Early Middle Devonian Research Articles

Contrasting styles of Taconian, Eastern Acadian and Western Acadian metamorphism, central and western New England

The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west‐central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back‐arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high‐T, low‐P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E‐W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well).The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P–T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400‐600° C were reached at pressures of about 4–6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by 40Ar/39Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600–725° C) and pressures (6.5–10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite–sillimanite boundary on a P–T diagram. There is little or no plutonism accompanying Western Acadian metamorphism.In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high‐T, intermediate‐P conditions for the highest grade rocks known in New England: T= 650–750° C, P= 4.5–6.5 kbar for granulite facies assemblages which apparently formed along an ‘anticlockwise’P–T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a ‘clockwise’ path with decompression cooling.Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445 Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe‐stage deformation and metamorphism in the Eastern Acadian zone appears to have ended by about 410 Ma. Tectonic thickening in the Western Acadian (including the western counterparts of the nappe‐stage deformation documented in the Eastern Acadian) must pre‐date attainment of peak metamorphic conditions dated at 395–385 Ma. Dome‐stage deformation clearly post‐dates peak metamorphism and deforms metamorphic isograds. The end of Western Acadian deformation is well constrained by 370‐375 Ma radiometric ages of late pegmatites and granitoids which cross‐cut all structures.

Evolution of high-latitude biotas

With the recent rapid improvement in the quality of the fossil record in both polar regions it has become possible to examine critically the development of high-latitude faunas and floras through time. Within arctic regions, distinctive biotas have been present, at least intermittently, since the early Permian, and in antarctic regions they can be traced back to the early Middle Devonian. It would appear that, whenever continents (and continental shelves) moved into the high latitudes they were the sites of significant biotic differentiation.To investigate the ways in which taxa may have accumulated in the high latitudes through time, three simple global models have been proposed. In the first of these it is envisaged that major groups of plants and animals arose in the low latitudes and disseminated subsequently into the polar regions. Here they would become relicts and the latter would be regarded as refugia. The second model is almost exactly the reverse of the first in that it proposes that certain taxa arose in the polar regions and then spread, through time, into the low latitudes. Such a process may at first sight seem less likely, but the Antarctic fossil record in particular is providing important evidence to suggest that certain major groups of both plants and animals had a high-latitude origin. The third model differs substantially from the previous two in that it makes no a priori assumptions about either centres of origin or the process of dispersal. It merely states that, at certain times in the past, widespread distribution patterns have been disrupted in the low latitudes to form disjunct populations in the northern and southern hemispheres. It results in the classic bipolar pattern, which can now be traced back to at least the Early Jurassic period.A major phase of Jurassic - Cretaceous bipolarity can be attributed to tectonic vicariance as the Pangean supercontinent disintegrated, and a major late Paleogene - early Neogene one can be related to climatic vicariance. Such hypotheses are attractive as they are testable by cladistic analysis. Although data available so far are limited, both fossil and living benthic molluscs offer considerable scope for future work.

Sequence Stratigraphy of Early-Middle Devonian Carbonate Shelf to Basin Transitions, Northeast British Columbia

Sequence Stratigraphy of Early-Middle Devonian Carbonate Shelf to Basin Transitions, Northeast British Columbia

Lower and middle devonian miospore-based stratigraphy in Libya and its relation to the megafloras and faunas

A review of miospores, faunas and megafloras of the Lower and Middle Devonian in Libya allows new conclusions to be drawn. The application of the Ardenne-Rhenish Devonian miospore zonation in Libya gives the following results. In the North Hammadah Basin, the base of the Tadrart Formation might well be strongly diachronous depsite the lithostratigraphical correlations shown on the base of “Gamma-Ray/Neutron” logs. At least the lower part of the Tadrart Formation in borehole MG-1 is within the lowermost part of the Lochkovian not at its base. In the southern margin of the Hammadah Basin, the Tadrart and Ouan-Kasa formations are probably not older than late Emsian, maybe early Eifelian. The Ouenine Formation I is often absent there or strongly reduced. The isochroneity of the (discordant) lower limit of the Tadrart Formation across the Hammadah Basin is far to be demonstrated. The Caledonian age of the discordance on the southern margin of the Hammadah Basin can obviously be challenged. In the eastern Murzuk area, the plant-bearing Tadrart-Emi Magri formations have a Middle Devonian age according to the plant fossils themselves. The discordance which separates these formations from the Acacus Sandstone at Dor el Goussa has to be late Early Devonian or Middle Devonian because the Acacus Sandstone “Psilophytes” cannot be older than Pragian and can even be younger (Emsian). There is obviously no available argument which could counteract the fossil floral data and prove that the discordance in the eastern Murzuk has the same age as the Silurian/Devonian transitional beds in the northern Hammadah Basin.

Petrology of eclogites and clinopyroxene-garnet metabasites from the Cabo Ortegal Complex (northwestern Spain)

High-pressure, subduction-related metamorphism is recorded in two structural units of the Cabo Ortegal complex of Spain: (i) an upper thrust unit (Concepenido-La Capelada) composed of granulites, eclogites and metaperidotites and (ii) a structurally underlying unit (Chimparra-Banded) formed of gneisses with minor metabasic intercalations. In the upper unit, a metamorphic episode of Lower Ordovician age caused the formation of eclogites from basic rocks with N-MORB compositions, while high-pressure, garnet-clinopyroxene granulites were formed from basic to intermediate protoliths with calc-alkaline affinities. Garnet replaces spinel in ultramafics of the same unit and may have originated simultaneously. The PT-conditions were c. 800°C at 13.5 kbar in the granulites, whereas similar temperatures and higher pressures ( > 17 kbar) prevailed in the eclogites. The migmatitic Chimparra and Banded gneisses contain boudins of garnet and omphacite-bearing rocks, with or without plagioclase. The basic protoliths of these rocks are similar to LREE-enriched basalts with alkaline affinities. The Silurian metamorphism in this gneissic unit took place at c. 700°C or lower and at pressures around 15 kbar. This metamorphism could have occurred when a volcano-sedimentary complex, formed in a thinned-crust environment, was subducted beneath an overlying active margin/volcanic arc complex. An ophiolitic unit, made up of the N- and T-type MORB metabasites of the Candelaria and Purrido-Peña Escrita Formations, underlies the gneisses. Metamorphism in this unit took place during the Early-Middle Devonian, mainly giving rise to amphibolite-facies associations. Sporadic coronitic metagabbros/dolerite and garnet-clinopyroxene metabasites indicate that amphibolite-granulite transitional conditions ( > 700°C, 8 kbar) were attained locally. Retrograde amphibolitic assemblages in the upper units are more or less coeval with this episode, which suggests that it may have been related to the cessation of obduction and the stacking of the overlying high-grade units.

The destruction of Iapetus and Tornquist's Oceans

The Cambro‐Ordovician Iapetus and Tornquist's Oceans formed a Pacific‐type ocean basin rimmed by volcanic island arcs and marginal basins. By the latest Ordovician to earliest Silurian this ocean basin was beginning to close, to become a Mediterranean‐type ocean basin. This was caused by the collision between a microcontinent (comprising England, Wales, much of Ireland and parts of north‐west Europe), called Eastern Avalonia, and the North American super‐continent, Laurentia, which resulted in no oceanic crust remaining in the region of present‐day central Newfoundland. Marine basins, however, persisted into the Middle Silurian. Throughout the Silurian and early Devonian, some 40–45 million years, various terranes continued to collide with the North American margin, predominantly under major left‐lateral strike‐slip until the remaining seaways were eliminated in the early Middle Devonian, to be replaced by terrestrial environments of the Old Red Sandstone.

Two new Devonian spine-bearing pleurotomariacean gastropod genera from Alaska

Two new spine-bearing gastropods,Chlupacispira spinosan. gen. and sp. andSpinulrichospira cheeneetnukensisn. gen. and sp., are described from the late Early Devonian (Emsian) and early Middle Devonian (Eifelian), respectively, of west-central Alaska. These represent the earliest reported spiny pleurotomariacean gastropods. Otherwise, spinose pleurotomariaceans are known from strata no older than Carboniferous age.Spinulrichospira cheeneetnukensisn. gen. and sp. appears to represent a more highly ornamented derivative ofUlrichospiraDonald. Both new genera are part of the more highly ornamented fauna which occurred in warm equatorial waters of the Old World Realm during the Early and Middle Devonian, in contrast to more weakly ornamented shells of the Eastern Americas Realm and even more weakly ornamented (almost totally “plain”) shells of the Malvinokaffric Realm. The latter two realms are thought to represent subtropical to warm temperate and cool temperate to cool polar conditions, respectively.

Paleomagnetism of devonian units from Miguasha, Gaspe, P.Q., Canada.

The area investigated is located at longitude 66°30′W and latitude 48°N, on the northern shore of Chaleur Bay. Some 124 specimens (46 samples) were collected from 9 sites in the following geological formations: La Garde (10 samples, late Early to early Middle Devonian), Pirate Cove (11 samples, early Middle Devonian), Fleurant (15 samples, early Late Devonian) and Escuminac (10 samples, early Late Devonian). These formations are composed of conglomerates, silty and calcareous sandstones deposited in a continental environment. The Upper Devonian geological units are amongst a few found in Canada to the east of Alberta. The main goal of this study is a better definition of the Apparent Polar Wander Path of the Appalachians of North America. This study allowed the identification of geological formations favourable to paleomagnetic sampling. Remanent magnetizations and magnetic susceptibilities were measured; both alternating field and thermal demagnetizations were carried out. After demagnetization, 4 components (A, B, C and D) were isolated and several components were found in each formation. Fold and conglomerate tests were performed; the first is significantly negative except for component B and the second is positive in the Fleurant Formation. The mean direction of the components is: 156°, +31° for A, 52°, -46° for B, 167°, -21° for C and 223°, +25° for D. The corresponding paleopoles are 22°N 141°E, 6°S 77°E, 51°N 137°E and 17°N 83°E. Based on the fold test, degree of alteration and unblocking temperatures, it was found that components A, C and D are secondary. Component B is either primary or secondary. The relative age of acquisition of magnetization is the following: B>D>A>C. The paleolatitudes vary from 17°S to 10°N going from A to B (tilt corrected), to D and to C. The results are discussed in relation to the Devonian Apparent Polar Wander Path of North America.

Devonian palaeontological data and the Armorica problem

Various palaeogeographic ideas about the relation between Gondwana and Euramerica during the Devonian Period are formalised into four competing hypotheses involving three areas (Euramerica, Gondwana, Armorica) separated by two marine faunal barriers. Palaeontological evidence from Devonian vertebrates is summarised to show support for the hypothesis that Gondwana and Euramerica were contiguous by the Middle Devonian, and that the intervening ocean had been occluded by the Late Devonian. Palaeontological data provide no support for the existence of a wide Devonian ocean separating Euramerica from Gondwana, nor for a discrete continental block (“Armorica”) which may have traversed this ocean during the Devonian. The Armorican region of southern Europe was apparently part of Gondwana during the Early Devonian. Biostratigraphic evidence of discrepancies in the first appearance of various invertebrate taxa between southern Europe, north Africa and North America are consistent with a biotic dispersal model involving shallow marine invertebrates and vertebrates during the early Middle Devonian, and continental faunas and floras in the Late Devonian. The latter event may have occurred at or near the Frasnian-Famennian boundary.

Shark teeth from the Early-Middle Devonian Cravens Peak Beds, Georgina Basin, Queensland

The oldest shark teeth so far recorded from Australia are described from the Early-Middle Devonian of western Queensland. The teeth show characters in common with those of the Jurassic-Recent hexanchoid sharks, and the possible relationship to these so-called ‘living fossils’ is discussed. The teeth are ascribed to a new species, Mcmurdodus whitei, of a genus found previously in the Middle-Late Devonian of Antarctica. Scales and prismatic cartilage found in the Cravens Peak Beds probably belong to this shark.

Metallogeny and tectonic development of the Tasman Fold Belt System in New South Wales

The northwestern corner of New South Wales consists of the paratectonic Late Proterozoic to Early Cambrian Adelaide Fold Belt and older rocks, which represent basement inliers in this fold belt. The rest of the state is built by the composite Late Proterozoic to Triassic Tasman Fold Belt System or Tasmanides. In New South Wales the Tasman Fold Belt System includes three fold belts: (1) the Late Proterozoic to Early Palaeozoic Kanmantoo Fold Belt; (2) the Early to Middle Palaeozoic Lachlan Fold Belt; and (3) the Early Palaeozoic to Triassic New England Fold Belt. The Late Palaeozoic to Triassic Sydney—Bowen Basin represents the foredeep of the New England Fold Belt. The Tasmanides developed in an active plate margin setting through the interaction of East Gondwanaland with the Ur-(Precambrian) and Palaeo-Pacific plates. The Tasmanides are characterized by a polyphase terrane accretion history: during the Late Proterozoic to Triassic the Tasmanides experienced three major episodes of terrane dispersal (Late Proterozoic—Cambrian, Silurian—Devonian, and Late Carboniferous—Permian) and six terrane accretionary events (Cambrian—Ordovician, Late Ordovician—Early Silurian, Middle Devonian, Carboniferous, Middle-Late Permian, and Triassic). The individual fold belts resulted from one or more accretionary events. The Kanmantoo Fold Belt has a very restricted range of mineralization and is characterized by stratabound copper deposits, whereas the Lachlan and New England Fold Belts have a great variety of metallogenic environments associated with both accretionary and dispersive tectonic episodes. The earliest deposits in the Lachlan Fold Belt are stratabound Cu and Mn deposits of Cambro-Ordovician age. In the Ordovician Cu deposits were formed in a volcanic are. In the Silurian porphyry CuAu deposits were formed during the late stages of development of the same volcanic are. Post-accretionary porphyry CuAu deposits were emplaced in the Early Devonian on the sites of the accreted volcanic arc. In the Middle to Late Silurian and Early Devonian a large number of base metal deposits originated as a result of rifting and felsic volcanism. In the Silurian and Early Devonian numerous SnW, Mo and base metal—Au granitoid related deposits were formed. A younger group of MoW and Sn deposits resulted from Early—Middle Carboniferous granitic plutonism in the eastern part of the Lachlan Fold Belt. In the Middle Devonian epithermal Au was associated with rifting and bimodal volcanism in the extreme eastern part of the Lachlan Fold Belt. In the New England Fold Belt pre-accretionary deposits comprise stratabound Cu and Mn deposits (pre-Early Devonian): stratabound Cu and Mn and ?exhalite Au deposits (Late Devonian to Early Carboniferous); and stratabound Cu, exhalite Au, and quartz—magnetite (?Late Carboniferous). S-type magmatism in the Late Carboniferous—Early Permian was responsible for vein Sn and possibly AuAsAgSb deposits. Volcanogenic base metals, when compared with the Lachlan Fold Belt, are only poorly represented, and were formed in the Early Permian. The metallogenesis of the New England Fold Belt is dominated by granitoid-related mineralization of Middle Permian to Triassic age, including SnW, MoW, and AuAgAs Sb deposits. Also in the Middle Permian epithermal AuAg mineralization was developed. During the above period of post-orogenic magmatism sizeable metahydrothermal SbAu(W) and Au deposits were emplaced in major fracture and shear zones in central and eastern New England. The occurrence of antimony provides an additional distinguishing factor between the New England and Lachlan Fold Belts. In the New England Fold Belt antimony deposits are abundant whereas they are rare in the Lachlan Fold Belt. This may suggest fundamental crustal differences.

The geometry of listric growth faults in the Devonian basins of Sunnfjord, W Norway

Thick Devonian clastic sequences accumulated in fault-bounded basins in W Norway at the close of the Caledonian orogeny and mark the beginning of a phase of crustal extension. Traditionally, the eastern boundary fault to the Devonian basins has been regarded as a thrust but cross-sections based on the use of a branch-line reveal it to be an extension fault. There is clear omission of structural section across the boundary and by matching up metamorphic rocks in the hanging- and foot-walls of the boundary fault, over 40 km of extensional dip slip can be measured at the N end of the fault. The boundary fault is a flat-lying listric fault with rollover anticlines in the hanging-wall. The fault also cuts up and down section along strike to form lateral ramps parallel to the movement direction. These ramps appear as wrench faults in outcrop. The ramp-flat geometry of the fault may have been inherited from a precursor Caledonian thrust fault which reversed its sense of slip in early Middle Devonian times to form the basins. Over 25 km of onlapping sediment accumulated in the largest basin during faulting in 14 Ma, with an average slip rate on the fault of 3 mm yr -1 .

Character diversification and patterns of evolution in early vascular plants

Available data on the stratigraphic ranges of latest Silurian and Devonian vascular plant macro-fossils (sporophytes) and spores provide insights into the tempo and mode of early tracheophyte evolution. Patterns of diversification, origination, and extinction conform in general to the predictions of Sepkoski's kinetic model of diversification. Rates of generic origination and extinction vary not only through time but also between organ systems for a single time interval. This fact, coupled with data on longevity and turnover and comparative morphological observations, can be used to document mosaic evolution in early vascular plant history. Mosaic evolution is an important theme in plant evolution; indeed, what we recognize as macroevolutionary events often correlate with brief periods of pronounced mosaicism. Such evolutionary patterns reflect the developmental biology of tracheophytes in which individual organs often have life spans that are considerably shorter than the life of the whole plant. Under these conditions, individual organs or organ systems can respond to different sets of evolutionary pressures.The major period of early vascular plant diversification occurred during the late Early and early Middle Devonian Period, 30 Myr or more after the origin of the group. Such lags in diversification are not uncommon in the fossil record. Sometimes they reflect extrinsic controls on diversification, but in other cases they appear to be a consequence of intrinsic rates of origination and extinction.

Stratigraphy and Facies Across Lower Devonian-Middle Devonian Boundary, Central Nevada

A regressive episode near the end of the Early Devonian caused a nearshore quartzose carbonate facies to shift to the edge of the carbonate shelf in central Nevada. Transgression followed in the latest Early Devonian and continued in the early Middle Devonian. Seaward of the shelf-wide unconformity produced by regression, a dolomitized, dark-gray lime wackestone-packstone with abundant crinoidal debris formed on the inner slope of a silled outer shelf basin. This dolomite is here named the Sadler Ranch Formation. The dolomite front forms the facies boundary between the Sadler Ranch and the upper Coils Creek limestone, which was deposited in the shallow outer shelf basin. Dark lime muds that characterize Sadler Ranch and upper Coils Creek sediments probably collected only uring low stands of sea level when the outer sill of the basin was emergent. Crinoid meadows flourished on the ramp when sea level rose. Conodonts in the Sadler Ranch Formation date the regression as serotinus Zone and the subsequent transgression as spanning the upper part of the serotinus Zone, the patulus Zone, and the lower part of the costatus costatus Zone. The transgression correlates closely with the Onondaga Limestone of New York and the Couvinian-Eifelian of western Europe and is of probable eustatic origin.

Paleotectonic Controls on Sedimentation in Northern Williston Basin Area, Saskatchewan: ABSTRACT

The Williston basin lies within the so-called stable cratonic interior and would not be expected to have had the same intensity of tectonic activity as is generally considered to be characteristic of cratonic margin sedimentary basins. Local paleotectonic controls on sedimentation are much more subtle, thus sediment distribution patterns are commonly related to major regional structural elements such as the ancestral Williston basin and the Sweetgrass arch. From time to time, however, other structural features appear to have been effective controls on sediment distribution patterns. In southern Saskatchewan, one of the most active of these was the Swift Current platform. This feature appears to have been sufficiently positive during early Paleozoic time to have caused a d stinct thinning of those sediments over it. But it is in early Middle Devonian time that it plays a more prominent role. During the time the Winnipegosis Formation was being deposited, the platform acted as a core for an accreting carbonate wedge that forms the southwestern margin of the Elk Point basin. Progressive inundation of the platform during the remainder of the Devonian made it a part of a broad, shallow shelf sea. However, it was a site of active local salt solution during that time, and may have been as well the locale for repeated accumulations of anhydrite deposits, particularly during the time the Duperow Formation was being laid down. The platform was mildly positive during other periods of sedimentation, as well as during periods of erosion. It was a site of widespread sa t solution during Mesozoic time, which was also its time of major tectonic fluctuation, as well as being the period when it had the most significant influence on sedimentation. Southeastern Saskatchewan is the locale for some significant regional gravity and magnetic anomalies which appear related to exposed structural zones in the Precambrian Shield. A major gravity anomaly on the extreme eastern side of the province is on trend with the Nelson River zone of Manitoba and a magnetic anomaly (Camfield-Gough conductor zone) can be traced to the Wollaston trend in north-central Saskatchewan. The existence of these two trends suggests that the regions proximal to them may have been tectonically active during various geologic periods. The Camfield-Gough zone is particularly significant in that it lies along the axis of the Hummingbird trough, an area affected by basement-controlled early salt solution, and it extends southward into the United States, where it is lanked by a number of local multizone oil-producing structures in North Dakota and Montana. The proximity of these structures to that conductor, which is thought to be a suture between two Proterozoic plates, suggests there is a relationship between them. Isopach and isochron maps have been used to show that these structures have been active during various geologic periods. Similar mapping techniques may show that similar structures are present in southeastern Saskatchewan. End_of_Article - Last_Page 1345------------

The Middle Devonian shoreline in North Devon, England

The Hollowbrook Formation is an early Middle Devonian regressive succession of sandstones and heterolithic facies marking a passage from shallow marine shelf muds, through a sandy shoreface to continental, ephemeral stream sheet sands. Ten facies are recognized, arranged in two sequence types. The shelf sequence consists of alternations of bioturbated muddy sandstone (fair weather sediment) and shelly concentrates or parallel laminated fine sandstone (storm deposits). The nearshore and foreshore sequences coarsen upwards, and pass from bioturbated muddy sandstone (shelf) to lenticular beds (inner shelf), then either wavy and flaser beds (fair weather lower shoreface) or hummocky beds and multistorey parallel laminated sands with bioturbated tops (lower shoreface storm shoals). Upper shoreface wave ripple cross laminated sandstones follow, then parallel and planar cross laminated sandstones (foreshore) and parallel laminated lithic sandstone (continental sheet flood). The lower parts of the shoreface sequence and inner shelf are burrowed by Skolithos. Arenicolites and Chondrites . The sequence differs from the high energy 'Oregon Coast' shore in the absence of a megarippled facies and presence of much bioturbation, but closely matches the sequence found in modern low energy, wave influenced sandy shores such as Sapelo Island, Georgia.

The Devonian System in China

SummaryThe Devonian System of China occurs in the Tianshan–Hingan* Geosyncline of North China, the Yangtze Paraplatform of South China and in the geosynclinal areas of West China. Devonian formations are entirely absent from the Sino-Korean Paraplatform and the Tarim Platform, but along the border of the Tarim Basin and the slopes of Qilianshan (Chilienshan) are continental sequences rich in vertebrate and plant fossils. On the basis of differences in biostratigraphic characteristics the Devonian is divisible into 8 regions: 1. Junggar–Hingan (Dzungar–Khingan), 2. South Tianshan (South Tienshan), 3. Qilianshan (Chilienshan), 4. Longmenshan (Lungmenshan), 5. South China, 6. South-eastern China, 7. West Sichuan–North Xizang (West Szechuan-North Tibet), 8. Himalaya–West Yunnan.Using both lithology and fossil content, five principal facies (types) may be further differentiated: 1. Hingan type (eugeosynclinal facies), 2. Baoxin type (miogeosynclinal facies), 3. Xiangzhou type (platform near-shore facies), 4. Nandan type (platformal off-shore facies), 5. Cuifengshan (Chuifeng shan) type (continental facies).The Devonian of South China occurs in various facies, but has a well-defined principal stratotype. It has been divided by Chinese geologists into eight stages, two in the Upper Devonian, two in the Middle Devonian and four in the Lower Devonian.The Devonian sequences of the geosynclinal regions of West China are characterized by facies which contain graptolites and tentaculites, and have largely been affected by regional metamorphism. The Devonian biota of the Junggar–Hingan Region is essentially an endemic one, but it is associated with a few European and North American elements. Marine pyroclastics are the principal rocks. General stratigraphical columns of those regions are given with a correlation with other parts of China. The continental Devonian of China is unique especially in the light of its vertebrate content. Red sandstones are widespread in the Lower Devonian, and intercalated with argillaceous limestone and mudstone. This sequence is rich in endemic East Asiatic faunal elements including agnatha (Galeaspida, Polybranchiaspida), Antiarchi, etc. Middle and Upper Devonian lithostratigraphic units consist principally of light coloured quartzose sandstone formations in Southeastern China, with some red beds in South and North China, while their fauna and flora are cosmopolitan in character. Their exact geological ages vary, however, as clearly shown in the South China region, especially towards the top of the Upper Devonian (with Asterolepis) and in the early Middle Devonian (with Bothriolepis).

Middle Paleozoic Sedimentation and Paleogeography of Southern Great Basin: ABSTRACT

Based on detailed analysis of conodont distribution, middle Paleozoic rocks in the southern Great Basin have been divided into refined time intervals. These include late Middle through Late Ordovician; early to middle Early Silurian; late Early Silurian; Late Silurian; and Early to early Middle Devonian. Rocks of these intervals consist of fine-grained limestone, fine to medium-grained dolostone, and lesser amounts of silty limestones and impure calcareous shales. Based on the distribution of lithofacies, deposition occurred on an eastern craton margin and within inner- and outer-shelf regions on the Cordilleran miogeocline. Offset of these regions can be used to estimate movement along major faults (e.g., the Death Valley-Furnace Creek fault zone). The craton margin was a region of supratidal to very shallow subtidal environments during part of this time interval. Regression and erosion of the craton margin during Early and Late Silurian time provided a source of carbonate mud as well as silt to sand-sized quartz which were redeposited on the shelf areas in the west. An unconformity separates Upper Ordovician rocks from overlying Lower Devonian rocks. Intertidal to subtidal environments with nearly continuous deposition existed on the inner shelf, providing evidence of regional regression during early and middle Early Silurian and Late Silurian time. The outer shelf was an area of complex environments including platforms and local basins. Uplift and erosion in Late Silurian time affected part of the outer shelf. In the northern nyo Mountains an unconformity separates upper Lower Silurian rocks from overlying Lower Devonian rocks. End_of_Article - Last_Page 418------------

New occurrences of trimerophytes from the Devonian of eastern Canada

Pertica dalhousii n.sp. is described from the late Lower or early Middle Devonian of New Brunswick. The plant is known from a central axis with spirally arranged, mostly dichotomous lateral branches. Some lateral branches terminate in erect clusters of 32–128 fusiform sporangia. Spores are circular, trilete, with a detachable outer sculptured layer, and resemble the dispersed spore genus Apiculiretusispora Streel. A trimerophyte from Gaspé is described and provisionally designated as cf. Pertica sp.; the specimens are too incompletely preserved to be assigned to any established species, but they add further information about morphologic variation in the genus Pertica.With the addition of new plant types referable to the trimerophytes, distinctions between genera and species are becoming less readily apparent, supporting the suggestion that the trimerophytes are a group of closely related plants in which considerable evolution was occurring in late Lower and Middle Devonian times. Additionally, these plants appear to represent an early stage in the differentiation of a distinct main axis – lateral branch type of organizaiton that probably led to the later evolution of megaphyllous leaves.

The Thomson Orogen of the Tasman Orogenic Zone

The Thomson Orogen forms the northwestern segment of the Tasman Orogenic Zone. It was a tectonically active area with several episodes of deposition, deformation and plutonism from Cambrian to Carboniferous time. Only the northeastern part of the orogen is exposed; the remainder is covered by gently folded Permian and Mesozoic sediments of the Galilee, Cooper and Great Artesian Basins. Information on the concealed Thomson Orogen is available from geophysical surveys and petroleum exploration wells which have penetrated the Permian and Mesozoic cover. The boundaries of the Thomson Orogen with other tectonic units are concealed, but discordant trends suggest that they are abrupt. To the west, the orogen is bordered by Proterozoic structural blocks which form basement west of the northeast-trending Diamantina River Lineament. The most appropriate boundary with the Lachlan and Kanmantoo Orogens to the south is an arcuate line marking a distinct change in the direction of gravity trends. The north-northwest orientation of the northern part of the New England Orogen to the east cuts strongly across the dominant northeast trend of the Thomson Orogen. The Thomson Orogen developed as a tectonic entity in latest Proterozoic or Early Cambrian time when the former northern extension of the Adelaide Orogen ∗ was truncated along the Muloorinna Ridge. Early Palaeozoic deposition was dominated by finegrained, quartz-rich clastic sediments. Cambrian carbonates accumulated in the southwest and a Cambro-Ordovician island arc was active in the north. Along the western margin of the orogen, sediments were probably laid down on downfaulted blocks of deformed Proterozoic rocks, with oceanic crust further to the east. A mid- to Late Ordovician orogeny which affected the whole of the Thomson Orogen marked the climax of its precratonic (orogenic) stage. The northeast structural trend of the orogen (parallel to its western boundary with the Precambrian craton) was imposed at this time and has controlled the orientation of later folding and faulting. Up to three generations of folding have been recognized and fine-grained metasediments exhibit a prominent slaty cleavage. Metamorphism was to the greenschist and amphibolite facies, the highest grade rocks being associated with synorogenic granodiorite batholiths in the north. Following deposition of Late Ordovician marine sediments at the eastern margin, emplacement of post-tectonic Late Silurian or Early Devonian batholiths ended the precratonic history of the Thomson Orogen. The subsequent transitional tectonic regime was characterized by deposition of Devonian to Early Carboniferous shallow marine and continental sediments including widespread red-beds and andesitic volcanics. The maximum marine transgression occurred in the early Middle Devonian. Localized folding affected the easternmost part of the Thomson Orogen at the end of Middle Devonian time and was followed by intrusion of Devono-Carboniferous granitic plutons. However, the terminal orogeny which deformed all Devonian to Early Carboniferous rocks of the orogen was of mid-Carboniferous age. It produced northeast-trending open folds and normal and high-angle reverse faults which are considered to reflect basement structures. The cratonization of the Thomson Orogen was completed with the emplacement of Late Carboniferous granites and the eruption of comagmatic volcanics in the northeast, permian and Mesozoic sediments accumulated in broad, relatively shallow down warps which covered most of the former orogen.