Conodont Colour Alteration Index Research Articles

Significance of conodont data for explaining geosystem perturbations during the Middle Devonian Kačák Episode

The present review of published data as well as the new results demonstrate the versatility of conodonts in documenting and explaining global environmental fluctuations related to the Kačák Episode (KE) in the latest Eifelian. Although the conodont zonation of the KE interval is ambiguous and requires revision, the compilation of conodont stratigraphic ranges shows their potential for a precise worldwide correlation of relevant marine strata. Conodont biofacies may serve to document environmental changes connected with KE, in particular the sealevel rise at its beginning, followed by a regressive trend. Nevertheless, the familiar Icriodus/Polygnathus ratio should be carefully applied as an indication of water depth and nearshore vs. offshore position, being controlled also by other factors, such as paleolatitude and/or climate. Oxygen isotopes in conodont apatite, studied using secondary ion mass spectrometry technique evidence a warming at the onset of KE, based on the new data from the open marine facies of the Prague Basin. At the same time, they indicated climate-controlled salinity fluctuations in the epeiric Belarusian Basin. The present investigations as well as previous results suggest caution when analyzing thermally altered conodonts which may result in biased oxygen isotope signatures. The present experience suggests the conodont colour alteration index CAI 3 as a boundary value above which the caution is necessary.

Lower and Upper Ordovician conodont biostratigraphy and revised lithostratigraphy in the fault and fold zones of the Boothia Uplift, south-western Boothia Peninsula, Nunavut

The strata exposed near Pasley Bay on south-western Boothia Peninsula were previously named the Franklin Strait Formation, spanning Lower Ordovician through early Silurian and dissected by two disconformities. The interpretation of the age and correlation were based on limited fossil evidence. New detailed field investigations and sampling at the Pasley Bay section resulted in a finding of over 1000 identifiable conodont specimens representing 37 species among 28 genera. Five Lower Ordovician and four Upper Ordovician North American standard conodont Zone-equivalent faunas are documented from the section, namely: Lower Ordovician Cordylodus angulatus, Rossodus manitouensis, Acodus deltatus/ Oneotodus costatus, Oepikodus communis, Reutterodus andinus, and Upper Ordovician Belodina confluens, Oulodus velicuspis, Oulodus robustus, and Amorphognathus ordovicicus Zone-equivalent faunas. These faunas enable a new understanding of the age and stratigraphic position of the Franklin Strait Formation: (1) it is now correlated to the Turner Cliffs (upper part of lower member and upper member), Ship Point, Thumb Mountain, Irene Bay, and Allen Bay (lower member) formations straddling Lower to upper Upper Ordovician with the Middle and lower Upper Ordovician missing; (2) it biostratigraphically overlaps with the simultaneously named Netsilik Formation for the strata containing the Cordylodus angulatus, Rossodus manitouensis, Acodus deltatus/Oneotodus costatus, and lower Oepikodus communis Zone-equivalent faunas, and lithostratigraphically for the upper part of lower member and upper member of the Turner Cliffs Formation and lower Ship Point Formation. This study also provides stratigraphic support for the tectonic model that advocates the Boothia Uplift to be a deep-seated, east-dipping thrust block. Additionally, the conodont Colour Alteration Index is used to estimate the increased heat effects close to the fault zones in the environs of the Boothia Uplift.

Organic Matter and Generation Potential of Paleozoic Deposits in the Tal’bei Block of the Chernyshev Ridge (Timan–Pechora Petroleum Province)

Abstract—The study area is located within the Chernyshev Ridge, a fold–thrust structure, which is a promising area for oil and gas exploration in the Timan–Pechora province. Petroleum source rocks of Paleozoic (from Upper Ordovician to lower Carboniferous) deposits stripped by the Vorgamusyurskaya-1 well in the Tal’bei block of the Chernyshev Ridge were studied by lithological, petrographic, organic-petrology, and organic-geochemistry methods. Most of the section is characterized by low values of Corg (<0.5%), genetic potential (S1 + S2 < 0.5 mg HC/g rock), and hydrogen index (HI < 100 mg HC/g Corg). Petroleum source rocks with elevated values of Corg (up to 0.9%), S1 + S2 (up to 2.1 mg HC/g rock), and HI (up to 277 mg HC/g Corg) were identified at different stratigraphic levels of the section: Upper Devonian (D3tm–sr), upper Silurian (S2gj), and Upper Ordovician (O3mt). The maceral composition of organic matter (OM) and the distribution of hydrocarbons in the saturated fraction of bitumens indicate that petroleum source rocks include only marine OM (type II kerogen). The degree of catagenesis of OM was estimated based on the bituminite reflectance (RVeq), Rock-Eval pyrolysis (Tmax), and conodont color alteration index (CAI). The available RVeq (0.63–0.84%), Tmax (430–443 °C), and CAI (1.5–2.0) data indicate that the entire Paleozoic well section is in the oil window (MC1–MC3). The new data refined the catagenetic zoning of the sedimentary section, according to which the OM reached the following gradations: MC1 in the lower Carboniferous and Famennian deposits, MC2 in the interval from the Frasnian to the upper part of the lower Silurian, and MC3 in the lower part of the lower Silurian and in the Upper Ordovician. The results of the study of petroleum source rocks and the type and maturity of OM will be the basis for basin modeling, which will increase the reliability of reconstructions of oil and gas formation at the Chernyshev Ridge.

The impacts of diagenesis on the geochemical characteristics and Color Alteration Index of conodonts

The conodont Color Alteration Index (CAI) has been widely used to determine the maximum temperature in carbonate rocks, despite recognition that conodont colour can be affected by other factors, such as diagenesis. Measurements of trace element characteristics in conodonts of varying CAI (1.5–6.0) from the Canadian Cordillera show that those specimens with the most anomalous CAI with respect to independent estimates of maximum temperature also have the highest concentrations of iron (> 3000 ppm). The adsorption of transition metals such as iron onto bioapatite crystals and permineralization by their oxides in conodont elements are herein proposed as mechanisms for the modification of CAI during diagenesis. Furthermore, the trace element characteristics of conodonts, primarily those of the lanthanide, or rare earth element (REE) series, have frequently been used as a proxy for palaeoceanographic conditions, including anoxic events. However, the impact of early diagenetic processes post-burial obscures this marine signal, and instead, the trace element characteristics of conodonts likely reflect the characteristics of pore waters or diagenetic fluids. Several geochemical ‘tools’ have been proposed to test for such overprinting of palaeoceanographic information, including Y/Ho vs. ΣREE, MREE/MREE*, U concentration, and La/Yb. However, REE and Raman structural characteristics in a suite of conodonts from the Canadian Cordillera indicate that these ‘tools’ cannot be systematically applied. The majority of conodont specimens analysed in this study appear to have been affected by post-burial diagenetic alteration using one or more of the ‘tools’, but seem to be unaltered when investigated using other ‘tools’. Additionally, several specimens which appear to be diagenetically unaltered when using the metrics of the geochemical ‘toolbox’ exhibit diagenetically induced structural changes. The geochemical ‘tools’ are thus unable to discriminate between diagenetic alteration induced before and after burial, and they should therefore be accompanied by structural analyses if REE characteristic of conodonts are used to infer palaeoceanographic conditions.

Conodont thermochronology of exhumed footwalls of low-angle normal faults: A pilot study in the Mormon Mountains, Tule Springs Hills, and Beaver Dam Mountains, southeastern Nevada and southwestern Utah

(U-Th)/He thermochronology is a well-established dating technique used to understand the temperature-time histories of rocks in a wide range of geologic settings. The technique is presently restricted to rocks that contain specific accessory minerals, such as apatite or zircon. Marine carbonates and shales typically lack these accessory phases in quantities and sizes practical for (U-Th)/He dating and thus present a challenge for application of the method. Here, we explore the utility of biogenic apatite from conodonts as a (U-Th)/He thermochronometer at a well-studied calibration site located in eastern Nevada and southwestern Utah.We performed (U-Th)/He thermochronometry, laser ablation inductively coupled plasma mass spectrometry, X-ray micro-computed tomography, and scanning electron microscopy (SEM) on specimens with conodont color alteration indices (CAI) of 1.5–3, extracted from carbonate rocks in the footwalls of low-angle normal faults in the Mormon Mountains, Tule Spring Hills, and Beaver Dam Mountains. Conodont (U-Th)/He (CHe) dates have high scatter; dates are commonly reproducible to 20% of sample means but can deviate up to 150%. All CAI 1.5–2.5 conodonts produce CHe dates younger than 240 Ma, consistent with thermal resetting of samples; however, most CAI 3 conodonts give ages 2–6× older than Mississippian and Permian deposition. Average U, Th, and rare earth element (REE) concentrations depend on porosity and permeability differences between albid and hyaline conodont tissue and range from <10 to 100 s of ppm in concentration. Parent isotope concentrations are especially low in CAI 3 conodonts, commonly <1 ppm, and there is an inverse relationship between these concentrations and CHe dates. The majority of parent U, Th, and Sm and REEs are concentrated within the outer 5 μm of the conodont elements and consistently show 5–10× enrichment relative to cores. Margin enrichment is also depressed with increasing CAI. SEM imaging shows a shift in the orientation of apatite microcrystallites from perpendicular to parallel to the major axes of the conodont elements at CAI 3 and corrosion and recrystallization features, likely associated with burial and diagenesis, on the surfaces of some CAI 2.5 and 3 conodonts.We propose that microstructural changes associated with increasing CAI influence CHe dates. Parent isotope loss occurs during the post-cooling stage, either in the outcrop or in the laboratory. Our hypothesis is that the double-buffered formic acid procedure for dissolving dolomitized carbonates may accelerate this loss in thermally altered, higher CAI conodonts.

Tectonothermal analysis of a major unit of the Cantabrian Zone: the Ponga unit (Variscan belt, NW Spain)

The paleothermometric evolution of the Ponga unit at the core of the Cantabrian Zone, the arcuate foreland fold and thrust belt of the Variscan orogen in NW Spain, is analyzed using the conodont Color Alteration Index and the Kubler Index of illite. The results indicate that the distribution of the thermal maturity patterns is independent of the stratigraphy and the structure of the unit. These include the occurrence of dominant diagenetic values and very low-to-low-grade metamorphism in the southern area. The main thermal event is interpreted to be related to hydrothermal fluids ascending through late-Variscan extensional faults. The anomalous high heat flow in the southern sector of the Ponga unit is part of a previously identified large band in the Cantabrian Zone, where the common occurrence of low-grade metamorphic rocks, hydrothermal dolostones, and ore mineralizations is related to an extensional orogenic episode that started near the Carboniferous–Permian boundary, soon after the closure of the Asturian arc.

Conodont geothermometry in pyroclastic kimberlite: constraints on emplacement temperatures and cooling histories

Kimberlite pipes from Chidliak, Baffin Island, Nunavut, Canada host surface-derived Paleozoic carbonate xenoliths containing conodonts. Conodonts are phosphatic marine microfossils that experience progressive, cumulative and irreversible colour changes upon heating that are experimentally calibrated as a conodont colour alteration index (CAI). CAI values permit us to estimate the temperatures to which conodont-bearing rocks have been heated. Conodonts have been recovered from 118 samples from 89 carbonate xenoliths collected from 12 of the pipes and CAI values within individual carbonate xenoliths show four types of CAI distributions: (1) CAI values that are uniform throughout the xenolith; (2) lower CAIs in core of a xenolith than the rim; (3) CAIs that increase from one side of the xenolith to the other; and, (4) in one xenolith, higher CAIs in the xenolith core than at the rim. We have used thermal models for post-emplacement conductive cooling of kimberlite pipes and synchronous heating of conodont-bearing xenoliths to establish the temperature–time history of individual xenoliths within the kimberlite bodies. Model results suggest that the time-spans for xenoliths to reach the peak temperatures recorded by CAIs varies from hours for the smallest xenoliths to 2 or 3 years for the largest xenoliths. The thermal modelling shows the first three CAI patterns to be consistent with in situ conductive heating of the xenoliths coupled to the cooling host kimberlite. The fourth pattern remains an anomaly.

Conodont biostratigraphy and carbon isotope stratigraphy of the Middle Ordovician (Darriwilian) Komstad Limestone, southern Sweden

The Komstad Limestone forms a distal part of the widespread ‘orthoceratite limestone’ of the Baltoscandian continent. In this paper, we present an integrated conodont biostratigraphy and carbon isotope stratigraphy for a major part of this formation and address its significance for evaluating Middle Ordovician correlation and overall stratigraphy. Four conodont zones are distinguished, including the Lenodus antivariabilis Zone, Lenodus variabilis Zone, the Yangtzeplacognathus crassus Zone, and the Eoplacognathus pseudoplanus Zone. Carbon isotopes have previously not been published from the Komstad Limestone. The data herein show that carbonate strata may retain a comprehensive pristine δ13C signal despite relatively strong thermal influence during diagenesis (Conodont Color Alteration Index 4–5).

Upper Ordovician – Upper Silurian conodont biostratigraphy, Devon Island and southern Ellesmere Island, Canadian Arctic Islands, with implications for regional stratigraphy, eustasy, and thermal maturation

The conodont biostratigraphy for the Upper Ordovician – Upper Silurian carbonate shelf (Irene Bay and Allen Bay formations) and interfingering basinal (Cape Phillips Formation) facies is established for parts of Devon and Ellesmere islands, central Canadian Arctic Islands. Revisions to the interpreted regional stratigraphic relationships and correlations are based on the stratigraphic distribution of the 51 conodont species representing 32 genera, identified from over 5000 well-preserved conodonts recovered from 101 productive samples in nine stratigraphic sections. The six zones recognized are, in ascending order, Amorphognathus ordovicicus Local-Range Zone, Aspelundia fluegeli Interval Zone, Pterospathodus celloni Local-Range Zone, Pt. pennatus procerus Local-Range Zone, Kockelella patula Local-Range Zone, and K. variabilis variabilis – Ozarkodina confluens Concurrent-Range Zone. These provided a more precise dating of the members and formations and, in particular, the range of hiatuses within this stratigraphic succession. The pattern of regional stratigraphy, facies changes, and hiatuses is interpreted as primarily related to the effects of glacioeustasy associated with the terminal Ordovician glaciation and smaller Early Silurian glacial phases, the backstepping of the Silurian shelf margin, and the geodynamic effects of the collision with Laurentia by Baltica to the east and Pearya to the north. Conodont colour alteration index values (CAI 1–6.5) from the nine sections complement earlier graptolite reflectance data in providing regional thermal maturation data of value in hydrocarbon exploration assessments.

Tectonothermal evolution of the northeastern Cantabrian zone (Spain)

A tectonothermal study of the northeastern sector of the Cantabrian zone (Ponga-Cuera and Picos de Europa units) using the conodont color alteration index (CAI) and Kubler index (KI) methods shows a variation from diagenetic to anchizonal conditions. The latter are illustrated in geological maps and cross sections. The greater part of the studied area has CAI values lower than 2, caused by two long periods of burial separated by a compressive interval (corresponding to the Variscan deformation), in which thrusts were the dominant structures. Tectonic superimposition was balanced by intense and fast erosion and had little effect on the CAI values. In contrast, a high thermal gradient was produced in the southern outcrops that led to recognize the transition between diagenetic and anchizonal conditions through CAI values near the basal thrust of the Picos de Europa unit. The diagenesis/anchizone boundary appears a little further south through the determination of the Kubler index in phyllosilicate minerals (KI = 0.42). CAI isograds cut the Variscan structures as a consequence of a thermal episode that occurred near the Carboniferous–Permian boundary at the beginning of an extensional regime. The thermal source was located further south, in the adjacent Pisuerga-Carrion unit. Alterations in the CAI values, as well as dolomitization and ore deposits, locally resulted from Permian–Mesozoic hydrothermal activity. Microtextural analysis of the conodonts allowed us to relate several types of apatite overgrowth to diagenetic conditions and recrystallization to anchizonal ones, whereas dissolution was common during hydrothermalism. Alpine deformation hardly produced any changes in the previous thermal pattern, but was responsible of the northwards tilting of the structure and CAI isograds.

Tectonothermal evolution in the core of an arcuate fold and thrust belt: the south-eastern sector of the Cantabrian Zone (Variscan belt, north-western Spain)

Abstract. The tectonothermal evolution of an area located in the core of the Ibero-Armorican Arc (Variscan belt) has been determined by using the conodont colour alteration index (CAI), Kübler index of illite (KI), the Árkai index of chlorite (AI) and the analysis of clay minerals and rock cleavage. The area is part of the Cantabrian Zone (CZ), which represents the foreland fold and thrust belt of the orogen. It has been thrust by several large units of the CZ, what resulted in the generation of a large number of synorogenic Carboniferous sediments. CAI, KI and AI values show an irregular distribution of metamorphic grade, independent of stratigraphic position. Two tectonothermal events have been distinguished in the area. The first one, poorly defined, is mainly located in the northern part. It gave rise to very-low-grade metamorphism in some areas and it was associated with a deformation event that resulted in the emplacement of the last large thrust unit and development of upright folds and associated cleavage (S1). The second tectonothermal event gave rise to low-grade metamorphism and cleavage (S2) crosscutting earlier upright folds in the central, western and southern parts of the study area. The event continued with the intrusion of small igneous rock bodies, which gave rise to contact metamorphism and hydrothermal alteration. This event was linked to an extensional episode due to a gravitational instability at the end of the Variscan deformation. This tectonothermal evolution occurred during the Gzhelian–Sakmarian. Subsequently, several hydrothermal episodes took place and local crenulation cleavage developed during the Alpine deformation.

Conodont color alteration index and upper Paleozoic thermal history of the Amazonas Basin, Brazil

The conodont color alteration index (CAI) was determined in elements from core samples of the Frasnian Barreirinha Formation (one well) and of the Pennsylvanian–Permian Tapajós Group (twenty three wells and one limestone quarry) in the Amazonas Basin. The thermal history of the basin is analyzed using the CAI value distribution represented in maps and stratigraphic sections through correlation schemes, and in conjunction with previously published data. The pattern of palaeotemperatures for CAI values of 1.5–3 is coincident with organic matter maturation under a sedimentary overburden providing diagenetic conditions in the oil/gas window. Locally, conodonts show metamorphism (CAI value of 6–7) in relation to the intrusion of diabase bodies in beds including high geothermal gradient evaporites. Microtextural alteration on the surface conodonts commonly shows several types of overgrowth microtextures developed in diagenetic conditions. Locally, recrystallization in conodonts with a high CAI value is congruent with contact metamorphism in relation to Mesozoic intrusions. The CAI values of 1.5 or 2 observed close to the surface in several areas of the basin may be interpreted in relation to a high thermal palaeogradient derived from the magmatic episode or/and to the local denudation of the upper part of the Paleozoic succession prior to this thermal event.

Strontium isotope (87Sr/86Sr) stratigraphy of Ordovician bulk carbonate: Implications for preservation of primary seawater values

The present study on bulk carbonate 87 Sr/ 86 Sr stratigraphy represents a companion work to earlier research that presented a conodont apatite-based Ordovician seawater 87 Sr/ 86 Sr curve for the Tremadocian–Katian Stages (485–445 Ma). Here, we directly compare the curve based on conodont apatite (including some new data not published in earlier work) with a new curve based on 87 Sr/ 86 Sr results from bulk carbonate from the Tremadocian–Sandbian Stages. We sampled eight Lower to Upper Ordovician carbonate successions in North America to assess the reliability of bulk carbonate to preserve seawater 87 Sr/ 86 Sr and its utility for 87 Sr/ 86 Sr chemostratigraphy. A high-resolution 87 Sr/ 86 Sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87 Sr/ 86 Sr ( 87 Sr/ 86 Sr seawater ). In total, 230 bulk carbonate samples that are paired to conodont samples were measured for 87 Sr/ 86 Sr in order to determine the conditions under which 87 Sr/ 86 Sr seawater is preserved in bulk carbonate. Results indicate that well-preserved bulk carbonate can faithfully record the 87 Sr/ 86 Sr seawater trend, but that its 87 Sr/ 86 Sr values are commonly more variable than those of conodont apatite. On average, bulk carbonate samples of the same age vary by 10–20 × 10 −5 , compared to 5–10 × 10 −5 for conodont apatite. The amount of isotopic alteration of bulk carbonate from seawater 87 Sr/ 86 Sr (Δ 87 Sr/ 86 Sr) was determined by taking the difference between 87 Sr/ 86 Sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87 Sr/ 86 Sr values. Cross plots comparing Δ 87 Sr/ 86 Sr values to bulk carbonate Sr concentration ([Sr]) and conodont color alteration indices (CAI; an estimate of the thermal history of a rock body) indicate that bulk carbonate is most likely to preserve 87 Sr/ 86 Sr seawater (minimally altered) when either: (1) bulk carbonate [Sr] is greater than 300 ppm, or (2) carbonate rocks experienced minimal thermal alteration, with burial temperatures less than ~150 °C. Carbonates with intermediate [Sr] (e.g., between 130 and 300 ppm) can also yield 87 Sr/ 86 Sr seawater values, but results are less predictable, and local diagenetic conditions may play a greater role. Modeling results support the argument that seawater 87 Sr/ 86 Sr can be preserved in bulk carbonates with low [Sr] if pore water:rock ratios are low ( 87 Sr/ 86 Sr is similar to the seawater 87 Sr/ 86 Sr value preserved in limestone. Bulk carbonate samples that meet these criteria can be useful for high-resolution measurements of 87 Sr/ 86 Sr seawater , with a sample variation on par with fossil materials ( −5 ), particularly for successions where well-preserved fossil material (i.e., conodonts or brachiopods) is not available, such as Precambrian strata, sequences recording mass extinction events, or otherwise fossil-barren facies. These criteria and model predictions based on bulk carbonate [Sr] must be considered in the context of whether a limestone accumulated under calcite seas (e.g., Ordovician), with relatively high seawater Sr/Ca, or aragonite seas, in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non-seawater Sr.

Dojrzałość termiczna utworów dewonu, karbonu i triasu masywu małopolskiego na podstawie wskaźnika CAI ([i]Colour Alteration Index[/i

THERMAL MATURITY OF DEVONIAN , CARBONI FEROUS AND TRIASSIC ROCKS IN THE CENTRAL PART OF THE MAŁOPOLSKA MASSIF FROM CONODONT COLOUR ALTERATION INDEX Abstract. A paleothermal study of Devonian, Carboniferous and Triassic carbonate rocks was carried out in eight boreholes from the central part of the Malopolska Massif by using data on thermal alteration of organic matter contained in conodonts. The maturity of organic matter expressed as the CAI (Colour Alteration Index) values was used to calculate palaeotemperatures that affected the conodont-bearing rocks. In the Wegrzynow IG 1 borehole, the CAI values for the Upper Givetian, Frasnian, Famennian and Lower Carboniferous rocks range from 3 (Givetian) to 2 (Lower Carboniferous). It suggests heating of rocks (at temperatures from 60 to 110°C) in the area of high palaeogeothermal gradient, close to the Variscan front. In the Upper Devonian sequence of the Pągow IG 1 borehole, the CAI value of 4.5 proves heating of rock at a temperature exceeding 230°C as a result of higher heat flow near a fault zone. The low CAI values (1‒1.5) of Triassic rocks provide the evidence for heating of rocks in the range of 50 to 90°C. The relatively low maturity of organic matter in the Triassic rocks indicates that the Devonian and Carboniferous sediments must have reached their thermal maturity in the Late Carboniferous prior to the Variscan orogeny. The signs of crude oil and natural gas observed in the Devonian and Carboniferous rocks in the central part of the Malopolska Massif attest their potential for hydrocarbon exploration.

Conodonts recovered from the carbonate xenoliths in the kimberlites confirm the Paleozoic cover on the Hall Peninsula, Nunavut

Hall Peninsula, located on southeastern Baffin Island, Nunavut, hosts the newly discovered Chidliak kimberlite province. Presently, this area lacks Phanerozoic sedimentary cover, except for the unconsolidated glacial deposits; however, Late Ordovician and Early Silurian microfossil conodonts have been recovered from carbonate xenoliths preserved in the Late Jurassic – Early Cretaceous kimberlites. Over 1300 conodont specimens were recovered, among which 32 species representing 23 genera are recognized, with four elements indeterminate. The well-preserved conodont faunas provide reliable evidence on the Hall Peninsula for (i) reconstructing the Lower Paleozoic stratigraphic units, including the Upper Ordovician Frobisher Bay, Amadjuak, Akpatok, and Foster Bay formations, and the Lower Silurian Severn River Formation, (ii) estimating a total of 270–305 m in thickness of Lower Paleozoic sedimentary cover prior to the emplacement of the kimberlites, (iii) tracing the erosion history after the emplacement of the kimberlites, and (iv) calculating a minimum erosion rate of 2 m/Ma. The conodonts have a wide range of conodont Color Alteration Index (CAI) values between 1.5 and 8, which is the largest range recorded in any known suite of xenoliths entrained in kimberlites.

Tectonothermal evolution of a major thrust system: the Esla–Valsurbio unit (Cantabrian Zone, NW Spain)

AbstractThe tectonothermal evolution of a unit in the foreland fold-and-thrust belt of the Iberian massif is established using the conodont colour alteration index (CAI). The unit consists of two parts with different tectonothermal histories – the Esla nappe region and the Valsurbio region – separated by a synorogenic Carboniferous basin (Guardo–Valderrueda basin). The Esla nappe region evolved in diacaizonal conditions (corresponding to the diagenetic conditions of the pelites) whose palaeotemperatures were controlled by rock burial. Maximum values were reached before the emplacement of the thrust nappes, so tectonic superimposition is not registered by the CAI. Overburial due to the emplacement of the thrust units was prevented by simultaneous intense erosion. The geothermal gradient obtained for burial wasc. 35 °C km−1and the temperature reached by the older Cambrian rocks wasc. 210–230 °C. The Valsurbio region was affected by an extensional tectonothermal post-orogenic event that gave rise to metamorphism with ancaizonal or epicaizonal conditions (corresponding to anchizone or epizone of the pelites). The most common maximum palaeotemperatures reached in this event fall within the range 305–415 °C, although higher palaeotemperatures could be reached locally as a consequence of contact metamorphism. This event gave rise to subhorizontal cleavage that cuts the main Variscan folds. Coal rank data indicate an increase in maximum palaeotemperatures eastwards from the Esla nappe region to the Valsurbio region through the Guardo–Valderrueda basin.

Conodont thermal alteration patterns in Devonian and Carboniferous rocks of the Ahnet and Mouydir basins (southern Algeria)

Conodont colour alteration indices (CAI) have been used to recognize the thermal maturation of the Middle Devonian to Late Carboniferous sedimentary infill of the Ahnet and Mouydir basins in southern Algeria. The study is based on a large set of conodont samples obtained from 69 sections and outcrops. In the Ahnet Basin, the Middle and Upper Devonian rocks at the surface show a uniform thermal maturity level characterized by the CAI values of 2, which suggests maximum burial temperatures about 50 °C. In contrast, the Carboniferous exposures yield a wide range of CAI values from 1 to 3.5. Besides, the general tendency in maturity decreases upward in the sequence, there are anomalies related to heating by small local magmatic bodies. In the Mouydir Basin, the CAI values in the Middle and Upper Devonian outcrops range from 2 to 3.5. The pattern indicates that the majority of Middle Devonian rocks have experienced burial temperatures of about 120 °C, while thermal maturation of the Upper Devonian strata is clearly lower, with maximum heating temperatures about 50 °C at the edges of the basin. The observed thermal maturity has been mostly achieved during Late Carboniferous time, prior to the Variscan inversion, and only locally during the Mesozoic. It is a product of loading by Carboniferous overburden succession of which part is now removed, approx. 200–1300 m in the Ahnet Basin and up to 1200 m in the Mouydir Basin. The analysis of palaeotemperatures suggests an average geothermal gradient of 30 °C/km during Carboniferous time in the Ahnet–Mouydir area. In terms of hydrocarbon exploration the studied Devonian succession is predominantly mature for oil and gas. In contrary, the Carboniferous rocks are immature. Higher maturities are likely to occur in the subsurface.

Changhsingian (Late Permian) conodonts from Son La, northwest Vietnam and their stratigraphic and tectonic implications

Late Permian conodonts are for the first time reported from Vietnam. Pa, Sa, Sb, Sc and M elements of the Changhsingian conodont species Hindeodus julfensis (Sweet) are reported from a 40cm thick limestone in the middle part of the Yenduyet Formation near Son La, NW Vietnam. The occurrence of H. julfensis indicates a Changhsingian age that is consistent with an interpreted early Changhsingian age for a brachiopod fauna slightly higher in the sampled section. The Son La section is located in the Song Da Rift Zone and overlies basaltic volcanics considered equivalent to the Emeishan large igneous province basalts that are plume related. The Permian–Triassic boundary in Vietnam is yet to be precisely located biostratigraphically but proxy chemostratigraphic data indicate its likely position in sections at Nhi Tao and Lung Cam, N. Vietnam and correlation with the Global Stratotype Section and Point at Meishan, South China. The recovered conodonts have a Conodont Colour Alteration Index of 5 and have been heated to c. 600°C but they do not show any evidence of textural alteration due to regional metamorphism such as micro-folding or stretching that would indicate any direct effects of the compressional Indosinian Orogeny.

Diagenetic alteration of the structure and δ18O signature of Palaeozoic fish and conodont apatite: Potential use for corrected isotope signatures in palaeoenvironmental interpretation

The oxygen isotopic compositions of Carboniferous conodonts and fish microfossils (ichthyoliths), from identical samples, were analysed in tandem in order to test whether these phosphatic media can be reliably used for palaeoclimatic reconstruction. The structure of conodonts and of most ichthyoliths analysed are somewhat analogous to the enamel and dentine/bone tissue, respectively, of modern mammals. Therefore, the diagenetic susceptibilities of the taxa analysed may provide important clues to other palaeoclimatic studies utilising a variety of biogenic apatite.Thermal maturation indices and scanning electron microscopy were used to characterise the preservation of biogenic apatite. Despite a high conodont colour alteration index (CAI) of ~5.5, conodont elements appear to have been only mildly affected by diagenetic alteration. In contrast, ichthyoliths were commonly recrystallised and exhibited diagenetic apatitic overgrowths containing amorphous pyrolytic carbon, interpreted as indicating that at least some overgrowth material derived from the original biogenic apatite.Diagenetic alteration has resulted in ichthyolith δ18O values being systematically lower by, on average, 2.9‰ (1σ=±0.3) relative to conodont apatite. Conodont samples yielded regionally correlatable isotope values, which can be interpreted in terms of more palaeoenvironmentally sensible palaeotemperatures relative to ichthyolith values. Densely crystalline, enamel-like conodont elements are interpreted as the more robust phosphatic medium to preserve original isotopic compositions. The strong correlation (r=0.8) of the δ18O values of the more structurally pristine conodont and commonly recrystallised ichthyolith apatite, indicates that (i) despite diagenetic lowering of absolute isotope values, original palaeoenvironmental records may be preserved and (ii) diagenetic overprinting may result in a stable offset, and therefore be correctable, locally.

Conodont alteration and tectonothermal evolution of a diagenetic unit in the Iberian Variscan belt (Ponga-Cuera unit, NW Spain)

AbstractTectonothermal analysis of a mainly carbonate unit located in the external part of the Variscan orogen in NW Spain is dealt with using the conodont colour alteration index (CAI) and the study of textures of these microfossils. The Kübler index of the illite (KI) is used as a complementary method. The area is characterized by a great uniformity in the CAI values, which in most cases are &lt;2, indicating diagenetic conditions. In spite of the low CAI values, textures show great variety and were mainly originated by diagenetic processes of apatite dissolution and precipitation. The conodonts underwent a long period of heating (probably from the Pennsylvanian to the Cenozoic) to low temperatures (&lt;100°C) to reach the low CAI values measured. Assuming a normal geothermal gradient, these temperatures required an overburden &lt;3 km that in part was due to burial and in part to tectonic superimposition. Minor local anomalies in the CAI values and some textural alterations, related to dissolution and precipitation of authigenic minerals, could be due to epithermal activity that gave rise to various ore deposits in the studied area mainly during Permian times.