Maximum Burial Depth Research Articles

The geological history of the Ballydeenlea Chalk Breccia, County Kerry, Ireland

The Ballydeenlea Chalk Breccia, Co. Kerry occurs as an outlier surrounded by Namurian rocks and is composed dominantly of Namurian mudstone clasts, with Cretaceous (Campanian) chalk acting as a matrix. A new model is proposed for the formation of the deposit in which it is considered to be the remnant of a suite of sediments deposited at the foot of a submarine fault scarp, active during the Campanian. In this model, Namurian mudstones comprised the active scarp and were redeposited by rockfall events. These were mixed with unlithified chalk ooze derived from slumping from the upthrown side of the fault. An organic maturation study of the Ballydeenlea Chalk Breccia indicates a maximum burial depth of between 1 and 1.5 km, suggesting uplift and erosion during the early Tertiary.

Late Eocene‐Oligocene Te Kuiti Group at Mount Roskill, Auckland, New Zealand

A 592 m deep water bore drilled at Mount Roskill in central Auckland, New Zealand, intersected products of the Auckland Volcanic Field (0–12 m), pumiceous sediments of the Tauranga Group (12–23 m), interbedded sandstone and mudstone of the Waitemata Group (23–475 m), and Te Kuiti Group sediments (475–592 m). This first record of Te Kuiti Group in the central Auckland area comprises erosionally truncated Glen Massey Formation beneath the Waitemata Group, a complete section through Mangakotuku Formation, and an incomplete Waikato Coal Measures section. Drilling stopped in Waikato Coal Measures, probably less than 30 m short of Paleozoic or Mesozoic basement. Vitrinite reflectance measurements indicate a lignite rank for coal fragments collected from the coal measures, and suggest a maximum burial depth of c. 800 m. Six Te Kuiti Group samples examined for palynomorphs and foraminifers gave ages ranging from Runangan to early Whaingaroan and show a transition from a predominantly terrestrial late Eocene environment to a shallow marine setting in the early Oligocene. This result helps substantiate previous inferences about the continuity of Te Kuiti Group deposition between Northland and Waikato, and supports the suggestion that the southern limit to the Northland Allochthon lies north of Auckland.

Application of organic matter and clay mineral studies to the tectonic history of the Abruzzo-Molise-Sannio area, Central Appenines, Italy

The Abruzzo-Molise-Sannio region is part of the Central Apennines foreland fold and thrust belt, a structure that formed during the Neogene as a result of the continental collision between the European and Adria plates. Certain areas of this crust experienced anomalously high temperatures due to deep burial, while others did not. This burial has been investigated using optical indicators of organic matter (OM) maturity and clay mineralogy. The maximum depths of burial of Tertiary surface sequences have been established based on the vitrinite reflectance ( R o) and thermal alteration index (TAI) of OM dispersed in synorogenic sediments and from clay mineralogy of pre-thrusting clayey deposits. The synorogenic deposits show low levels of organic maturity having R o values less than 0.6% indicating early mature to mid-mature hydrocarbon generation. The pre-thrusting deposits are shales characterised by a very high percentage of clay minerals with lesser amounts of quartz, K-feldspar, plagioclase and calcite. The dominant minerals are highly smectitic interstratified illite/smectite (RIS). Both R o and I/S ratios were used to calibrate burial and thermal evolution of this chain sector by numerical modelling. The results indicate that the structural units which outcrop in the study area (particularly the Molise Unit) were never overthrust by significant thicknesses of rocks during mountain building, at least before Pliocene times; the hypothesis that more internal units overthrust the presently exposed units also seems to be unlikely.

Extrusion tectonics and elevation of lower crustal metamorphic rocks in convergent orogens

In the proposed model, weak zones of rheologically homogeneous crust, continually compressed between rigid lithospheric indenting plates, can be exhumed upward by extrusion. The rate of exhumation is governed by the rate of convergence of approaching plates and by the width of the weak deformable zone. Modeled extrusion results in near-isothermal decompression during the rapid elevation history in narrow orogenic belts for realistic plate velocities. Such pressure-temperature-time paths exhibit distinct collapsed geochronologies and record maximum metamorphic temperatures ( T max ) at the maximum burial depth ( P max ).

Flow of Formation Waters, Aquifer Characteristics, and Their Relation to Hydrocarbon Accumulations, Northern Alberta Basin

Based on the wealth of data generated by the oil industry, the regional-scale characteristics of rocks, the flow of formation waters, and their relation to hydrocarbon accumulations were analyzed for the northern Alberta basin. The flow of formation waters in several aquifers and aquifer systems separated by intervening aquitards is at steady state and is driven by the present-day topography both on a regional and a local scale. The flow is generally from a recharge area in the southwest at the fold belt and Bovie Lake fault, to discharge in the northeast at the Great Slave Lake, the lowest point in the basin. The flow in Devonian aquifers is in open systems from recharge to discharge areas, whereas the flow in Carboniferous and Cretaceous aquifers is in semi-open systems, discharging into adjacent aquitards. Very high porosity and permeability in places in the Devonian Elk Point aquifer system are due to reefs, fracturing, dolomitization, and karst processes. Very high permeability probably leads to relatively high flow rates along the Presqu'ile barrier reef, resulting in local advective effects on the terrestrial heat transport to the surface. On a regional scale, all of the aquifers are underpressurized due to upstream propagation through high-permeability zones of low hydraulic heads at discharge elevations. The flow pattern is corroborated by salinity distributions, with comparatively lower salinity in each aquifer at recharge in the southwest and at discharge in the northeast caused by mixing with fresher meteoric water, and higher salinity between recharge and discharge areas. Salinity distributions show that the aquifers are not completely flushed of the original formation waters. Dissolution of salt and anhydrite from adjacent strata leads to high salinity in the Elk Point aquifer system and Beaverhill Lake aquifer. Hydrocarbons generated in the southwest in Devonian and Carboniferous strata, at maximum burial depth during the Laramide orogeny, migrated updip to the northeast driven by their own buoyancy and entrained by the flow of formation waters. Unless stratigraphically trapped by reefs and at the edge of semi-open aquifers, migration to the discharge areas led to loss of the volatile components and biodegradation into altered bitumens. The hydrostratigraphy and direction of the flow of formation waters in the northern part of the Alberta basin indicate that hydrocarbons generated in this region did not contribute to the formation of the giant Athabasca oil sand deposit located southeast of the study area.

Determining apparent exhumation from Chalk outcrop samples, Cleveland Basin/East Midlands Shelf

Porosity measurements of 22 Upper Cretaceous Chalk samples, and mean Chalk porosities derived from sonic logs in three wells, were used to quantify apparent exhumation (height above maximum burial-depth) in the onshore Cleveland Basin/East Midlands Shelf. Late Cretaceous/Tertiary exhumation of the East Midlands Shelf resulted in the removal of 1.2 km of section near the coast and more than 2 km of section inland, to the west. The southern margin of the Cleveland Basin was exhumed by 2 km, and exhumation increases northwards towards the recognized inversion axis running east–west along the basin’s centre. The northwards increasing exhumation associated with the east–west trending inversion axis of the Cleveland Basin is superimposed upon the regional westward trend of increasing exhumation of eastern England. These two trends control the outcrop distribution of the Upper Cretaceous Chalk in the Cleveland Basin/East Midlands Shelf.

Diagenetic processes influencing porosity in sandstones from the Triassic Buntsandstein of the Iberian Range, Spain

Sandstones of the Triassic Buntsandstein of Spain were studied in an attempt to quantify effects of diagenetic processes such as compaction, cementation and leaching on reservoir properties. Samples were taken from Shell well Sigüenza 44-3 some 110 km northeast of Madrid, drilled on the Sigüenza High in the Iberian Range. The sandstones are arkoses to sublitharenites of a typical Triassic redbed facies characteristic of the whole region. The present-day depth of the base Buntsandstein is 538 m, whilst a maximum burial depth of ca. 2600 m was estimated from vitrinite reflectance (0.94 to 1.14), kaolinite temperature (76°C) and illite crystallinity (Kuebler index 4.9). From an estimated average Initial Porosity of 38%, porosity was reduced to the present value of 13%. The strongest process to reduce porosity was found to be mechanical compaction causing a loss of 15%, followed by chemical compaction (pressure solution) with a loss of about 6%. The cement (11.4% R.V.) apparently reduced the Initial Porosity by about only 5%. Early carbonate and sulphate cements are interpreted to have stabili sed the rock framework working against mechanical compaction and pressure solution. This is supported by an increase in sutured grain contacts where early cements are absent or rare. The later quartz cement, interpreted to be largely the product from pressure solution, has reduced porosity in a ratio of 0.5:1. This suggests that compaction continued after the formation of the late quartz cement. Although leaching was observed especially in feldspar grains, clay and K-feldspar neomorphosis is interpreted to have counteracted the formation of substantial secondary porosity. Permeability in the Buntsandstein is generally low (average 17.4 mD) and has been strongly affected by the alteration of feldspars to clay causing a wide range of permeability from 0.11 to 73.0 mD.

STRATIGRAPHY AND RESERVOIR POTENTIAL OF THE MESOZOIC KHORAT GROUP, NE THAILAND: Part 2: Diagenesis and Reservoir Quality

The Mesozoic Khorat Group in NE Thailand consists of a series of continental red‐beds, divided here into five formations, which unconformably overlie the lithologically‐similar Nam Phong Formation. The reservoir quality of the Khorat Group decreases progressively with increasing age, owing to burial compaction and diagenesis. Compactional fabrics suggests a maximum burial depth of more than seven km, although the Sao Khua Formation (in the middle of the Khorat Group) shows early calcite cements which stabilised the sediment fabric at burial depths of about one km. Porosities vary from 11% in the uppermost part of the Khorat Group (Khok Kruat Formation) to 4.9% in the Nam Phong Formation. The ratio of secondary grain‐dissolution porosity to primary porosity increases with age (and depth). Authigenic minerals consist of quartz (1.3 ‐ 9.3%), calcite (0 ‐ 26.5%) and kaolinite (0 ‐ 4.0%). The occurrence of detrital clays (0.8–9.6%) in particular causes downgrading of the porosity of many of the sandstone samples examined.

Thermal History of the 1.1-Ga Nonesuch Formation, North American Mid-Continent Rift, White Pine, Michigan

The Nonesuch Formation, a black siltstone, was deposited in the Lake Superior portion of the mid-continent rift system during middle Proterozoic rifting. Despite its age of nearly 1.1 Ga, the Nonesuch Formation contains liquid petroleum and solid pyrobitumen. Numerical modeling techniques were used in this study to constrain the thermal history of the Nonesuch Formation at White Pine, Michigan. Thermal modeling addresses two issues: (1) the utility of illite-smectite expandability as a limiting parameter for describing the thermal history of ancient (Proterozoic) sedimentary rocks, and (2) the potential for hydrocarbon maturation within the Nonesuch Formation at White Pine. A range of potential burial histories for the Nonesuch Formation was constructed based on geologica evidence. Time-temperature histories were calculated for each burial history model assuming one-dimensional, transient, conductive heat flow. Results of numerical time-temperature models were then evaluated on the basis of organic and inorganic thermal maturity indicators including Rock-Eval® pyrolysis data and biomarker indices (correlated to equivalent vitrinite reflectance values), in addition to illite-smectite expandability. Illite-smectite expandability values appear generally consistent with measured organic thermal maturity values. The preferred set of cases also produces calculated illite-smectite expandability and vitrinite reflectance values consistent with thermal maturity indicators (that do not include vitrinite) collected in the field. Modeling results demonstrate tha a combination of elevated basal heat flow and rapid burial during the Proterozoic is required to produce the observed thermal maturities at White Pine, Michigan. More specifically, modeling indicates that maximum temperatures for the Nonesuch Formation, 110 to 125°C, were reached at about 1075 Ma, coincident with a maximum burial depth of about 6 km, although 4 km represents a more plausible value. The results support a thermal history consistent with in-situ oil generation at White Pine; however, thermal modeling alone cannot rule out the possibility that oil was generated and expelled elsewhere and migrated into the area with fluids expelled during an episode of postrift compression.

Structural and Thermal Evolution of the Siwaliks of Western Nepal

The piedmont of the Himalayas is formed in Western Nepal by: a) Siwalik sediments affected by folds, thrust and back­ thrust structures and b) intra-belt basins (Duns) that are dis laced piggyback above the thrust sheets. Vitrinite reflectance values (VRo) are found between 0.3% and 0.5% in Middle Siwalik sediments and between 0.6 and 1% in Lower Siwaliks. The thermal maturity of the organic matter agrees with maximum burial depth (3500 m for Middle Siwaliks and 6000 m for Lower Siwaliks) that do not strongly exceed the stratigraphic thickness of the Siwaliks Group. Intense erosion concomitant with deformation balances closely tectonic thickening and prevent burial of the Siwalik sediments at great depth. Nonetheless, Duns developed above the steeper part of the basal decollement and/or ahead of back-thrusts prevent the exhumation of rock and could lead to greater burial depth.

95/04430 Influence of chemical modification of a bituminous coal on its behaviour during thermomechanical analysis

A set of 14 samples—both extracted and unextracted from the Toarcian of the Paris Basin have been investigated using Curie-point pyrolysis-mass spectrometry and Curie-point pyrolysis gas chromatography mass spectrometry. The relative amount of n-alkenes and n-alkanes in the pyrolyzates increases with increasing maximum burial depth of the samples. Comparison of the pyrolysis data of extracted and unextracted samples shows that generation of hydrocarbons from the kerogen starts at a maximum burial depth of ~ 1000m. The increase of pristane and phytane in the extracts of the deeper samples is correlated with the gradual decrease of the characteristic pyrolysis product prist-1-ene. Three samples yield pyrolyzates with high relative amounts of aromatic compounds. This phenomenon probably reflects a different type of contributing organic matter and/or a different environment of sedimentation.

Deep crustal growth of quartz, kyanite and garnet into large‐aperture, fluid‐filled fractures, north‐eastern Connecticut, USA

Abstract A petrographic and petrological analysis of exceptionally well‐preserved hydrothermal veins from the Merrimack synclinorium, north‐eastern Connecticut, has been carried out in order to place new field‐based constraints on fracture aperture dimensions and porosity in the lower continental crust. The veins preserve substantial open space today in outcrop, and contain mineral assemblages including subhedral to euhedral crystals of quartz, kyanite and almandine‐rich garnet. Textural evidence indicates unequivocally that the vein minerals grew into macroscopic (mm‐ to cm‐scale) open space between the vein walls. The veins are interpreted to have been large‐aperture fractures along which significant advective fluid infiltration and chemical reaction occurred. The porosity of the rock mass due to open space between fracture walls today is c. 0.3%, but it could have been as large as several percent when the flow system was active. Quantitative thermobarometry results from vein mineral assemblages indicate that the fractures formed at pressures and corresponding crustal depths of c. 0.8 GPa and c. 30km, and temperatures of 550–600° C. The depth of fracture formation corresponds to published estimates of the maximum burial depth of the Merrimack synclinorium during the Acadian orogeny. The formation of large‐aperture fractures could increase significantly the transient permeability of the deep crust, and therefore influence metamorphic heat and mass transfer.

Hydrochemistry of the saline groundwaters of the lower Mersey Basin Permo-Triassic sandstone aquifer, UK

Saline groundwater occurs at depth in the Triassic Sandstones of the northern end of the Cheshire Basin in northwest England. Major and minor ion data suggest that the origin of the salinity is dissolution of evaporite deposits; the salinity probably developed in an interval following the Tertiary inversion of the Basin which would have allowed faulted contact between the sandstones and the overlying evaporite sequence of the Mercia Mudstone Group. Isotope evidence suggests the saline waters to have been considerably diluted by presumably dispersive mixing with a meteoric water recharged under cool, and therefore Quaternary, climatic conditions. Comparison with other UK Permo-Triassic Basin brines suggests the Merseyside waters are not unusual, except in terms of the importance of exchange reactions and dilution by Quaternary recharge waters. These factors are due to the shallow maximum burial depth of the aquifer and the preservation of smectites, and also to the geometry of the aquifer and its low-permeability faults. In the light of the water chemistry data it is suggested, in a slight modification of the proposal of Naylor et al. Q. Geol. Soc. Lond., 146: 685–699, 1988), that the Cheshire Basin mineralization resulted from mixing of the relatively oxidized, SO4-rich evaporite brines with the reduced, SO4-poor, metal-rich, Barich, Coal Measures brines which were expelled through faults from the Coal Measures during Basin inversion: some degree of oxyhydroxide-stripping of the sandstones by the Coal Measures brines would have enhanced metal concentrations. The water resulting from the mixing of brines would have been far from equilibrium, and the barite and sulphide precipitation followed along the faults.

Hydrochemistry of the saline groundwaters of the lower Mersey Basin Permo-Triassic sandstone aquifer, UK

Saline groundwater occurs at depth in the Triassic Sandstones of the northern end of the Cheshire Basin in northwest England. Major and minor ion data suggest that the origin of the salinity is dissolution of evaporite deposits; the salinity probably developed in an interval following the Tertiary inversion of the Basin which would have allowed faulted contact between the sandstones and the overlying evaporite sequence of the Mercia Mudstone Group. Isotope evidence suggests the saline waters to have been considerably diluted by presumably dispersive mixing with a meteoric water recharged under cool, and therefore Quaternary, climatic conditions. Comparison with other UK Permo-Triassic Basin brines suggests the Merseyside waters are not unusual, except in terms of the importance of exchange reactions and dilution by Quaternary recharge waters. These factors are due to the shallow maximum burial depth of the aquifer and the preservation of smectites, and also to the geometry of the aquifer and its low-permeability faults. In the light of the water chemistry data it is suggested, in a slight modification of the proposal of Naylor et al. Q. Geol. Soc. Lond., 146: 685–699, 1988), that the Cheshire Basin mineralization resulted from mixing of the relatively oxidized, SO 4-rich evaporite brines with the reduced, SO 4-poor, metal-rich, Barich, Coal Measures brines which were expelled through faults from the Coal Measures during Basin inversion: some degree of oxyhydroxide-stripping of the sandstones by the Coal Measures brines would have enhanced metal concentrations. The water resulting from the mixing of brines would have been far from equilibrium, and the barite and sulphide precipitation followed along the faults.

Quantification of Tertiary Exhumation in the United Kingdom Southern North Sea Using Sonic Velocity Data

Sonic velocities from the Upper and Middle Chalk (Upper Cretaceous), the Bunter Sandstone, and the Bunter Shale (both Lower Triassic) were used to independently quantify apparent exhumation (height above maximum burial depth) in the United Kingdom (UK) southern North Sea. Apparent exhumation is the displacement, on the depth axis, of a given velocity/depth trend from the normal (unaffected by exhumation) trend. Apparent exhumation results derived from the Upper and Middle Chalk, the Bunter Sandstone, and the Bunter Shale are statistically similar. The consistency of results from carbonate and clastic units suggests that, at a formational and regional scale, overcompaction (i.e., anomalously high sonic velocity) in all three units analyzed reflects previously greater buria depth, rather than sedimentological and/or diagenetic processes, and validates the use of lithologies other than shale in maximum burial depth studies. The consistency of results from units of Early Triassic to Late Cretaceous age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that maximum Mesozoic-Cenozoic burial depth in the southern North Sea was attained prior to Tertiary exhumation. The proposed magnitudes of exhumation are generally greater than those previously published for the southern North Sea, but they are consistent with recent estimates from apatite fission track analysis. The amount of exhumation of the rock column is greatest on the recognized inversion axes (i.e., Sole Pit-Cleveland), where it reaches approximately 2.5 km. However, there was a regional component of 1.0-1.5 km exhumation during the Tertiary that affected structurally uninverted areas, and on which the more localized inversion-related exhumation was superimposed. Cretaceous-Tertiary burial prior to exhumation must have been of greater magnitude and more rapid than suggested by the preserved stratigraphy. The effect of this extra burial and subsequent exhumation on sedimentary rock decompaction procedure and thermal maturation modeling is illustrated for the Cleethorpes-1 and 44/7-1 wells, and must also be incorporated in modeling reservoir diagenesis. The regional, Tertiary tectonic uplift associated with exhumation must have had a thick-skinned origin.

Tertiary structuration and erosion of the Inner Moray Firth

Abstract Seismic profiles and field data show that the Inner Moray Firth (IMF) experienced significant structural modification during Early Tertiary times with the development of inversion, strike-slip and extensional oblique-slip geometries as well as uplift and erosion at a mid-late Danian unconformity. Seismic reflection profiles across the IMF also show progressively older stratigraphic subcrop towards the west. Analysis of sonic velocities and vitrinite reflectance demonstrate that up to 1.5 km of basin fill has been removed from the IMF. The height of the sequences above maximum burial depth (apparent erosion) is at a maximum in the northwestern part of the basin, where inversion geometries are found, and decreases to zero in the Outer Moray Firth. However, if post-erosional burial is taken into account, the actual amount of erosion during Early Tertiary exhumation (total erosion) is shown to be more evenly distributed, and of greater magnitude throughout the IMF. Incorporation of the effects of Tertiary erosion into analysis of basin development requires much greater post-rift burial than if Tertiary erosion is ignored. It seems most likely that the Early Tertiary deformation of the IMF occurred in response to NE Atlantic (Thulean) and Alpine events. Late Cretaceous-Early Tertiary times saw the change of the predominant stress field of North West Europe from one of extension to one of NW-SE compression (Ziegler, P. A. 1987, Tectonophysics , 137 , 1–5 & 389–420). The present-day stress field of Western Europe is described by a NW-SE direction of maximum principal stress (Müller, B. et al. 1992, Journal of Geophysical Research , B97 , 11 783–11 803). The present-day-, and by inference palaeo-, intraplate stress field of Western Europe can be attributed to plate-driving forces acting on the boundaries of the Eurasian plate. On average, the orientation of present-day maximum stress in western Europe is subparallel to the direction of relative plate motion between Africa and Europe (Müller, B. et al. 1992, Journal of Geophysical Research , B97 , 11 783–11 803). A combination of stresses associated with Alpine collision between Europe and Africa, and those associated with opening of the North Atlantic are considered responsible for the Late Cretaceous-Early Tertiary NW-SE compressional stress field of North West Europe.

A Modified Approach to Estimating Coal and Coal Gas Resources: Example from the Sand Wash Basin, Colorado

The significant increase in United States gas production from coal beds over the past 5 yr has encouraged exploration and development of coal gas resources worldwide. Accurate assessment of coal and coal gas resources and delineating areas within basins containing the largest resources are important aspects of resource development. Previous resource studies may have overestimated or underestimated coal gas resources because the ash and density terms of resource equations were inconsistently or inappropriately considered. In basins where coal analysis data are sparse, coal gas resources are best calculated on an ash-free basis. The density contrast between ash-forming minerals and organic matter is large enough that the weight percent ash is much larger than the corresponding volume percent. Therefore, a correction factor relating weight percent ash-free coal and ash yield (determined from proximate analysis) to ash-free coal volume is required to accurately calculate coal gas resources. Rather than using one density value, as has been done in previous studies, our calculations require that bulk coal density (including mineral matter) be distinguished from ash-free coal density. Coal and coal gas resources of the Williams Fork and Fort Union formations in the Sand Wash basin, determined from modified resource equations, are 291 billion tons (short tons) (264 billion t [metric ton]) and 79 Tcf(FOOTNOTE *) (2.2 Tm3). These resources are significantly higher than previous estimates of basin resources of 34.5 billion tons (31.3 billion t) of coal and 14 Tcf (0.4 Tm3) of coal gas. The Williams Fork Formation contains 79% of the coal and 95% of the coal gas resources, reflecting greater maximum burial depth and higher gas contents. The Fort Union Formation contains 21% of the coal, but only 5% of the coal gas resources. Coal gas resources are greatest in the central part of the basin where Williams Fork resources approach 70 Gcf(FOOTNOTE *)/mi SUP>2 (0.8 Gm3/km2). In comparison, the San Juan basin, the nation's most prolific coal gas-producing basin, has maximum coal gas resources approaching 35 Gcf/mi2 (0.4 Gm3/km2). The high resource density in the Sand Wash basin is due to greater net coal thickness rather than high gas content.

Determination of maximum paleotemperatures of burial (MPTB) of sedimentary rocks from pyrolysis data on the associated organic matter: basic principles and practical application

The Balazuc 1 borehole drilled in the frame of the “Géologie Profonde de la France” programme and the work carried out by various specialists provided the samples and the independent paleotemperature assessment data both needed to check a model aiming to the straightforward determination of maximum paleotemperatures of burial (MPTB) from Rock-Eval® pyrolysis data. Basically, the model supposes that laboratory (e.g., Rock-Eval®) pyrolysis resumes the successive elimination of kerogen moieties of increasing thermal stability where it had stopped during burial diagenesis. Then, the temperature Tmin determined at the onset of the S2 Rock-Eval® pyrolysis peak — supposed to correspond to a peculiar kerogen moiety which was just undergoing cracking when maximum burial depth was reached — might be used to calculate the corresponding MPTB, taking into account the different values of the experimental and natural thermal gradients. The good agreement between calculated MPTB-values and results of fission-track and fluid-inclusion studies carried out on Balazuc 1 borehole samples supports the validity of the model. According to fundamental considerations the application of this approach is limited to samples having at least reached the onset of the metagenetic stage. Other possible limitations and the causes of uncertainty on the obtained results are discussed. A detailed operating procedure is presented.

Quantification of Tertiary erosion in the Inner Moray Firth using sonic velocity data from the Chalk and the Kimmeridge Clay

Sonic velocities for the Chalk and Kimmeridge Clay were analysed to determine the importance and magnitude of Tertiary erosion for 26 wells in the Inner Moray Firth. Apparent erosion (height above maximum burial depth) was derived by the displacement, along the depth axis, of a given velocity-depth trend from the normal (undisturbed) trend. The similarity in apparent erosion results derived from the Cretaceous Chalk and Late Jurassic Kimmeridge Clay successions of the Inner Moray Firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and/or diagenetic process. Resultant apparent erosion estimates are variable across the Inner Moray Firth. Such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern Inner Moray Firth. However, the actual magnitude of erosion (equal to the sum of apparent erosion and post-erosional Tertiary burial) was approximately 1 km throughout the whole Inner Moray Firth area, indicating that Early Tertiary erosion was regional and not simply restricted to the inverted basin margins. However, a well defined mechanism sufficient to have generated the regional event is still sought.

Thermal comparison of mississippi valley-type lead-zinc deposits and their host rocks using fluid inclusion and conodont color alteration index data

Fifteen North American Mississippi Valley-type districts, distributed from the Arctic archipelago to Arkansas and in host rocks ranging in age from Late Cambrian to early Carboniferous, were selected for abroad test survey of Mississippi Valley-type district thermal histories. For each district in which data were collected, histograms of homogenization temperatures for the ore minerals were compared with the temperature range of the corresponding host rocks, as indicated by color alteration index (CAI) determinations on conodonts in the host-rock carbonates. Although a special effort was made to collect conodonts from the Upper Cambrian Bonneterre Formation, host rock to the Southeast Missouri district, all samples were devoid of conodonts. Thus only 14 districts were used in the final analysis.Comparison of fluid inclusion homogenization temperatures in main-stage ore minerals with color alteration index-determined host-rock temperatures reveals that a majority of Mississippi Valley-type districts (Pine Point, Newfoundland Zinc, Mascot-Jefferson City, Copper Ridge, Sweetwater, Central Missouri, Northern Arkansas, and Tri-State) are representative of group 1 (i.e., they are in thermal equilibrium with their host rocks insofar as could be determined by the color alteration index method). Ore minerals in group 2 districts (Upper Mississippi Valley, Polaris, and central Tennessee) are significantly hotter than surrounding host rocks and, as such, define a positive thermal anomaly. Host rocks to the Austinville-Ivanhoe, northern Newfoundland, and Robb Lake deposits (group 3) revealed the highest color alteration index values of North American Mississippi Valley-type districts, with host-rock temperatures greatly exceeding ore mineral fluid inclusion temperatures (negative thermal anomaly).With respect to major tectonic elements of North America, group 3 deposits (cool ore in hot rocks) are positioned within orogenic belts but on the continental side. Some group 1 deposits (ore temperature = host-rock temperature) are well within the continental interior but others are adjacent to orogenic belts on the continental side. Group 2 deposits (hot ore in cool rocks) are all within the continental interior.Current hypotheses regarding the formation of Mississippi Valley-type deposits and simultaneous heating of their host rocks invoke the migration of hot brines set into motion craton-ward by the hydraulic gradient produced by orogenic uplift at the craton edge. Once the ore-bearing fluids entered the foreland carbonate platforms, they traveled through sedimentary aquifers of the continental interior raising the temperature of the thinly covered carbonate rocks. The fluids left, as evidence of their passage, elevated color alteration index values and group 1 Mississippi Valley-type deposits distributed with decreasing homogenization temperatures away from the orogenic front. The correspondence between host-rock and ore temperatures suggests that group 1 deposits were deposited essentially within their own aquifers.Group 2 deposits were formed when the same, or genetically similar, fluids rose from the regional aquifer(s) into structurally controlled conduits in the continental interior, possibly developed as a result of synorogenic doming and arching. This upward channeling of the fluids resulted in rapid and localized fluid flow which precluded or at least inhibited wide-spread heating of the host rocks by ore fluids; consequently, color alteration index values in group 2 districts record only maximum burial depths of the host rocks and bear little relation to temperatures of the ore fluids. Although the cause of the unusually high color alteration index values (4-5) of group 3 districts is not fully understood, it is noted that these districts occur near the extreme outward edge of foreland platforms, near or within the leading margins of orogenic belts. Whether these group 3 host rocks were heated by simple tectonic burial or by passage of heated fluids under shallower conditions is not known.