Illite Age Analysis Research Articles

Unravelling multiple thermotectonic events accommodated by the Highland Boundary Fault: Insights from K–Ar dating

Multiple reactivation episodes and long slip histories lead to complex fault structures, whose unravelling remains challenging in the absence of absolute time constraints. We apply K–Ar isotopic dating of illitic fault rocks, coupled with X-ray diffraction and microstructural analyses to constrain, for the first time, timing of illite-producing brittle fault movements accommodated by the Highland Boundary Fault (HBF), Scotland. Illite Age Analysis (IAA) plots indicate multiple fault reactivation events on the HBF. IAA plots for the red foliated chaotic fault-breccia conform to a ‘normal’ IAA pattern with younger ages recorded for ‘authigenic’ 1 M illite (306-276 and 300-272 Ma) and older dates for ‘detrital’ 2M1 illite (554-502 and 471-427 Ma). Conversely, IAA plots for the superimposed blue fault-gouge reverse the ‘normal’ trend, with older ‘authigenic’ 1 M illite (348-314 and 276-25 Ma) and younger ‘detrital’ 2M1 illite (258-234 and 250-226 Ma) ages. We propose this results from heterogeneous shearing and strain localisation. In localised pods of the blue fault-gouge, strain accelerated clay mineral growth, increased crystallite-size, and facilitated polytypic transformation from 1 M to 2M1 illite via defect migration. Elsewhere in the blue fault-gouge, and in the red foliated chaotic fault-gouge, low strain regimes allowed the 1 M polytype to remain unaltered.

Extraction of 40Ar-39Ar Ages from a Multicomponent Mixture: A Case Study From the Tatra Mountains, Poland

AbstractExtraction of meaningful information on the timing of fault activity from clay gouges using radiometric dating methods, such as those based on the K-Ar system, can be challenging. One of the factors complicating interpretation of the radiometric dating results is the presence of multiple K-bearing components in the gouge material. In the current study, an attempt was made to develop a new interpretative method for K-Ar and 40Ar-39Ar dating, capable of handling a three-component mixture. In addition, the mineral composition of clay gouges from the Tatra Mountains (Poland), which has not been investigated before, is reported. The mineral compositions of the bulk clay gouge material and separated size fractions were determined by X-ray diffractometry and Fourier-transform infrared spectroscopy. The gouge samples were composed of quartz, dioctahedral mica (as a discrete phase and as a component of mixed-layered illite-smectite), and chlorite, commonly with plagioclase and more rarely with K-feldspar, dioctahedral smectite, calcite, anatase, or trace kaolinite. One feldspar-free sample containing three mica polytypes (1Md, 1M, and 2M1) was chosen for dating with the 40Ar-39Ar method. The results of 40Ar-39Ar dating were interpreted using three concepts: Illite Age Analysis (IAA), a method based on the MODELAGE software, and a newly developed three-component concept. The age values obtained with IAA were −14 Ma ± 31 Ma and 180 ± 91 Ma for authigenic (1Md) and inherited (1M + 2M1) components, respectively. The MODELAGE-based approach returned –4 ± 40 Ma and 165 ± 62 Ma. The three-component approach returned age values of polytypes as follows: 1Md, 15 ± 37 Ma; 1M, 135 ± 57 Ma; 2M1, 121 ± 56 Ma based on the medians and the interquartile ranges of non-normal distributions of Monte Carlo-simulated age values. The results obtained indicated that the 1Md polytype was probably formed during the most recent stage of fault activity, while 1M and 2M1 polytypes are of equal age, roughly.

Illite-Age-Analysis (IAA) for the Dating of Shallow Faults: Prerequisites and Procedures for Improvement

Fault age determination using the illite-age-analysis (IAA) method for fault gouges has played a key role in providing absolute age information in tectonic evolution studies for the last 20 years. The accuracy and precision of the IAA method depend on (1) how to reasonably quantify the relative content of 1M/1Md illite generated from fault activity compared to detrital 2M1 illite in the size fractions of the fault gouge, and (2) how to minimize the error factors in K-Ar or Ar-Ar dating analysis. XRD-based quantitative analysis of illite polytype has made great progress in accuracy by generating a simulated XRD pattern of 1M/1Md polytype using WILDFIRE© and full-pattern-fitting it with the XRD pattern measured from size fractions of the fault gauge. Nevertheless, the results of quantitative analysis of illite polytypes may vary depending on the sample state of the size fractions for XRD analysis, especially the preferred orientation due to the layered crystal structure of illite. In addition, the radiometric dating results may be distorted depending on the error factor of the dating method itself and on the mineral composition of the size fractions, that is, the presence of K-containing minerals such as biotite and K-feldspar other than illite. In this study, we reviewed previous studies that determined fault activity ages by applying IAA to fault gouges. From this, the prerequisites and recommendations for each of the five steps (particle size separation process, XRD analysis process, polytype quantification, radiometric dating, IAA plot) for improving the IAA method are summarized and presented. The continuous application of the improved IAA is expected to greatly contribute to the study of tectonic evolution through geological time.

K-Ar fault gouge dating of Neogene thrusting: The case of the siliciclastic deposits of the Trasimeno Tectonic Wedge (Northern Apennines, Italy)

The Northern Apennines (NA) are a characteristic example of foreland fold-and-thrust belt progressively migrating towards its foreland. Their tectonic evolution has been quite tightly constrained in time by microfossil biostratigraphy applied to syn-orogenic deposits within foreland basins. This makes the NA well suited to test the reliability of K-Ar illite dating of Neogene deformation affecting siliciclastic sequences. We sampled two top-to-the-ENE thrusts, whose well defined cores are defined by scaly gouge formed at the expense of the pelitic component of the host rock. X-ray diffraction (XRD) and K-Ar isotopic analysis of multiple grain-size fractions of the gouge, allowed us to discriminate between syn-kinematic and inherited illite crystals in the fault rocks. Illite age analysis (IAA) constrains fault slip along the thrusts to 15.2 ± 7.6 Ma and 15.4 ± 16.6 Ma. The results, in spite of their analytical uncertainty, are fully consistent with the local evolution of the NA as constrained by the independent biostratigraphic studies and confirm the general suitability of this geochronological approach to Neogene deformation.

Clay mineral dating of displacement on the Sronlairig Fault: implications for Mesozoic and Cenozoic tectonic evolution in northern Scotland

AbstractTemporary excavations during the construction of the Glendoe Hydro Scheme above Loch Ness in the Highlands of Scotland exposed a clay-rich fault gouge in Dalradian Supergroup psammite. The gouge coincides with the mapped trace of the subvertical Sronlairig Fault, a feature related in part to the Great Glen and Ericht–Laidon faults, which had been interpreted to result from brittle deformation during the Caledonian orogeny (c. 420–390 Ma). Exposure of this mica-rich gouge represented an exceptional opportunity to constrain the timing of the gouge-producing movement on the Sronlairig Fault using isotopic analysis to date the growth of authigenic (essentially synkinematic) clay mineralization. A series of fine-size separates was isolated prior to K–Ar analysis. Novel, capillary-encapsulated X-ray diffraction analysis was employed to ensure nearly perfect, random orientation and to facilitate the identification and quantification of mica polytypes. Coarser size fractions are composed of greater proportions of the 2M1 illite polytype. Finer size fractions show increasing proportions of the 1M illite polytype, with no evidence of 2M1 illite in the finest fractions. A series of Illite Age Analysis plots produced excellent R2 values with calculated mean ages of 296 ± 7 Ma (Late Carboniferous–Early Permian) for the oldest (2M1) illite and 145 ± 7 Ma (Late Jurassic–Early Cretaceous) for the youngest (1M) illite. The Late Carboniferous–Early Permian (Faulting event 1) age may represent resetting of earlier-formed micas or authigenesis during dextral displacement of the Great Glen Fault Zone (GGFZ). Contemporaneous WNW(NW)–ESE(SE) extension was important for basin development and hydrocarbon migration in the Pentland Firth and Moray Firth regions. The Late Jurassic–Early Cretaceous (Faulting event 2) age corresponds with Moray Firth Basin development and indicates that the GGFZ and related structures may have acted to partition the active extension in the Moray Firth region from relative inactivity in the Pentland Firth area at this time. These new age dates demonstrate the long-lived geological activity on the GGFZ, particularly so in post-Caledonian times where other isotopic evidence for younger tectonic overprints is lacking.

K–Ar age determinations on the fine fractions of clay mineral ‘Crystallinity Index Standards’ from the Palaeozoic mudrocks of southwest England

AbstractClay mineral ‘Crystallinity Index Standards’ (CIS) composed of Palaeozoic mudrocks from southwest England were investigated systematically in five sub-fractions per sample for the first time. X-ray diffraction was used to determine mineral assemblages, calibrated 001 illite full-width-at-half-maximum (FWHM) values and illite polytype compositions, in addition to K–Ar isotopic analyses of all fine fractions. The FWHM results of the <2 µm fraction are consistent with previous studies and reflect the range of diagenetic to epizonal grades covered by the sample set SW1 to SW7 (~0.61–0.26°2θ). Diagenetic and lower anchizone samples also show significant broadening of 001 illite reflections in the finer fractions and contain mixtures of authigenic 1M+ 1Mdillite and detrital 2M1white mica polytypes suitable for illite age analysis. The estimated end-member ages of the Bude (SW1-1992) and younger Crackington (SW3-2000) mudstones yield detrital ages of Late Cambrian to Middle Ordovician (493–457 Ma) and a broad range of 1M+ 1Mdillite ages between Middle Permian and Early Jurassic (271–190 Ma). The detrital age of the stratigraphically older Crackington Formation mudrock (SW2-1992) is Late Devonian (384–364 Ma) with 1M+ 1Mdillite ages between Late Triassic and Early Jurassic (219–176 Ma). The origin of Mesozoic 1M+ 1Mdillite ages may represent neocrystallized illite associated with Mesozoic hydrothermal events or similar events that thermally reset older authigenic illite with partial loss of radiogenic argon and no renewed crystal growth. In contrast, upper anchizonal and epizonal Devonian slates (SW3-2012, SW4-1992, SW6-1992 and SW7-2012) contain only the 2M1polytype, with K–Ar ages younger than the stratigraphic age. The three finest fractions of SW4-1992 yield consistent Late Carboniferous ages (331–304 ± 7 Ma) that are considered to date the neocrystallized 2M1mica. Most fractions of epizonal slate (SW6-1992, SW7-2012) yield Early Permian ages (293.6–273 Ma) corresponding to published cooling ages of the Tintagel High-Strain Zone and the intrusion of the Bodmin granite (291.4 ± 0.8 Ma). These first K–Ar age constraints for the fine fractions of the CIS should provide useful reference values for testing analytical procedures of illite age analysis.

Fault gouge dating: history and evolution

ABSTRACTRadiometric dating of fault gouges has become a useful tool for regional tectonics studies and for exploring and understanding fault and earthquake processes. Methods to define the absolute age of faults achieved a solid scientific foundation almost 25 years ago when the development and application of illite age analysis for investigating sedimentary burial and thermal histories found a new potential application – defining the age of fold-and-thrust development. Since then, the methods have benefitted from further development and incorporation of the 40Ar/39Ar micro-encapsulation method and quantitative clay mineral evaluation to distinguish polytypes (Wildfire). These refinements to the methods have improved their application in fold-and-thrust terrains and have opened up applications in normal and strike-slip fault environments. Another important development is the use of absolute dating methods in retrograde clay gouges in which clays in a fault develop from igneous or metamorphic wall rocks that contain no clays. In addition, the method has also been shown to be useful at dating folds in fold-and-thrust belts. We think the method is now an established part of the geological toolkit, look forward to future fault structural and tectonic studies that incorporate fault ages and hope that researchers continue to probe and discover ways that the method can assist fault process studies, including earthquake fault studies.

인제단층의 중생대 이 후 재활동 연대

Recently developed illite-age-analysis(IAA) approach was applied to determine the fault-reactivated events for the Inje fault that cut through Precambrian biotite granitic gneiss with NNE-SSW trend in the middle of Korean peninsula. Three distinct fault-reactivated events of shallow crustal regime were recognized using the combined approach of optimized illite-polytype quantification and K-Ar age-dating of clay fractions separated from 4 fault clay samples: 87.0±0.12 Ma, 65.5±0.05 and 66.6±1.38 Ma, 45.6±0.15 Ma, respectively. As well, 2M1 illite ages of 193~196 Ma and 254.3±6.96 were discernible, which may be related to the fault-activated time in the relatively deep crust. The study results suggest that the Inje fault would be firstly formed at 254.3°ae6.96 Ma and sporadically reactivated in shallow regime since about 87 Ma. These reactivation events in shallow regime might be due to the Bulguksa orogeny that would be strongly influenced in Korean peninsula at that time.

K–Ar illite dating to constrain multiple events in shallow crustal rocks: Implications for the Late Phanerozoic evolution of NE Asia

A recently developed illite-age-analysis (IAA) approach was applied to determine the multiple events for the Chugaryeong fault belt, Korea. Each event was determined by a combined approach of the optimized illite-polytype quantification and the K–Ar age-dating of clay fractions separated from the fault clays. The Late Cretaceous to Paleogene events (76.5±0.8, 69.1±0.6, 59.3±0.7, and 48.2±0.7Ma) were recognized by calculating the authigenic 1M/1Md illite ages on the IAA plots of the fault clays. The Early Cretaceous ages (121.7–124.7 and 112.4±1.5Ma) were also obtained from the convergent intercepts of 100% 2M1 illite on the IAA plots. The absence of the 2M1 illites in the host-rock indicates that the Early Cretaceous ages represent the timings of high-T hydrothermal events of >280°C. The 2M1 illites in the fault clays should be pre-formed by a fluid-rock interaction under a relatively high-T subsurface condition, and be mechanically reworked into the near surface along the fault by post-tectonic events. This is the first report determining the absolute age constraints of multi-activated tectonic events from a fault. These geochronological determinations of the multiple events recorded in the Chugaryeong fault belt are crucial to establish the tectonic evolution of the Korean Peninsula since the Late Cretaceous.

Fault gouge dating in the Southern Appalachians, USA

Illite age analysis (IAA), the method of comparing radiometric ages of successive size fractions with varying percentages of detrital illite, has been successfully applied to several rock types, including fault gouge, shales and argillaceous limestones. IAA results are presented for fi ve fault rocks, including four clay gouges and one cataclasite, from the exhumed Southern Appalachian foreland fold-thrust belt (eastern United States). Determining detrital versus authigenic illite is now an established procedure, utilizing X-ray analysis to quantify illite polytypes. Less established, however, is how to interpret 40 Ar/ 39 Ar ages as a function of diagenetic grade. Both a total gas age, incorporating the recoiled argon fraction after irradiation, and a retention age (omitting the recoiled fraction) are obtained for a sample. We relate their respective use to diagenetic grade and, specifi cally, the crystallite thickness of illite. When measured crystallite sizes are equal to or smaller than that calculated from Ar recoil we use total gas ages (here ~5 nm), while for crystallites greater than ~10 nm we use retention ages. The four clay gouge ages are all the same within error and much younger than that of the cataclasite. We conclude that a major period of frontal Appalachian fault activity occurred during the early Permian (Cisuralian, 276–280 Ma) and that these foreland faults were active simultaneously as part of an internally deforming, regional thrust wedge.

일라이트 폴리타입: 그 특성과 단층 활동연대 결정에의 활용

일라이트에서 적층방식을 달리하여 나타나는 폴리타입은 대부분 1M과 <TEX>$2M_1$</TEX>이며, 적층 Disordering에 따른 <TEX>$1M_d$</TEX> 또한 흔히 관찰된다. 본 해설에서는 일라이트의 폴리타입을 구분하고 정량적으로 해석할 수 있는 광물학적 이론적 근거와 방법을 소개하고, 소규모 열수환경에 준하는 단층대 내 생성시기 및 조건을 달리하는 일라이트 폴리타입 혼합물에의 적용을 통한 단층 활동 및 재활동연대 결정에의 활용 가능성을 검토하였다. 단층연대해석의 정확도와 신빙도를 높이기 위해서는 단층암 대상시료 내 일라이트 폴리타입의 생성환경에 대한 정보, 동정 및 광물학적 특성에 대한 규명이 이루어져야 한다. 또한 WILDFIRE(C)시뮬레이션을 이용한 정량분석에서, <TEX>$1M_d$</TEX> 폴리타입의 Disordering 정도 및 I/S 광물의 팽창도에 기인한 (hkl) 회절선들의 Broadening 효과 등 일라이트 폴리타입 정량분석법의 오차요인을 최소화시키는 하기 방법을 고려해야 할 것이다. The 1M and <TEX>$2M_1$</TEX> stacking sequences are the most frequently encountered in the illite species among the possible 6 polytypes. The <TEX>$1M_d$</TEX>, derived from the 1M polytype which exhibits a variable degree of disorder in the stacking sequence, is also observed in illite samples. In this paper, the author introduces and reviews the theoretical background of the quantitative analysis method of illite polytypes, and considers the possibility to determine the fault age and its reactivation age using K/Ar age-dating based on the quantification of illite polytypes in the fault system. For the increase of the accuracy and precision of the illite age analysis method, the occurrence, identification, and mineralogical characterization of illite polytypes should be defined in detail. The broadening effect of (hkl) reflections, due to disordering of 1M polytype and the presence of I/S minerals with expandability, are also considered as the main parameters controlling the quantification of illite polytypes using the WILDFIRE(C)simulation.

Links between orogenic wedge deformation and erosional exhumation: Evidence from illite age analysis of fault rock and detrital thermochronology of syn-tectonic conglomerates in the Spanish Pyrenees

We present new geochronologic data illuminating the tectonic and erosional history of the orogenic wedge exposed in the south-central Pyrenees, Spain. We interpret illite-age analyses from four fault gouges that record thrust-belt development and document the importance of out-of-sequence thrusting. Fault activity occurs in pulses, with slip occurring contemporaneously on multiple faults throughout the wedge. New apatite fission-track data from syn-orogenic sediments of the Sis Conglomerate body reveal ages of 48 to 42 Ma, with no consistent variation upsection in strata deposited from 41 to 30 Ma. We interpret these data, as well as thermal modeling of track-length distributions, to imply rapid cooling in the interior of the Pyrenean wedge during the Middle Eocene. The record of fault activity and erosion suggests that orogenic wedges may not evolve in a steady fashion, but generally exhibit significant changes in rates of deformation and exhumation. The observed correlation in the timing of tectonism and erosional exhumation provides evidence for links between tectonic and surface processes.

Comparison of K-Ar Ages of Diagenetic Illite-Smectite to the Age of a Chemical Remanent Magnetization (CRM): An Example from the Isle of Skye, Scotland

AbstractThe clay fractions of Jurassic marls in the Great Estuarine Group in southern Isle of Skye are composed of mixed-layered illite-smectite (I-S) with large percentages (&gt;85%) of illite layers, kaolinite, and generally smaller amounts of chlorite. These marls have not been buried to the depths normally required to convert smectite to illite-rich I-S, so it is possible that the conversion was in response to heat and hydrothermal fluids from nearby early Tertiary igneous activity ∼55 Ma ago. The large percentages of illite layers in I-S, the Środoń intensity ratios, and the Kübler index values appear to be consistent with the formation of diagenetic I-S as a result of relatively brief heating caused by igneous activity. The Jurassic rocks in southern Skye contain a secondary chemical remanent magnetization (CRM) that resides in magnetite and formed at approximately the same time as the Tertiary igneous rocks on Skye. K-Ar age values for I-S based on illite age analysis have been determined to test the hypothesis that the CRM was acquired coincidently with the smectite-to-illite conversion. However, linear extrapolation of K-Ar age vs. percentage of 2M1 polytype (detrital illite) from one marl (EL-6) yields an estimate for the age of diagenetic illite of 106 Ma, which is close to the measured age of the finest subfraction (108 Ma). These estimated and measured age values, however, could be substantially greater than the true age of the diagenetic illite in I-S because of the presence of detrital 1Md illite that was recycled from early Paleozoic shales and whose abundance relative to the diagenetic I-S may have been enhanced because the diagenetic fluid had a low K/Na ratio, limiting the amount of diagenetic illite formed. Nevertheless, most of the illite in the Elgol marls (80% or more in the finest fractions) must be diagenetic and probably formed in response to the early Tertiary magmatism.

Fault dating in the Canadian Rocky Mountains: Evidence for late Cretaceous and early Eocene orogenic pulses

Fault rocks from the classic Rocky Mountain foreland fold-and-thrust belt in southwestern Canada were dated by Ar analysis of clay grain-size fractions. Using X-ray diffraction quantification of the detrital and authigenic component of each fraction, these determinations give ages for individual faults in the area (illite age analysis). The resulting ages cluster around 72 and 52 Ma (here called the Rundle and McConnell pulses, respectively), challenging the traditional view of gradual forward progression of faulting and thrust-belt history of the area. The recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges, which result in geologically reasonable strain rates for the area. In addition to regional considerations, this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper-crustal kinematics and regional tectonic analysis of ancient orogens.

Reservoir quality and K-Ar age dating of the pre-Khuff section of Kuwait

ABSTRACT Based on studies of petrographic thin sections from core and cutting samples, the pre-Permian siliciclastics in four deep wells in southern Kuwait were found to be tight. Three of these wells are located on the crestal region of the Burgan Arch, and one on the Umm Gudair anticline. These clastics were encountered beneath a thin brick-red shale of unknown thickness, immediately below the pre-Khuff unconformity at the base of the Permian-Triassic Khuff Formation. The pre-Khuff clastics range in thickness from a few tens of feet to more than 4,000 ft, and overlie a Proterozoic argillite (Economic Basement). Based on Illite Age Analysis (IAA) of samples from cores, the depositional K-Ar age of the pre-Khuff clastics is estimated to be younger than 509 Ma (90 percent confidence interval: 544–481 Ma, i.e. Cambrian-Early Ordovician). The argillite was uplifted through the 300°C isotherm at about 611 Ma (90% confidence interval: 635-588 Ma, i.e Late Proterozoic); its deposition and metamorphism preceded this date. During the Paleozoic, the pre-Khuff clastics were buried to depths of 10,000–15,000 ft, but were subsequently uplifted in the Late Paleozoic. IAA diagenetic K-Ar ages of the Economic Basement (421 Ma; 90 percent confidence interval: 442-397 Ma; Late Ordovician-Early Devonian) and pre-Khuff clastics (369 Ma; 90 percent confidence interval: 404–337 Ma, i.e. Devonian-Early Carboniferous) indicate that by these times the pre-Khuff section was already deposited and undergoing burial diagenesis. The interpretation of gravity data indicates that in Paleozoic basinal regions (e.g. between the Burgan Arch and Umm Gudair Anticline), the Paleozoic sedimentary section is likely to be more complete and may exceed 10,000 ft in thickness.

Illite polytype quantification for accurate K-Ar age determination

PolyQuant, a new method to quantify illite polytypes in physical mixtures, has been coupled with Illite Age Analysis (IAA, Pevear 1992) to determine accurate K-Ar ages of the individual polytypes. K-Ar radioisotope dates from illite in shales and sandstones are typically meaningless because these rocks are mixtures of diagenetic and detrital components in unknown proportions. IAA uses linear extrapolation to unmix measured K-Ar ages and establish the end-member ages. This procedure requires samples to have a diagenetic Illite-Smectite (I-S) component mixed with a detrital discrete illite component because they are separable and quantifiable using X-ray diffraction (XRD) of oriented aggregate samples. We extend IAA for use in mixtures of diagenetic and detrital illite with no I-S component by coupling a genetic algorithm (GA) with the WILDFIRE programs (1993) of Reynolds in a new computer program, PolyQuant, which quantifies illite polytypes in a natural sample. Based on first principles of XRD, WILDFIRE is a forward model that calculates hkl reflections by varying parameters such as rotational disorder, octahedral cation occupancy (i.e., cis- and transvacancies), chemical substitutions (K and Fe), orientation, and crystallite thickness. PolyQuant executes several forward models, using the GA to vary the parameters and weight percents of each polytype to automatically achieve a “best fit” with an experimental XRD pattern. Sensitivity tests reveal that changes in the weight percent of an illite polytype, disorder (P 0 , P cv ), and the orientation function affect the pattern more than other parameters. In addition, PolyQuant successfully quantified illite components in prepared mixtures of 1M illite and 2M 1 muscovite and 1M d illite and 2M 1 muscovite. We show examples to validate IAA from prepared mixtures of end-members with known ages and also to obtain end-member ages of highly illitic clay fault gouge. Using K-Ar ages and PolyQuant to quantify the 1M and 2M 1 components from mixtures of RM-30 (25 ± 1 Ma) and a 2M 1 muscovite from Wards (428 ± 9 Ma), IAA determined end-member ages of 19.9 Ma (0-44 Ma) and 429 Ma (408-459 Ma)-in reasonable agreement with the measured results. In addition, we use clay gouge from the Canadian Rockies to date movement along thrust faults. Regional geologic data indicate movement along these faults occurred between 73 and 48 Ma. IAA determined the ages of faulting to be from 79.2 to 53.5 Ma, in good agreement with the known range of fault dates. Our results indicate that neoformation of illite in the fault zone must have modified the ages to younger dates.

Comparison of the Mineralogical and Chemical Composition of 2 Shales from the Western Canada Sedimentary Basin and the United States Gulf Coast

AbstractThe mineralogy and geochemistry of shales reflect the composition of the initially deposited precursor mud, subsequently modified by diagenetic processes. To see if significant geochemical differences exist between shales that mainly owe their present-day composition to either deposition or diagenesis, we compare the published mineralogical, bulk and clay-fraction geochemical, and clay-fraction O-isotopic compositions of 2 shales. One shale is from the Western Canada Sedimentary Basin (WCSB), and its composition mainly reflects primary (depositional) chemical and mineralogical variations (smectitic to illitic illite/smectite) within this unit. The other shale is from the United States Gulf Coast (USGC), and its composition mainly reflects mixed-layer illite/smectite (I/S) diagenesis of deposited smectitic clay material. The chemical and mineralogical trends of WCSB and USGC shales, including one of increasing illite content in I/S with depth or maturity, are essentially indistinguishable, in both bulk shale and clay fraction, despite the contrasting genetic interpretations for the origin of the contained I/S. Thus, similar mineralogical and chemical trends with depth or temperature can result either from inherited depositional compositional heterogeneity of the sediment, from burial metamorphism of shale or a combination of both. Interestingly, the O-isotopic compositions of the clay fractions from the WCSB and USGC are significantly different, a fact that reflects original clay formation from source material and water of quite different isotopic compositions. The discrimination between depositional and diagenetic contributions to shale composition continues to pose challenges, but a combination of bentonite, illite polytype, clay isotopic and trace and rare earth elemental analyses together with illite age analysis holds promise for future work.

The Tale of Three Basins: Illite Age Analysis, Shale Diagenesis, and Interpretation of Diagenetic History: ABSTRACT

The Tale of Three Basins: Illite Age Analysis, Shale Diagenesis, and Interpretation of Diagenetic History: ABSTRACT