Younger Assemblages Research Articles

Review of Archean supracrustal assemblages of the southern Abitibi greenstone belt in Ontario, Canada: products of microplate interaction within a large-scale plate-tectonic setting

Previously formulated plate-convergent models account for many aspects of the large-scale regularity and parallelism between different subprovinces of the Archean Superior Province. Such models do not, however, appear to adequately explain the “within-subprovince” evolution of 2.75-2.68 Ga supracrustal assemblages of the Late Archean southern Abitibi greenstone belt. Principal supracrustal assemblage classes include: (l) Assemblages dominated by tholeiitic metabasalt and metakomatiite with varying amounts of tholeiitic intermediate to felsic metavolcanic rocks. These assemblages probably formed in extensional settings. The ratio of “basalt to komatiite” and the ratio of “basalt + komatiite to intermediate+felsic” metavolcanic rocks may be a function of mantle temperature and crustal thickness at the site of extension, respectively. (2) Assemblages dominated by intermediate to felsic fragmental and/or effusive rocks are generally of calcalkalic affinity and likely formed in island-arc settings. (3) Assemblages containing a substantial component of iron formation may represent older supracrustal assemblages that form structural and/or stratigraphic basement to some of the younger supracrustal assemblages. (4) Assemblages consisting almost wholly of turbidite facies metasedimentary rocks appear to postdate volcanic activity related to the previous assemblages. Deposition of these sediments broadly coincides with the onset of regional deformation. These assemblages may represent distal deposits related to an emerging collisional orogen. (5) Assemblages consisting of alluvial-fluvial metasedimentary rocks with or without spatially associated alkalic metavolcanic rocks are associated with crustal-scale shear zones and are the youngest assemblages within the greenstone belt. Alluvial-fluvial assemblages likely represent the final emergence of the consolidating orogen. Geochronological data and assemblage distribution suggest that the supracrustal assemblages of the southern Abitibi greenstone belt may represent the amalgamation of products of diverse and intricate microplate-tectonic settings repeated in time and space prior to the impingement of the large-scale plate-tectonic regime that dictated the final large-scale configuration of the Archean Superior Province.

Archaeocyaths from Yorke Peninsula, South Australia and archaeocyathan Early Cambrian zonation

Zhuravlev, A. Yu., & Gravestock, D. I., 1994:03:28. Archaeayaths from Yorke Peninsula, South Australia and archaeocyathan Early Cambrian zonation. Alcheringa 18, 1–54. ISSN 0311-5518. Two assemblages of archaeocyaths are documented from Lower Cambrian outcrops and drillholes on Yorke Peninsula. South Australia. The older assemblage (11 species) occurs in the uppermost Kulpara Formation and conformably overlying basal Parara Limestone, and is equivalent to Lower Faunal Assemblage II in the Flinders Ranges. The younger assemblage (28 species plus Acanthhcyathus and Rodiocyathus) occurs in the Koolywurtie Member near the top of the Parara Limestone. Equivalent taxa are widespread in the Flinders Ranges, western New South Wales and Antarctica. Archaeocyathan distribution is now sufficiently well known to propose three assemblage zones and two informal assemblages for regional correlation. No new taxa have been added, but Erugatocyathus scutatus (Hill) and Pycnoidocyathus latiloculatus (Hill), hitherto known only from Antarctica, are found in the upper assemblage on Yorke Peninsula. Irregular archaeocyathan systematics are discussed, the ontogeny of Archaeopharetra irregularis (Tylor) clarified, and Kruseicnema Debrenne. Gravestock & Zhuravlev, represented by K. gracilis (Gordon), is fully described.

New Caradoc brachiopods from the Iberian Chains (northeastern Spain) and their stratigraphic significance

New collections of Caradoc brachiopods from the lower half of the Fombuena Formation in the Iberian Chains (northeastern Spain) have yielded rich fossil associations that, in spite of the endemism of most of their species, permit detailed correlations with other sequences in southwestern Europe, Bohemia, and the British Isles. The oldest association studied occurs at the top of the basal oolitic ironstone of the Fombuena Formation, where 10 brachiopod species have been recorded; the youngest association occurs in sandstones in the middle part of the unit, having yielded seven brachiopod species. Herein, 11 forms are described including four new species (Gelidorthis carlsi, Saukrodictya tormoensis, Reuschella herreraensis, andRostricellula marciali) and one new subspecies (Aegiromena aquila intermedia). The stratigraphic setting of the oolitic ironstone and its brachiopods suggest a correlation with similar beds that crop out at various localities in the Iberian Peninsula and northwestern France, as well as with the Zdice-Nucice Iron Ore, placed at the base of the Vinice Formation in Bohemia. The regressive–transgressive cycle, whose point of maximum shoaling coincides with the deposition of the apparently widespread ferruginous bed, is restricted to the Mediterranean Province and may have been caused by regional epeirogenic movements. The stratigraphic gap marked by the ironstone had been assessed at several localities in Iberia and Armorica; it ranges from late Costonian to Soudleyan, an age not in disagreement with the occurrence of a new species ofReuschellaimmediately above the ironstone of the Iberian Chains, sinceReuschellais not known before the Soudleyan.Dalmanella unguis unguis, which has been recorded in the youngest assemblage and which in Wales is restricted to the Marshbrookian, is another important guide form in the problematic correlation between the Mediterranean Caradoc and the British type sequence.

The geology and regional correlation of the Proterozoic Okwa Inlier, western Botswana

The small Okwa inlier exposes four unconformity-bounded Proterozoic lithostratigraphic assemblages in the Kalahari region of central southern Africa, where the Precambrian geology is otherwise largely obscured by Phanerozoic strata. The inlier provides the only exposures in the Okwa basement, a structural block between the Archaean Kaapvaal-Zimbabwe Craton and the Late Proterozoic Damara orogen. Felsites and sericitic quartzites of the Okwa basement complex were intruded by varied granites between 2.1 and 2.0 Ga ago. Deformation of the granites to form gneisses at ∼ 1.8 Ga (D 1) was accompanied by metamorphism of pre-tectonic mafic dykes in the epidote amphibolite facies. In the Mid-Proterozoic, felsic tuffs and red sandstones of the lower Okwa group were intruded by dolerite sills, and a contemporary northeasterly dyke-swarm intruded the basement granitoids. A fracture cleavage was imposed on the lower Okwa group during folding and greenschist/low amphibolite facies metamorphism (D 2) at ∼ 1.15 Ga. The red-bed sediments of the upper Okwa group suffered only very mild deformation (D 3). The youngest assemblage is assigned to the Late Proterozoic Nama Group. It mainly consists of gently dipping, fine-grained sandstones. The Okwa basement complex is confirmed as a segment of Eburnian-aged crust which was accreted to the Archaean craton at about 1.8 Ga, probably adjacent to epicratonic sediments allied to the Magondi Supergroup of Zimbabwe or to the Olifantshoek Sequence of South Africa, although such sediments are nowhere exposed in the Okwa sector. The lower Okwa group is thought to be equivalent to volcano-sedimentary sequences such as the Sinclair Group of Namibia. The upper Okwa group can be correlated either with immediately pre-Damaran units such as the Auborus Formation of the Sinclair area, or with the Nosib and Ghanzi Groups in the early Damara Supergroup. The occurrence of Nama sediments overlying the Okwa basement seems to confirm their suspected presence beneath the Karoo Supergroup in the Ncojane-Nosop Basin of western Botswana.

Volcanic history and tectonic development of the Central High Cascade Range, Oregon

Two distinct volcanic assemblages characterize the central Cascades of Oregon. The earlier, larger, and longer lived of the two is part of a calc‐alkaline, continental margin andesite belt exposed in the Western Cascades. The younger assemblage, conventionally known as the “High Cascades,” overlies the eastern part of the older assemblage and extends eastward to the Deschutes Valley. The High Cascades are also calc‐alkaline, but mafic components of a broad lava flow platform surround isolated silicic centers. A short‐lived episode of subsidence produced a graben within the central High Cascades, Subsequent mafic eruptions largely filled the graben, while the silicic centers continued to be active. This twofold volcanic development followed by extreme localized crustal extension is ascribed to the influence of slow circulating currents in the sub‐Cascade mantle, induced by drag against the subducted slab. Circulation of the lithosphere was enhanced by prolonged metasomatism and loss of strength in the mantle wedge; similar circulation may be a common or even essential component of volcanism above other subduction systems. The rates of convergence and subduction have decreased in the Cascade system to a degree that has permitted conductive heating of the slab and adjacent mantle. Circulation has migrated to higher levels within the mantle wedge, causing a more direct interaction with the crust, development of a new mafic‐dominated volcanic assemblage, and graben subsidence.

Shape analysis of two sympatric coral species: Implications for taxonomy and evolution

Fourier shape analysis of Pleurodictyum americanum and P. dividua, favositid tabulate corals, from the Middle Devonian Hamilton Group of New York State, indicates that colony growth form is variable within species between environments, but that the range of variability in growth form is static through time. Pleurpdictyum dividua displays a columnar morphotype; however, P. dividua and P. americanum cannot be discriminated from one another based on colony shape alone. A major prediction of the punctuated equilibrium model of evolution is morphological stasis throughout the duration of a species. Harmonic means of assemblages of P. americanum show no persistent temporal trends. The range of harmonic means present among the oldest assemblages (Centerfield Limestone) are comparable to those among the youngest assemblages (Windom Shale) and to those in the intervening intervals. Character stasis in colony growth form lasted throughout a 2.5-3.5 my interval. In tabulate corals, where reproductive modes such as fragmentation have not been demonstrated, growth form may be useful in interpreting ancient environments. In addition, the tempo of evolution of growth form can be analyzed when sufficient morphological data are available from a wide distribution of environments. Thus, Fourier shape analysis of growth form provides a powerful tool for paleontologists to interpret the ecology and evolution of colonial marine animals.

Late Cretaceous Kossmaticeratidae (Ammonoidea) from the Magallanes Basin, Chile

Even though ammonites are not very abundant in Patagonia, a large number of specimens were collected by geologists from the Empresa Nacional del Petroleo (ENAP), Chile, during the course of several years of field work in the Magallanes Basin. This study describes the ammonite family Kossmaticeratidae present in that collection, which is a typical element of most southern hemisphere Late Cretaceous assemblages. The species described include: Kossmaticeras (Kossmaticeras?) fasciculatum n. sp., K. (Karapadites) constrictus n. sp., K. (Natalites) cecioni n. sp., K. (Natalites) hauthali (Paulcke), K. (Natalites) sp., Neograhamites kiliani Spath, N. taylori Spath, Gunnarites elegans n. sp., G. bhavaniformis (Kilian and Reboul), Grossouvrites gemmatus (Huppe), and Maorites densicostatus (Kilian and Reboul).The stratigraphic information on this fauna indicates that it is distributed in four assemblages. The oldest (assemblage A), possibly late Santonian to early Campanian in age, includes the species Kossmaticeras (Kossmaticeras?) fasciculatum, Neograhamites taylori, and possibly N. kiliani. This is followed by assemblage B (possibly middle to late Campanian), with K. (Karapadites) constrictus, K. (Karapadites) centinelaense (Blasco, Nullo, and Proserpio), K. (Natalites) hauthali, K. (Natalites) cecioni, Neograhamites morenoi Riccardi, N. taylori, N. kiliani, K. (Natalites) sp., Gunnarites elegans, G. bhavaniformis, and possibly Pseudokossmaticeras paulckei Collignon. The third assemblage (C), possibly late Campanian to early Maastrichtian in age, contains G. elegans, G. bhavaniformis, Grossouvrites gemmatus, and K. (Natalites) hauthali. The youngest assemblage (D) is possibly late Maastrichtian in age and is composed of the species Maorites densicostatus and Grossouvrites gemmatus.

Palynology of Kamthi Formation in Godavari Graben

Five palynological assemblages have been recognized in the Kamthi Formation of Godavari Graben. The assemblages are characterized by overall abundance of striate disaccate pollen. The associated palynofossils, however, distinguish the palynoassemblages at various levels of the formation. The Lower Member containing coal seams shows unequivocal resemblance with the Raniganj palynoflora of Damodar Valley and Son Mahanadi coalfields District change in lithology at the beginning of the Middle member is marked by the appearance of Parasaccites rich assemblage simulating a cooling phase akin to that observed during the Talchir period. The younger sediments of the Middle Member contain Corisaccites Guttulapollenites Assemblage and Densipollenites Assemblage in order of succession representing the uppermost Permian palynoflora in Godavari Graben. The youngest assemblage also indicates close proximity to the Permian Triassic, thus making the Kamthi Formation a time transgressive unit.

Chitinozoaires du Caradoc supérieur – Ashgill inférieur du Québec, Canada

Chitinozoans from samples of the Trenton, Utica, and Lorraine groups of the St. Lawrence Lowlands are described.The studied assemblages contain well-known upper Caradocian and Ashgillian species and also new species, such as Conochitina pygmaea and Hercochitina cristata.The stratigraphic and geographic distribution of the microfossils allows identification of taxa with stratigraphic value.The assemblages so characterized also allow a link to be made between younger assemblages from the Ashgillian sequence of Anticosti Island, previously described by the author, and older ones described by Martin from Caradocian beds of the St. Lawrence Lowlands.

Palynological correlation of coal seams in Godavari Graben, Andhra Pradesh, India

The palynological study of various coal seams from Yellandu, Kothagudem, Belampalli, Ramakrishnapuram and Ramagundam coalfields of Godavari Graben has been done. The distributional pattern of various palyno-taxa has suggested the occurrence of six miospore assemblages. Miospore Assemblage E is marked by the association of Parasaccites and Scheuringipollenites. Assemblage D is characterised by the dominance of Brevitriletes. In assemblages C1, C2 and C3, Brevitriletes is associated with Hennellysporites. Indotriradites, although rare, occurs only in this assemblage. Horriditriletes and Latosporites characterise Assemblage B in association with Brevitriletes Primuspollenites gains significance only at this stage of palynofloral succession. In the youngest Assemblage A all the triletes decrease appreciably giving way to nonstriate-disaccates, Scheuringipollenites, so as to take up the dominance. The sporological succession shows only one change after Assemblage E (Upper Karharbari) was deposited. Miospore assemblages D to A represent the Lower Barakar palynoflora.

Quaternary glaciomarine deposits and foraminifera from Edgeøya, Svalbard

Late Quaternary marine deposits from Edgeoya (eastern part of the Svalbard archipelago) were analysed lithologically and micropalaeontologically. The studied sections show coarsening upward developments which reflect shallowing of the depositional area connected to Holocene land uplift. Four foraminiferal assemblages are recognized and are referred to successively decreasing water depths. The oldest assemblage is dominated by Elphidum excavatum and was formed under high glacial influence at a depth of c. 75 m in late Younger Dryas time. The youngest assemblage is dominated by Elphidium frigidum and reflects Holocene shallow water to beach conditions. Transverse ridges (mostly of cross bedded gravel) occurring in the outer parts of the valleys are interpreted as submarine ice front deltas accumulated during a period with permanent position of the glacier fronts. This stop in glacial retreat, called the Visdalen event, is referred to early Preboreal time.

Late villafranchian and early mid-pleistocene faunas in Italy

Typical Upper Villafranchian assemblages survive with slight changes up to the end of the Lower Pleistocene. One of these, near Imola, can be correlated with a marine fauna attributed to the Upper Calabrian by Gignoux (1916), and overlies the Plio-Pleistocene boundary by more than 1,000 m. Another faunal assemblage from Farneta, southern Tuscany, is perhaps the youngest assemblage with Villafranchian affinities in Italy. The beginning of the Cromerian is marked by a spectacular faunal revolution. In the surroundings of Rome the Cromerian can be correlated with the marine Sicilian. Other Cromerian faunas are recorded from Tuscany (San Gimignano and Farneta) and from Piedmont (Cogtiglione Monferrato).

Correlation of Silurian Strata Between Georgia, Alabama, and Florida Based on Chitinozoan Biostratigraphy: ABSTRACT

Silurian chitinozoans have been described previously from 4 wells in north-central Florida. These wells also have been correlated previously with each other as well as with known Silurian sections. The youngest assemblage is of Ludlovian age; the oldest is of late Llandoverian age. Outcrop samples were collected from the Silurian Red Mountain Formation in Alabama and Georgia. This formation has been dated as Llandoverian on the basis of megafossil evidence. Four Red Mountain Formation sections are divided into biostratigraphic zones on the basis of chitinozoan assemblages. Attempts at correlating the well and outcrop samples on the basis of frequency of chitinozoan taxa proved unsuccessful, because only a few taxa are abundant throughout the sections. Instead, correlations are made on the first or last occurrence of certain taxa. A comparison of the assemblages in the oldest (Ludlovian) Florida well section and the youngest (late Llandoverian) Red Mountain Formation section indicates that they have only 2 taxa in common. A comparison between the other parts of the Florida and Alabama-Georgia sections was not feasible because of the great age differences. The general aspect of the Florida assemblages is quite different from those in Georgia and Alabama. It is concluded that the sections in Georgia and Alabama are of ages different from those in Florida. The rocks are not different faunal facies of isochronous strata because chitinozoans are planktonic and are therefore not lithofacies dependent. One problem encountered in this study was that 3 wells did not penetrate very deeply into Silurian strata. Three of the Florida wells only penetrated the Upper Silurian, and only one went as deep as the Lower Silurian. Future work in correlating the Silurian rocks from these 3 states must depend on new wells being drilled to greater depths. End_of_Article - Last_Page 1784------------

Silurian Chitinozoans from Florida Well Samples: ABSTRACT

End_Page 2036------------------------------Chitinozoans of Silurian age were recovered from four wells in Florida: Cone No. 1, Tillis No. 1, Hilliard No. 1, and Kie Vining No. 1. An attempt was made to establish a correlation between the four wells using the chitinozoan evidence, and the results are presented. The youngest assemblage probably is of Ludlovian age; the oldest is of late Llandoverian age. End_of_Article - Last_Page 2037------------

Composition and range of some devonian spore assemblages of Canada

Two distinct spore assemblages occur in Lower and (?)early Middle Devonian rocks of Gaspé Bay, Quebec. The older one, which has been dated faunally as Gedinnian, Siegenian and Early Emsian, is associated in its upper part with the so-called Psilophyton flora and contains predominantly small, acamerate and azonate spores with a reticulate, radially ribbed or minutely sculptured exine. Most of the spores are different from any previously described. The younger assemblage, not dated faunally, succeeds the other abruptly. It contains Ancyrospora spp., Dibolisporites spp., Rhabdosporites langiRichardson, and others of relatively large size which also occur in Eifelian and Early Givetian rocks of Scotland and the U.S.S.R.In the Canadian Arctic, five successive assemblages of spores have been recognized in the Bird Fiord, Hecla Bay and Griper Bay Formations of the southern Queen Elizabeth Islands. They contain a number of species that also are present in Givetian to Famennian rocks of western Canada, Spitsbergen, Scotland, the U.S.S.R. and Australia. Where faunal control is available, the stratigraphic ranges of many of these species, for example Archeeoperisaccus spp. and Lophozonotriletes cristifer (Luber) Kedo, are in general agreement with the ranges given by Chibrikova (1963), Kedo (1955, 1957), Naumova (1953), and others.

Some Lower Devonian microfloras from Southern Britain

A new spore assemblage of Siegenian age from Llanover, south Wales is described, and compared with an assemblage of Gedinnian age recently described by Chaloner and Streel (1966) from Newport, south Wales. These are compared with other new assemblages from the Faringdon borehole, the Brightling No. 1 borehole and the Canvey Island borehole, all in the southern half of England and known from other fossils to be probably of Lower Devonian age. All five assemblages are dominated by relatively simple spore types. The Siegenian assemblage shows only a small increase in morphographical variety over the Gedinnian assemblage, but a considerable increase in the mean maximum size of the spores studied. The borehole assemblages show approximately the same size range as the Siegenian assemblage, but differ in a progressive increase in the variety of morphography represented. All are concluded to be younger, and are placed in an order according to age. Spores with bifurcate spines appear in the Canvey Island material. The youngest assemblage, from Brightling, shows a marked increase in variety over the others, and includes saccate and zonate forms as well as those with bifurcate spines. This is regarded as consistent with an Emsian age.

Upper Cretaceous Formations and Faunas of Southern California

Rocks of Upper Cretaceous age crop out in three areas in the vicinity of Los Angeles, southern California. The section in the northern Santa Ana Mountains consists of 2,600 feet (more or less) of conglomerates, shales, and sandstones. These are divided into three formations, of which the upper two are further divisible into two members each. The rich faunas from the shales and sandstones are divisible into two major faunal assemblages that have but few species in common. The older assemblage is subdivisible into two minor faunal divisions. The younger assemblage is similarly divisible into three minor faunal divisions. The Cretaceous beds of the Santa Monica Mountains consist of more than 8,000 feet of coarse clastics. These are nearly unfossiliferous with the exception of a 300-foot sandstone bed near the top of the section. This bed contains a fauna that is correlated with the highest faunal subdivision of the Santa Ana Mountains. The Simi Hills Cretaceous consists of more than 6,000 feet of shales and sandstones. Only the lowermost 500 feet plus of this section has yielded fossils. The fossiliferous beds are correlated with the uppermost two minor faunal divisions of the Santa Ana Mountains.