Middle Ordovician curves can be correlated, the Great Basin sections show clear evidence of overprinting by local variables related to both diagenesis (dolomitization) and platform restriction.

δ13C chemostratigraphy in the upper Tremadocian through lower Katian (Ordovician) carbonate succession of the Siljan district, central Sweden; pp. 277–286

Based on delta C-13 data from two drillcores recovered from the Siljan district, we present a first continuous carbon isotope record of the upper Tremadocian-lower Katian limestone succession of central Sweden. New names for some isotopic carbon excursions from the Cambrian-Ordovician boundary through the basal Darriwilian are introduced. The Mora 001 core from the western part of the Siljan impact structure ranges through the Lower-Middle Ordovician, whereas the Solberga 1 core from its eastern part ranges through the Middle-lower Upper Ordovician. Upper Tremadocian and Floian units are extremely condensed and include extensive stratigraphic gaps. Multiple hardgrounds, sometimes with minor karstic overprint, imply recurrent periods of erosion and/or non-deposition. Like in other parts of Sweden, the Dapingian and Darriwilian succession is characterized by a relatively complete sedimentary record and low sedimentation rates. (Less)

High resolution trilobite stratigraphy of the Lower–Middle Ordovician Öland Series of Baltoscandia

AbstractThe first ever list of the regional Öland Series (Tremadocian to mid Darriwilian) trilobites and agnostids from the whole of Baltoscandia is compiled. The study includes revision of systematics as well as vertical and horizontal distribution. This is necessary because of the uneven state of knowledge, some faunas having being studied recently, others not for more than a century. Three successive faunal types are recognized: the Olenid Fauna surviving from the Cambrian in black bituminous shale facies, the immigratingCeratopygeFauna, and the stabilized Asaphid Fauna. The latter fauna collapsed in the middle of Darriwilian and gave way to a set of new post-Ölandian faunas.

Identification and distribution of marine hydrocarbon source rocks in the Ordovician and Cambrian of the Tarim Basin

Abstract A process for identifying marine hydrocarbon source rocks is established based on outcrop measurement, ground-penetrating radar survey, core analysis, logging evaluation, seismic interpretation, etc. It is used to identify the Cambrian and Ordovician marine hydrocarbon source rocks and to predict their spatial distribution in the Tarim Basin. Four suites of marine hydrocarbon source rocks are identified in the Tarim Basin, and they are distributed, respectively, in the Lower-Middle Cambrian of the Awati and Manjiaer sags, the Heituao Formation of the Lower-Middle Ordovician in the Tadong region, the Saergan-Tumuxiuke-Queerqueke formations of the Middle-Upper Ordovician from the Awati Sag to the Manjiaer Sag, and the Yin'gan and Lianglitage formations of the Upper Ordovician from the Awati Sag to the Tazhong area. The development of these source rocks was controlled by the sea level rise and they are characterized by different facies in the same stage, namely, different source rocks are deposited in sedimentary facies zones at the same stage. The formation of the source rocks shifted with the carbonate basin and was interrupted when there are large quantities of siliciclastic influx.

Tracking shallow marine red beds through geological time as exemplified by the lower Telychian (Silurian) in the Upper Yangtze Region, South China

Marine red beds occur frequently in China through geological time. Despite their complex environments, the red beds are found in three depositional settings: 1) oceanic, deep water, as in the Upper Cretaceous of southern Tibet; 2) outer shelf, deeper water, as in the Lower-Middle Ordovician of South China; and 3) inner shelf, shallow water, as in the Silurian and Triassic in South China. The Silurian marine red beds are recurrent in the lower Telychian, upper Telychian, and upper Ludlow. This paper is to document the marine nature of the lower Telychian red beds (LRBs) in the Upper Yangtze Region and to discuss the spatial and temporal distribution of the LRBs and their depositional environments. The LRBs are best developed on the north side of the Cathaysian Oldland, which can be interpreted as the source area. It is inferred that they were deposited during a marine regression, characterized by the lack of upwelling, low nutrition and organic productivity with a decrease of biodiversity and a high rate of sedimentation. The iron-rich sediments may have been transported by rivers on the oldland into the Upper Yangtze Sea, as rates of deposition were rapid enough to counteract normal reducing effect around sediment-water interface. The LRBs are different from the off-shore, deeper water red beds of lower Telychian in Avalonia and Baltica and further from the oceanic, deep water red beds of Upper Cretaceous in southern Tibet chiefly in palaeogeographic settings, biotic assemblages and marine environments.

280 Acritarch biostratigraphy of the Lower-Middle Ordovician in South China

280 Acritarch biostratigraphy of the Lower-Middle Ordovician in South China

Bryozoans of the Latorp and Volkhov horizons (Lower-Middle Ordovician) of the Leningrad Region

This monograph deals with the morphology of bryozoans of the orders Cystoporida and Trepostomida from the Latorp and Volkhov horizons (Lower-Middle Ordovician, Floian-Darriwilian stages) of the Leningrad Region. Diagnoses of some families belonging to these orders are revised. Based on morphological features of the bryozoans studied and on the succession of their taxa, four assemblages and three stages in their development are established. In addition, the stratigraphic and paleogeographic distributional patterns of bryozoans in the Lower and Middle Ordovician of the world are analyzed.

Scandinavian Isograptids (Graptolithina, Isograptidae): Biostratigraphy and Taxonomy

SUMMARY The Lower–Middle Ordovician of Scandinavia is largely dominated by endemic Atlantic Province graptolite faunas, which are useful only for local biostratigraphy and paleoecology, but are difficult for correlation on an inter-continental scale. Isograptids, regarded as essential for inter-continental biostratigraphic correlations in the Lower–Middle Ordovician due to their wide palaeobiogeographic distribution, have rarely been recorded from the region. The Scandinavian Tøyen Shale and Bogo Shale formations bear a number of isograptids that range in age from the late Floian to the early Darriwilian, and these provide a good tool for inter-continental correlations of this interval. Isograptus caduceus norvegicus Monsen and Isograptus spinosus Spjeldnaes are interpreted as possible tetragraptid species. The following biostratigraphically important Isograptus species from Scandinavia are described as new: Isograptus spjeldnaesi n. sp., Isograptus rigidus n. sp. Isograptus mobergi n. sp. and Scandinavian zonal names in the Dapingian are revised accordingly.

The Middle Ordovician litho- and biostratigraphy of northwestern Gorny Altai

The structure of the Lower–Middle Ordovician Tuloi and Karasa Formations, which occur widely in northeastern Gorny Altai, has been studied. A bed-by-bed description of a new section of the boundary sediments between these stratigraphic units has been given. The graptolite, trilobite, and brachiopod assemblages typical of the Tuloi and Karasa Formations have been analyzed. Graptolite zones and subzones have been recognized, which permit correlating the stratotype and key sections of these local stratigraphic units with the stages of the International Stratigraphic Chart (ISC). The Karasa Formation occupies a considerably larger stratigraphic volume than presumed: It spans the late Arenig and the entire Llanvirn (including the Llandeilo) of the British standard, which is used in Russia (=Upper Dapingian–Darriwilian of the new Ordovician standard in the ISC). Finally, the paper discusses the suggested changes in the names of regional stratigraphic units (horizons) in the Ordovician stratigraphic chart of the western Altai–Sayan fold belt.

Biodiversification of Early to Middle Ordovician conodonts: a case study from the Zitai Formation of Anhui Province, eastern China

The Lower–Middle Ordovician Zitai Formation of the South China palaeoplate consists of a succession of purple red, nodular argillaceous limestones. Palaeogeographically, it is distributed along the southeastern margin of the Yangtze Platform, and is of late Floian to Dapingian age, correlative with the Dawan Formation of the Middle and Lower Yangtze Platform. In Shitai County, Anhui Province, East China, the Zitai Formation is rich in conodonts, enabling the recognition of four biozones based on first appearance data. Detailed palaeontological and biostratigraphical study of these conodonts reveals that the Ordovician conodont radiation in the Lower Yangtze Platform attained its first diversity peak low in the Oepikodus evae Biozone. This diversification is generally consistent with macroevolutionary trends of brachiopods of South China and graptolites of the Upper Yangtze Platform, but was earlier than that of trilobites and acritarchs of the same palaeoplate. Correlation with the sea-level curve for South China suggests that conodont diversity change during the Ordovician radiation was mainly controlled by regional sea-level fluctuations.

Ecostratigraphical interpretation of lower Middle Ordovician East Baltic sections based on brachiopods

AbstractA detailed ecostratigraphical framework is established for the lower Middle Ordovician Kundan regional stage of the East Baltic area corresponding to theAsaphus expansus,A. ranicepsandA. eichwalditrilobite zones (lower Darriwilian). The study is based on approximately 6200 brachiopods collected bed by bed from limestone sections in northern Estonia (Harku Trench and Saka) and western Russia (Putilovo Quarry, Lava River canyon and Lynna River valley) with, in addition, the first detailed systematic assessment of the Kundan brachiopods of the East Baltic. These sections represent an oblique depth transect some 400 kilometres long, deepening eastwards. Five biofacies associations have been recognized using detrended correspondence and cluster analyses: a shallow-waterLycophoriaassociation, a transitionalGonambonitesassociation and two deeper-water associations, the soft-substrateOrthis callactisand the hard-substrateOrthambonitesassociations. A separate, fifth soft-substrate association is present in the marl beds at the main locality of Putilovo Quarry. The associations reflect a combination of palaeo-water depth and substrate. The biofacies facilitate an ecostratigraphical correlation along the transect, and third and fourth order sea-level curves are reconstructed, reflecting mainly eustasy. The sea-level was relatively low, early in the Kundan, but then rose significantly into theA. ranicepsBiozone. This corroborates the recent discovery of possible small early Darriwilian ice caps on Gondwana.