Devonian Basin Research Articles

Prograde epizonal clay mineral assemblages and retrograde alteration in tectonic basins controlled by major strike-slip zones (W Iberian Variscan chain)

AbstractWe have carried out optical microscopy, X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM) studies of phyllosilicates from black slates of very low to low-grade metamorphism. Such slates belong to a Middle/Late Devonian basin and an Early Carboniferous basin associated with the Porto–Tomar–Ferreira do Alentejo strike-slip shear zone (Ossa-Morena Zone, Portuguese Iberian Variscan Massif). These black slates are imbricated in an Upper Proterozoic substratum of higher metamorphic grade. Kübler Index values of white micas and mineral assemblages deduced from the XRD, SEM and TEM data (muscovite, chlorite and pyrophyllite) indicate high anchizonal and epizonal metamorphic conditions for slates from these basins. The b parameter and the phengitic contents of mica suggest the occurrence of low pressures (1–2 kbar) related to an extensional geotectonic setting. The dense fracture network shown by SEM images and the high density of crystal defects revealed by the TEM study in the eastern basin, adjacent to faults produced by shearing, suggest that their epizonal phyllosilicates were more affected during deformation than those belonging to the western basin, favouring the development of a retrograde association (siderite, kaolin group minerals and Al-smectite) on the epizonal paragenesis. Microcavities formed along phyllosilicate cleavage acted as channels for fluid transport favouring alteration under low-temperature conditions.

Sedimentary evolution of a Palaeozoic basin and ridge system: the Middle and Upper Devonian of the Ahnet and Mouydir (Algerian Sahara)

The Ahnet and Mouydir regions of southern Algeria are part of one of the world's largest, almost undeformed exposures of Palaeozoic rocks which exemplify a hitherto poorly known early Variscan development of a Devonian basin and ridge system. This area includes a series of intracratonic basins along the northern margin of the West African Craton which consists (from W to E) of the Reggane Basin, Azel Matti Ridge, Ahnet Basin, Foum Belrem Ridge and Mouydir Basin. The depositional and palaeogeographic interpretation is based on 71 sections in this region, which for the first time were biostratigraphically calibrated by means of conodonts, goniatites and brachiopods. The structural evolution during Devonian times was probably controlled by reactivation of ancient N–S- to NW–SE-running faults in the Precambrian basement, which caused differential subsidence and uplift of a previously largely unstructured siliciclastic shelf. A hiatus during Emsian times indicates widespread emergence during this interval. The entire area was flooded during the earliest Eifelian, when the first vestiges of the Azel Matti Ridge become evident by stratigraphic condensation. The palaeogeographic differentiation is most apparent during the Givetian, when a shoal with reduced carbonate sedimentation was established on the Azel Matti Ridge passing towards the west and east into basinal environments of the Reggane and Ahnet basins, respectively. The Foum Belrem Ridge is distinguished by increased subsidence during the early Givetian and by revived uplift during the late Givetian. In the Mouydir Basin further east, up to 1000 m of shales were deposited during the Givetian. The early Frasnian is marked by the ubiquitous sedimentation of black shales and bituminous styliolinites. These lithologies occur repeatedly already during the Middle Devonian and document intermittent anoxic conditions. The basin and ridge topography is levelled by the shallowing-up sequence of up to 1400 m thick upper Frasnian and Famennian shales which grade into a deltaic sequence of uppermost Famennian/Tournaisian sandstones. The up to now only vaguely discriminated lithostratigraphic formations of the Devonian have been biostratigraphically defined in suitable type sections.

Stratigraphic variation of complex impurities in platform limestones and possible significance of atmospheric dust: a study with emphasis on gamma-ray spectrometry and magnetic susceptibility outcrop logging (Eifelian-Frasnian, Moravia, Czech Republic)

Mineral dust and other constituents of Devonian atmospheric aerosols together with certain amounts of aquatic suspensions of riverine detrital origin, colloidal particle dispersions and seawater solutes were embedded in ~95–98% (or purer) limestones on a consistently subsiding isolated carbonate platform where they formed very complex impurity systems. Very low Th/U values, relative abundance of Fe but a slight excess of K typically characterize these ultrafine impurities which are mineralogically dominated by smectite–illite and mica (sericite) together with goethite. In vertical section, these impurities are arranged like incremental series of light and dense bands. The combined method of natural gamma-ray spectrometric and magnetic susceptibility measurements (GRS–MS) together with background of major and trace element analyses provided first systematic insights into fine-scale variability of these impurities in a pile of pure carbonate platform beds, mostly in inner platform Amphipora limestone facies. These carbonates were deposited close to sea level but far from river mouths, and they represented a certain sort of a relatively “independent” medium that was primarily sensitive to climatically (and by rare events) controlled input of atmospheric dust. A remarkable similarity of MS stratigraphic patterns that reflect the quantity and quality of embedded impurities in very distant and paleogeographically separated Devonian basins might be seen as supporting this viewpoint. A long composite stratigraphic section of the Moravian Karst ranges from mid-Eifelian to end-Frasnian levels and yields a number of time characterizing GRS–MS variations that can potentially serve as templates for high-resolution stratigraphic correlations over long distances.

Geometry and kinematics of the Boulonnais fold-andthrust belt (N France): implications for the dynamics of the Northern Variscan thrust front

Recent structural data obtained within the Boulonnais thrust belt along the Northern Variscan thrust front provide a new insight into the deformation history of the Variscan belt. The zone under study recorded the latest increments of inversion of the Rheno-hercynian basin in Late Westphalian—Early Stephanian times (ca 305 Ma). Thrusting within the southern margin of the “Old Red Sandstones Continent” propagated primarily in sequence up to the Brabant massif which acts as a significant buttress. This buttress effect, which we suggest to be due to the southward activation of deep-seated inherited Caledonian thrusts, resulted in the out-of-sequence dislocation of the thrust front. The associated localized thrust stack induced a sharp flexure of the underthrust block moulding the border of the molassic coal-bearing foreland basin. During this process, strain markers at the thrust front document an evolution of thrusting from a dominantly NNE to NE direction whereas more internal parts of the belt undergo dextral transcurrent deformations (e.g. the Bray fault zone). Such a partition of the deformation argues for a general transpressional setting of the SE England-Boulonnais orogenic belt acting as a relay zone between more frontal parts of the Variscan orogen (Ardenno-Rhenish and SW England segments). This mechanism highlights the segmented nature of the Variscan thrust front and points out the importance of the Devonian basin pre-structuration in the present-day arcuate shape of the Variscan belt. Late Variscan reactivation of the thrust network occured within a transtensional setting associated to a dominant NNW-SSE directed extension. This event induced the formation of local intramontane basins and contributed to the thinning and thermal restoration of the thickened external Variscan crust in Upper Stephanian—Middle Permian times.

Reducing Gas-Production Decline Through Dewatering: A Case History From the Naturally Fractured Aguarague Field, Salta, Argentina

Summary The Aguarague field is located in the Devonian basin, northwest of the province of Salta, Argentina. Gas production is obtained from clastics of the Huamampampa formation through an intense network of natural macro- and microfractures. The reservoir is in a north/south-trending symmetrical anticline. Five high-deliverability gas wells watered out between 1991 and 1994; as a result, the field gas production decreased substantially. This triggered a study in 1995 to try to find a means of extending the life of the reservoir. The study indicated that without any modifications, gas production could be expected to continue, at most, until 1999. This paper describes how a detailed fracture characterization led to a dewatering project using gas lift, which significantly reduced the gas decline of the reservoir. The project also brought back to gas production some of the wells that had previously watered out. The 1995 forecast is compared with the actual production performance to date.

Fluids and hydrothermal alteration assemblages in a Devonian gold-bearing hot-spring system, Rhynie, Scotland

ABSTRACTHydrothermal alteration at Rhynie, Aberdeenshire, Scotland, is concentrated along a fault zone, which juxtaposes surface deposits and the mineralised feeder zone to the Rhynie hotspring system. Mineralisation consists of breccias and veins filled with quartz, chert, calcite, K-feldspar and pyrite. Associated pervasive alteration comprises a high-temperature K-feldsparquartz-illite facies (formed at 250–350°C), a medium-temperature mixed layered illite/smectitequartz-K-feldspar-chlorite-calcite facies (formed at 150–200°C) and a low-temperature mixed layered illite/smectite-chlorite-calcite facies (formed at 100 to +150°C). The fluids responsible for mineralisation were mainly moderate- to high-temperature (Th =91–360°C), low-salinity (<0·2 to 2·9 wt.% NaCl eq.) H2O-NaCl-heated meteoric fluids comparable to modern and ancient hot-spring systems. The migration of these fluids was mainly restricted to a major fault zone bounding the Devonian basin. Fluids responsible for mineralisation, alteration and cementation elsewhere in the basin were low-temperature (Th 57 to 161°C), low- to high-salinity (<0·2 to 18 wt.% NaCl eq.) H2O-NaCl fluids, which resemble basinal brines.

Changing Silurian–Devonian relative plate motion in the Caledonides: sinistral transpression to sinistral transtension

The late Silurian to mid- or late Devonian interval in the Caledonides was a period dominated, sequentially, by sinistral transpression, strike-slip and transtension during the development of mainly non-marine ‘red-bed’ basins following the Ludlow–PrÍdolÍ transition from marine to terrestrial sedimentation. The tectonic event that led to and generated the sinistral Devonian basins was the highly oblique sinistral closure of the Iapetus Ocean between Laurentia and Baltica and between Laurentia and Avalonia. We examine the diachronous closure of Iapetus, the contrasting tectonic modes arising from that closure, and the nature and origin of subsequent Devonian deformation north and south of the Iapetus Suture in the context of progressively changing, sinistrally dominated relative plate motion between Laurentia and Avalonia–Baltica. We suggest that, from about 435 to 395 Ma, there was about 1200 km of sinistral strike-slip relative motion between Laurentia and Baltica. Our lower and upper estimates of Silurian–Devonian relative plate motion rates of 30 mm a −1 and 67 mm a −1 based upon geological data, are similar to present rates.

Dating the exhumation of UHP rocks and associated crustal melting in the Norwegian Caledonides

U–Pb and Rb–Sr dating was undertaken in combination with P–T estimates to (1) constrain the time of ultrahigh-pressure (UHP) eclogite formation in the Stadlandet UHP province of Norway, (2) date later crustal melting–migmatization of the eclogite country gneisses, and (3) temporally trace post-migmatite cooling and retrogression under amphibolite facies metamorphic conditions. In contrast to earlier U–Pb studies which used accessory minerals from the gneisses, we focused on the direct dating of minerals defining the HP assemblage. For the eclogite, rutile and omphacite fractions were analyzed for U–Pb, and from an adjacent migmatite leucosome titanites and K-feldspar. For Rb–Sr dating, phengite was measured for the eclogite, and biotite for two leucosome layers of the migmatite–eclogite complex. A U–Pb age of 389±7 (2σ) Ma is obtained if the full set of 12 rutile and five omphacite analyses is regressed (MSWD: 16), and 389±2 Ma for those nine data which strictly satisfy isochron conditions (MSWD: 0.78). The 389-Ma age is interpreted to date equilibration and freezing of the eclogite paragenesis at maximum temperatures of 770 °C, reached during decompression to 1.8 GPa. Decompression from 2.8 to 1.8 GPa occurred in the partial melting domain of granitic crust, with the migmatites being dated at 375±6 Ma by titanite and K-feldspar from an eclogite-adjacent granitic leucosome. This titanite age also shows that the U–Pb chronometer in rutile is very robust to high temperatures—it remained a closed system for at least 14 million years, at temperatures in excess to 650 °C. After decompression and migmatization, exhumation is accompanied by rapid cooling to reach the 300 °C isograde by 357± 9 Ma, determined by a biotite isochron for a leucosome in a slightly shallower structural level. In considering that the time of maximum pressure is bracketed by early zircon crystallization during subduction and later omphacite–rutile equilibration in the eclogites, an exhumation rate of 5 mm/year is deduced for initial exhumation, occurring between 394 and 389 Ma. For subsequent cooling from 770 to 600 °C, we obtain a rate of 2.3±1.3 mm/year. First stages of exhumation most likely occurred under an overall compressional regime, whereas Devonian basin formation is associated to detachment movements during 389–375 Ma exhumation. This period of extension is followed by a much younger, decoupled thermal phase at 327±5 Ma, occurring under static conditions within very restricted zones, most likely in association with the circulation of fluid phases along old discontinuities. Initial isotopic signatures of Sr and Pb substantiate Paleo- to Meso-Proterozoic crust formation times of the Stadlandet UHP province precursor lithologies.

Crustal structure of northwestern Svalbard and the adjacent Yermak Plateau: evidence for Oligocene detachment tectonics and non-volcanic breakup

SUMMARY In 1999 new seismic refraction data were collected off northwestern Svalbard and the adjacent Yermak Plateau. A 260 km long profile provides detailed velocity information for the northeastern edge of the Eurasian Continent and the adjacent Yermak Plateau. North of Forlandsundet Graben the depth of the Moho varies between 23 and 28 km, and remains at this depth to the northern edge of the profile at 81°N. The crustal lithology off western Svalbard can be related to the basement province west of the Raudfjorden Fault Zone. Off the northern shoreline of Svalbard the structure of the Tertiary Danskoya Basin is mapped. Below this, a Late Silurian/Early Devonian basin, with seismic velocities between 5.1 and 5.8 km s−1 and a thickness of up to 8 km is present. A Palaeozoic sequence of up to 6 km thickness is expected below the Tertiary cover north of the Danskoya Basin. An earlier suggestion, that Oligocene rift processes affected the southern Yermak Plateau, is confirmed. A detachment structure is situated below the Palaeozoic Basin below Danskoya Basin, which is probably a consequence of simple shear tectonics. The middle crust exhibits low seismic velocities above the detachment fault. The lowermost crust beneath is slightly contaminated by mantle-derived melts, which is deduced by the slightly elevated velocities of the lowermost crust. These melts can be attributed to decompressive melting caused by modest uplift of the Moho during stretching. The velocity–depth model provides no evidence for large magmatic activity, which implies a non-volcanic rifted margin history. This leads to the assumption that the proposed Yermak Hotspot during the breakup of Svalbard from northern Greenland did not exist.

Partitioning pre-, syn- and post-Variscan deformation in the Holy Cross Mountains, eastern Variscan foreland

Abstract In this study we demonstrate how a combined structural, sedimentological and palaeomagnetic approach provides a new perspective on the tectonic evolution of the Holy Cross Mountains. In the field, we performed a structural and sedimentological analysis of Palaeozoic rocks. Our analysis was complemented by a palaeomagnetic study and by the restoration of balanced cross sections in Palaeozoic and Mesozoic rocks. Different steps of deformation were restored for a c. 350 Ma period. (1) The extensional tectonics of the Devonian basin was unravelled: the resulting normal fault system constituted the fundamental structural control for the later Variscan tectonic inversion and Alpine deformations. (2) The style of Variscan folding is characterized and quantified by way of a cross section across the Holy Cross Mountains. (3) The role of the reactivation of Variscan faults during the Permo-Triassic initiation of the Polish Basin was examined. (4) The localized Alpine compressive deformation was quantified and shown to contribute only to a minor degree to the present-day state of deformation in the Holy Cross Mountains. The Holy Cross Fault zone is the product of the interplay of changing transtensional and transpressional settings during the Variscan diastrophic cycle, with the final effect of the Variscan evolution being the flower-like structure of the Holy Cross Fault zone.

Multistage extensional evolution of the central East Greenland Caledonides

Recent field investigations in the central East Greenland Caledonides (72°–74°N) resulted in the identification of an orogen‐scale extensional fault system called the Fjord Region Detachment (FRD). Previous geochronologic constraints on this deformation indicated that the FRD was active circa 430–425 Ma, a time when the Baltica‐Laurentia collision was thought to be occurring, and continued to be active for up to 80 million years. We present new40Ar/39Ar thermochronologic data from an E‐W transect that cuts across two splays of the FRD. Our data demonstrate that at least two distinct episodes of faulting were responsible for extension in the East Greenland Caledonides: an earlier phase (circa 425–423 Ma) that was synorogenic and penetrated to middle‐crustal levels, followed by a post‐Caledonian phase of reactivation (∼414 to 380 Ma) that affected even deeper structural levels. Furthermore, we present in situ UV laser40Ar/39Ar data for pseudotachylite collected along the deepest splay of the FRD that indicate this fault was active again as recently as ∼357 Ma (coeval with Devonian basin formation). Altogether, our data suggest that rather than being active continuously for 80 million years, the FRD consisted of multiple splays that were active for shorter intervals over discrete time periods separated by as much as 60 million years. Finally, our data provide evidence that young extensional deformation associated with postorogenic collapse in East Greenland was not restricted to the formation of sedimentary basins in the far eastern part of the orogen, but also resulted in deformation of the Archean‐Paleozoic crystalline basement.

Evolution of the southeastern Lachlan Fold Belt in Victoria

The Benambra Terrane of southeastern Australia is the eastern, allochthonous portion of the Lachlan Fold Belt with a distinctive Early Silurian to Early Devonian history. Its magmatic, metamorphic, structural, tectonic and stratigraphic histories are different from the adjacent, autochthonous Whitelaw Terrane and record prolonged orogen‐parallel dextral displacement. Unlike the Whitelaw Terrane, parts of the proto‐Benambra Terrane were affected by extensive Early Silurian plutonism associated with high T/low P metamorphism. The orogen‐parallel movement (north‐south) is in addition to a stronger component of east‐west contraction. Three main orogenic pulses deformed the Victorian portion of the terrane. The earliest, the Benambran Orogeny, was the major cratonisation event in the Lachlan Fold Belt and caused amalgamation of the components that comprise the Benambra Terrane. It produced faults, tight folding and strong cleavage with both east‐west and north‐south components of compression. The Bindian (= Bowning) Orogeny, not seen in the Whitelaw Terrane, was the main period of southward tectonic transport in the Benambra Terrane. It was characterised by the development of large strike‐slip faults that controlled the distribution of second‐generation cleavage, acted as conduits for syntectonic granites and controlled the deformation of Upper Silurian sequences. Strike‐slip and thrust faults form complex linked systems that show kinematic indicators consistent with overall southward tectonic transport. A large transform fault is inferred to have accommodated approximately 600 km of dextral strike‐slip displacement between the Whitelaw and Benambra Terranes. The Benambran and Bindian Orogenies were each followed by periods of extension during which small to large basins formed and were filled by thick sequences of volcanics and sediments, partly or wholly marine. Some of the extension appears to have occurred along pre‐existing fractures. Silurian basins were inverted during the Bindian Orogeny and Early Devonian basins by the Tabberabberan Orogeny. In the Melbourne Zone, just west of the Benambra Terrane, sedimentation patterns in this interval, in particular the complete absence of material derived from the deforming Benambra Terrane, indicate that the two terranes were not juxtaposed until just before the Tabberabberan Orogeny. This orogeny marked the end of orogen‐parallel movement and brought about the amalgamation of the Whitelaw and Benambra Terranes along the Governor Fault. Upper Devonian continental sediments and volcanics form a cover sequence to the terranes and their structural zones and show that no significant rejuvenation of older structures occurred after the Middle Devonian.

Geological setting of the Early Devonian Rhynie cherts, Aberdeenshire, Scotland: an early terrestrial hot spring system

Abstract: The drilling of nine cored boreholes, including a deep hole (233 m) to a gabbroic basement, in the vicinity of the Rhynie chert locality has resulted in a major revision of the structure and stratigraphy of the area. The main new structural element recognized is a low angle extensional fault system which defines the western edge of a half graben containing the Early Devonian succession. The fault system was the main conduit for the fluids that fed the hot springs, and a heat source to the southeast of Rhynie is indicated. Rapid and unexpected lateral and vertical variations in lithology, together with new lithological units, have been identified. The latter include a thick unit of intensely altered lapilli tuffs which are unique to the Rhynie basin and to other Devonian basins in NE Scotland. Analysis of these lithologies in conjunction with the new structural model allows the succession at Rhynie to be correlated with the succession in the rest of the basin and a model of basin evolution and hot spring development to be constructed. Siliceous sinters were deposited by multiple episodes of hot spring activity and are variably interbedded with shales, sandstones and minor tuffs in a unit about 35 m thick. Sinter deposition was ultimately controlled by repeated subsidence along the basin margin. Geothermal activity was probably widespread in Northern Britain in the Early Devonian but the surface deposits are unlikely to be preserved. The Rhynie deposit has survived due to a fortuitous combination of circumstances.

The Cambrian to mid Devonian basin development and deformation history of Eastern Avalonia, east of the Midlands Microcraton: new data and a review

Abstract A review is given of recently published and new data on Avalonia east of the Midlands Microcraton. The three megasequences from Cambrian to mid Devonian described in Wales and Welsh Borderland are also present east of the Midlands Microcraton (Brabant Massif, Condroz, Ardennes, Remscheid and Ebbe inliers, Krefeld high). The three megasequences are caused by a tectonic driving mechanism and are explained by three different geodynamic contexts: an earlier phase with extensional basins or rifting and rather thick sequences, when Avalonia was still attached to Gondwana; a second phase with a shelf basin with moderately thin sequences when Avalonia was a separate continent and a later phase with a shelf or foreland basin development and thick sequences. Deformation of the megasequences 1 and 2 or 1 to 3 varies between areas. In Wales and the Lake District the Acadian phase is long-lived and active from early to mid Devonian. In the Ardennes inliers a deformation is active between the late Ordovician and the Silurian (Ardennian Phase), with a similar intensity as the core of the Brabant Massif, when present erosion levels are compared. The Brabant Massif is partly deformed by the long-lived Brabantian Phase from late Silurian till early mid Devonian. Both the Ardennes inliers and the Brabant Massif are not classic orogenic belts, only slate belts where no more than the epizone is reached at present erosion levels. Areas supposedly close to the microcraton or basement are nearly undeformed (SW Brabant Massif and central Condroz). A model of anticlockwise rotation of Avalonia of about 55° from Caradoc to Emsian is proposed to explain the deposition setting of megasequence 3 and the subsequent Acadian and Brabantian deformation. Immediately after the Avalonian microcontinent touched Baltica in Caradoc times it created a short-lived subduction magmatic event from The Wash to the Brabant Massif and soon after the magmatism ended a foreland basin developed. Possibly during and after that development a long-lived and slow compressional event occurred, leading to the deformation of the Anglo-Brabant Deformation Belt. In the early Devonian, contemporaneous with the shortening of the Anglo-Brabant Deformation Belt, extension occurred in the Rheno-Hercynian Zone, possibly caused by the same slow rotation of Avalonia. More evidence emerges that Avalonia east of the Midlands Microcraton comprises not one but probably two terranes: the remnant of the palaeocontinent Avalonia, and what is called the palaeocontinent Far Eastern Avalonia; the latter is only occasionally observed in the few deep boreholes into the Heligoland-Pomerania Deformation Belt, in southern Denmark, NE Germany and NW Poland, with scant available indirect data in between indicating only Proterozoic basement and no Caledonian deformation. For Far Eastern Avalonia a similar palaeogeographical history is postulated as Avalonia, with rifting from Gondwana in Arenig or earlier times, collision with Baltica before the mid-Ashgill and deformation between the late Ordovician and latest Silurian. The Avalonia concept might need to be expanded to an ‘Avalonian Terrane Assemblage’ with cratonic cores and small short-lived oceans as in the Armorican Terrane Assemblage.

Comprehensive constraint on the tectono-sedimentary setting of Late Paleozoic turbidites of the Kamuste area, eastern Junggar, Xinjiang

Petrography and geochemistry, combined with sedimentation analyses allow for a thorough evaluation of the tectono-sedimentary setting of late Paleozoic turbidites of the Kamuste area, eastern Junggar. Sandstones of the Alabiye1) Formation are composed mostly of volcanic and sedimentary detritus with lesser amounts of plagioclase and quartz. They were derived from an undissected magmatic-arc provenance. The geochemistry of sandstone-mudrock suites indicates a fesic-intermediate igneous provenance, and constrains the Alabiye Formation to have derived from a differentiated oceanic-continental margin island-arc tectonic setting. Likewise, geochemistry and sandstone petrography of the Kamuste Formation reflect a mixed provenance signature dominated by magmatic arc, basement uplift, and subduction-complex sources of a differentiated continental-island arc. Sedimentation analysis indicates that the Alabiye and Kamuste formations are two sets of turbidite sequences deposited on a submarine slope and a submarine fan and basin plain respectively. In conclusion, submarine slope turbidite deposition of the Alabiye Formation records the main sedimentary response to the development of early Devonian back-arc basins of the northern Junggar tectonic belt. Submarine fan and basin plain turbidite and background hemipelagic deposition of the Kamuste Formation record the main sedimentary response to the late Early Carboniferous development of an inter-arc relict ocean basin of the eastern Junggar composite terrane.

The nature of the Variscan basement in southeast England: evidence from integrated potential field modelling

The Variscides of southeast England are buried beneath post-Carboniferous cover. Interpretations of the basement are based mainly on deep boreholes. Geophysical signatures from the basement are contained within the regional gravity and magnetic data. A gravity stripping exercise has been undertaken to remove the gravitational effect of the post-Variscan cover to generate a residual gravity map. This map is interpreted along with integrated potential field modelling along four long interconnected profiles and compared with a revised pre-Permian subcrop map. The magnetic evidence suggests that Precambrian magnetic basement of the Midlands Microcraton has been buried southwards by north-vergent Variscan thusting over the foreland. North of the Variscan Front, short-wavelength anomalies superimposed upon this deep Precambrian source are due to shallower Silurian and Carboniferous volcanic rocks. Many residual gravity lows within the Rhenohercynian zone may be related to thick, low-density Devonian basins. In the English Channel a change in geophysical signature occurs north of the Portland–Wight Fault, coinciding with phyllites in the basement. Models are presented in which the English Channel magnetic anomalies originate within the pre-Permian basement. Comparisons with anomalies in the Southwestern Approaches suggest that the Portland–Wight Thrust is a terrane boundary, possibly a subduction-related suture, implying southerly directed Variscan subduction.

Deformation and metamorphism of Devonian rocks in the outer Solund area, western Norway: implications for models of Devonian deformation

Many modern accounts of the Devonian rocks of western Norway, which emphasise the extensional processes by which the basins were formed, tend to ignore or gloss over the deformation and metamorphism which these rocks have suffered during contractional deformation. The present account describes extensive ductile folding, penetrative cleavage formation and low-grade regional metamorphism developed in the Devonian strata of the outer Solund region. Although the deformation in this area is more intense than in other Devonian basins, it belongs to the same movement plan. It will also be shown that the Devonian rocks of the Scandinavian Caledonides apparently share a common tectonothermal development. This regional contractional event, conceptually, may relate either to a Mid-Late Devonian arc-continent collision, or to a transpressional strain regime.

The Devonian of western Canada -- aspects of a petroleum system: Introduction

This special volume of the Bulletin of Canadian Petroleum Geology has two objectives. Firstly, it gathers together contributions which represent recent advances in our understanding of the Devonian of western Canada. Secondly, and perhaps more importantly from the perspective of the petroleum geologist, it summarizes our current knowledge of different aspects of the western Canadian Devonian as a petroleum system. As such, it seeks to augment previous contributions to this discipline, including the most recent Symposium on the Devonian System (McMillan et al., 1989). As editors, we sought to collect as broad a range of papers on western Canadian petroleum geology as possible. While not exhaustive in scope, we believe this volume covers enough ground to be of interest to earth scientists and petroleum geoscientists in particular. As Canadian geoscientists, we can take pride in the amount, and quality, of work that has been done on our Devonian petroleum system. The availability of high quality data, both in the subsurface and outcrop, makes these strata an important laboratory for scientific concepts and, given the region’s status as a world-class hydrocarbon basin, serves as a data-rich analogue for explorationists pursuing carbonate plays in other basins. The contribution by Root adds to our knowledge of the tectonic history of our Devonian basin. It provides data on basin evolution which, until now, has mostly been the subject of speculation. It will also allow us to see the role of Antler tectonism north of the 49th parallel in a different light. Potma et al. describe in detail the sequence stratigraphic model for the late Givetian-Frasnian of the Alberta Basin (Fig. 1⇓) they first presented in 1992 (e.g. Wong et al., 1992). Third-order depositional sequences are correlated across the basin, from the subsurface to the mountain outcrop …

The middle Devonian basins of western Norway: sedimentary response to large-scale transtensional tectonics?

The Devonian basins of western Norway were formed during late- to post-orogenic extension of overthickened Caledonian crust. The basins are situated in the hanging wall of the extensional Nordfjord–Sogn Detachment Zone (NSDZ) and display extensional half-graben geometries in sections parallel to the local direction of principal extension. Based on overall facies configurations, paleocurrent patterns and intrabasinal structures, we infer an anticlockwise rotation of the syndepositional extension direction from NW–SE in the south (Solund basin) to WSW–ENE in the north (Hornelen basin). The axes of folds that are roughly parallel to the local extension direction are rotated correspondingly. The Kvamshesten basin is located between the Solund and Hornelen basins. Sedimentological and structural data show evidence of an early, southeastwards tilt direction followed by a more eastwards tilt and associated E–W flowing paleodrainage. Correspondingly, NW–SE trending folds and reverse faults are superposed by E–W trending ones at low to intermediate stratigraphic levels. The variations in apparent tilt direction for the basins together with variations in intrabasinal structure is interpreted to reflect an anticlockwise rotation of the regional syndepositional strain field. The above observations and inferences indicate that the Devonian basins in western Norway formed in a strain field dominated by regional transtension, accommodated by extension along the NSDZ and sinistral strike–slip along orogen-parallel shear zones and faults to the north of the basins; alternatively, NW-directed extension preceded the introduction of a sinistral strike–slip component. The models are in accordance with recent work carried out in the footwall of the NSDZ and illustrates the tectono-sedimentary response to a complex interplay between extension and strike–slip that appears to have been fundamental in the late-stage disintegration of the Caledonian orogen.

Basin evolution in western Newfoundland: New insights from hydrocarbon exploration

The Humber zone is the most external zone of the Appalachian orogen in western Newfoundland. It records multiphase deformation of the Cambrian-Ordovician passive margin and of the Ordovician to Devonian foreland basins by the Taconian, Salinian, and Acadian orogenic events. The recent phase of exploration drilling has provided new evidence for structural, stratigraphic, reservoir, and source rock maturation models of western Newfoundland. The first well, Port au Port 1, supported the hypothesis that the Round Head thrust had an earlier extensional history prior to the Acadian compressional inversion that created the present-day structural high of the Port au Port Peninsula. The well tested a small anticline formed in a footwall shortcut fault of the Round Head thrust. The second well, Long Point M-16, was drilled at the northern tip of Long Point to test a triangle zone identified by previous workers. This well demonstrates that the frontal monocline at the western edge of the triangle zone is elevated by a stack of imbricate thrusts composed of rocks of the Taconian allochthon and compressional basement-involved faults that have uplifted the Cambrian-Ordovician carbonate platform. The structural model developed in the Port au Port area with the aid of these wells has been extended throughout the Humber zone in western Newfoundland. Changes in structural style illustrated by regional cross sections suggest that prospective trap geometries are only developed in the southern and central parts of the region. The reservoir model proposed invokes exposure and karsting of the footwalls of extensional faults formed as the carbonate platform collapsed during a Middle Ordovician hiatus, the St. George unconformity. Structural relief became more pronounced as extensional collapse continued through the Middle Ordovician. (Begin page 394) These structurally high fault footwalls became the foci for dolomitizing and mineralizing fluids that used major faults as fluid conduits during the Devonian. Fluids deposited sulphide ores and created zebra and sparry dolomite and some sucrosic hydrothermal dolomites in the St. George Group and the Table Point Formation. The reservoir model, maturity and source rock data, and the structural models have been combined with seismic and onshore surface geology. This enables the prospectivity of the western Newfoundland Cambrian-Ordovician play trend to be evaluated for further exploration.