Helvetic Domain Research Articles

Geochemistry of clumped isotopologues of CH4 within fluid inclusions in Alpine tectonic quartz fissures

Petroleum-bearing fluid inclusions are small encapsulations of oil and gas that persist in the rock record long after their parent fluids have moved on. The chemical and isotopic compositions of inclusions offer a unique way to investigate hydrocarbon formation pathways in deep Earth environments and in ancient geological times. Here we use stable isotope compositions of methane, including clumped isotopologues 13CH3D and 12CH2D2, to evaluate the formation conditions of methane within fluid inclusions in Alpine tectonic quartz fissures. We observe that this CH4 is relatively 13C and 2H enriched (δ13C = −26 to −39‰; δ2H = −126 to −146‰), consistent with the formation by advanced catagenesis of organic matter, and has clumped isotope compositions indicating intramolecular isotopic equilibrium at catagenetic to low-grade (sub-greenschist) metamorphic temperatures (TΔ13CH3D: 120-300°C; TΔ12CH2D=2120-260°C). Our findings, combined with an analysis of the geographic distributions of inclusion-bearing fissures and fluid inclusion PVT behaviors, provide direct evidence of reservoirs of thermogenic methane trapped beneath the Alps during Mid-Miocene tectonic nappe emplacement (c.a. 25 to 15 Ma). The gas was sequestered in a concentrated form that likely favored the accumulation of up to 50,000-150,000 Megatons (Mt) of CH4 across the Helvetic domain in Switzerland. Following the subsequent tectonic uplift during the period 17 to 10 Ma, we suggest that the trapped methane-rich fluids migrated to shallower depths along faults, leading to the release of Alpine metamorphic methane into near-surface aquifers and the atmosphere at rates of 0.01 to 0.03 Mt⋅y−1.

Cretaceous syn-sedimentary faulting in the Wildhorn Nappe (SW Switzerland)

During Cretaceous time, the area of the future Helvetic nappes (Central Alps, south-western Switzerland) was part of a large ramp-type carbonate depositional system on the European margin, in which the area of the Wildhorn Nappe was transitional to the more distal and relatively deeper Ultrahelvetic basin. The Wildhorn Nappe includes an Upper Cretaceous succession bearing clear evidence for syn-sedimentary normal faulting, such as syn-sedimentary geometries related to well oriented NE-striking faults, sedimentary dykes, lateral variations in the thickness and facies of formations, anomalous and discordant contacts corresponding to palaeo-escarpments, and slump folds. Four stages of syn-sedimentary fault activity have been recognized. (1) Post-Cenomanian disruption and exhumation of the Schrattenkalk platform related to distributed normal faulting, which contributed to the initiation of karst erosion on topographic highs and sedimentation in topographic lows. (2) Turonian–Santonian marine transgression accompanied by localized normal faulting, creating growth-fault structures, differential subsidence and slope instability. A transition from distributed to more localized faulting is observed, related to a final stage in the evolution of the Cretaceous extensional process. (3) Early Maastrichtian faulting. The facies and thickness of subsequent sediments reflect a passive adaption to the pre-existing topography of the sea floor, established during the earlier tectonic movements. (4) Post-Maastrichtian north-directed tilt and erosion. In the Wildhorn Nappe, palaeo-fault activity most probably ended in the Early Maastrichtian rather than continuing into the Eocene. Until now, the regional importance and magnitude of Late Cretaceous extension has not been recognized in the Helvetic domain. This widespread event may be related to post-breakup extensional tectonics along the European margin or, alternatively but less likely, to lateral gravitational collapse of the margin.

Earthquake focal mechanisms and stress orientations in the eastern Swiss Alps

This study presents an updated set of earthquake focal mechanisms in the Helvetic and Penninic/Austroalpine domains of the eastern Swiss Alps. In eight cases, based on high-precision relative hypocentre locations of events within individual earthquake sequences, it was possible to identify the active fault plane. Whereas the focal mechanisms in the Helvetic domain are mostly strike-slip, the Penninic/Austroalpine domain is dominated by normal-faulting mechanisms. Given this systematic difference in faulting style, an inversion for the stress field was performed separately for the two regions. The stress field in the Penninic/Austroalpine domain is characterized by extension oriented obliquely to the E–W strike of the orogen. Hence, the Penninic nappes, which were emplaced as large-scale compressional structures during the Alpine orogenesis, are now deforming in an extensional mode. This contrasts with the more compressional strike-slip regime in the Helvetic domain towards the northern Alpine front. Relative to the regional stress field seen in the northern Alpine foreland with a NNW–SSE compression and an ENE–WSW extension, the orientation of the least compressive stress in the Penninic/Austroalpine domain is rotated counter-clockwise by about 40°. Following earlier studies, the observed rotation of the orientation of the least compressive stress in the Penninic/Austroalpine region can be explained as the superposition of the regional stress field of the northern foreland and a uniaxial extensional stress perpendicular to the local trend of the Alpine mountain belt.

Deciphering the message of Early Cretaceous drowning surfaces from the Helvetic Alps: What can be learnt from platform to basin correlations?

AbstractDrowning unconformities are a frequent feature of carbonate platforms and generally express the incapacity of shallow‐marine ecosystems to adapt to abrupt sea‐level rise and/or palaeoenvironmental change. During the late Hauterivian and early Barremian, the Helvetic carbonate platform experienced a major drowning phase documented by the phosphate and glauconite‐rich Altmann Member. It has been shown that a drowning unconformity in the form of a hardground at the base of the Altmann Member on the Pilatus summit (central Switzerland) results from a complex, polyphased diagenetic history that includes two main phases of phosphogenesis. Using carbon‐isotope stratigraphy, biostratigraphy and sequence stratigraphy, the correlation of this drowning unconformity from the Helvetic domain with more distal and more complete sedimentary archives allows reconstruction of the sequence‐stratigraphic context of the drowning surface, and also an estimation of the amount of time represented by the episodes of condensation. Unconformities associated with the Altmann Member drowning phase developed during transgressive episodes, when strong currents arrived onto the previously subaerially exposed platform, and periods of phosphogenesis may have lasted up to 2·8 Myr. Variations in trophic levels and hitherto less well‐known intervening emersion phases played an essential triggering role for the Altmann drowning phase. High‐resolution studies of drowning unconformities unravel the diversity of palaeoenvironmental parameters involved in the unfolding of such crises during the evolution of carbonate platforms.

The pioneer work of Bernard Kübler and Martin Frey in very low-grade metamorphic terranes: paleo-geothermal potential of variation in Kübler-Index/organic matter reflectance correlations. A review

Low-temperature metamorphic petrology occupies the P–T field between sedimentary and metamorphic petrology. Two important pillars of low-temperature metamorphism are coal petrology and clay mineralogy. When low temperature petrology was established bridging a hiatus between the two classical geological disciplines of sedimentary geology and metamorphic petrology, geologists faced a need for the usage of different terminology tenets. Martin Frey and Bernard Kubler were two pioneers in low-grade metamorphic petrology. They focused their research on clarifying the relationships of clay mineralogy and organic petrology to metamorphic pressure (P) and temperature (T) conditions. The ultimate aim of M. Frey and B. Kubler was to establish a correlation between clay indices and organic parameters for different geodynamic setting and therefore for various pressure–temperature (P–T) conditions occurring in low grade metamorphic terranes. For this purpose, a special attention was addressed to the correlation between the Kubler-Index (KI) and vitrinite reflectance (VR). All these efforts are dedicated to estimate the P–T conditions and thus to gain insight into the geodynamic evolution of low-grade metamorphic terranes. B. Kubler and M. Frey honored here concentrated their studies to the Helvetic Central Alps area. The very low-grade Helvetic domain is therefore of basic interest of this paper. Ensuing the extensive compilation of data from the Helvetic domain, a reinterpretation of Kubler and Frey’s research is presented in the light of last decade’s scientific progress. A comprehensive dataset available enables to discriminate many factors influencing the Kubler-Index and organic-matter reflectance alongside to time, temperature and pressure. The correlation is restricted to the KI and organic matter reflectance (mostly VR) because most of the studies used both methods. Organic matter reflectance (OMR) includes data from vitrinite reflectance and bituminite reflectance measurements. Geodynamics has important control on the KI/VR (OMR) correlation. Tectonic units having a similar geodynamic evolution are featured by the comparable KI/OMR trends, related to the particular paleo-geothermal conditions. Obviously the KI/OMR correlations provide a mean to characterise geothermal gradients and metamorphic very-low-grade pressure–temperature conditions. In terranes where high deformations rates are reported, exceeding the high anchizone conditions, strain promotes the kinetic effects of temperature and pressure on the KI versus OMR ratio.

Géologie du karst de Tsanfleuron

The Tsanfleuron karren field belongs to the Helvetic domain of the Alps. It is formed by several nappes made of Mesozoic and Paleogene (Nummulitic) rocks, among which limestones take an important place. This area mainly belongs to the Diablerets nappe and for a small part to the overlying Mont Gond nappe, which are the two lower subdivisions of the Wildhorn group of nappes. An isoclinal syncline connects these two nappes, which are gently folded by a large antiform. The axial plunge of the whole structure is between 10 and 15° to the north-east. The fractures certainly influenced the karstic erosion but more observations are currently necessary to prove this link. Cross-sections also show the groundwater pathways and the connections between infiltration points and springs.

Distribution of authigenic albites in a limestone succession of the Helvetic Domain, eastern Switzerland

A new occurrence of authigenic albite was found in a Jurassic sedimentary succession of the Glarus Nappe near Walenstadt (eastern Switzerland). The euhedral shape and the chemical purity of these albites are evidence for their authigenic origin. The crystals are irregularly distributed in the sediment, highlighting the importance of the host rock composition for albite authigenesis. The crystals occur exclusively in limestones with a carbonate content higher than 80 wt-%. A diagenetic or hydrothermal origin of albite authigenesis is discussed for the studied region. Clay mineral transformation appears to be an important source of ions for albite formation.

Oxidation of methane at the CH 4/H 2O–(CO 2) transition zone in the external part of the Central Alps, Switzerland: Evidence from stable isotope investigations

With the aim of understanding the mechanisms that control the metamorphic transition from the CH 4– to the H 2O–(CO 2)-dominated fluid zone in the Helvetic domain of the Central Alps of Switzerland, fluid inclusions in quartz, illite “crystallinity” index, vitrinite reflectance, and the stable isotope compositions of vein and whole rock minerals and fluids trapped in quartz were investigated along four cross-sections. Increasing temperature during prograde metamorphism led to the formation of dry gas by hydrocarbon cracking in the CH 4-zone. Fluid immiscibility in the H 2O–CH 4–(CO 2)–NaCl system resulted in cogenetic CH 4- and H 2O-dominated fluid inclusions. In the CH 4-zone, fluids were trapped at temperatures ≤ 270 ± 5 °C. The end of the CH 4-zone is marked by a sudden increase of CO 2 content in the gas phase of fluid inclusions. At temperatures > 270 ± 5 °C, in the H 2O-zone, the total amount of volatiles within the fluid decreased below 1 mol% with no immiscibility. This resulted in total homogenization temperatures of H 2O–(CO 2–CH 4)–NaCl inclusions below 180 °C. Hydrogen isotope compositions of methane in fluid inclusion have δD values of less than − 100‰ in the CH 4-zone, typical for an origin through cracking of higher hydrocarbons, but where the methane has not equilibrated with the pore water. δD values of fluid inclusion water are around − 40‰, in isotopic equilibrium with phyllosilicates of the whole rocks. Within the CH 4 to H 2O–(CO 2) transition zone, δD(H 2O) values in fluid inclusions decrease to − 130‰, interpreted to reflect the contribution of deuterium depleted water from methane oxidation. In the H 2O-zone, δD(H 2O) values increase again towards an average of − 30‰, which is again consistent with isotopic equilibrium with host-rock phyllosilicates. δ 13C values of methane in fluid inclusions from the CH 4-zone are around − 27‰, in isotopic equilibrium with calcite in veins and whole rocks. The δ 13C(CH 4) values decrease to less than − 35‰ at the transition to the H 2O-zone and are no longer in equilibrium with the carbonates in the whole rocks. δ 13C values of CO 2 are variable but too low to be in equilibrium with the wall rock fluids, compatible with a contribution of CO 2 from closed system oxidation of methane. Differences in isotopic composition between host-rock and Alpine fissure carbonate are generally small, suggesting that the amount of CO 2 produced by oxidation of methane was small compared to the C-budget in the rocks and local pore fluids were buffered by the wall rocks during precipitation of calcite within the fissures.

Eocene-Pliocene time scale and stratigraphy of the Upper Rhine Graben (URG) and the Swiss Molasse Basin (SMB)

We present a general stratigraphic synthesis for the Upper Rhine Graben (URG) and the Swiss Molasse Basin (SMB) from Eocene to Pliocene times. The stratigraphic data were compiled both from literature and from research carried out by the authors during the past 6 years ; an index of the stratigraphically most important localitites is provided. We distinguish 14 geographical areas from the Helvetic domain in the South to the Hanau Basin in the North. For each geographical area, we give a synthesis of the biostratigraphy, lithofacies, and chronostratigraphic ranges. The relationships between this stratigraphic record and the global sea-level changes are generally disturbed by the geodynamic (e.g., subsidence) evolution of the basins. However, global sea-level changes probably affected the dynamic of transgression–regression in the URG (e.g., Middle Pechelbronn Beds and Serie Grise corresponding with sea-level rise between Ru1/Ru2 and Ru2/Ru3 sequences, respectively) as well as in the Molasse basin (regression of the UMM corresponding with the sea-level drop at the Ch1 sequence). The URGENT-project (Upper Rhine Graben evolution and neotectonics) provided an unique opportunity to carry out and present this synthesis. Discussions with scientists addressing sedimentology, tectonics, geophysics and geochemistry permitted the comparison of the sedimentary history and stratigraphy of the basin with processes controlling its geodynamic evolution. Data presented here back up the palaeogeographic reconstructions presented in a companion paper by the same authors (see Berger et al. in Int J Earth Sci 2005).

Erosion in the Western Alps (Dora Baltea basin): 1. Quantifying sediment provenance

Sourced in the M. Bianco, the highest mountain of the Alps, the Dora Baltea River cuts across the arc of the Western Alps, drains all major Alpine tectonic units from the Helvetic domain to the South alpine domain, and finally joins the Po River. Petrographic composition of clastic detritus carried by the Dora Baltea River and its tributaries faithfully mirrors the complex geology of the Western Alps. High-resolution modal analysis of Dora Baltea sands allowed us to distinguish seven end-member metamorphoclastic provenances. River sands from the M. Bianco External Massif are dominated by quartz, feldspars, and hornblende. Detritus from basement and cover rocks of the Briançonnais zone includes low-grade metasedimentary lithic fragments, epidote, chloritoid, and garnet. Sands from the Internal Penninic Massifs include quartz and subordinate feldspars, garnet, and epidote. Abundant metabasite and serpentine–schist lithic grains, along with epidote, actinolite, and few pyroxenes, characterize detritus from the calcschists and meta-ophiolites of the oceanic Piemonte zone. Sands from the Austroalpine zone include quartz, abundant metamorphic lithic fragments, garnet, and locally significant sillimanite (Dent Blanche kinzigites). The relative contribution of each end member to the Dora Baltea sediment flux was assessed with forward modelling of end-member mixing. Our estimates, calculated from the entire petrographic and mineralogical data set, indicate that the main sources of bed load in the Dora Baltea mountain basin are the granitoid to gneissic rocks exposed in the M. Bianco Massif and in the Austroalpine zone. In order to make allowance for the contribution of a particular bedrock constituent in a compound source area to the derivative sand, the original concept of sand generation index (SGI) was modified to assess the relative contribution of the seven end-member sources of detritus. The highest total-basin SGI indices are calculated for the M. Bianco Massif (SGI=16.4) and Austroalpine Domain (SGI=1.4). The M. Bianco Massif, which represents only 3% of the whole catchment, has the highest capacity of sand-sized bed-load production, owing to its granitoid lithology and extreme relief. This actualistic provenance study demonstrates that sediment composition can be markedly influenced by source rocks that occupy only a minor portion of the drainage area.

Stress field variations in the Swiss Alps and the northern Alpine foreland derived from inversion of fault plane solutions

This study is devoted to a systematic analysis of the state of stress of the central European Alps and northern Alpine foreland in Switzerland based on focal mechanisms of 138 earthquakes with magnitudes between 1 and 5. The most robust feature of the results is that the azimuth of the minimum compressive stress, S3, is generally well constrained for all data subsets and always lies in the NE quadrant. However, within this quadrant, the orientation of S3 changes systematically both along the structural strike of the Alpine chain and across it. The variation in stress along the mountain belt from NE to SW involves a progressive, counterclockwise rotation of S3 and is most clear in the foreland, where it amounts to 45°–50°. This pattern of rotation is compatible with the disturbance to the stress field expected from the indentation of the Adriatic Block into the central European Plate, possibly together with buoyancy forces arising from the strongly arcuate structure of the Moho to the immediate west of our study area. Across the Alps, the variation in azimuth of S3 is defined by a progressive, counterclockwise rotation of about 45° from the foreland in the north across the Helvetic domain to the Penninic nappes in the south and is accompanied by a change from a slight predominance of strike‐slip mechanisms in the foreland to a strong predominance of normal faulting in the high parts of the Alps. The observed rotation can be explained by the perturbation of the large‐scale regional stress by a local uniaxial deviatoric tension with a magnitude similar to that of the regional differential stress and with an orientation perpendicular to the strike of the Alpine belt. The tensile nature and orientation of this stress is consistent with the “spreading” stress expected from lateral density changes due to a crustal root beneath the Alps.

Emplacement at granulite facies conditions of the Sesia–Lanzo metagabbros: an early record of Permian rifting?

The evolution of the pre-Alpine Corio and Monastero metagabbros points to strong chemical and mineralogical similarities with that of other Permian gabbro bodies of the Alps, which are concentrated in the Southalpine and Austroalpine domains. The structural and metamorphic pre-Alpine evolution of these gabbros records a re-equilibration following the emplacement in the deep crust ( P=0.6–0.9 GPa and T=850±70 °C), exhumation through amphibolite facies conditions ( P=0.5–0.35 GPa and T=570–670 °C), followed by a greenschist facies imprint (0.25≤ P≤0.35 GPa and T<550 °C). This retrograde P– T evolution suggests that the exhumation occurred in a high thermal gradient regime, such as that induced by upwelling of an asthenospheric plume during continental rifting. This would be consistent with the crustal thinning known to have occurred in both the Southalpine and Austroalpine domains during Permian times. The gabbros and their country acid granulites are spatially associated with the serpentinised subcontinental mantle of the Lanzo Massif. This lithologic association and the metamorphic evolution is similar to that of the Fedoz gabbro (Austroalpine Domain of the Central Alps) and completely different from that observed in passive margins, where no remnants of the lower crust occur and the upper granitic crust directly overlies the serpentinized lherzolites. The location of Permian gabbro bodies in the Austroalpine and Southalpine domains and their absence in the Helvetic domain is evidence for asymmetric rifting.

Evolution of the western Swiss Molasse basin: structural relations with the Alps and the Jura belt

Abstract The tectonic evolution of the NW alpine front and foreland basin is reviewed in the light of new structural and chrono-stratigraphical data. Seismic-reflection profiles from the Jura fold thrust belt and Molasse basin, surface-geology and thrust-system considerations lead to a complete cross-section of the NW Alpine front including the Helvetic domain. Restoration of this section places individual Cenozoic formations in their approximate palaeogeographic position. ‘Geohistory’ plots are constructed for five profiles along a SE-NW transect. Thrust front, onlap and forebulge advanced at high rates of 10–20 km/Ma −1 at the onset of foreland basin formation in the late Eocene/ early Oligocene (40–30 Ma). In these early stages, the foreland basin is an underfilled flexural trough with about 100 km width, less than 600 m water depth at the deepest point and less than 200 m of total accumulated sediments. From 30 to 22 Ma, thrust front and ‘pinch-out’ migrate at a decreased rate of about 5 km/Ma −1 northwestward. The basin width remains constant at around 100 km; an increased total subsidence ( c. 2.7 km) is compensated by sedimentation. At around 22 Ma, the thrust front seems to come to a halt southeast of Lausanne, whereas a strong subsidence trend prevails. After the Serravallian ( c. 12 Ma) the Alpine thrust front jumps by about 100 km northwestward from a position southeast of Lausanne to the external Jura leading to thrust related uplift, deformation and concommitant erosion of the entire basin fill. No new flexural foreland basin in response to the modified thrust- and load-geometry has yet been developed. The present-day Molasse basin is only a small remnant of a much larger foreland basin in a very advanced stage of its evolution.

Cenomanian ammonites from the condensed deposits of the Helvetic Domain (western Alps, France and Switzerland)

The Cenomanian ammonites of the Helvetic Domain (French-Swiss Western Alps) are revised and illustrated for the first time since the middle of the Nineteenth Century. The faunas occur in remarkable condensed hemipelagic glauconite-rich biomicrites that also contain planktic foraminifera and small stromatolites. Thirty-three ammonite species referred to 15 genera are described; the assemblage extends from the lower part of the Lower CenomanianMantelliceras mantelliZone (Neostlingoceras carcitanenseSubzone) to the lower part of the Middle CenomanianAcanthoceras rhotomagenseZone (Turrilites costatusSubzone). The assemblage comprises 57 percent trachyostracous forms, 41 percent heteromorphs of the standard N.W. European sequence, and 2 percent leiostracous forms, which inhabited an external shelf biome. The biogeographic affinity of this fauna is essentially cosmopolitan (69 percent), but 31 percent of the specimens belong to the European Boreal biota, with no Tethyan taxa.

Les dépôts à phosphorites du Crétacé moyen de la plate-forme helvétique: Rôle de l'eustatisme et des criculations océaniques

During the middle Cretaceous (late Aptian — middle Cenomanian) silicoclastic shelf deposits overlained the shallow-water Urgonian limestones of the Helvetic domain. In the southern part of this area, both biostratigraphic analysis and sequence stratigraphy give evidence for four depositional sequences within the mid-Cretaceous deposits: S1 (Upper Aptian) starts with transgressive bioclastic deposits containing phosphatic allochems. These are overlained by argillaceous siltstones which pass upwards to calcarenaceous sands. This vertical evolution represents the build-up of a bioclastic and silicoclastic shelf with a shallowing upward trend. The sequence S1 is interrupted by an important unconformity which erodes locally down to the Urgonian substratum. S2 (late Upper Aptian — Middle Albian) and S3(Upper Albian pro parte) arerepresented by thin bioturbated phosphate-rich greensands. During the formation of these two sequences, sedimentation was partly controlled by the morphology of the basal unconformity. For instance, the sequence S2 begins with an allochem-rich sandstone which was deposited only in erosional scours. S4 (late Upper Albian — middle Cenomanian) iscomposed of highly condensed hemipelagic glauconiterich biomicrites which are usually capped by ministromatolites. This last mid-Cretaceous sequence is covered by the pelagic limestones of the Seewen formation (late Cenomanian — Santonian). Except for the sequence S1, all the mid-Cretaceous sequences show low sedimentation rates (5 cm.Y −1 to 1 m.Y −1) which cannot be explained only by eustatic control. The regional distribution of mid-Cretaceous condensed deposits compared to published palaeoceanographic reconstructions suggests that the mid-Cretaceous sedimentation on the North Tethyan Margin may have been controlled by a subsurface oceanic current.

Relation between the condensed Albian deposits of the Helvetic domain and the oceanic current-influenced continental margin of the northern Tethys

Other| September 01, 1988 Relation between the condensed Albian deposits of the Helvetic domain and the oceanic current-influenced continental margin of the northern Tethys M. Delamette M. Delamette Author Inst. Sci. Terre, Geneva 1211, Switzerland Search for other works by this author on: GSW Google Scholar Author and Article Information M. Delamette Author Inst. Sci. Terre, Geneva 1211, Switzerland Publisher: Société Géologique de France First Online: 03 Mar 2017 Online Issn: 1777-5817 Print Issn: 0037-9409 GeoRef, Copyright 2012, American Geosciences Institute. Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique, Vandoeuvre-les-Nancy, France Bulletin de la Société Géologique de France (1988) IV (5): 739–745. https://doi.org/10.2113/gssgfbull.IV.5.739 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. Delamette; Relation between the condensed Albian deposits of the Helvetic domain and the oceanic current-influenced continental margin of the northern Tethys. Bulletin de la Société Géologique de France 1988;; IV (5): 739–745. doi: https://doi.org/10.2113/gssgfbull.IV.5.739 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin de la Société Géologique de France Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Detrital Pumpellyite in the Peri-Alpine Molasse

ABSTRACT Pumpellyite, seldom recognized as a detrital mineral, is a widespread detrital constituent of the Oligocene-Miocene Alpine foreland molasse. Electron-probe microanalyses show the presence of three different types of pumpellyite in the sediments: with low-Mg and low-Fe content; high-Mg and low-Fe content, and with high-Mg and high-Fe content. Their overall chemistry covers a wide compositional field which can be related to different metamorphic conditions throughout the Alps. The earliest appearance of pumpellyite was detected in the lower part of the Upper Oligocene. It is associated with index minerals from high-pressure rocks of the Penninic domain, which indicates a common parentage. In the Burdigalian upper-marine molasse of western and central Switzerland, pumpellyite was commonly encountered in a variety of sedimentary facies. The provenance of the majority of the grains can be traced to the meta-ophiolites of the Upper Pre-Alpine nappes and to those of the Platta happy (southeast Switzerland), respectively. There is no appreciable evidence of pumpellyite being eroded from the low-grade metamorphic Taveyannaz type of sandstone of the Helvetic domain, suggesting that it was still under burial during the Burdigalian.