Conodont Colour Alteration Index Values Research Articles

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.

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.

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 <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 (<100°C) to reach the low CAI values measured. Assuming a normal geothermal gradient, these temperatures required an overburden <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.

Migrated oil on Novaya Zemlya, Russian Arctic: Evidence for a novel petroleum system in the eastern Barents Sea and the Kara Sea

In this article, we present evidence for the migration of Cretaceous or younger oil into Paleozoic strata on the islands of Novaya Zemlya, Russian Arctic, originating from the surrounding basins of the Barents Sea and the Kara Sea. We thus suggest a novel petroleum system for the greater Barents Sea and Kara Sea regions. Organic geochemical data reveal that medium mature petroleum is hosted in overmature Paleozoic rocks onshore Novaya Zemlya. This petroleum likely migrated from the surrounding offshore sedimentary basins of the Kara and Barents seas into the onshore strata on a broad front, covering 400 km (248 mi) along the archipelago. Thus, the findings describe the remnants of petroleum migration on a regional scale. High maturities of the examined onshore strata (vitrinite reflectance [Ro] of 1.9–4.0% and conodont color alteration index values of 4–6) render the indigenous origin of the medium mature petroleum unlikely. Organic geochemical characteristics of the migrated hydrocarbons lead us to suggest the existence of a hitherto undescribed marine shale source rock with some contribution of terrigeneous organic matter. The 13C isotope values varying within a range of 5 for bulk extracts as well as for individual n-alkanes and isoprenoids suggest isotopic heterogeneity of the organic source facies. The source rock units are inferred to have released hydrocarbons mostly at oil window maturities (higher than 0.6% Ro). The occurrence of n-alkanes and isoprenoids, as well as abundant and partially degraded tricyclic terpanes indicates mixed undegraded and heavily biodegraded oil. This suggests several petroleum charges with varying degrees of microbial degradation. Occurrences of oleanane, high ratios of C28 to C29 steranes and elevated 24-norcholestane and 24-nordiacholestane ratios suggest Cretaceous or Tertiary source rocks.

Calculation of the conodont Color Alteration Index (CAI) for complex thermal histories

Abstract A simple model for the Arrhenius reaction conodont Color Alteration Index (CAI) that can be implemented easily in a computer program is introduced. The model, named to as EasyCAI, envisages the overall process of conodont alteration as a series of 12 parallel pseudo-first-order reactions. Our approach is especially amenable to a spreadsheet program and can solve complex geological histories involving variable heating and cooling rates. With EasyCAI, a profile of CAI values versus time can be obtained for a given stratigraphic level if the time–temperature history for that level has been estimated. EasyCAI can provide additional constraints to optimize thermal history models by computing profiles of CAI values with depth and time; thus providing a new tool for quantitative CAI palaeothermometric analysis.

Conodont palaeothermometry of contact metamorphism in Middle Ordovician rocks from the Precordillera of western Argentina

AbstractThe Yerba Loca Formation (Middle–Upper Ordovician), exposed in the Western Precordillera, San Juan, Argentina, is made up of clastic–carbonate turbidites, and basic–ultrabasic rocks. It is affected by regional Siluro-Devonian very low-grade metamorphism that locally reaches greenschist facies. At Ancaucha creek, 45 conodont samples were taken from two sections that include 30 to 50 m thick sills. In order to analyse the thermal alteration patterns produced by these intrusive bodies, conodont Colour Alteration Index (CAI) is contrasted with optical petrography and X-ray diffraction analyses of clay minerals. The intrusions are dated as post-Darriwilian (Da2), as determined by conodont biostratigraphy of the host rock, which indicates theParoistodus horridusSubzone of theLenodus variabilisZone. The distribution of CAI values defines a thermal aureole of about 2.5 times intrusion thickness that prevailed over the later very low-grade metamorphism. Metasomatism at Ancaucha creek is recorded by CAI values of 4 to 7, particularly restricted to a few layers close to the intrusions, as indicated by conodont textures and rock fabric. One-dimensional thermal computer simulation conforms to empirical data indicating temperatures greater than 600 °C for the contact zone, although it points out slightly narrower thermal aureoles. The clay mineral assemblage of most of the analysed samples (chlorite, illite, smectite and I/S mixed-layers) is complex and probably derives from several superimposed processes, thus representing non-equilibrium assemblages. In turn, KI values (0.27–0.32) indicate anchizone metamorphism, in agreement with regional CAI values of 4; consequently, the occurrence of smectite and I/S probably resulted from retrograde diagenesis processes.

Conodont CAI and magnetic mineral unblocking temperatures: implications for the Western Canada Sedimentary Basin

The laboratory unblocking temperature of a magnetic component ( T UB) is dependent on the time duration and temperature of its acquisition and thus is similar in concept to the conodont color alteration index (CAI). The CAI provides an independent determination of the past thermal history that can be related to the observed T UB. We present simple CAI versus T UB plots that can be used to determine the possibility of a thermal remagnetization in specific magnetic minerals, given the measured T UB, or alternatively, to estimate the minimum T UB necessary for a magnetization component to have been unaffected, given a specific CAI value. Failure to correctly correlate CAI values in the Cordillera on the western side of the Western Canada Sedimentary Basin may have led to secondary uplift remagnetizations being interpreted as primary remanent magnetizations in earlier paleomagnetic studies.

Transition from Diagenesis to Metamorphism Near the Front of the Variscan Regional Metamorphism (Cantabrian Zone, Northwestern Spain)

Distribution of paleotemperatures has been studied in Paleozoic rocks of the Variscan fold and thrust belt of the NW Cantabrian Zone (Spain) by a combined application of illite crystallinity (IC) and conodont color alteration index (CAI). Results show that the diagenetic‐metamorphic grade increases with the increase in rock age, as marked by decreasing IC values and increasing CAI values. Moreover, the CAI isogrades roughly reflect the structural traits of the area, indicating that maximum temperatures were attained during sedimentary burial previous to folding. The geothermal paleogradient obtained suggests conditions near the boundary between low and middle P/T conditions. There is evidence of at least two thermal events during the tectonothermal development of the belt. The main stage is interpreted as the result of the burial of the rocks characterized by late diagenesis conditions that occurred under a geothermal gradient of ∼35°C km−1. The narrow zone with low anchizonal conditions in the western part of the area could represent the front of the Variscan orogenic metamorphism. This stage was followed by thrusting that juxtaposed higher‐grade rocks of the internal zone (Westasturian‐Leonese Zone) over lower‐grade rocks of the external one (Cantabrian Zone), giving rise to an inverted metamorphic pattern. Subsequently, rocks in the southern part of the area were affected by contact metamorphism associated with emplacement of granitoid bodies. This late‐Variscan thermal event led to local enhancement of CAI values and to changes in the clay mineralogy by metasomatism.

Geothermal palaeogradients and metamorphic zonation from the conodont colour alteration index (CAI)

The conodont colour alteration index (CAI) is potentially valuable in thermal history investigations, but there are problems in its use. An approach is proposed herein to overcome the problem of establishing reliable geothermal palaeogradients in areas with burial metamorphism, based on knowledge of the stratigraphic succession thickness, the boundaries of chronostratigraphic units, and the age and duration of deformation episodes. Although also problematical that a correlation between CAI and Kübler index (KI) values does not exist, there are areas where CAI and KI values are notably consistent for the anchizone boundaries; however, anomalous correlations between CAI and KI values have also been found in some areas. A CAI anchizone (or ancaizone) is defined in order to enable CAI values to be used independently to establish a metamorphic zonation.

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.

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.

Simultaneous use of thermal indicators to analyse the transition from diagenesis to metamorphism: an example from the Variscan Belt of northwest Spain

The transition from diagenesis to very low-grade metamorphism has been characterized by conodont colour alteration index (CAI), illite crystallinity (IC), clay-mineral assemblages and rock fabric in a transect (Cape Peñas–Cape Torres) located in the Cantabrian Zone (northwest Spain). IC values show a northwest to southeast change from epizonal to diagenetic values. CAI values show a general congruence for low- and high-grade areas. This correlation indicates that CAI values for the epizone are >5.5, the anchizone 5.5≥CAI>4, and the diagenetic zone ≤4. Cleavage develops in incompetent rocks with CAI values greater than 3, mainly by pressure solution. Temperature increases towards the inner parts of the orogen (towards the northwest) and is considered to be related to the tectonic superposition caused by thrusting and folding. A gradient of 35°C/km is inferred for the transect. In the southeastern sector of the section there are several thermal anomalies that are attributed to the presence of epithermal fluids.

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.

Thermal maturation of the Lower Carboniferous strata of the Northumberland Trough and Tweed Basin from conodont colour alteration index (CAI) data

SUMMARY Conodont colour alteration (CAI) values for the Lower Carboniferous strata of the Northumberland Trough and Tweed Basin, northern England, can be attributed to three causes. On a regional scale, CAI values of 1.5 – 2 in the Solway and Northumberland basins are due to depth of burial. High values in excess of CAI 5 are localised in the thermal aureoles of Permo-Carboniferous intrusions. In southern Northumberland, CAI values for Brigantian strata are higher than those for Tournaisian and Chadian to Asbian strata. It is suggested that this was caused by the onset of hydrothermal circulation and release of hot brines from the lower crust during Late Carboniferous transpression. This heating event was short lived; it occurred during the earliest Stephanian and was coeval with the intrusion of the Whin Sill and the early phases of Pb-Zn mineralisation in the North Pennine Orefield.

Conodont geothermometry in Palaeozoic carbonate rocks of Tasmania and its economic implications

Conodont colour alteration index (CAI) values from Early to Middle Palaeozoic marine carbonate rocks of the West Tasmania Terrane range from 1.5 to 8. Highest CAI values are found in hornfels adjacent to a Jurassic dolerite intrusion. CAI of 5 indicate temperatures of 300–480°C and are found either adjacent to Jurassic dolerite intrusions or in an arcuate belt around the west and northwest of the island rimming the Precambrian rocks of the Tyennan and Rocky Cape regions. CAI values of 1.5 are found in the southwest of the state and values of 2 are common in central southern Tasmania. Low vitrinite reflectance values from the unconformably overlying Late Carboniferous‐Late Triassic Parmeener Supergroup, suggest that the major heat input into the conodonts was in the Late Devonian‐Early Carboniferous associated with regional metamorphism and the widespread intrusions of granitoids. High CAI values are not due to thick overburden as these are very unlikely in many parts of Tasmania. It is probable that high ...

Conodont colour alteration patterns in Devonian rocks of the eastern Anti-atlas, Morocco

The thermal and burial history of the eastern Anti-Atlas during Late Palaeozoic time is investigated using conodont colour alteration data and the electron spin resonance technique (ESR). Both the conodont colour alteration index (CAI) patterns and the results of ESR investigations indicate that the Devonian rocks of the eastern Anti-Atlas attained their present-day maturation level during Carboniferous time prior to Variscan uplift. The observed CAI values from 3 to 5 reflect heating to temperatures of 120°C and 310°C respectively. The conodont CAI pattern observed in the Devonian rocks is a product of burial and loading by thick Carboniferous cover but the maturation level is too high to be accounted for by burial beneath the known stratigraphical overburden under normal heat flow conditions. The analysis of palaeotemperatures inferred from conodont CAI values shows variation in heat flow during Carboniferous time, with an average geothermal gradient of 30°C/km in the Béchar Basin and higher gradients in the region of the mader and Tafilalt. The style of the observed CAI patterns suggests that depositional trends in the Carboniferous cover, now largely removed by erosion, followed the Devonian subsidence patten.

Evidence for Post-Cementation Migration of High-Temperature, High-Pressure Fluids--Siluro-Devonian Helderberg Group, Central Appalachians: ABSTRACT

Fluid inclusion data and fracture-filling cements in carbonate and siliciclastic rocks of the Siluro-Devonian Helderberg Group, central Appalachians, indicate post-cementation, late Paleozoic migration of high-pressure, high-temperature fluids. Void-filling quartz and calcite cements contain secondary, 2-phase fluid inclusions that give freezing temperatures of -20 to -25°C (-4 to -13°F) (salinity > 22 wt.% NaCl). Homogenization temperatures are 200 to 300 + °C (392 to 572°F) (temperatures calibrated and pressure corrected) and greatly exceed maximum paleotemperatures (120 to 160°C; 248 to 320°F) given by conodont color-alteration index values or calculated from known sedimentary overburden. High homogenization temperatures suggest rapid movement of metamorphic fluids so that ambient burial temperature was not raised for long enough periods of time to affect conodont CAI values. These fluids probably came from Blue Ridge-Piedmont thrust sheets that were undergoing metamorphism during late Pal ozoic deformation. These fluids migrated more than 75 km (47 mi) during thrusting. Well-cemented sandstone and limestone have multiple crosscutting trains of secondary hydrocarbon inclusions. Some trains crosscut cement-filled fractures. Hydrocarbons also occur as thin films along cement crystal boundaries and as secondary inclusions trapped along calcite deformation twins. These inclusions indicate geopressured fluids moved along intercrystalline boundaries and along deformation twin planes in calcite under deep burial conditions either during or after deformation. Rare fractures contain transported skeletal grains, exotic clasts, recemented clasts of fracture-filling cement, and mud. Cement clasts contain included mud and skeletal grains, and indicate several episodes of particle transport, cementation, and refracturing prior to final fracture filling. Primary(?) 2-phase fluid inclusions in vein-filling calcite give homogenization temperatures of 120 to 150°C (248 to 302°F). Coarse-grained clastic fracture fills indicate migration of rapidly moving fluids capable of transporting clasts through fracture conduits under deep burial conditions. End_of_Article - Last_Page 471------------