Fossiliferous Sedimentary Rocks Research Articles

Life and land engulfed in the late Early Jurassic Karoo lavas of southern Gondwana

AbstractThe rock record from the late Early Jurassic in southern Africa encompasses the history of voluminous continental flood basalt outpourings associated with the magmatic events in the Karoo–Ferrar Large Igneous Province (LIP) in southern and eastern Gondwana. This multiphase magmatism produced one of Earth’s largest continental flood basalt successions volumetrically and is assumed to have been a main driving mechanism in late Early Jurassic global environmental perturbations, including mass extinctions and changes in climate. In southern Africa, these Lower Jurassic flood basalts are interbedded with fossiliferous sedimentary rocks, which in turn host the last signs of ‘Karoo life’ in the form of fossil plants, invertebrates and vertebrates, including the trackways of hopping mammals and the ultimate Karoo dinosaurs. The sedimentology and palaeontology of the interbeds archived depositional and biotic processes in running water as well as in and around shallow, up to ∼10 m deep freshwater lakes and ponds in the late Early Jurassic. This study explains how a complex freshwater palaeo-habitat prevailed – albeit temporarily – in this extremely stressful environment, which was unlike any modern volcanic system. The evidence collectively points to seasonally wet, warm temperate climatic conditions during the early phases of Karoo volcanism. Moreover, the evidence in the rocks also suggests that the dynamic volcanic conditions resulted in shifting habitats that likely facilitated the migration of the ultimate Karoo biota towards the north and west, away from the main Karoo land of fire, just before Gondwana started to disassemble. This refinement of the environmental dynamics in southern Gondwana presented herein lays the groundwork for future high-resolution volcanological, geochronological and chemostratigraphical studies aimed at the nuanced understanding of the global environmental effect of the Karoo–Ferrar LIP.

Depositional age and provenance of high-grade paragneisses from the Mérida Andes, Venezuela: Implications for the Ediacaran–Cambrian tectonic setting of northwestern Gondwana

Isotopic, geochemical, and geochronological data are provided to constrain sedimentary sources and depositional ages of high-grade paragneisses of the Iglesias Complex in the Mérida Andes (Venezuela). U-Pb geochronology of detrital zircons suggests a maximum depositional age of 540–530 Ma, whereas age spectra reveal sources from Pan-African-Brasiliano belts, the Amazon Craton, and Oaxaquia-Putumayo basement. Reworking of such Gondwanan sources is also reflected in whole-rock Nd and Hf crustal residence ages of ~2.0–1.3 Ga. Post-depositional disturbance of the U-Pb system by metamorphism during the early Paleozoic and Permo–Triassic is assessed through in-situ analysis of zircon rims and discordant analyses. Field evidence, geochemical and isotopic signatures are consistent with pelitic-psammitic and volcanic-volcaniclastic compositions for sedimentary protoliths. The former show provenances from continental intermediate sources, whereas the latter are probably sourced in an immature volcanic arc, suggesting the initiation of subduction of the Iapetus oceanic crust beneath northwestern Gondwana around 530 Ma. Sedimentary precursors were likely deposited during the latest Ediacaran–earliest Cambrian in an extensive continental shelf, fed by detritus draining from the topographic highs of the Pan-African-Brasiliano belts, across the Amazon craton and into the Iapetus Ocean. Thus, the main direction of sediment flow was opposite to the present-day transport from the Andes Cordillera by the Pacific Ocean to the Amazon fan in the Atlantic Ocean. Chemical trends in gneisses suggest fluvial sedimentary protoliths and cold-climate weathering at high latitudes. Metasedimentary rocks in the Mérida Andes show similarities with equivalent units in Peri-Gondwanan crustal fragments such as the Acatlán Complex and the Santander Massif, but no correlation with Ediacaran metasedimentary rocks of the Maya Block. Ediacaran–Cambrian fossiliferous sedimentary rocks in the Eastern Venezuela Basin and the Colombian foreland basin are possible equivalents but these units remained unmetamorphosed, as its paleoposition was far from the subduction trench during the Early Ordovician climax of the Famatinian orogeny.

Large giraffids (Mammalia, Ruminantia) from the new late Miocene fossiliferous locality of Kemiklitepe-E (Western Anatolia, Turkey)

Kemiklitepe is a well-known locality with four recognised fossiliferous horizons, KTA to KTD, which have yielded a plethora of mammalian remains. Previous taxonomic studies indicate the presence of three giraffid taxa: Samotherium major and Palaeotragus rouenii from the uppermost three horizons, KTA, KTB and KTC, as well as Palaeotragus rouenii and Samotherium? sp. from the lowermost KTD horizon. In this study a new locality, Kemiklitepe-E, is presented for the first time. Kemiklitepe-E is located approximately 350 m NW of the classic Kemiklitepe locality. The fossiliferous sedimentary rocks at Kemiklitepe-E occur at the same stratigraphic level as localities KTA, KTB and KTC. The preliminary faunal list includes representatives of Proboscidea, Chalicotheriidae, Equidae, Bovidae and Giraffidae. Comprehensive descriptions and comparisons of the Kemiklitepe-E Giraffidae specimens suggest the co-occurrence of two large giraffids: Samotherium major and Helladotherium duvernoyi. Samotherium major, previously documented from this region, is the most common taxon at Kemiklitepe. Helladotherium duvernoyi is rare at Kemiklitepe and here reported for the first time. The two taxa coexisted during the middle Turolian in Greece and Western Anatolia. In addition, it is suggested that specimens of Samotherium? sp described from KTD possibly belong to Samotherium neumayri. Based on the stratigraphic position of fossiliferous rocks, as well as the faunal data presented herein, the newly discovered locality is considered to be of middle Turolian (MN12) age.

LATE CRETACEOUS STRATIGRAPHY OF THE SOUTHEASTERN CHACO - PARANÁ BASIN (“NORTE BASIN” - URUGUAY): THE MAASTRICHTIAN AGE OF THE CALCRETIZATION PROCESS

In the last 40 years, successive revisions have been introduced to the lithostratigraphy of the Late Cretaceous of the Norte Basin, whose record is formed by predominantly siliciclastic continental fossiliferous sedimentary rocks. A factor that contributed to the terminology proliferation was the misplaced importance attributed to the epigenetic processes and products (calcretes and ferricretes, occasionally fossiliferous) affecting those siliciclastic rocks. Based on field work and lithological logs from a number of key areas, the validity of the original stratigraphic column showing, from base to top, the Guichón, Mercedes, Asencio and Queguay formations is supported. For those particular units, the lithostratigraphic scope given by the original definitions is correct and, with minimum adjustments, they should be restored for their practical usefulness as geological mapping units. An evolutionary scheme for the Late Cretaceous of the Norte Basin is herein presented, with the aim of contributing to a greater understanding of the succession of sedimentary and epigenetic processes. A Maastrichtian age is proposed for the carbonatic cementation and/or substitution that affected the Mercedes and Asencio formations, thus originating the Queguay Formation, based on absolute age of the calcite cement (~ 72 Ma; U-Pb), paleontological data, and the correlation established with similar events recorded in the Marília Formation on the Bauru Basin (Brazil).

Primary relationships of basaltic intrusive rocks and siliceous sedimentary rocks in the Northern Chichibu Belt, Shikoku Island, southwest Japan

Understanding the primary relationships between basaltic rocks and fossiliferous sedimentary rocks provides direct information on the timing and geologic setting of past oceanic igneous activity. Though the basalts associate with cherts in the Northern Chichibu Belt have been petrographically identified as OIA/OIT or MORB, such primary relationships of them are not well understood.This paper provides the primary intrusive relation of basaltic rocks contact with chert and calcareous siliceous shale of Sugyo area in the Northern Chichibu Belt on Shikoku Island. The sediments are included within the basalt as xenoliths and in the contact zone with the basalt they have been recrystallized and contain metamorphic minerals possibly produced by contact metamorphism due to the basalt intrusion. These indicate that the basalts are sills or dykes that have intruded the sediments. The cherts are radiolarian chert which alternate with the calcareous siliceous shale and include minerals of oceanic volcanic origins. Asselian radiolarian fossils are yielded from a subangular chert xenolith that was intruded by the basalt possibly before the chert had undergone complete diagenesis. The petrography of the sills indicates they are alkali basalts. X-ray fluorescence analyses indicate that the basaltic rocks include subalkaline basalts and oceanic island tholeiite or oceanic island alkaline basalts.We have concluded that the basaltic dykes and sills were products of Asselian or younger intra-plate volcanism at an oceanic area covered by a pile of pelagic siliceous sediments. These Asselian cherts are earlier than the Sakmarian, Wordian and Capitanian limestones that overlie intra-plate basalts of Ohnogahara part of the Northern Chichibu Belt. The basalt sills may therefore represent the early stages of intra-plate igneous activity in the paleo-Pacific Ocean.

Geological setting of Earth's oldest fossils in the ca. 3.5 Ga Dresser Formation, Pilbara Craton, Western Australia

Detailed stratigraphic, petrographic, and zircon U–Pb geochronological data are provided for surface outcrops and newly obtained, unweathered drillcore intersections of Earth's oldest fossiliferous sedimentary rocks, the lowermost chert–barite unit of the ca. 3.5 Ga Dresser Formation, Warrawoona Group, Pilbara Craton. Results show that the ∼8 m thick unit at the drilling locality consists of thinly bedded, originally micritic carbonates deposited under quiet water conditions that are interbedded with volcaniclastic conglomerates and coarse polymict conglomerates (diamictites) deposited during periods of tectonic instability. The presence of rapid vertical and lateral facies changes, tilted bedding in some members, and internal erosional unconformities, combined with analysis of fault offsets, indicate that tectonically unstable periods were caused by growth faulting. Intense hydrothermal fluid flow accompanied episodes of growth faulting and resulted in pulsed, repeated precipitation of silica ± barite ± sphalerite that alternated with precipitation of pyrite. Clasts of hydrothermal minerals in sandstone and coarse, polymict conglomerate beds at several levels within the unit highlight the repeated nature of hydrothermal fluid circulation during sediment accumulation. Hydrothermal fluid circulation caused widespread acid–sulfate alteration of the footwall, extensive replacement of the newly deposited carbonate sediments by hydrothermal precipitates, and crystallization of hydrothermal chert–barite–pyrite in veins perpendicular to bedding in the footwall and parallel to bedding in the sedimentary unit. The combined evidence points to deposition of the chert–barite unit within an active volcanic caldera. A 10 cm thick bed of felsic volcaniclastic tuff within finely bedded carbonates near the top of the unit has yielded a maximum age of deposition of 3481.0 ± 3.6 Ma (2 σ uncertainty), confirming earlier Pb–Pb age data for the antiquity of these rocks. Putative signs of life are present as stratiform, columnar, domical, and coniform stromatolitic laminates at various levels throughout the unit. Petrographic observations show that red- and black-weathering stromatolitic laminates on the surface consist of pyrite in unweathered drillcore material. Observation of local relics of carbonate between pyrite crystals in these laminates indicates a carbonate protolith prior to replacement by hydrothermal pyrite, which provides support for a biological origin of stromatolitic laminates. Further support is provided by clasts of laminated carbonaceous material in thinly bedded, primary micritic carbonates. Textural analysis of jaspilitic “cherts” near the top of the unit reveal haematite as tiny crystals within recrystallized siderite/dolomite rhombs in carbonate beds affected by hydrothermal silica alteration. The presence of unaltered diagenetic pyrite crystals in the haematite-altered siderite indicates that alteration did not result from oxidizing fluids. Rather, haematite alteration is interpreted as the result of an increase in pH during diagenetic alteration by mildly reducing, silica-rich fluids associated with eruption of overlying basalts, possibly with the influence of microbial activity. This has important implications for the origin of jaspilitic cherts throughout the early Archean record and for atmospheric conditions of early Earth.

Early Neogene history of the Central American arc from Bocas del Toro, western Panama

A newly discovered sequence of lower to middle Miocene rocks from the eastern Bocas del Toro archipelago, western Panama, reveals the timing and environment of the earliest stages in the rise of the Isthmus of Panama in this region. Two new formations, the Punta Alegre Formation (lower Miocene, Aquitanian to Burdigalian) and the Valiente Formation (middle Miocene, Langhian to Serravallian), are here named and formally described. The Punta Alegre Formation contains a diagnostic microfauna of benthic and planktic foraminifera and calcareous nannofossils that indicate deposition in a 2000-m-deep pre-isthmian neotropical ocean from as old as 21.5-18.3 Ma. Its lithology varies from silty mudstone to muddy foraminiferal ooze with rare thin microturbidite layers near the top. The Valiente Formation, which ranges in age from 16.4 to ca. 12.0 Ma, lies with slight angular unconformity on the Punta Alegre Formation and consists of five lithofacies: (1) columnar basalt and flow breccia, (2) pyroclastic deposits, (3) coarse-grained volcaniclastic deposits, (4) coral-reef limestone with diverse large coral colonies, and (5) marine debris-flow deposits and microturbidites. These lithofacies are interpreted to indicate that after ca. 16 Ma a volcanic arc developed in the region of Bocas del Toro and that by ca. 12 Ma an extensively emergent archipelago of volcanic islands had formed. 3 9 Ar/ 4 0 Ar dating of basalt flows associated with the fossiliferous sedimentary rocks in the upper part of the Valiente Formation strongly confirms the ages derived from planktic foraminifera and nannofossils. Paleobathymetric analysis of the two new formations in the Valiente Peninsula and Popa Island, in the Bocas del Toro archipelago, shows a general shallowing from lower- through upper-bathyal to upper-neritic and emergent laharic and fluviatile deposits from ca. 19 to 12 Ma. The overlying nonconformable Bocas del Toro Group contains a lower transgressive sequence ranging from basal nearshore sandstone to upper-bathyal mudstone (ca. 8.1-5.3 Ma) and an upper regressive sequence (5.3-3.5 Ma). A similar paleobathymetric pattern is observed from the Gatun to Chagres Formations (12-6 Ma) in the Panama Canal Basin area and in the Uscari, Rio Banana, Quebrada Chocolate, and Moin Formations (8-1.7 Ma) in the southern LimOn Basin of Costa Rica.

An overview of the stratigraphy and tectonics of the Nepal Himalaya

Nepal can be divided into the following five east–west trending major tectonic zones. (i) The Terai Tectonic Zone which consists of over one km of Recent alluvium concealing the Churia Group (Siwalik equivalents) and underlying rocks of northern Peninsular India. Recently active southward-propagating thrusts and folds beneath the Terai have affected both the underlying Churia and the younger sediments. (ii) The Churia Zone, which consists of Neogene to Quaternary foreland basin deposits and forms the Himalayan mountain front. The Churia Zone represents the most tectonically active part of the Himalaya. Recent sedimentologic, geochronologic and paleomagnetic studies have yielded a much better understanding of the provenance, paleoenvironment of deposition and the ages of these sediments. The Churia Group was deposited between ∼14 Ma and ∼1 Ma. Sedimentary rocks of the Churia Group form an archive of the final drama of Himalayan uplift. Involvement of the underlying northern Peninsular Indian rocks in the active tectonics of the Churia Zone has also been recognised. Unmetamorphosed Phanerozoic rocks of Peninsular India underlying the Churia Zone that are involved in the Himalayan orogeny may represent a transitional environment between the Peninsula and the Tethyan margin of the continent. (iii) The Lesser Himalayan Zone, in which mainly Precambrian rocks are involved, consists of sedimentary rocks that were deposited on the Indian continental margin and represent the southernmost facies of the Tethyan sea. Panafrican diastrophism interrupted the sedimentation in the Lesser Himalayan Zone during terminal Precambrian time causing a widespread unconformity. That unconformity separates over 12 km of unfossiliferous sedimentary rocks in the Lesser Himalaya from overlying fossiliferous rocks which are >3 km thick and range in age from Permo-Carboniferous to Lower to Middle Eocene. The deposition of the Upper Oligocene–Lower Miocene fluvial Dumri Formation records the emergence of the Himalayan mountains from under the sea. The Dumri represents the earliest foreland basin deposit of the Himalayan orogen in Nepal. Lesser Himalayan rocks are less metamorphosed than the rocks of the overlying Bhimphedis nappes and the crystalline rocks of the Higher Himalayan Zone. A broad anticline in the north and a corresponding syncline in the south along the Mahabharat range, as well as a number of thrusts and faults are the major structures of the Lesser Himalayan Zone which is thrust over the Churia Group along the Main Boundary Thrust (MBT). (iv) The crystalline high-grade metamorphic rocks of the Higher Himalayan Zone form the backbone of the Himalaya and give rise to its formidable high ranges. The Main Central Thrust (MCT) marks the base of this zone. Understanding the origin, timing of movement and associated metamorphism along the MCT holds the key to many questions about the evolution of the Himalaya. For example: the question of whether there is only one or whether there are two MCTs has been a subject of prolonged discussion without any conclusion having been reached. The well-known inverted metamorphism of the Himalaya and the late orogenic magmatism are generally attributed to movement along the MCT that brought a hot slab of High Himalayan Zone rocks over the cold Lesser Himalayan sequence. Harrison and his co-workers, as described in a paper in this volume, have lately proposed a detailed model of how this process operated. The rocks of the Higher Himalayan Zone are generally considered to be Middle Cambrian to Late Proterozoic in age. (v) The Tibetan Tethys Zone is represented by Cambrian to Cretaceous-Eocene fossiliferous sedimentary rocks overlying the crystalline rocks of the Higher Himalaya along the Southern Tibetan Detachment Fault System (STDFS) which is a north dipping normal fault system. The fault has dragged down to the north a huge pile of the Tethyan sedimentary rocks forming some of the largest folds on the Earth. Those sediments are generally considered to have been deposited in a more distal part of the Tethys than were the Lesser Himalayan sediments. The present tectonic architecture of the Himalaya is dominated by three master thrusts: the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). The age of initiation of these thrusts becomes younger from north to south, with the MCT as the oldest and the MFT as the youngest. All these thrusts are considered to come together at depth in a flat-lying decollement called the Main Himalayan Thrust (MHT). The Mahabharat Thrust (MT), an intermediate thrust between the MCT and the MBT is interpreted as having brought the Bhimphedi Group out over the Lesser Himalayan rocks giving rise to Lesser Himalayan nappes containing crystalline rocks. The position of roots of these nappes is still debated. The Southern Tibetan Detachment Fault System (STDFS) has played an important role in unroofing the higher Himalayan crystalline rocks.

Late Jurassic (Kimmeridgian-Tithonian) macrofossil assemblage from Jason Peninsula, Graham Land: evidence for a significant northward extension of the Latady Formation

New exposures of fossiliferous sedimentary rocks at Cape Framnes, Jason Peninsula (65°57′S, 60°33′W) are assigned to the Middle–Late Jurassic Latady Formation of the south-eastern Antarctic Peninsula region. A sequence of fine to coarse-grained sandstones of unknown thickness has yielded a molluscan and plant macrofossil assemblage rich in the following elements: perisphinctid ammonites, belemnopseid belemnites, oxytomid, trigoniid and astartid bivalves, and bennettitalean fronds and fructifications. The overwhelming age affinities are with the Kimmeridgian–early Tithonian part of the Latady Formation, as exposed on the Orville and Lassiter coasts. The Cape Framnes sedimentary rocks help to constrain the age of a major sequence of acid volcanic rocks on Jason Peninsula, and show that the Latady Basin was geographically much more extensive than recognized previously. It was the principal depositional centre of Middle–Late Jurassic sedimentation in the Antarctic Peninsula back-arc region and in areal extent may have rivalled the essentially Cretaceous Larsen Basin.

Stratigraphy of the western Nepal Lesser Himalaya: A synthesis

The western Nepal Lesser Himalaya, lying between the Marsyangdi and Bheri Rivers, is generally free from crystalline nappes and exposes the sedimentary and metasedimentary rock sequences in a wide zone between the MBT and the MCT. Geologically, the area is complicated by the presence of a number of folds, thrusts and imbricate zones. Despite the complicated geology, excellent studies have been carried out in recent years on the stratigraphy of this region. However, individual work has been confined in areas separated by wide intervening parts that are not as well studied. Many researchers have proposed different stratigraphic nomenclatures in their respective study areas, and their stratigraphic subdivisions do not always correspond well with each other. Therefore, confusion and uncertainties have remained in the stratigraphic interpretation of the western Nepal Lesser Himalaya. An attempt has been made here to appraise and synthesize the work done so far to bring out a clearer picture of the problems and prospects in the stratigraphic investigations in western Nepal.
 For convenience, the stratigraphy of the Lesser Himalayan metasediments the younger group of fossiliferous sedimentary rocks and the Jajarkot Nappe are treated separately. An attempt has been made to extend the stratigraphic nomenclature of the Nawakot Complex of central Nepal established by Stocklin and Bhattarai (1977) and Stocklin (1980) to western Nepal. In spite of the type sections of these rocks in central Nepal and the rocks of western Nepal being separated by a large intervening area, there is a fair consistency in the stratigraphic sequence and rock assemblages in the two areas. The uniformity in the use of nomenclature of rock units will greatly help to build up a more systematic and reliable stratigraphy of Nepal Lesser Himalaya in the future.
 The low grade metamorphic and the sedimentary rocks of the Lesser Himalaya are divided into the older Nawakot Unit and the younger Tansen Unit. Only the Tansen Unit is fossiliferous. Whereas the Nawakot Unit is largely of Precambrian age, the Tansen Unit ranges in age from Permo-Carboniferous to Lower Tertiary. The Jajarkot Nappe, tectonically overlying the Lesser Himalayan metasediments, consists of medium grade metamorphic rocks such as garnet bearing schists, feldspathic schists and quartzites. These rocks are succeeded conformably by unmetamorphosed carbonate rocks of possibly Cambro­ Ordovician age and stratigraphically and tectonically comparable to the Phulchauki Group of the Kathmandu Nappe of central Nepal.

Discussion on a reassessment of the early Ordovician 'Celtic' brachiopod province

R. B. Neuman, D. A. T. Harper & C. R. van Staal write: The results of several recent geological studies relevant to early Ordovician palaeogeography are not mentioned in this recent paper by Cocks & McKerrow (1993). Contributions of particular significance include the recognition that the Dunnage Zone of Newfoundland consists of two distinct parts, the Notre Dame and Exploits subzones of contrasting Ordovician stratigraphy, lithology, structure, metamorphism, plutonism, lead isotope signatures, mineralization, geophysics, and faunas ( Williams et al. 1988 ). The boundary between these subzones, named the Red Indian Line in Newfoundland, is probably a suture. Fossiliferous sedimentary rocks are rare in the Notre Dame Subzone, but Arenig-Llanvirn graptolites occur in shales at three places and at one place conodonts have been extracted from carbonate clasts in volcanic breccias. All these fossils are considered to have Laurentian affinities (Nowlan & Thurlow 1984; Williams 1992 ; Williams et al. 1992 ). Fossiliferous Lower Ordovician rocks are more abundant in the Exploits subzone. In addition to the several occurrences of Celtic assemblage brachiopods (Cl-C6 of Neuman & Harper 1992 ; Harper 1992 ; Cocks & McKerrow 1993), Gondwanan trilobites and graptolites (Cyclopyge and Undulograptus) are known in the Exploits subzone of central Newfoundland ( Williams et al. 1992 ) from shale that overlies a Lower Ordovician ophiolite fragment near an Occurrence of Celtic brachiopods (C5 of Neuman & Harper 1992 ). Palaeomagnetic studies of Newfoundland rocks near the Red Indian Line showed that these two subzones were separated by about 30° of latitude (van der Pluijm et al. 1990;

U–Pb geochronologic constraints on the volcanic evolution of the Mira (Avalon) terrane, southeastern Cape Breton Island, Nova Scotia

New U–Pb ages for late Precambrian volcanic and associated plutonic units in the Mira (Avalon) terrane of southeastern Cape Breton Island indicate that volcanic suites were erupted over a span of at least 100 Ma. The oldest dated rock is a quartz–feldspar rhyodacitic porphyry from the unit that hosts the Mindamar Zn–Pb–Cu–Ag–Au deposit in the Stirling belt, which has an age of [Formula: see text]. The most widespread volcanism and plutonism occurred at ca. 620 Ma in the East Bay Hills and Coxheath Hills belts, and probably the Sporting Mountain belt, as indicated by U–Pb ages and U–Pb maximum ages for rhyolite flows and U–Pb and Ar–Ar ages of crosscutting plutons, as well as stratigraphic constraints. Younger volcanic rocks occur in the Coastal belt, from which a rhyodacitic crystal tuff is dated at [Formula: see text] and a pluton is dated at 574 ± 3 Ma. A rhyolite flow from the contiguous Main-à-Dieu sequence yields a maximum age of ca. 563 Ma, and a minimum age for this sequence is indicated by overlying latest Precambrian to Cambrian fossiliferous sedimentary rocks. Middle Devonian plutonism in the Mira terrane is confirmed by an age of [Formula: see text] from the Lower St. Esprit granodiorite in the Coastal belt. The range of ages of volcanic and plutonic rocks in Mira terrane is similar to that in other parts of Avalon terrane in eastern Newfoundland and southern New Brunswick. Many of the dated rocks contain xenocrystic zircons of Middle Proterozoic ages which suggest a South American source.

Geology and geochronology of the line islands

Geological and geophysical studies along the entire length of the Line Islands were undertaken in order to test the hot spot model for the origin of this major linear island chain. Volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls. Volcanic rocks from the Line Islands are alkalic basalts and hawaiites. In addition, a tholeiitic basalt and a phonolite have been recovered from the central part of the Line chain. Microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high TiO2 (1.0–4.0 wt %) and Al2O3 (3.4–9.1 wt %) and are of alkaline to peralkaline affinities. Major element compositions of the Line Islands volcanic rocks are very comparable to Hawaiian volcanic rocks. Trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas; the Line Islands lavas are very much unlike typical mid‐ocean ridge or fracture zone basalts. Dating of these rocks by 40Ar‐39Ar, K‐Ar, and paleontological methods, combined with Deep Sea Drilling Project data from sites 165, 315, and 316 and previously dated dredged rocks, provide ages of volcanic events at 20 localities along the chain from 18°N to 9°S, a distance of almost 4000 km. All of these dates define mid‐Cretaceous to late Eocene edifice or ridge‐building volcanic events. Eocene volcanic events took place from 15°N to 9°S, and Late Cretaceous events took place from 18°N to 9°S. In the southern Line Islands both Cretaceous and Eocene events took place on the same edifice or ridge, indicating recurrent volcanism at a single locality. The irregular distribution of atolls in the chain, the fact that Late Cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern Line Islands, and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t1/2 related subsidence path. Magnetic surveys of 11 seamounts show that four of the seamounts, from the central Line chain, give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of Cretaceous seamounts. These four poles agree with other paleomagnetic data of middle‐late Eocene‐early Oligocene age from the Pacific. One of these four seamounts yielded a 40Ar‐39Ar total fusion age of 39 Ma. Because the poles of all four seamounts fall into a tight group we infer that they are probably of middle‐late Eocene age to early Oligocene age. The other three seamounts as well as one seamont from the Line Islands analyzed previously all give virtual geomagnetic poles which agree with Late Cretaceous paleomagnetic data from the Pacific. Of these four seamounts, three give Late Cretaceous 40Ar‐39Ar ages ranging from 71 to 85 Ma; another, in the southern Line Islands, is interpreted, on paleontological evidence, to be of Late Cretaceous age. The origin of the Line Islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots. The single hot spot model cannot account for all of the volcanic edifices in the Line Islands, although it does explain a general age progression of 9.6±0.4 cm/yr from north to south along the chain derived from a number of dated edifices. The four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data. The petrologic data argue against a mid‐ocean ridge or transform fault origin proposed by earlier workers. The complex volcanic province represented by the Line Islands remains a challenge to existing models for the origin of midplate volcanism in the Pacific. An atoll drilling program which could determine edifice building histories, paleolatitudes of seamount formation, and subsidence rates and patterns in the Line Islands is needed.

Insights on Why Graphic Correlation (Shaw's Method) Works

In 1964 A. B. Shaw presented a method of correlating fossiliferous sedimentary rocks based on interpretation of graphic plots of first- and last-occurrences of taxa. Because there is no way to determine the true total ranges of fossil taxa, it is instructive to test the accuracy of the method using hypothetical datasets. The dataset used here consists of 16 taxa in six sections with differing known rates of rock accumulation. In all graphs, a single straight-line correlation was a reasonable interpretation. The resulting ranges after the first and third rounds of compositing reproduce the "true" ranges but with small errors. Slight errors in the positioning of individual correlation lines are more likely to lengthen ranges artificially than to shorten them. Shaw's method works well because, whereas actually sampled ranges will be shorter than true ranges, errors in correlation will be likely to extend some ranges. This or any exercise using simulated data is useful only if the hypothetical situation resembles real geologic situations and if insights derived from the hypothetical dataset provide insights into real situations. The method is only as good as the available data.

Geology and biology of Oceanographer submarine Canyon

We investigated Oceanographer Canyon, which is on the southeastern margin of Georges Bank, during a series of fourteen dives in the “Alvin” and “Nekton Gamma” submersibles. We have integrated our observations with the results of previous geological and biological studies of Georges Bank and its submarine canyons. Fossiliferous sedimentary rocks collected from outcrops in Oceanographer Canyon indicate that the Cretaceous—Tertiary boundary is at 950 m below sea level at about 40°16′N where at least 300 m of Upper Cretaceous strata are exposed; Santonian beds are more than 100 m thick and are the oldest rocks collected from the canyon. Quaternary silty clay, deposited most probably during the late Wisconsin Glaciation, veneers the canyon walls in many places, and lithologically similar strata are present beneath the adjacent outer shelf and slope. Where exposed, the Quaternary clay is commonly burrowed by benthic organisms that cause extensive erosion of the canyon walls, especially in the depth zone (100–1300 m) inhabited by red crabs ( Geryon) and/or jonah crabs ( Cancer). Bioerosion is minimal on high, near-vertical cliffs of sedimentary rock, in areas of continual sediment movement, and where the sea floor is paved by gravel. A thin layer of rippled, unconsolidated silt and sand is commonly present on the canyon walls and in the axis; ripple orientation is most commonly transverse to the canyon axis and slip-faces point downcanyon. Shelf sediments are transported from Georges Bank over the eastern rim and into Oceanographer Canyon by the southwest drift and storm currents; tidal currents and internal waves move the sediment downcanyon along the walls and axis. Large erratic boulders and gravel pavements on the eastern rim are ice-rafted glacial debris of probable late Wisconsinan age; modern submarine currents prevent burial of the gravel deposits. The dominant canyon megafauna segregates naturally into three faunal depth zones (133–299 m; 300–1099 m; 1100–1860 m) that correlate with similar zones previously established for the continental slope epibenthos. Faunal diversity is highest on gravelly sea floors at shallow and middle depths. The benthic fauna and the fishes derive both food and shelter by burrowing into the sea floor. In contrast to the nearby outer shelf and upper slope, Oceanographer Canyon has not been extensively exploited by the fishing industry, and the canyon ecosystem probably is relatively unaltered.

AGE OF THE GRANITIC ROCKS OF NOVA SCOTIA

The ages of biotite, muscovite, and potassium feldspar in granitic rocks from 27 localities in Nova Scotia, as determined by A/K and Sr/Rb methods, fall in the range 300 to 400 m.y. At two localities, where nearby intrusive contacts with upper Lower Devonian fossiliferous sedimentary rocks are established, the best estimate of the age of the intrusive rocks is 365 ± 20 m.y. Biotite and potassium feldspar in granite from Boisdale Hills (Cape Breton Island), associated with nearby Middle Cambrian sedimentary rocks, have an estimated age of 500 ± 20 m.y. A single age analysis of biotite in the metamorphosed Meguma series gives 340 m.y. Radiation-damage ratios for zircon at nine localities where the age has been determined show one concordant result and eight which are lower than the A/K and Sr/Rb ages by 16 to 33 per cent. The evidence of the mineral ages in the granitic rocks investigated thus far in Nova Scotia strongly suggests a protracted period of intrusion in the Devonian, with the upper Lower Devonian older than 365 m.y. Middle Cambrian is tentatively believed to be older than 500 m.y. Holmes9 B time scale, with Cambrian limits of 430 and 510 m.y. and Devonian limits of 255 and 313 m.y., would require an extension of 10-25 per cent to meet the Nova Scotia data.

CONTRIBUTION TO THE GEOLOGY OF THE ISLANDS MARGARITA AND CUBAGUA, VENEZUELA

Research Article| May 01, 1957 CONTRIBUTION TO THE GEOLOGY OF THE ISLANDS MARGARITA AND CUBAGUA, VENEZUELA H. G KUGLER H. G KUGLER POINTE-A-PLERRE, TRINIDAD, B.W.I. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1957) 68 (5): 555–566. https://doi.org/10.1130/0016-7606(1957)68[555:CTTGOT]2.0.CO;2 Article history received: 30 Aug 1956 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation H. G KUGLER; CONTRIBUTION TO THE GEOLOGY OF THE ISLANDS MARGARITA AND CUBAGUA, VENEZUELA. GSA Bulletin 1957;; 68 (5): 555–566. doi: https://doi.org/10.1130/0016-7606(1957)68[555:CTTGOT]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Tertiary formations of the Islands Margarita and Cubagua comprise fossiliferous sedimentary rocks of Eocene, Oligocene, Miocene, and Pliocene age. Remnant blocks of limestone with Midwayan fossils are also reported. The orbitoid-bearing Punta Carnero Formation is about 2000 m thick. The Oligocene Los Vagres limestone contains a coral fauna which is related to that of the San Luis limestone of Falcón. About 300 m of mottled graywacke in Cubagua borehole No. 1 is considered to be Oligocene and is correlated with the San Lorenzo Formation of eastern Falcón. Miocene clays and sands are about 1600 m thick in Cubagua borehole No. 2. On Cubagua and Margarita these rocks are unconformably overlain by the Cubagua Formation with fossils known from the Caujarao, La Vela and Ojo de Agua Formations of Falcón. Pliocene is represented on Cubagua by about 10 m of fossiliferous sandy marl with Lyropecten arnoldi, a marker fossil of the Cabo Blanco Formation.The Eocene and the lower Oligocene rocks are strongly folded. The upper Oligocene Los Vagres limestone appears to rest unconformably on vertical dipping lower Eocene. In the deep structural trough of Cubagua the lower Miocene was found also to be compressed. The middle and upper Miocene was less affected by orogenic movements, of which at least two phases are recognized by unconformities at the base of the Caujarao and of the younger La Vela Formation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Discoveries in Antarctica

A DISPATCH from the British Graham Land Expedition under the leadership of Mr. J. Rymill published in The Times of December 12 announces an important change in the map of Graham Land. In December 1928 Sir Hubert Wilkins in his flight over Graham Land reported the existence of two straits, Casey Channel and Stefansson Strait, in about latitudes 69 ° 45'S. and 71 ° S. respectively, across the area known as Graham Land which previously was thought to be united to the Antarctic Continent. Between the two straits he mapped roughly the Finley Archipelago. The British expedition, sledging along the west coast of Graham Land, to lat. 72 ° S., long. 67 ° W., or ninety miles south-west of where Sir H. Wilkins turned, found no trace of these two straits and believes that low glaciers must have been mistaken for ice-covered straits. Thus Graham Land south of Crane Channel in the Antarctic Circle is restored to its former continuity with the continent. A further discovery of importance is that Alexander I Land, discovered by Bellingshausen in 1821 and sighted by Charcot in 1909, is not a small island but a large land area at least two hundred and fifty miles from north to south and separated from Hearst Land by an ice-filled cleft-like strait fifteen miles wide and 200 miles long. The expedition charted the eastern coast line of this island. The strait appears to be a fault feature with the eruptive rocks of Graham Land on the east and fossiliferous sedimentary rocks on the west or island side.