Famennian Time Research Articles

Exhumation of high-grade rocks in the Saxo-Thuringian Belt: geological constraints and geodynamic concepts

Abstract The Saxo-Thuringian Belt on the northern flank of the European Variscides resulted from SE-ward subduction of a Cambro-Ordovician rift basin under the Teplá-Barrandian (Bohemian) margin. It contains ultra-high-pressure (UHP) metamorphic rocks, which are now exposed in different tectonic settings, and were exhumed in different modes, under different thermal regimes. Eclogites of the upper allochthon (tectonic klippen of Münchberg, Wildenfels and Frankenberg) originated from early Devonian ( c. 400 Ma) subduction. Cooling ages in the klippen, combined with clast spectra and ages of detrital minerals in the foreland flysch record exhumation in Famennian time. The high-pressure (HP) rocks rose in a narrow corridor along the suture zone, were retrogressed under amphibolite facies conditions, and rapidly recycled into the foreland flysch. Rocks of the lower allochthon are exposed in dome structures emerging from under the relative autochthon. The HP Saxonian Granulites were formed at c. 20 kbar/1050 °C, and the Erzgebirge contains diamond-bearing quartzo-feldspathic rocks, associated with eclogites. Peak temperatures of the lower allochthon were attained at c. 340 Ma. Immediately afterwards, these rocks were emplaced under the floor of the relative autochthon, which, at this time, was a foreland basin receiving clastic sediments from the orogen adjacent to the SE. Emplacement of the lower allochthon is most clearly documented in the Saxonian Granulite Dome, in which HP granulites are juxtaposed against low-pressure-high-temperature (LP-HT) rocks of the hanging wall. The interface is a zone of HT shear, which cuts out ⩾ 60 km of crustal thickness. In more internal parts of the belt (Erzgebirge), the newly emplaced lower allochthon was subsequently reworked by thrusting and polyphase refolding. Emplacement of the hot, low-viscosity lower allochthon was probably driven by buoyancy and the hydraulic gradient between the crustal root to the SE and the lower crust of the foreland. Unlike the earlier HP rocks exposed in the upper allochthon, the 340 Ma HP rocks of the lower allochthon were thermally softened, and, instead of piercing their cover, intruded into the foreland. Therefore, the 340 Ma rocks do not appear in the clastic record of the flysch. The two contrasting mechanisms of exhumation observed in the upper and lower allochthon are apparently due to different thermal regimes.

Flexural cantilever models of extensional subsidence in the Munster Basin (SW Ireland) and Old Red Sandstone fluvial dispersal systems

Abstract Flexural cantilever (2D) computer modelling of the palinspastically restored Mid-Late Devonian Munster Basin has been used to appraise quantitatively extensional subsidence and Old Red Sandstone (ORS) stratigraphic geometries. One-dimensional decompaction and (Airy isostatic) backstripping were carried out to constrain syn-rift forward models; these specify the Late Palaeozoic rifting history of the region. Forward modelling showed that listric faults and detachments fail to reproduce restored ORS sediment geometries, but instead indicated that multiple planar, upper-crustal faults are necessary to achieve the correct order of syn-rift subsidence across the basin. Modelled (non-unique) sections transverse to the basin bounding fault (Dingle Bay-Galtree Fault Zone) replicated ORS geometries with cumulative extensions of 27 km in the east (stretching factor β = 1.3) and 59 km in the west (β = 1.48), with effective elastic thicknesses of 7 and 8 km, respectively, starting from a 40 km thick post-Acadian crust. Resultant peak heat-flow anomalies predict well the location of known syn-rift volcano-magmatic centres. Modelling indicated that significant offshore faults are required to achieve subsidence in the west Cork region, implying that the basin continues offshore. Observed ORS (<0.85 km thick) sections on the regional footwall, considered to be the result of thermal (post-rift) subsidence, are not accounted for by modelling, whereas c . 1 km of post-rift ORS is modelled over 5 Ma in the central-southern regions of the basin. These sections buried the principal rift faults during late Famennian time. The ORS of the Munster Basin is dominated by two large-scale transverse fluvial dispersal systems that were largely insensitive to deflection by any extension faults that propagated in the syn-rift fill. A third major system entered in the SW, demarcated by antithetic extension faults south of the depocentre.

Extension of lithofacies and conodont biofacies models of Late Devonian to Early Carboniferous carbonate ramp and black shale systems, southern Canadian Rocky Mountains

Uppermost Devonian and Lower Mississippian strata in the Rocky Mountains of southwestern Canada and northwestern Montana record widespread oceanographic changes during middle to late Paleozoic time associated with the termination of a carbonate ramp system, the onset of a deep-water, low-oxygen event and possible marginal tectonism, and the later reestablishment of a carbonate ramp. Integrated lithofacies and conodont biofacies developed previously for these strata between the Bow Valley and the international border have been extended northward to the Athabasca region of the Alberta Rocky Mountains. During early-middle Famennian time, the southern Canadian Rocky Mountains region was the site of a westward-deepening and westward-thickening carbonate ramp system (Palliser Formation). By late Famennian time carbonate ramp deposition ended and was followed by widespread deposition of organic-rich, low-oxygen facies in shelf to basinal environments (Exshaw Formation and correlative units). The overlying Banff Formation consists of anaerobic to marginally aerobic, starved-basin to deep-ramp lithofacies succeeded by shallower water carbonates; this sequence records basinward (westward) progradation of the Banff ramp in middle to late Tournaisian time. Distinct conodont biofacies representative of shallow-ramp to deep-basin settings that were previously recognized in the southernmost Canadian Rocky Mountains and Montana have also been identified to the north between the North Saskatchewan and Athabasca valleys. Upper Palliser carbonates contain low-diversity conodont faunas of indigenous to transported palmatolepid-, polygnathid-, and apatognathid-dominated assemblages. Exshaw deposits contain indigenous and reworked palmatolepid- and bispathodid-dominated assemblages and reworked or transported polygnathids. Lower Banff biofacies include transported and indigenous assemblages of siphonodellids, polygnathids, and pseudopolygnathids representative of the deep-middle Banff ramp. Polygnathid-hindeodid biofacies of shallower middle-ramp environments occur higher in the Banff Formation in the North Saskatchewan and Athabasca valleys.

Radiometric evidence for Middle Devonian inversion of the Hill End Trough, northeast Lachlan Fold Belt

The publication of a new geological time‐scale by the Australian Geological Survey Organisation and radiometric dates from the Hill End goldfield have prompted the re‐examination of the timing of deformation of the Hill End Trough to determine whether it occurred in Middle Devonian or Early Carboniferous time. Palaeontological evidence from the western trough margin and the Capertee High dates the end of deposition in the trough as late Emsian or early Eifelian (385–382 Ma). After a mid‐Devonian hiatus of at least 15 million years, paralic sedimentation commenced on the Molong and Capertee Highs in late Frasnian or early Famennian time (367–363 Ma). No Upper Devonian sedimentary formations occur in the Hill End Trough. Structural relationships indicate that the oldest mineral veins at Hill End preceded cleavage formation in the deformed trough sedimentary rocks. Early vein muscovites have Middle Devonian 40Ar/39Ar dates of 380–370 Ma. Regional metamorphic biotites from Hill End have well constrained 40Ar/39Ar closing ages of 360–358 Ma (mid‐Famennian). The metamorphic (thermal) maximum which outlasted penetrative deformation, is estimated here by modelling to have been about 370 Ma (latest Givetian). This clearly places the earlier main deformation in the Middle Devonian. Deformation probably began by terminating trough deposition in latest Emsian to early Eifelian time and ended in early Givetian time at about 375 Ma ago. Published pressure and temperature data from the Hill End goldfield suggest that deformation thickened the 6 km sediment column to around 11 km. The thermal model suggests there was post‐deformation erosion of about 4 km and little if any further erosion occurred during Late Devonian to Early Carboniferous time. The shortening accompanying the inversion of the northern Hill End Trough may have been taken up in the region to the south, both east and west of the Copperhannia Thrust, and east of the southern termination of the Capertee High. The general conclusions are not affected by a new Devonian time‐scale discussed in Appendix 2.

Taxonomy, phylogeny and biogeography of the late Famennian conodont genus <i>Mashkovia</i>

Abstract. Mashkovia is one of the provincial conodonts which developed during late Famennian time in the cratonic regions of Russia. In this study, the taxonomy of this genus is revised, based on diagnostic characters of the Pa elements, such as the morphology of the anterior part of the platform, the ornamentation and the shape of the secondary keels. As a consequence, four species, including M. silesiensis n. sp. now discovered in Upper Silesia of southern Poland, are distinguished. The apparent absence of Mashkovia from North America, Variscan Europe, Australia and Africa cannot be simply explained by using temperature or other global climatic factors as a reason for the provincialism. Currents and/or local palaeoecologic factors were probably more important in controlling the distribution of these conodonts.

Synsedimentary tectonics, mud-mounds and sea-level changes on a Palaeozoic carbonate platform margin: a Devonian Montagne Noire example (France)

The Devonian sedimentary succession of the southern flank of the Montagne Noire (France) was deposited along a divergent margin. This paper is a contribution to describe and evaluate biogenic, sedimentary, geochemical and micropalaeontological features as indicators of sea-level changes and global history of the Devonian in this area. Following transgression and shallow-water environments during Early Devonian time (Lochkovian to early Emsian), biogenic mud-rich mounds with stromatactis developed during latest Emsian at the platform margin. The depth of the Devonian sea was increasing and the seafloor passed below the photic zone and the lower limit of storm wave base during the Emsian. Growth and seismic faults affected the mounds and created Neptunian cracks and crevices, quickly filled with sedimentary material (pisoids) and cements (Neptunian dykes and veins). Light and CL-microscopy, and stable isotope geochemistry show that stromatactis, cements of Neptunian dykes, veins and pisoid cortices are early marine, whereas the red finely crystalline material that forms the bulk of the mound has been cemented in the near-surface diagenetic environment, after the early marine cementation of stromatactis and Neptunian dykes and veins, by meteoric or hydrothermal fluids. The sedimentary rocks overlying the stromatactis mounds exhibit regularly condensed iron and manganese-rich layers, interrupted by the Kellwasser hypoxic horizon. These condensed deposits developed up to the Famennian in a context of carbonate gravity sedimentation and became more and more rhythmic and frequent up section. The occurrence and irregular distribution of large-scale submarine mass flows during Frasnian and Famennian times can be related to block faulting on which Lower Devonian stromatactis mounds could have been uplifted by this block faulting to form seamounts. The sea-level fluctuations detected in the southern flank of Montagne Noire are compared to the Devonian eustatic sea-level curve of Johnson et al. (1985). [Johnson, J.G., Klapper, G., Sanberg, C.A., 1985. Devonian eustatic fluctuations in Euramerica. Geol. Soc. Am. Bull. 96, 567–587.]

First record of a large Callixylon trunk from the late Devonian of Gondwana

A 5 m long permineralized trunk from the Upper Kellwasser member of southern Morocco represents the first record of a large trunk of an identifiable species of Callixylon, C. erianum, from Gondwana. This occurrence constitutes the most reliable evidence based on plant megafossils for a floral connection between Laurussia and Gondwana in late Devonian times and for a proximity of these continents in Famennian times. The potential of this trunk for studies of the architecture and growth patterns of the earliest trees with a gymnospermous type of arborescent habit is discussed.

Construction of the Silurian and Devonian seawater 87Sr/ 86Sr curve

The estimated path of seawater 87Sr/ 86Sr during the Silurian and Devonian is based on 86 analyses (84 whole rocks) presented here and in other published results. The seawater 87Sr/ 86Sr rose steadily throughout the Silurian from a poorly constrained value near 0.70793 (Δsw −114) at the Ordovician–Silurian boundary. The 87Sr/ 86Sr value at the Silurian–Devonian boundary is near 0.70871 (Δsw −36) based on an earliest Devonian limestone suite from New York. The 87Sr/ 86Sr falls through the Early Devonian to reach a low near 0.70772 (Δsw −135) in the Eifelian and Givetian and rises to near 0.70803 (Δsw −104) in Famennian time. The seawater 87Sr/ 86Sr at the Devonian–Mississippian boundary is estimated to be near 0.70805 (Δsw −102). The middle Paleozoic sample suite is judged less suitable for defining seawater 87Sr/ 86Sr than those from other Paleozoic periods. The middle Paleozoic seawater 87Sr/ 86Sr variation indicates domination of continental Sr contributions during the Silurian and later Devonian. The high ratio (continental) Sr contribution is controlled by the climate and the area, age and composition of exposed crystalline rocks. The dominance of the oceanic (low ratio) Sr during the Early and part of Middle Devonian is coincident with a reported increase in continental volcanism. The estimated rise and fall of sealevel, leading to flooding or exposure of the continents, does not appear to control seawater 87Sr/ 86Sr during the middle Paleozoic.

Electron microprobe study of detrital amphiboles from Famennian synorogenic clastic sediments of the Saxothuringian belt (Erbendorf Paleozoic, NE-Bavaria, Germany): Consequences for provenance and geotectonic development

Detrital amphiboles from the Famennian, Saxothuringian greywackes of the “Erbendorf Paleozoic” in Bavaria were analysed with an electron microprobe. The results were compared with recalculated literature data of amphiboles from potential source rocks to obtain more detailed information about the provenance of the oldest preserved synorogenic sediments in the Saxothuringian belt.All of the detrital amphiboles show very similar, homogeneous chemical compositions. All of them are Ca-amphiboles (with (Ca+Na)B ≥: 1.34; NafB < 0.67; (Na+K)A < 0.50 and Ti < 0.50), mostly inagnesio-hornblende and tschermakitic hornblende. The comparison with amphiboles from rocks of potential provenance areas reveals that the Randamphibolit-Series of the Miinchberg Massif – or an equivalent, already eroded unit – can be regarded as source rock of the detrital amphiboles.The presented data are the first evidence, that the Randamphibolit of the Miinchberg Massif was exhumed as early as in the Famennian, shortly after its metamorphism. It can be concluded that before Famennian time the complete Saxothuringian oceanic crust and large parts of the Saxothuringian continental crust had been subducted at the active margin. This implies that the collision of the Saxothuringian plate with the Tepla-Barrandian microplate, leading to the accretion of the Saxothuringian plate, happened 15–20 Ma earlier than previous authors had supposed. Accordingly, the collisional stage must have been reached not later than in the middle Devonian (approx. 380 Ma).

Rift-related Devonian sedimentation and basin development in South China

During Devonian times South China lay to the north of the Palaeo-Tethyan ocean, the boundary being a passive continental margin. A shallow sea covered the southern parts of the continent while northern areas, forming the Huanan Landmass, were emergent. At the beginning of the Devonian most of South China was above sea level. Subsequent transgression from the south gave rise to an irregular coastline with the development of many fault-controlled gulfs. Further transgression led to the development of an epicontinental sea with reefs forming along the margins of the submerged gulfs and black shales deposited within them. By Emsian time a widespread carbonate platform was established, while anoxic deposition continued in the troughs. The marine transgression peaked in the Frasnian Stage. During Famennian time widespread regression occurred and much of South China became once more emergent. Peneplanation of the Huanan Landmass led to the partial infilling of many of the older fault-bounded depressions. Throughout the Devonian the local distribution of sediments was strongly controlled by NE-SW trending transtensional faults that bounded NW-SE trending normal faults. These structures continued to influence sedimentation in the Late Palaeozoic, the Mesozoic and possibly the Tertiary in the offshore Beibu Gulf Basin.

Conodont biofacies and taphonomy along a carbonate ramp to black shale basin (latest Devonian and earliest Carboniferous), southernmost Canadian Cordillera and adjacent Montana

Uppermost Devonian – lowermost Carboniferous strata in the southernmost Canadian Cordillera and adjacent Montana record the onset and termination of low-oxygen conditions in carbonate-dominated epicontinental and shelf seas. Several distinct conodont biofacies representative of shallow-ramp to deep-basin settings are recognized on the basis of conodont distribution and preservation patterns.During early and middle Famennian time, the region was the site of a westward-deepening carbonate ramp (Palliser Formation) that was bordered to the west by a deep, shale basin (Lussier syncline strata). Palliser carbonates contain low-diversity conodont faunas of indigenous to transported Palmatolepis-, Polygnathus-, and Apatognathus-dominated assemblages. Basinal deposits yield a pelagic palmatolepid biofacies. Middle to late Famennian time was marked by termination of carbonateramp sedimentation and flooding of the margin with oxygen-depleted water. Deposition of organic-rich facies began in the expansa Zone in shelf to basin environments (Exshaw Formation and correlative units). These deposits contain indigenous pelagic Palmatolepis- and (or) Bispathodus-dominated assemblages; reworked or transported fragments are primarily polygnathids and icriodontids.Sedimentation of anaerobic to aerobic, deep-water, lower Banff facies occurred intermittently until middle Tournaisian and, locally, late Tournaisian time prior to westward progradation of younger carbonate deposits. Middle Tournaisian biofacies include transported and indigenous assemblages of siphonodellids (deep–middle ramp). Late Tournaisian biofacies parallel lithofacies changes associated with shallowing of the Banff sequence and are characterized by scaliognathid–doliognathid (basin to deep ramp), polygnathid and polygnathid–bactrognathid (deep to middle ramp), and bactrognathid–hindeodid (middle to shallow ramp) indigenous and displaced biofacies. The spatial relations of these Famennian and Tournaisian biofacies are generally consistent with models developed for correlative strata elsewhere.

Environmental record of Devonian-Mississippian carbonate and low-oxygen facies transitions, southernmost Canadian Rocky Mountains and northwesternmost Montana

Uppermost Devonian and Lower Mississippian strata in the southernmost Canadian Rocky Mountains and adjacent Montana record part of a widespread low-oxygen episode in middle Paleozoic epicontinental and shelf seas and provide critical information concerning the oceanographic setting of the western continental margin of Euramerica at this time. During early-middle Famennian time, the region was the site of a westward-deepening carbonate ramp (Palliser Formation) that was bordered to the west by a shale basin (Lussier region). Sedimentation patterns changed in middle-late Famennian time with the termination of carbonate ramp sedimentation and ultimate deposition of organicrich sediments in an oxygen-stressed, deep-water setting (Exshaw Formation). A model for the widespread deposition of these sediments entails the initial flooding of epicontinental/shelf seas with water derived from an expanded and intensified oxygen-minimum zone during a late Famennian transgression. Sedimentation of anaerobic to marginally aerobic, deep-water, lower Banff facies occurred into middle Tournaisian and, locally, late Tournaisian time prior to the westward progradation of carbonate ramp sediments of the middle and upper Banff. Quartzofeld-spathic clastic rocks in the Exshaw and Banff reflect the periodic influx of detritus from a likely western (Antler-age) orogenic source from late Famennian into late Tournaisian time. Degree of bottom-water oxygenation during deposition of the Exshaw and Banff low-oxygen facies was recognized using ichno-fabric, organic C and S relationships and degree of pyritization (DOP). Anaerobic, laminated black shales with high DOP values lack a macrobenthos, with the notable exception of rare concentrations of inarticulate brachiopods on bedding planes. Dysaerobic to marginally aerobic facies have intermediate DOP values and a gradient of increasing degree of bioturbation.

Stromatoporoids from the Famennian (Devonian) Wabamun Formation, Normandville oilfield, north–central Alberta, Canada

Stromatoporoids are the principal framebuilding organisms in the patch reef that is part of the reservoir of the Normandville field. The reef is 10 m thick and 1.5 km2 in area and demonstrates that stromatoporoids retained their ability to build reefal edifices into Famennian time despite the biotic crisis at the close of Frasnian time. The fauna is dominated by labechiids but includes three non-labechiid species. The most abundant species is Stylostroma sinense (Dong) but Labechia palliseri Stearn is also common. Both these species are highly variable and are described in terms of multiple phases that occur in a single skeleton. The other species described are Clathrostroma cf. C. jukkense Yavorsky, Gerronostroma sp. (a columnar species), and Stromatopora sp. The fauna belongs in Famennian/Strunian assemblage 2 as defined by Stearn et al. (1988).

Stromatoporoids from the Famennian (Devonian) Wabamun Formation, Normandville oilfield, north–central Alberta, Canada

Stromatoporoids are the principal framebuilding organisms in the patch reef that is part of the reservoir of the Normandville field. The reef is 10 m thick and 1.5 km2in area and demonstrates that stromatoporoids retained their ability to build reefal edifices into Famennian time despite the biotic crisis at the close of Frasnian time. The fauna is dominated by labechiids but includes three non-labechiid species. The most abundant species isStylostroma sinense(Dong) butLabechia palliseriStearn is also common. Both these species are highly variable and are described in terms of multiple phases that occur in a single skeleton. The other species described areClathrostromacf.C. jukkenseYavorsky,Gerronostromasp. (a columnar species), andStromatoporasp. The fauna belongs in Famennian/Strunian assemblage 2 as defined by Stearn et al. (1988).

An oblique subduction and transform faulting model for the evolution of the Broken River Province, northern Tasman Orogenic Zone

Formation of the Broken River Province, an important element of the northern Tasman Orogenic Zone, can be rationalized in terms of oblique subduction and strike‐slip faulting. Its two broadscale terranes, the smaller southwestern Graveyard Creek Subprovince and larger Camel Creek Subprovince, are regarded as Silurian‐Devonian forearc basinal, and subduction complex, assemblages, respectively. They are divided by the Gray Creek Fault, interpreted as a dextral strike‐slip structure of Siluro‐Devonian age. Part of the multiphase structural history of the Camel Creek Subprovince is ascribed to the partitioning of shortening and shear stress regimes known to characterize continental margins experiencing oblique‐slip subduction. Late Devonian (Famennian) crustal shortening is ascribed to a change in the direction of plate motion and an increase in its rate. A sheet of basement is thought to have been thrust eastwards to cover most of the forearc basinal assemblage with the southwestern part of the Burdekin River Fault developing as a tear structure at its southern margin. Development of the Clarke River Fault Zone, regarded as a transform offsetting the continental margin in a sinistral sense, is also inferred for Famennian time. Northeasterly trending folds characteristic of the Graveyard Creek Subprovince and recognized as a discrete fold phase in the southern Camel Creek Subprovince are considered to have been induced by shear adjacent to the Clarke River Fault Zone and to wedging of the continental margin sedimentary assemblage between basement blocks. Oblique subduction appears to have resumed in Early Carboniferous (Tournaisian) time. Two previously separate structures, the southwestern Burdekin River Fault and part of the older Gray Creek Fault, united to form a single dextral strike‐slip structure. Movement on this composite fault, the Burdekin River Fault of current nomenclature, sponsored the development of a deep Early Carboniferous pull‐apart molasse basin which retained stratigraphic and structural continuity with the preceding forearc basin. Extensive shallow Early Carboniferous molasse basins developed in response to vertical movements on the Gray Creek and Clarke River Fault Zones. With subsequent vertical movement coupled with erosion, the Burdekin River Fault has come to mark the eastern limit of overthrust basement.

Effect of the Frasnian-Famennian extinction event on the stromatoporoids

Although Paleozoic stromatoporoids were drastically reduced in abundance and diversity by the Frasnian-Famennian (F/F) mass extinction event, at least 20 genera (mostly labechiids and clathrodictyids) are known from latest Devonian rocks. Some typical Frasnian genera survived the F/F crisis to die out in early Famennian time, and others took part in the Strunian resurgence of the stromatoporoids. Small stromatoporoid reefs are known from Famennian and Strunian rocks. The history of the group suggests a large, rapid, but progressive environmental change at the F/F boundary and the survival of genera better adapted to cool water.

Late Devonian rugose corals and the Frasnian–Famennian crsis

All well-dated occurrences of Upper Devonian septate corals (subclass Rugosa) are analyzed at species and higher taxonomic levels. Platform-dwelling corals are differentiated from the less numerous basin-dwelling corals.Through early and middle Frasnian time, evolution outpaced the extinction of rugose corals, so that late Frasnian faunas were more diverse than the earliest Frasnian faunas. This gradual increase of genera was terminated abruptly by an almost complete extinction of late Frasnian platform-dwelling Rugosa in uppermost gigas to lower Palmatolepis triangularis Zone time. None of the 151 species, and probably as few as two or three, at most five, of the 47 genera of late Frasnian shallow-water corals survived this event. The loss of coral biomass was equally devasting. Very few early Famennian platform-dwelling corals are known, although platform carbonates of this age were deposited in several regions. Platform faunas, comprising 66 species and 27 genera, reappeared in late Famennian time. These faunas were unrelated phylogenetically to known Frasnian faunas but were forerunners of shallow-water Carboniferous faunas.Basin-dwelling Rugosa were affected little by the late Frasnian extinction event: all 12 Frasnian genera survived it.Rugose coral data provide compelling evidence for a major late Frasnian extinction of shallow-marine benthos without suggesting a unique cause of it. The data are consistent with results expected of an asteroid impact or a rapid decline of ocean temperature, especially in regard to the very much higher survival rate among basin-dwelling corals. However, the data are not controlled by a sufficiently refined time scale to prove the geological instantaneity of such an event.

Late Devonian glaciation in South America

An ice age in Famennian (Late Devonian) time is documented by the presence of diamictites with striated, faceted and polished pebbles; rhythmites with dropstones; erratic boulders; and striated pavements and deformed sandstones. The glacigenic beds occur in three huge intracratonic basins (Solimões, Amazonas and Parnaìba) and in one pericratonic basin (Acre) in northern Brazil. Other possible Late Devonian diamictites may occur elsewhere in Brazil and Africa which, if confirmed, would extend considerably the area affected by this glacial age. The onset of the Late Devonian glaciation caused a global narrowing of the warm climatic belts, sharp regression, increased clastic sedimentation, massive biotic extinction, reduced biochemical deposition and unconformities. This resulted in important facies changes in the world geological record. Ice centers moved from northern Africa to southwestern South America from Late Ordovician to Early Silurian time. From Mid-Silurian to early Late Devonian time no record of glaciation is known. In late Late Devonian time intermittent glaciation began again in central South America and, from late Late Devonian to early Late Permian time, ice centers migrated towards Antarctica across South America and South Africa. The Devonian and Ordovician-Silurian glaciations together with the better known Permo-Carboniferous glaciations may all have primarily resulted from the shifting position of the Gondwana continent with respect to the South Pole.

FACIES AND TIME IN DEVONIAN TROPICAL AREAS

Summary Europe, North America and Australia are thought, largely on palaeo-magnetic evidence, to have been mostly within the tropics during Devonian times. The marine rocks of some of these areas are reviewed to elucidate the age, nature and extent of facies movements and transgressions of the period, especially in relation to cratonic centres. It is shown that a pattern of increasingly successful transgressions followed the late Silurian Caledonian orogeny and culminated in the transgressions of the late Devonian. Evidence for the international correlation of these movements is analyzed. Lower Devonian movements, broadly general in northern Europe, can tentatively be recognized in the Appalachian area, but evidence does not permit convincing correlation elsewhere. The early mid-Devonian transgression, which brought a substantial facies change in Western Europe, is the date given for the major transgression across the Russian platform; of about the same age are the extensive equivalents of the Onondaga Limestone in the eastern U.S.A. There is broad evidence of transgressional Middle Devonian in western U.S.A. and Canada, but it is not so precisely dated. The transgression close to the Middle–Upper Devonian boundary is the clearest dated internationally and the onset falls in the early lunulicosta Zone almost everywhere; this is the Frasnian transgression of Russia and the Taghanic onlap of the U.S.A. Local areas show slightly later, or earlier commencement. Late Frasnian and early Famennian times together covered a general period of deepening and this movement terminated reef and massive limestone deposition almost everywhere except in Australia. Certain orogenic effects (Acadian and Antler movements, etc.) change local regimes but do not mask entirely the evidence of global eustatic change. Of all the explanatory mechanisms, that of volumetric changes at spreading plate margins seems the simplest cause for large-scale, and perhaps many small-scale, facies change patterns.

Ordovician to Devonian History of Northern Yukon and Adjacent District of Mackenzie

ABSTRACT The eastern half of the study area, the Mackenzie and Anderson River Plains area, is underlain by uniform sequences of carbonates and relatively minor shales, cherts and evaporites, and is marked by widespread, regional disconformities. Middle Ordovician, Upper Silurian, and at least the lower Lower Devonian sediments are completely absent from the area. The western portion that is, roughly the northern part of Yukon -- was, as a result of more local tectonic instability, characterized by more rapid facies changes, development of troughs, ridges and basins, and more localized disconformities and unconformities. The Richardson Trough, more or less conforming in outline to the area of the present Richardson Mountains, was a deeper-water, graptolite-shale trough from before earliest Ordovician through to at least early Emsian; was apparently positive between late Emsian and early Givetian; and, finally, was an area of rapid sedimentation throughout the remaining Devonian. South of but in linear continuity with the Richardson Trough, a structure recognized for the first time and termed the Bonnet Plume High may have been emergent throughout the Ordovician, Silurian and the earliest part, at least, of the Devonian. The area west of the present Richardson Mountains, north of the Ogilvie Mountains, and south of British and Barn Mountains, and termed the Yukon Stable Block, was one of platform-carbonate deposition. The Block was more or less diagonally dissected by an essentially NE-SW-trending structure, the Dave Lord High, which was positive from Late Ordovician through to at least mid-Early Devonian times. At times what was probably a subaqueous extension of this High joined with an area of carbonate development west of and beneath the Mackenzie River delta, and appears to have restricted the northern end of the Richardson Trough. The Neruokpuk Formation of the British Mountains, in the extreme northwestern portion of the region, is considered to be at least Early Ordovician to Late Silurian in age, because of correlation with rocks of the same formation in Alaska, and because of its lithologic similarity to rocks of that age range in the Barn Mountains. These rocks are interpreted as miogeosynclinal equivalents of the more stable platform suites to the south and southeast. Intrusion of two small, granitic masses is believed to have occurred at about Early Devonian time in the Barn Mountains area. End_Page 321------------------------ During late Middle and early Late Devonian times, the sedimentation pattern of the entire region changed, leading to the deposition of black, siliceous shale and localized reefs. This was followed during later Frasnian and Famennian time by strong uplifting in and northwest of the British and Barn Mountains regions, and the deposition of widespread and locally great thicknesses of clastic sediments to the southeast. These are characterized by coarse, greywacke-type sediments in the northwest, grading southeastward to finer sandstones and siltstones, and minor carbonates.