Colour Alteration Index Research Articles

Conodonts and corals in kimberlite xenoliths confirm a Devonian seaway in central Ontario and Quebec

Eighteen samples containing sedimentary rock xenoliths were obtained from cores drilled into eight Mesozoic kimberlite pipes in the Kirkland Lake area, Ontario, and in Ontario and Quebec near Lake Timiskaming. Nine samples from five pipes contained fossils that were used for age determinations. These fossils are Middle or Late Ordovician graptolites, inarticulate brachiopods, and conodonts; Silurian and (or) Devonian conodonts; Early Devonian colonial corals; a Devonian stromatoporoid; and Early to Middle Devonian conodonts. Regionally, conodonts are unaltered (conodont colour alteration index, CAI 1). Conodont CAI values from the xenoliths are elevated (CAI 2), and a few conodonts have surface colour changes, suggesting hydrothermal alteration. Age determinations allow stratigraphic correlation between xenoliths and Paleozoic outcrops. For the Ordovician and Silurian samples, correlations are made to exposures in the nearby Lake Timiskaming outlier. For the Devonian samples, the closest possible correlative outcrops are about 300 km away. These fossils provide the first physical evidence of a connection between a Lake Timiskaming "basin" and other Ontario basins during at least part of the Devonian. These strata persisted at least until the Mesozoic before they were removed by erosion.

Palaeozoic clay mineral sedimentation and diagenesis in the Dinant and Avesnes Basins (Belgium, France): relationships with Variscan tectonism

Abstract Clay mineral investigations have been performed on more than 500 limestones and shales sampled in Lower Devonian (Emsian) to Lower Carboniferous (Namurian) outcrops in the Dinant and Avesnes Basins (Ardenne Massif, NW Europe). Clay mineral data have been placed in the palaeoenvironmental and structural histories documented by previous lithological, stratigraphical, palaeontological, diagenetic and tectonic contexts. The clay associations are dominated by illite and chlorite derived partly from the erosion of land masses surrounding the marine domain. The geothermal gradient estimated from correlation with conodont colour alteration index ranges between 40 and 70°C/km. A diachronous northwards migration of the diagenesis/metamorphism interface links to uplift caused by Late Carboniferous compressional folding and overthrusting. Associated clay minerals include smectite, locally preserved from diagenetic changes mainly by early pore closure, that reflect lagoonal or quiet offshore marine conditions. Smectite and subordinate kaolinite abundances decrease upwards during the Devonian in three successive intervals suggesting alternations of sub-arid to drier climates. The local occurrence of corrensite (ordered chlorite–smectite mixed-layer) is attributed to the moderate diagenetic transformation of pre-existing smectite

Lower Palaeozoic stratigraphy and palaeontology, Karadere–Zirze area, Pontus Mountains, northern Turkey

Lower Palaeozoic rocks in the Karadere-Zirze area, east of Safranbolu (Pontides, northern Turkey), range from Early Ordovician to Silurian. Overlying the probably Tremadoc Bakacak Formation are Aydos Formation quartzites, followed conformably by the Karadere Formation, dated as Early Arenig to Early Llanvirn by means of graptolites which are assigned to seventeen genera and include three new forms: Eoglyptograptus bouceki, Prolasiograptus haplus praecursor and Undulograptus? mui. Late Arenig trilobites from the Karadere Formation include Bergamia, Cyclopyge, Dionidella?, Leioshumardia and Seleneceme. In the Limestone Member of the overlying Ketencikdere Formation, uncommon trilobites suggest only a mid- to late Ordovician age, but conodonts with Colour Alteration Index 5–6 indicate the Amorphognathus tvaerensis Biozone (early Caradoc). Macrofossils are rare in the Siltstone Member, but conodonts from the middle of the unit suggest the highest subzone of the A. tvaerensis Biozone; the youngest visible strata are, on acritarch evidence, at least as high as Caradoc, but the Ashgill is not confirmed and the contact with overlying Silurian rocks is unexposed. The Findikli Formation comprises: a Lower Member, black argillites with Llandovery graptolites and acritarchs; and an Upper Member, grey shales with late Wenlock graptolites, overlain unconformably by Devonian rocks. The succession differs significantly from contemporaneous deposits in southern Turkey and its affinities lie with western Europe, including the Welsh Basin.

Evolution of metamorphic fluids at the Variscan fold-and-thrust belt in eastern Belgium

Quartz veins in the southern, greenschist facies metamorphic border of the Stavelot–Venn Massif (Belgium) have been examined. A detailed petrographical study of these quartz veins indicates the presence of eleven quartz stages. A microthermometric analysis shows the expulsion of three main fluid types during the Palaeozoic. In the first quartz stage (I) of Caledonian age, mixed aqueous/gaseous (H 2 O–CO 2 –NaCl) fluid inclusions are present. The post-Caledonian and pre-Variscan second stage (II) is characterised by gaseous CO 2 –N 2 inclusions occurring along inter-crystal grain boundaries and by aqueous H 2 O–NaCl intra-crystal inclusions. The quartz stages III–VI, which formed during the Variscan period, contain primary mixed aqueous/gaseous (H 2 O–CO 2 –NaCl) inclusions, all with an equal bulk chemical composition. Post-Variscan quartz stages VII–XI could not be investigated microthermometrically. The combination of microthermometry, chlorite geothermometry and geothermal gradients reveals minimum trapping conditions of 200 MPa and 200°C for stage I quartz and 175–275 MPa and 385–435°C for quartz stages III–VI. The trapping conditions of stage II quartz could not be determined due to the strong recrystallisation of the quartz veins. At the lower-metamorphic Variscan thrust front, distinct temperature anomalies are associated with thrust faults, as indicated by conodont colour alteration index (CAI), vitrinite reflectance data, chlorite geothermometry and fluid inclusion microthermometry. The expulsion of metamorphic fluids generated at the southern part of the Stavelot–Venn Massif could have caused the distinct temperature anomalies along Variscan thrust faults. Alternatively, some of these anomalies could have been induced by deeply circulating meteoric fluids.

Age and provenance of limestone clasts in Lower Old Red Sandstone conglomerates: implications for the geological history of the Midland Valley Terrane

Abstract Conodont-bearing limestone clasts in Lower Old Red Sandstone conglomerates in the Lanark and Strathmore basins and the Pentland Hills Inlier, Midland Valley, Scotland, indicate a source in a cryptic arc terrane with a mid-Ordovician ( P. serra-P. anserinus Biozone) limestone cover. Simpson coefficients of similarity indicate that the faunas from the limestone clasts are closer to conodont faunas from the Holy Cross Mountains, Poland, and the Wrae Limestone in the Northern Belt of the Southern Uplands, than to those in coeval strata from the Laurentian margin including Girvan. Conodont colour alteration index values indicate separate thermal histories for the limestone clasts and coeval strata in the Girvan Inlier. The cryptic arc was located to north of the Northern Belt of the Southern Uplands during Ashgill time and to south of the Midland Valley in Late Silurian-Early Devonian time and clearly had a complex tectonic history.

Acritarchs from the Ordovician of the Oslo Region, Norway

Ordovician (Tremadoc–Early Llanvirn age) acritarchs are recorded for the first time from five samples in the Asker–Royken area, south of the Oslo Region, Norway. Two samples from the Nes–Hamar district in the north were barren. Permian contact metamorphic temperatures of the sampled area are in excess of 300°C and are consistent with the conodont Colour Alteration Index (CAI). This study confirms that suggested previous temperature thresholds (< 200°C) for the preservation of reliably identifiable acritarchs are too low. Comparison of the present assemblages with those elsewhere in Baltoscandia, Gondwana and China are consistent with the earliest Arenig age of the base of the Toyen Formation and the Arenig–Llanvirn age of the succeeding Huk Formation of the Oslo Region.

Structure of organic matter in conodonts with different colour alteration indexes

Devonian conodont samples from the Beechwood Member of the North Vernon Limestone, IN, USA and Garra Formation Wellington, NSW, Australia with Colour Alteration Index (CAI) values of 2 and 4, respectively, were analysed using X-ray photoelectron spectroscopy (XPS), pyrolysis gas chromatography mass spectrometry (py-GC/MS) and Fourier transform infrared emission spectroscopy (FTIRES). The results show the organic matter is different for the two conodont samples, but contrary to that predicted if only thermal processes were operating. The organic matter in the Beechwood Member conodonts appears to be reduced whereas conodonts from the Garra Formation appear to be oxidised. Pyrolysates from organic matter in the Beechwood Member conodonts are dominated by an n-alkane series of hydrocarbons from C 8 to C 19, and simple aromatic hydrocarbons, however pyrolysates of organic matter from conodonts in the Garra Formation are dominated by nitrogen compounds and ketones. It is concluded that CAI values should be checked spectroscopically since CAI values indicating oil potential will be misleading if the organic matter is oxidised.

Tectono-thermal evolution in a region with thin-skinned tectonics: the western nappes in the Cantabrian Zone (Variscan belt of NW Spain)

The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC). Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role of the burial on the metamorphism.

Incipient metamorphism between Ufa and Beloretzk, western fold-and-thrust belt, southern Urals, Russia

The low-temperature thermal history of Paleozoic and Precambrian shales and slates was studied in detail along a NW–SE transect between Ufa and Beloretzk in the western fold-and-thrust belt of the southern Urals, Russia. The aim of the investigations was to compare four thermal parameters, namely illite and chlorite crystallinity, vitrinite reflectance, and Conodont color alteration index (CAI), in order to quantify the finite (i.e. maximum) thermal grade. The transect extends from Devonian to Permian sedimentary units of the pre-Uralian foredeep, crossing Precambrian siliciclastic and carbonate units of the Bashkirian megaanticlinorium into the Paleozoic units of the Ural-Tau antiform. In general, the four methods indicate similar metamorphic grades of the samples. The finite thermal grade ranges from lower diagenesis (Ro=0.9%, CIS-FWHMill001ad=0.770 Δ°2Θ, CIS-FWHMchl002ad=0.447 Δ°2Θ, CAI=1.5) in the pre-Uralian foredeep to epizone (CIS-FWHMill001ad=0.243 Δ°2Θ, CIS-FWHMchl002ad=0.252 Δ°2Θ, CAI=6) in the eastern part of the Bashkirian mega-anticlinorium and the Zilair synclinorium. All parameters show a sudden change in value at thrust boundaries and an increase in metamorphic grade with stratigraphic age within structural units. In comparison, the westernmost Precambrian units of the Ala-Tau-anticlinorium are characterized by thermal grades of lower diagenesis. Magmatic dikes cause a wide variation of the thermal grade in the western part of the Bashkirian megaanticlinorium. Also in areas with an intensive cleavage development (Zilair synclinorium) the finite thermal grade shows a stronger relationship towards the deformation than the stratigraphic position.

Tectonic and regional metamorphic implications of the discovery of Middle Ordovician conodonts in cover rocks east of the Green Mountain massif, Vermont

Middle Ordovician (late Arenigian - early Caradocian) conodonts were recovered from a dolostone lens in carbonaceous schist 30 m below the base of the Pinney Hollow Formation in the Eastern Cover sequence near West Bridgewater, Vermont. These are the first reported fossils from the metamorphic cover sequence rocks east of the Green Mountain, Berkshire, and Housatonic massifs of western New England. The conodonts are recrystallized, coated with graphitic matter, thermally altered to a color alteration index (CAI) of at least 5, and tectonically deformed. The faunule is nearly monospecific, consisting of abundant Periodon aculeatus Hadding? and rare Protopanderodus. The preponderance of Periodon and the absence of warm, shallow-water species characteristic of the North American Midcontinent Conodont Province suggest a slope or basin depositional setting. The conodont-bearing carbonaceous schist is traceable 3 km southeast to the Plymouth area, where it had been designated the uppermost member of the Plymouth Formation, previously regarded as Early Cambrian in age. The age and structural position of the carbonaceous schist above dolostones of the Plymouth Formation but below the Pinney Hollow Formation (upper Proterozoic and Lower Cambrian?) suggest that this unit may be correlative or time transgressive with the Ira Formation, which underlies the Taconic allochthons in the Vermont Valley. Such a correlation supports the concept of placing the western limit of the root zone of the Taconic allochthons beneath the Pinney Hollow Formation. An approximate absolute age assignment for the conodont-bearing rock is between 470 and 454 Ma. This suggests that dynamothermal metamorphism during the Taconian orogeny on the east flank of the Green Mountains was younger than early Caradocian, which is in accord with the middle Caradocian age of the Ira Formation west of the Green Mountain massif.

Conodont colour alteration indices (CAI) of Upper Ordovician limestones from the Iberian Peninsula

Conodont colour alteration index (CAI) data in Upper Ordovician rocks from several areas of the Variscan domain in the Iberian Peninsula indicate conditions ranging from diagenesis to low-grade metamorphism. In most of the areas, where studies using other indicators, such as illite crystallinity (IC) or where vitrinite reflectance are lacking, the CAI method has permitted a preliminary estimation of the metamorphic grade. In the Almadén syncline (Central-Iberian Zone), where IC studies are available, the thermal conditions inferred from CAI data agree with those obtained by the IC method. In the Puertollano–Almuradiel syncline, the thermal interval obtained primarily from fluid inclusions (270–370°C) overlaps considerably with that obtained from CAI data (180–340°C). In general, cleavage in rocks is present in anchizonal or epizonal conditions, whereas in diagenetic conditions with CAI ≥ 2.5, cleavage is scarce. The conodont texture changes with increasing metamorphism, and apatite recrystallisation appears in general with CAI ≥ 5. Variation of CAI values within a single sample and/or within short stratigraphic distances observed at several localities is due to hydrothermal activity.

New information on the age and thermal history of a probable Early Triassic siltstone near Kaka Point, South Island, New Zealand

Abstract The conodont Neospathodus homeri s.l. supports a late Early Triassic, Malakovian, age for an ammonoid‐rich horizon in the Potiki Siltstone, 0.9 km south of Kaka Point, southeast Otago, rather than an alternative designation of Early to Middle Anisian. The conodont colour alteration index suggests a thermal history with temperature not exceeding c. 80°C. The Kaka Point structural belt appears to have had a tectonic and thermal history different from that of the adjacent Murihiku Terrane.

Thermal maturation of the Lower Palaeozoic strata in the southwestern margin of the Malopolska Massif, southern Poland: no evidence for Caledonian regional metamorphism

Conodont colour alteration index (CAI) values have been used for the assessment of the thermal history of Lower Palaeozoic strata in the southwestern margin of the Malopolska Massif, along the contact with the Upper Silesian Massif. The CAI data provide no evidence for a previously suggested greenschistgrade regional metamorphism in the Cracow-Myszkow zone during the Caledonian epoch. Near Zarki, the Silurian rocks display a relatively uniform thermal overprint (CAI values of 4) resulting from sedimentary burial during the early Late Carboniferous. The estimated maximum temperatures of 200–220°C can be explained by an elevated heat flow associated with the Cracow Fault system. This thermal maturation level was locally enhanced (CAI values up to 8) after the Westphalian, due to the magmatic activity caused by the Variscan regional extension.

Uppermost Cambrian and Lower Ordovician conodont biostratigraphy of the Survey Peak Formation (Ibexian/Tremadoc), Wilcox Pass, Alberta, Canada

The Survey Peak Formation (350 m thick) consists of calcareous shale, mudstone, siltstone, and limestone-pebble conglomerate, and is formally divided into four revised members, successively: Basal Silty Member, Putty Shale Member, Middle Carbonate Member, and Upper Carbonate Member. The Wilcox Pass section, Jasper National Park, Alberta, provides a standard stratigraphic section for the southern Canadian Rocky Mountains and yielded over 4,500 conodonts from 87 samples which are of high species diversity, moderately well preserved, with a conodont Color Alteration Index (CAI) value of 3. Over 50 multielement species representing some 20 genera are identified, and most are illustrated. Conodont zones recognized in the four members include: Basal Silty Member—Eoconodontus notchpeakensis, Cordylodus proavus, C. caboti, C. intermedins, and C. lindstromi; Putty Shale Member—lower C. angulatus; Middle Carbonate Member—upper C. angulatus and coeval Rossodus manitouensis, Colaptoconus priscus (new) and Scolopodus cf. S. rex; Upper Carbonate Member—Striatodontus lanceolatus-S. striatus. In total, ten conodont lineage and assemblage zones are recognized within the Survey Peak Formation. Most can now be precisely correlated with trilobite faunas and acritarch microfloras described from this section, and with conodont zones established for the Great Basin area, Utah-Nevada, and for western Newfoundland. The Survey Peak Formation is of uppermost Cambrian to lower Ibexian (Tremadoc) in age with the Cambrian-Ordovician boundary placed between 26 and 29 m above the base of the formation in the Basal Silty Member.

Conodont biostratigraphy of the attenuated Dent Group (upper Ordovician) at Hartley Ground, Broughton in Furness, Cumbria, UK

SUMMARY At Hartley Ground, Broughton in Furness, Cumbria, the Dent Group, a mixed clastic-carbonate succession at the base of the Windermere Supergroup, is attenuated (only 20 m thick) and rests unconformably on the Borrowdale Volcanic Group. This succession includes a debris-flow deposit (Lumholm Member), the age of which was thought previously to be either Ashgill or Llandovery. Five samples from the Hartley Ground succession have yielded an Amorphognathus ordovicicus Biozone conodont fauna, considered to be of mid- to late Rawtheyan (Ashgill) age. The conodont fauna occurs in strata below and within the matrix of the Lumholm Member, although reworking within the latter cannot be completely discounted. Conodont colour alteration index (CAI) values of 5 for all the samples suggest these strata have been heated to temperatures of 300–400°C. At normal continental geothermal gradients this would indicate a minimum depth of burial of approximately 10km. New multi-element reconstructions are proposed for Amorphognathus ordovicicus Branson &amp; Mehl and Birksfeldia circumplicata Orchard; Hamarodus europaeus (Serpagli) is considered to be a member of the family Periodontidae.

Burial history of Palaeozoic strata in Belgium as revealed by conodont colour alteration data and thickness distributions

Thickness distributions of Devono-Carboniferous formations and their relationship to conodont colour alteration indices (CAIs) from over 500 sample locations in Belgium and northern France have been studied to reconstruct the thermal alteration and burial history of Palaeozoic rocks. The depositional centre during Eifelian through Visean times was located in the subsiding southern part of the Dinant Basin, forming a thick sedimentary pile of probably more than 3.5 km. In order to explain conodont CAIs chiefly resulting from Upper Carboniferous sedimentation, two different depositional centres are considered, giving rise to approximately 4.5 km of sediment in the central part of the Dinant and Namur Basins. As a consequence of the northward shifting Variscan fold belt, the Upper Carboniferous succession in the southern part of the Dinant Basin and in the Rocroi area was reduced or absent. Conodont CAIs together with other thermal alteration data from the Condroz Massif indicate that this area was a structural “high” for most of the Devonian. The Brabant Massif was probably covered with approximately 1.5–2.0 km of Upper Carboniferous sediments in the south and less than 1.0 km in the central and northern parts. In the central Campine Basin Visean strata could have been buried by 3.5–4.5 km of Upper Carboniferous rocks.

Calibration of Conodont Color Alteration Using Color Image Analysis

A set of 40 polished sections of Silurian and Devonian conodonts with color alteration indices ranging from 1.0 to 8.0 was examined using Color Image Analysis. The color composition of each conodont specimen was determined by measuring the intensities of the red, green, and blue color components on several carefully selected areas. The data were evaluated using nested analysis of variance. The observed variance can be attributed to three different sources of error. Within the studied conodont population, the variance due to rank amounts to an average 90%. The variance due to taxonomic differences or type of conodont element is about 3% and, thus, of minor importance. The heterogeneity within single specimens amounts to 6% and is also a minor source of variance. The application of Color Image Analysis helps characterize samples of different rank but similar optical properties. Color Image Analysis helps reduce the subjectivity of conodont color alteration studies.

Exploring for Hydrocarbons in Geothermally and Hydrothermally Complex Areas--A Southern Nevada Example: ABSTRACT

Time-based isograd maps using conodont color alteration indices (CAI) have been compiled and interpreted for a large area in southern Nevada that includes Yucca Mountain, the Nevada Test Site, and the Nellis Air Force Bombing and Gunnery Range. These maps were produced to evaluate the controversy about possible important mineral and (or) energy resources near Yucca Mountain, the potential burial site for high-level nuclear waste. The hydrocarbon potential of the Yucca Mountain area has been likened to that of the Railroad and Pine Valley areas, 200 km to the northeast where 35 million barrels of oil have been produced from Paleozoic and lower Tertiary strata. In 1991, two companies with no previous drilling experience in Nevada drilled three oil exploration wells within 20 km of Yucca Mountain and within or close to the Timber Mountain caldera system. No shows of oil or gas were found in these wells. The deepest well was drilled to 5,000 feet and penetrated 2,200 feet of upper Tertiary valley-fill deposits and volcanic rocks overlying an overturned sequence of Upper Cambrian and Lower Ordovician rocks having conodonts with CAI values of 5. Our new conodont sampling, however, has targeted some thermally favorable areas for hydrocarbons eastmore » of Yucca mountain, but their maturation history suggests that the potential for oil is substantially lower than in the Railroad and Pine Valley areas. Cambrian through Triassic rocks in the vicinity of Yucca Mountain have experienced temperatures too high for oil to be preserved, except for a narrow zone (20 x 100 km) northeast of Yucca Mountain, where Mississippian through Triassic rocks are just within the upper limit of the oil generating window. Most of this zone, however, lies on Federal lands that are, for now, inaccessible for a variety of security and environmental reasons.« less

Conodont colour alteration indices: Palaeotemperatures and metamorphism in the Northern Calcareous Alps ? a general view

The middle and eastern parts of the Northern Calcareous Alps (NCA) can be subdivided into two distinct units with a lateral boundary marked by abrupt changes in the conodont colour alteration index (CAI-values). The first of these is a northern unit (Tirolikum) with a relatively homogeneous distribution of no or low grade conodont alteration (CAI 1.0–2.0). The thermal overprint is thought to be relatively young and related to a heat flow from the Tauern crystallization. The second unit consists of the Juvavic nappe system (Juvavikum), which is distributed along the southern rim of the NCA but also covers some of the northern parts of the Tirolikum. With respect to its CAI-distribution the Juvavikum is more heterogeneous on a regional and local scale, with some local CAI-inversions. The Juvavikum additionally shows distinctly different sets of CAI-values one with weak (CAI 1.0–1.5) and another with strong alteration (CAI 5.5–7.0) — at present the highest known thermal overprint measured in the NCA. The metamorphism is relatively old as it predates the Late Jurassic—Early Cretaceous gravity tectonic emplacement of the Juvavikum onto the Tirolikum. The high CAI-values of parts of the Juvavic nappe system are though to be related to tectonic burial in an accretionary wedge formed parallel to the closure of the ‘Vardar’ Ocean. The low CAI values of the Tirolikum apparently exclude a direct juxtaposition of the two units at the time of metamorphism.

Conodont colour alteration indices: Palaeotemperatures and metamorphism in the Northern Calcareous Alps ? a general view

Conodont colour alteration indices: Palaeotemperatures and metamorphism in the Northern Calcareous Alps ? a general view