Late Early Silurian Research Articles

Late Ordovician-Early Silurian extension of the northern margin of the Upper Yangtze Platform (South China) and its impact on organic matter accumulation

Breakthroughs in shale gas exploration in the Upper Ordovician-Lower Silurian strata of the Upper Yangtze Platform have attracted interest in its sedimentary-tectonic evolution, but the tectonic background of the northern margin of the Upper Yangtze Platform remains unclear. In this paper, the Wufeng-Longmaxi formations on the northern margin of the Upper Yangtze Platform were investigated. Based on geochemical and mineralogical analyses of the tuffs/K-bentonites of the Wufeng Formation and the barite in the Longmaxi Formation, as well as previous research results, it was concluded that the northern margin of the Upper Yangtze Platform was in an extensional tectonic background during the Late Ordovician-Early Silurian. Detailed analysis revealed that, (1) the U–Pb zircon age of the tuff in the Bajiaokou section in South Qinling is 443.91 ± 0.92 Ma. The Zr/TiO2–Nb/Y diagram of the tuffs/K-bentonites indicates that their protoliths were alkaline-subalkaline basalt and andesite series rock. Based on the Th–Hf/3-Ta, Th–Tb*3-Ta*2, and TiO2–Nb/3-Th diagrams, there are undiscovered intraplate tension calc-alkaline basalts in the northern Yangtze Platform or the southern Qinling region, which provided volcanic clastic materials to the Ziyang, Lan'gao, Chengkou, Yichang and other regions. (2) Scanning electron microscopy revealed that the barite crystals in the Longmaxi Formation exhibit dissolution features and have a large particle size. Energy spectrum analysis of these barite crystals revealed that they have C, O, S, and Ba contents of 8.48 wt%, 22.98 wt%, 13.09 wt% and 55.44 wt%, so they are speculated to have been formed via cold methane seep genesis in a weak extensional tectonic setting. The 87Sr/86Sr ratios of the barite revealed that different types of barite were simultaneously formed in this area under the influences of hydrothermal and cold methane seeps. (3) The analysis of the heavy minerals in the Lower Silurian strata in the Bajiaokou section revealed that the provenance in the South Qinling area changed significantly during the late Early Silurian. Based on the above analyses, the northern margin of the Upper Yangtze Platform was in an extensional tectonic setting during the Late Ordovician-Early Silurian. The distribution of the total organic carbon content indicated that the extensional tectonic background provided good conditions for the enrichment and preservation of organic matter. The results of this study provide an understanding of the regional sedimentary-tectonic pattern and evolution of the Yangtze Platform during this period, as well as a reference for future shale gas exploration in this region.

Zircon U–Pb geochronology and Hf isotopic constraints on the petrogenesis of the Late Silurian Shidonggou granite from the Wulonggou area in the Eastern Kunlun Orogen, Northwest China

ABSTRACTThe Wulonggou area in the Eastern Kunlun Orogen (EKO) in Northwest China is characterized by extensive granitic magmatism, ductile faulting, and orogenic gold mineralizations. The Shidonggou granite is located in the central part of the Wulonggou area. This study investigated the major as well as trace-element compositions, zircon U–Pb dates, and zircon Hf isotopic compositions of the Shidonggou granite. Three Shidonggou granite samples yielded an average U–Pb zircon age of 416 Ma (Late Silurian). The Late Silurian Shidonggou granite is peraluminous, with high alkali contents, high Ga/Al ratios, high (K2O + Na2O)/CaO ratios, and high Fe2O3T/MgO ratios, suggesting an A-type granite. The Shidonggou granite samples have zircon εHf(t) values ranging from −7.1 to +4.4. The Hf isotopic data suggest that the Late Silurian granite was derived from the partial melting of Palaeo- to Mesoproterozoic juvenile mantle-derived mafic lower crust. Detailed geochronological and geochemical data suggest that the Late Silurian granite was emplaced in a post-collisional environment following the closure of the Proto-Tethys Ocean. Combining data of other A-type granitic rocks with ages of Late Early Silurian to Middle Devonian, such post-collisional setting related to the Proto-Tethys Ocean commenced at least as early as ~430 Ma (Late Early Silurian), and sustained up to ~389 Ma (Middle Devonian) in the EKO.

Detrital zircon evidence for the reactivation of an Early Paleozoic syn-orogenic basin along the North Gondwana margin in South China

The Early Paleozoic Lian-Shao Basin, South China, is a syn-orogenic basin that developed on the northern margin of Gondwana in response to the Kwangsian orogeny. U–Pb detrital zircon age patterns from Middle Ordovician to Lower Silurian sandstones around the Jiangnan Uplift within the basin provide evidence for exhumation and erosion of the basin basement. The majority of the samples from both east and west of the Jiangnan Uplift display similar age distributions with a dominant peak between 1000 and 900Ma and subordinate peaks at ~2500Ma, ~1700Ma and 600–500Ma. However, the zircon age spectrum of one sample from the west side of the Uplift is distinct with a main peak at 880Ma and lacking grains younger than 650Ma. Combined geochronological and paleocurrent data suggest that detritus within the syn-orogenic basins was derived from the northern segment of East Gondwana but with additional input of detritus from the Jiangnan Uplift by the late Early Silurian. This information together with analysis of time equivalent strata along strike to the northeast suggests that the Jiangnan Uplift was not a continuous barrier to sediment transport from the southeast but represented part of a discontinuous series of islands supplying local detritus that was mixed with sediment transported from northern Gondwana.

Collisional accretion of a Late Ordovician oceanic island arc, northern Tasman Orogenic Zone, Australia

A distinctive Late Ordovician volcano-sedimentary terrane, embracing the Carriers Well Formation and Everetts Creek Volcanics and dismembered slivers now structurally intercalated in the adjoining Wairuna Formation, is located within the Broken River Province of the northern Tasmanides. It abuts a basement of mafic–ultramafic rocks (Gray Creek Complex) and overlying Early Ordovician deep marine sedimentary and volcanic strata (Judea Formation) which host tonalitic plutons. The terrane lies at the western, inboard-margin of the Camel Creek Subprovince, a broad tract of multiply deformed mid-Paleozoic turbidites with minor basalt and chert variously interpreted as the infill of a backarc basin or an accretionary wedge. U–Pb dates from detrital zircon indicate a maximum Late Silurian age for siliciclastic rocks from the previously undated Wairuna Formation. Geochemistry of volcanic rocks from the volcano-sedimentary terrane show them to be largely of mafic to intermediate compositions of calc-alkaline affinity, comparable with broadly coeval Macquarie Arc volcanic suites of the southern Tasmanides. Trace-element systematics identify a subduction relationship for the volcanic suite and V/Ti employed as a discrimination tool identifies the terrane as representing an oceanic island arc, consistent with its sedimentary facies which include volcaniclastic mass flow deposits, limestone, and radiolarian chert. Continent-derived sandstone in the sedimentary assemblage, confirmed by the ages of detrital zircon from a sandstone sample from the Carriers Well Formation, indicates that the oceanic island arc developed proximal to the Late Ordovician continental margin of East Gondwana. Its nature and location bear on the tectonic setting of the entire Camel Creek Subprovince, for which interpretation as an Early Silurian–Early Devonian accretionary wedge is favoured. Collision of the island arc with the continental margin, and associated deformation of part the intervening oceanic crustal tract, now represented by the Gray Creek Complex and its sedimentary cover (Judea Formation) registers the initiation of subduction accretion in late Early Silurian (Llandoverian) time. It marks early-stage orogenesis in the Broken River Province, accurately timed by stratigraphic relationships in the basinal succession developed in the Graveyard Creek Subprovince immediately to the west of the arc assemblage. Tectonism was regionally developed in north Queensland at this time, coeval with the Benambran Orogeny of the Lachlan Orogen in which the Macquarie Arc was likewise accreted to the East Gondwana margin. Benambran orogenesis marks a general phase of shortening, and removal by subduction, of oceanic crust and inversion of continent-derived overlying sedimentary cover along the East Gondwana margin.

Palynology and isotope geochronology of the Upper Ordovician–Silurian successions (Ghelli and Soltan Maidan Formations) in the Khoshyeilagh area, eastern Alborz Range, northern Iran; stratigraphic and palaeogeographic implications

Surface samples from the Ghelli and Soltan Maidan Formations in the Khoshyeilagh area of the eastern Alborz Range, northern Iran, were analyzed to determine the age and the stratigraphic relationship of these two units. The samples contained rich palynomorph assemblages, dominated by acritarchs (36 species recognized, distributed among 28 genera) and chitinozoans (nine species identified, distributed among seven genera). Cryptospores and scolecodonts as well as a few graptolite remains were also observed, although not studied in detail. Based on the restricted stratigraphic range of chitinozoan and acritarch species, a Late Ordovician (late Katian–Hirnantian) age is assigned to the Ghelli Formation. Palynological samples from the Soltan Maidan Formation yielded acritarch assemblages characterized by species commonly found in Upper Ordovician sediments together with typical middle Silurian forms. Considering reworking of the Upper Ordovician species, the age of the investigated part of the Soltan Maidan Formation is not younger than Gorstian (early Ludlow, early late Silurian). U–Pb zircon ages of 434.4 ± 6.4 Ma (Telychian, late Llandovery, late early Silurian) obtained for granitic clasts collected at the base of the Soltan Maidan Formation, are consistent with the inferred palynological age and indicate that granite emplacement, cooling, exhumation, erosion, transportation and deposition of its clasts took place in a time interval estimated to be 5 to 10 million year long. Based on the presence of diagnostic chitinozoan taxa, the Ghelli Formation can be assigned to the Armoricochitina nigerica, Ancyrochitina merga, and Spinachitina oulebsiri chitinozoan biozones. Chitinozoan assemblages reflect a clear palaeobiogeographic affinity with the previously defined ‘North Gondwana Domain’. The composition of acritarch assemblages also appears to be consistent with newly proposed hypotheses of a Late Ordovician phytoplanktonic biogeographical differentiation between a Laurentian/Baltica realm, and a Gondwanan realm to which the present assemblages belong. The presence of cryptospores together with taxonomically diverse chitinozoan, acritarch, and scolecodont assemblages, suggest a relatively shallow marine, platformal depositional environment for the Ghelli Formation.

Volcanology, geochemistry and structure of the Ordovician Cargo Volcanics in the Cargo – Walli region, central New South Wales

The Middle to lower Upper Ordovician Cargo Volcanics occur in a structural outlier immediately west of the contiguous Molong Volcanic Belt of the Ordovician Macquarie Arc. The sequence of basaltic to dacitic lavas, lava breccias and associated volcaniclastic rocks with medium-K calc-alkaline affinities has been interpreted as the subaqueous portion of a major intra-oceanic arc stratovolcano. The oldest part of the succession consists of massive to pillowed, poorly vesicular, aphyric to moderately plagioclase + clinopyxroxene-phyric basalt and andesite lavas and hypabyssal sills. The main part of the interpreted volcanic edifice is composed of massive and pillowed vesicular andesite and dacite, flanked by lenses of hyaloclastite, pillow breccias and interbedded with debris-flow and turbiditic deposits of crystal-rich and pebbly volcanic sandstones, siltstone and minor conglomerate. Uplift and erosion of the edifice prior to deposition of the overlying Eastonian Barrajin Group limestones is indicated by local aprons of thickly bedded subaqueous volcanic conglomerate and pebbly sandstone which appear to be separated from both the underlying andesite-dominant pile and overlying limestones by angular unconformities. The Cargo Volcanics, and the neighbouring Walli Volcanics further south, are clearly distinguished from other Macquarie Arc lavas by their high (>25) Zr/Nb values. However, the Walli Volcanics are readily differentiated from the Cargo Volcanics by their higher P2O5 contents and likely high-K calc-alkaline to shoshonitic affinities. The Cargo Volcanics are intruded by Cu – Au mineralised, plagioclase + hornblende + quartz-phyric dacites with medium-K calc-alkaline affinities. These dacites have lower TiO2 and higher MgO contents at any SiO2 level compared to the Cargo Volcanics, and are compositionally similar to Late Ordovician to Early Silurian (453 – 441 Ma) dacites at Copper Hill and in the Narromine Complex. However, at Cargo, clasts derived from the dacites are locally abundant in volcaniclastic conglomerate near the top of the Cargo Volcanics, indicating that the dacites were intruded and exhumed prior to deposition of the Barrajin Group limestones, which commenced at ca 454 Ma in this area. The dacites at Cargo are intruded by small, apparently unmineralised monzonitic intrusions with shoshonitic affinities, which also appear to pre-date deposition of the Barrajin Group. LA-ICPMS U – Pb dating of detrital zircons from the Ranch Member at the base of the Daylesford Limestone (basal Barrajin Group) revealed a dominant population with an average age of 453.0 ± 4.1 Ma (identical within error to the mid-Ea1 age for the host sediments based on fossil assemblages) and subordinate populations with average ages of 480 Ma and 505 Ma. The 453 Ma zircons were most probably derived from either the intrusive dacites or monzonites, suggesting that the Cargo region was rapidly exhumed following emplacement of the felsic intrusions. The angular unconformities near and at the top of the Cargo Volcanics and the broadly coincident change in magma chemistry suggest that this sector of the Macquarie Arc underwent major tectonic upheaval commencing in the latest Gisbornian – early Eastonian. The Cargo Volcanics are interpreted to have undergone major eastward translation at this time (from an original position now buried beneath the Cowra Trough). Their present juxtaposition with the contiguous Molong Volcanic Belt probably did not occur until the late Early Silurian during the final stages of the Benambran Orogeny.

Submarine gravity slides on the Paleozoic continental slope at the western edge of the Great Basin, east-central California: A mechanism for development of unconformities in slope environments

The middle Paleozoic continental slope, represented by rocks exposed near Badger Flat in the northwestern Inyo Mountains, at the western edge of the Great Basin in east-central California, failed by submarine gravity sliding twice during Silurian and Devonian time. Each time amajor unconformity was developed between the surface exposed beneath the slides and the much younger rocks deposited after the slope failures. The first slope-failure event took place between the late Early Silurian and the end of the Late Silurian when much of the upper member of the Late Ordovician-Early Silurian Ely Springs Dolomite was detached and displaced as a coherent slab ~1km down slope. The second event occurred in the MiddleDevonian when rocks of the Ely Springs Dolomite were again detached, this time forming a rockslide that traveled about 10km down slope to the northwest where it was deposited as a chaotic breccia >250m thick. These events took place as the regionwas undergoing substantial geomorphic change. The first detachment resulted from an abrupt steepening of the slope, an event previously recognized as having occurred all along the continental margin from eastern California to central Idaho. The second event probably was triggered by a pronounced drop in sea level on the already rather steep slope. At least one similar detachment resulting in an unconformity has been recognized along the middle Paleozoic margin in central Nevada. Recognition of these slope-failure events in an area of unusually complete exposure of middle Paleozoic slope deposits provides an excellent example of amechanism other than submarine erosion or nondeposition to explain the development of unconformities in ancient slope sequences.

Palaeomagnetism and geochemistry of Early Palaeozoic rocks of the Barrandian (Teplá-Barrandian Unit, Bohemian Massif): palaeotectonic implications

The Barrandian area (the Teplá-Barrandian unit, Bohemian Massif) provided palaeomagnetic results on Early Palaeozoic rocks and chemical data on siliciclastic sediments of both Middle Cambrian and Early Ordovician to Middle Devonian sedimentary sequences; an outcoming interpretation defined source areas of clastic material and palaeotectonic settings of the siliciclastic rock deposition. The siliciclastic rocks of the earliest Palaeozoic sedimentation cycle, deposited in the Cambrian Přı́bram-Jince Basin of the Barrandian, were derived from an early Cadomian volcanic island arc developed on Neoproterozoic oceanic lithosphere and accreted to a Cadomian active margin of northwestern Gondwana. Inversion of relief terminated the Cambrian sedimentation, and a successory Prague Basin subsided nearby since Tremadocian. Source area of the Ordovician and Early Silurian shallow-marine siliciclastic sediments corresponded to progressively dissected crust of continental arc/active continental margin type of Cadomian age. Since Late Ordovician onwards both synsedimentary within-plate basic volcanics and older sediments had been contributing in recognizable proportions to the siliciclastic rocks. The siliciclastic sedimentation was replaced by deposition of carbonate rocks throughout late Early Silurian to Early Devonian period of withdrawal of the Cadomian clastic material source. Above the carbonates an early Givetian flysch-like siliciclastic suite completed sedimentation in the Barrandian. In times between Middle Cambrian and Early/Middle Devonian boundary interval an extensional tectonic setting prevailed in the Teplá-Barrandian unit. The extensional regime was related to Early Palaeozoic large-scale fragmentation of the Cadomian belt of northwestern Gondwana and origin of Armorican microcontinent assemblage. The Teplá-Barrandian unit was also engaged in a peri-equatorially oriented drift of Armorican microcontinent assemblage throughout the Early Palaeozoic: respective palaeolatitudes of 58°S (Middle Cambrian) and 17°S (Middle Devonian) were inferred for the Barrandian rocks. The Middle Devonian flysch-like siliciclastics of the Prague Basin suggest a reappearance of the deeply dissected Cadomian source area in a proximity of the Barrandian due to early Variscan convergences and collisions of the Armorican microcontinents. Significant palaeotectonic rotations are palaeomagnetically evidenced to take place during oblique convergence and final docking of the Teplá-Barrandian microplate within the Variscan terrane mosaic of the Bohemian Massif.

What caused the Early Silurian change from mafic to silicic (S‐type) magmatism in the eastern Lachlan Fold Belt?

One of the most significant, but poorly understood, tectonic events in the east Lachlan Fold Belt is that which caused the shift from mafic, mantle‐derived calc‐alkaline/shoshonitic volcanism in the Late Ordovician to silicic (S‐type) plutonism and volcanism in the late Early Silurian. We suggest that this chemical/isotopic shift required major changes in crustal architecture, but not tectonic setting, and simply involved ongoing subduction‐related magmatism following burial of the pre‐existing, active intraoceanic arc by overthrusting Ordovician sediments during Late Ordovician — Early Silurian (pre‐Benambran) deformation, associated with regional northeast‐southwest shortening. A review of ‘type’ Benambran deformation from the type area (central Lachlan Fold Belt) shows that it is constrained to a north‐northwest‐trending belt at ca 430 Ma (late Early Silurian), associated with high‐grade metamorphism and S‐type granite generation. Similar features were associated with ca 430 Ma deformation in east Lachlan Fold Belt, highlighted by the Cooma Complex, and formed within a separate north‐trending belt that included the S‐type Kosciuszko, Murrumbidgee, Young and Wyangala Batholiths. As Ordovician turbidites were partially melted at ca 430 Ma, they must have been buried already to ∼20 km before the ‘type’ Benambran deformation. We suggest that this burial occurred during earlier northeast‐southwest shortening associated with regional oblique folds and thrusts, loosely referred to previously as latitudinal or east‐west structures. This event also caused the earliest Silurian uplift in the central Lachlan Fold Belt (Benambran highlands), which pre‐dated the ‘type’ Benambran deformation and is constrained as latest Ordovician — earliest Silurian (ca 450–440 Ma) in age. The south‐ to southwest‐verging, earliest Silurian folds and thrusts in the Tabberabbera Zone are considered to be associated with these early oblique structures, although similar deformation in that zone probably continued into the Devonian. We term these ‘pre’‐ and ‘type’‐Benambran events as ‘early’ and ‘late’ for historical reasons, although we do not consider that they are necessarily related. Heat‐flow modelling suggests that burial of ‘average’ Ordovician turbidites during early Benambran deformation at 450–440 Ma, to form a 30 km‐thick crustal pile, cannot provide sufficient heat to induce mid‐crustal melting at ca 430 Ma by internal heat generation alone. An external, mantle heat source is required, best illustrated by the mafic ca 430 Ma, Micalong Swamp Igneous Complex in the S‐type Young Batholith. Modern heat‐flow constraints also indicate that the lower crust cannot be felsic and, along with petrological evidence, appears to preclude older continental ‘basement terranes’ as sources for the S‐type granites. Restriction of the S‐type batholiths into two discrete, oblique, linear belts in the central and east Lachlan Fold Belt supports a model of separate magmatic arc/subduction zone complexes, consistent with the existence of adjacent, structurally imbricated turbidite zones with opposite tectonic vergence, inferred by other workers to be independent accretionary prisms. Arc magmas associated with this ‘double convergent’ subduction system in the east Lachlan Fold Belt were heavily contaminated by Ordovician sediment, recently buried during the early Benambran deformation, causing the shift from mafic to silicic (S‐type) magmatism. In contrast, the central Lachlan Fold Belt magmatic arc, represented by the Wagga‐Omeo Zone, only began in the Early Silurian in response to subduction associated with the early Benambran northeast‐southwest shortening. The model requires that the S‐type and subsequent I‐type (Late Silurian — Devonian) granites of the Lachlan Fold Belt were associated with ongoing, subduction‐related tectonic activity.

The microfossil record of early land plants.

Dispersed microfossils (spores and phytodebris) provide the earliest evidence for land plants. They are first reported from the Llanvirn (Mid-Ordovician). More or less identical assemblages occur from the Llanvirn (Mid-Ordovician) to the late Llandovery (Early Silurian), suggesting a period of relative stasis some 40 Myr in duration. Various lines of evidence suggest that these early dispersed microfossils derive from parent plants that were bryophyte-like if not in fact bryophytes. In the late Llandovery (late Early Silurian) there was a major change in the nature of dispersed spore assemblages as the separated products of dyads (hilate monads) and tetrads (trilete spores) became relatively abundant. The inception of trilete spores probably represents the appearance of vascular plants or their immediate progenitors. A little later in time, in the Wenlock (early Late Silurian), the earliest unequivocal land plant megafossils occur. They are represented by rhyniophytoids. It is only from the Late Silurian onwards that the microfossil/ megafossil record can be integrated and utilized in interpretation of the flora. Dispersed microfossils are preserved in vast numbers, in a variety of environments, and have a reasonable spatial and temporal fossil record. The fossil record of plant megafossils by comparison is poor and biased, with only a dozen or so known pre-Devonian assemblages. In this paper, the early land plant microfossil record, and its interpretation, are reviewed. New discoveries, novel techniques and fresh lines of inquiry are outlined and discussed.

New paleontological and geological data on the Ordovician and Silurian of Bolivia

The oldest vertebrates of South America are from the thick Anzaldo (=Cuchupunata) Formation in central Bolivia. At the scale of the basin, the Anzaldo is overlain by the San Benito, Tokochi, Cancañiri, Llallagua and Kirusillas/Uncía formations. The Anzaldo was classically dated Caradoc (early Late Ordovician), but recent paleontological data have suggested a Llanvirn age (early Middle Ordovician). The only significant fossil invertebrates recently collected in the Anzaldo, viz., lingulid brachiopods, give an age not older than Late Ordovician. Fossils from the Tokochi suggest a Caradoc age. The microfossils (acritarchs and foraminifers mainly) collected in the Cancañiri and Kirusillas/Uncía formations indicate an Ashgill to Wenlock age (late Late Ordovician to late Early Silurian) for these formations. A Caradoc (or perhaps older) age thus seems more correct for the Anzaldo Formation. These new paleontological data have major implications on our knowledge of the Ordovician-Silurian basins of Bolivia: 1) transition from a Middle Ordovician marine foreland basin to a Late Ordovician-Llandovery glacial-marine to turbidite trough in the Altiplano occurred in the (late?) Caradoc; 2) a major sea-level rise developed around the Llandovery-Wenlock boundary; 3) a fossiliferous limestone member of shallow origin and early Wenlock age is present approximately between Cochabamba and Santa Cruz.

The stratigraphy, depositional environments and periods of deformation of the Wajid outcrop belt, southwestern Saudi Arabia

The Wajid outcrop belt in southwestern Saudi Arabia is an area of over 22,000 square kilometers where strata of Middle(?) Cambrian through to Jurassic crop out. The Early Palaeozoic sediments in southwestern Arabia have been placed in the Wajid sandstone. This paper elevates this unit to group status and recognizes its former members as formations. The remaining Palaeozoic successions are placed in the Qalibah and Khuff formations. The Middle(?) Cambrian through to Early(?) Ordovician Dibsiyah Formation, Wajid Group, correlates with the Saq sandstone of central and northern Saudi Arabia. The ‘lower’ Dibsiyah buries a topography developed on the Arabian shield during the Late Precambrian Pan-African and/or the late Early Cambrian periods of uplift. The late Early Cambrian period of deformation is documented in southwestern Saudi Arabia and adjacent Oman by an angular unconformity which separates the Dibsiyah Formation correlative from the underlying Late Precambrian-Early Cambrian Lower Haima Group. The ‘lower’ Dibsiyah was deposited in a northward prograding fluvial apron. The ‘upper’ Dibsiyah was deposited in intertidal to shallow subtidal marine settings during a period of coastal onlap. The Late(?) Ordovician Sanamah Formation overlies the Dibsiyah Formation in angular and/or erosional unconformity. This unconformity was developed during a period of regional uplift which began near the close of the Middle Ordovician. The Sanamah Formation was deposited in broad valley complexes excavated in fluvial settings. The Early Silurian Qusaiba Member of the Qalibah Formation unconformably overlies the Sanamah. The Qalibah Formation was deposited in shallow marine to mid-shelf settings during a major period of coastal onlap. The angular unconformity to disconformity, which separates the Qalibah Formation from underlying units, resulted from diastrophism of Late Ordovician to earliest Silurian age. The Early Devonian to Middle(?) Carboniferous Khusayyayn Formation unconformably overlies the Qalibah Formation. The Khusayyayn Formation was deposited in shallow marine, lagoonal, eolian and fluvio-deltaic settings. The unconformity separating the Khusayyayn Formation from the Qalibah Formation documents a period of regional diastrophism of late Early Silurian to late Early Devonian age. Unconformably overlying the Khusayyayn is the Middle Carboniferous through Early Permian Juwayl Formation. This unit was deposited in broad valley complexes excavated in fluvial settings. The base Juwayl unconformity was produced by the Hercynian orogeny. The Permian Khuff Formation unconformably overlies the Juwayl and the former deposited in shallow marine settings. The base Khuff Formation unconformity resulted from a period of major coastal onlap. The Early Triassic Sudair shale succeeds the Khuff and the former was deposited during a period of coastal offlap in lagoonal to coastal plain settings. Near the end of the Early Triassic the Arabian Peninsula underwent another period of tectonism. In central and southwest Saudi Arabia this event is reflected by the erosion of the Sudair shale and older units from the higher portions of reactivated fault blocks and the syndepositional thickening of the Jilh Formation on the down thrown sides of these fault blocks.

Silurian volcanism in the Wollondilly Basin, eastern Lachlan Fold Belt, New South Wales

The Wollondilly Basin, east of Goulburn in the Southern Highlands of New South Wales, contains a mafic to silicic volcanic succession together with limestone and clastic deposits. The oldest unit, the Gundary Formation, includes clastic deposits, silicic pyroclastic flows, and mafic‐intermediate lavas and was emplaced, at least in part, in a terrestrial environment. Fresh hornblende separates from two samples of dacitic ignimbrite have an average K‐Ar age of 426 ± Ma (late Early Silurian). The mafic‐intermediate flows have a shoshonitic geochemical signature that reflects the nature of their source material. The Gundary Formation is in faulted contact with the marine Boxers Creek Formation which conformably underlies a Late Silurian turbidite unit, the Towrang Formation. Cessation of volcanic activity is reflected by decreasing input of volcanic detritus up section and the increase in detritus derived from uplifted Ordovician basement rocks. The succession demonstrates that rocks with shoshonitic affinity...

Ordovician emplacement of the Mount Dingley Diatreme, Western Ranges of the Rocky Mountains, southeastern British Columbia

External and internal morphologies are well shown by a newly discovered diatreme that is exceptionally well exposed in a cirque within the north face of Mount Dingley. The diatreme contains abundant brecciated host rocks mixed with highly altered, fine-grained, light-green igneous fragments (minerals include muscovite, chlorite, quartz, carbonate, and some remnant K-feldspar). The diatreme cuts Lower Ordovician rocks of the McKay Group. Olistostromes and other volcaniclastic rocks that are directly associated with the diatreme are bevelled beneath a regional unconformity below the Upper Ordovician Beaverfoot Formation. Lower Ordovician gastropods are present just below the volcaniclastic rocks and within what appears to be a lens of sediment within one of the olistostrome beds. These occurrences indicate a mid-Early Ordovician time of intrusion, but there is the possibility that the pipe was emplaced later within the interval mid-Early to early Late Ordovician. In the Western Ranges, three other episodes of emplacement of diatremes have been documented previously as within the intervals early Middle to early Late Ordovician, latest Early Silurian to early Middle Devonian, and Late Permian. Many of the diatremes are broadly contemporaneous with widespread, but volumetrically small, Ordovician and Lower Paleozoic volcanic and intrusive rocks found throughout the Canadian Cordillera. These volcanic and intrusive rocks have been interpreted as evidence of continued Lower Paleozoic extensional tectonism and some are associated with large base-metal deposits.

A paleoclimate simulation of the Wenlockian (Late Early Silurian) world using a general circulation model with implications for early land plant paleoecology

The Silurian Period (439–409 Ma) is known for its extensive organic-rich, graptolitic, black shales and graptolitic, shelf carbonates. Physical conditions drive paleoclimate and control the zonal deposition of lithotopes. Moreover, the paleoclimate created a paleoceanic environment favorable for widespread generation, deposition, and preservation of plankton. The relationship between the paleogeographic framework, including paleotopography, and the resultant bio- and lithostratigraphy are suitable for study with a general circulation model (GCM). For this study we chose the Wenlockian Stage (430–424 Ma), the late Early Silurian. The Wenlockian northern hemispheric surface was dominated by a large ocean, the southern hemisphere by the giant Gondwanan continent. Much of Gondwana's extensive margin was in the mid-latitudes. Laurentia and Baltica occupied a tropical position, and Siberia and Kazakh laid in warm temperate latitudes to the north. Silurian stratigraphy fits a paleoatmosphere with elevated greenhouse conditions. Estimated Silurian atmospheric CO 2 values vary between wide limits. We used 1120 ppm CO 2 4x that of the pre-industrial level and at the lower end of the range of estimates. The paleoclimate was forced by the paleogeography of each hemisphere's circulation. The northern hemisphere Silurian simulation is dominated by strong zonal circulation in all seasons. In contrast, the continental southern hemisphere reacts to the summer heating and winter cooling of Gondwana. This simulation furnishes paleoclimatic conditions that help explain the distribution of early land plants. This paleoclimate simulation supports a humid coastal paleoenvironment for early Silurian land plants. Furthermore, these results imply that relative humidity was more important than precipitation rates, and that intercontinental spore dispersal between Laurentia/Europe and Gondwana was not wind-aided. The GCM provides a good match with Silurian lithological and paleobiological data.

Major Paleozoic land plant evolutionary bio-events

Adaptive radiations and extinctions affecting tracheophytes, the only group of land plants with a well-documented fossil record, occur during time intervals distinct from those affecting marine invertibrates. This distinctive pattern is recognizable at both Era and Period levels. Discordance between tracheophyte appearance, diversification and extinction and contemporary marine bio-events has been reflected in the recognition of ‘planetary’ floras (Palaeophytic, Mesophytic, Cenophytic), the counterparts of marine invertibrate-based Eras of the Planerozoic (Paleozoic, Mesozoic, Cenozoic). Some have recognized significant levels of land-plant evolution, Epochs, within these planetary floras. With the discovery in the past 20 years of a major group of Middle Ordovician/Lower Silurian post-algal/pre-tracheophytic land plants, the concept of the Palaeophytic and its Epochs must be redefined. Here, the redefined Palaeophytic extends from the mid-Permian downward into the mid-Ordovician and three ‘evolutionary land floras’ are recognized: Eoembryophytic (mid-Ordovician to pre-latest Early Silurian); Eotracheophytic (latest Early Silurian through early Gedinnian); and Eutracheophytic (late Gedinnian to mid-Permian). The Silurian/Devonian boundary stressed in prevous Palaeophytic subdivisions is considered of limited importance as a major benchmark in land plant evolution. A newly defined Era, the Epeirophytic, a continental counterpart of Kryshtofovich's marine Thalassophytic, is recognized with an upper boundary at the mid-Ordovician and extending down indefinitely into the Precambrian. One major evolutionary flora, the Phycomycophytic, first recognized by Kryshtofovich for the Thalassophytic, is recognized for the continental habitat as well.

Late Early Silurian (Wenlockian) general circulation model-generated upwelling, graptolitic black shales, and organic-rich source rocks—An accident of plate tectonics?

Through an evolutionary process of plate tectonics, the Wenlockian physical world was composed of an oceanic northern hemisphere and a mainly continental southern hemisphere dominated by the giant continent of Gondwana. The high-latitude position of Gondwana placed much of its extensive margin in the middle latitudes. Laurentia and Baltica occupied an equatorial position; Siberia and Kazakh were to the north. The global paleoclimate was created and driven by the diverse paleogeography of the two hemispheres. In the model, the northern hemisphere is dominated by strong zonality in all seasons. In contrast, the continental southern hemisphere reacts to wide temperature ranges of summer heating and winter cooling of Gondwana. The extensively flooded, ∼28000 km margin of Gondwana occupied the zone of the southern hemisphere westerlies. These westerlies produced intense surface upwelling and, by extension of present analogies, high primary biologic productivity. Gondwana's northern margin extended far enough toward the equator that significant winter sea ice did not form in the southern hemisphere: this allowed a vertically stratified sea to develop, as well as probable dysaerobic and anoxic conditions. A high organic flux of marine plankton accumulated on the sea floor, where it was subsequently buried and preserved. A strong positive correlation exists between upwelling generated by a general circulation model (GCM) on Gondwana's northern margin and the distribution of southern hemisphere basins with source rocks containing types I and II kerogens.

Age and correlation of the Honorat Group, southern Gaspé Peninsula

The Honorat Group of southern Gaspé consists of two formations: the Arsenault and the Garin. The Arsenault Formation, heretofore considered barren, has yielded a graptolite faunule indicative of a Llanvirn–Llandeilo age (early Middle Ordovician), suggesting a correlation with the lower Mictaw Group of Gaspé as revised by de Broucker. A hiatus of indeterminate magnitude, corresponding to the Taconic unconformity, probably separates the Arsenault Formation from the overlying Garin Formation. The Garin has yielded graptolites ranging from the upper Climacograptus spiniferus Zone to the Paraclimatograptus manitoulinensis Zone (late Middle to early Late Ordovician). The C. spiniferus Zone graptolites are identical to those of the upper γ sequence of the Cloridorme Formation of northern Gaspé and the Blind Brook Formation of the Munsungun Anticlinorium of Maine but differ somewhat from those from the upper Tetagouche Group of New Brunswick, which are closer to those from the Summerford and Exploits groups of north-central Newfoundland. The C. spiniferus zone has a wide distribution in eastern North America. It correlates with the Orthograptus amplexicaulis Zone of the southwestern United States and with the Climacograptus baragwathi Zone (Ea2) of the Pacific faunal province.The Matapédia Group, stratigraphically above the Honorat Group, has yielded both shelly fossils and a few graptolites. The ages of the graptolites now date it as Late Ordovician to late Early Silurian (Dicellograptus complanatus Zone – Monograptus sedgwickii Zone).

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------------