Broom Point Research Articles

Palaeoenvironmental changes in slope carbonates across the Late Cambrian–Early Ordovician in western Newfoundland

The Martin Point section (western Newfoundland, Canada) spans the uppermost Cambrian Broom Point and Martin Point members of the Green Point Formation (upper Furongian). The investigated interval (~90 m) consists of rhythmites of thinly‐bedded marine carbonates (lime mudstones) alternating with green and black shale and thin conglomerate beds. Samples were extracted from the micritic carbonates, and their preservation was evaluated by multiple petrographic and geochemical examinations. The δ13Corg profile exhibits a positive shift (~2‰) associated with the globally well‐documented negative δ13Ccarb HERB (Hellnmaria–Red Tops boundary) excursion and correlated with similar distinct shifts in the Al, Si, and ∑REE components, thus suggesting contributions from detrital organic matter relatively depleted in 12C induced likely by a eustatic sea‐level drop and enhancement of terrigenous inputs. Similarly, the δ15Norg variations are consistent with the proposed sea‐level drop. This is also correlated with positive shifts on the Fe and Mn profiles reflecting the overprint of terrigenous inputs rather than redox conditions.

Carbon-isotope stratigraphy of the uppermost Cambrian in eastern Laurentia: implications for global correlation

AbstractThe δ13C profile from an interval of the Martin Point section in western Newfoundland (Canada) spans the upper Furongian (uppermost Cambrian). The interval (~90 m) is a part of the Green Point Formation of the Cow Head Group and consists of the Martin Point (lower) and the Broom Point (upper) members. It is formed of slope marine carbonates alternating with shales (rhythmites) and conglomeratic interbeds. The preservation of the investigated micritic carbonates was meticulously evaluated by multiple petrographic and geochemical screening tools. The δ13C and δ18O values (−0.5 ± 0.8 ‰VPDB and −7.1 ± 0.3 ‰VPDB, respectively) exhibit insignificant correlation (R2= 0.002) and similarly the correlation of δ13C values with their Sr and Mn counterparts, which supports the preservation of at least near-primary δ13C signatures that can be utilized to construct a reliable high-resolution carbon-isotope profile for global correlations.The δ13C profile exhibits two main negative excursions, a lower broad excursion (~3 ‰) that reaches its maximum at ~70 m below the Martin Point / Broom Point members boundary and an upper narrow excursion (~2.5 ‰) immediately below the same boundary. The lower excursion can be correlated with the global latest Furongian HERB event (TOCE), which is also recognized in the C-isotope profile of the GSSP boundary section at Green Point whereas the upper excursion matches with that of the Cambrian‒Ordovician boundary in the same section. The peak of the HERB δ13C excursion is correlated with positive shifts on the Th/U and Ni profiles (redox and productivity proxies).

Redox conditions across the Cambrian–Ordovician boundary: Elemental and isotopic signatures retained in the GSSP carbonates

Lime mudstone samples (rhythmites) were collected at high resolution from outcrops of the Cambrian–Ordovician GSSP boundary section at Green Point (western Newfoundland, Canada). The sequence (~45m-thick) consists of slope carbonates with alternating shale and siltstone interbeds, and it spans the boundary located between the Martin Point and Broom Point members of the Green Point Formation (Cow Head Group). Samples were extracted from micritic rhythmites by microdrilling and subsequently screened using petrographic and geochemical criteria to evaluate their degree of preservation. Although the δ13Corg profile (−29.7 to −25.6‰ VPDB) shows insignificant variations, the TOC values (0.1 to 4.1%) exhibit a generally upward decreasing trend. A negative δ13Ccarb excursion, reflecting a sealevel rise, marks a geochemical anomaly that correlates with an increase in the N contents (0 to 2.9%) of organic matter and the δ15Norg values (−0.6 to +6.0‰), which suggests a change to more reducing oceanic conditions. The U contents vary from 0.1 to 3.0ppm and the δ238U values (−0.97 to −0.18‰) generally decrease with the negative δ13Ccarb excursion. The U isotopic variations suggest a widespread increase in reducing conditions associated with sealevel rise during this interval. The investigated sedimentary rocks were slope carbonates where dysoxic conditions likely dominated throughout the entire section. Therefore, the changes in the TOC, N, δ15Norg, and δ238U profiles across the boundary are not as sharp as would be expected by a local change from oxic shallow-water to dysoxic/anoxic deep-water settings.

High-resolution chemostratigraphy of the Cambrian–Ordovician GSSP: Enhanced global correlation tool

The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) represents the Global Stratotype Section and Point (GSSP) for the Cambrian–Ordovician boundary on Laurentia. The formation consists of the Martin Point (lower) and the Broom Point (upper) members, which constitute a thick (~170m) deep subtidal to slope marine carbonate sequence. Preservation of the micritic carbonates of the Green Point Formation was evaluated by multiple petrographic and geochemical screening tools. The δ13C and δ18O values of near-primary micrites range from −4.7‰ to +1.7‰ (VPDB) and from −8.7‰ to −5.5‰ (VPDB), respectively, with no significant correlation (R2=0.002). Similarly, the δ13C values show no significant correlation with Mn/Sr ratios or total REE contents, which suggest that the investigated carbonates retain their near-primary δ13C signatures that can be utilized to construct a high-resolution carbon-isotope profile for the GSSP. The δ13Ccarb profile about the GSSP trends generally positive into Bed 22 of the Green Point Formation. The top of the C. intermedius Zone marks the onset of a negative δ13Ccarb shift to about 6.0‰ culminating in the upper part of the zone. Subsequently, the δ13Ccarb profile trends positive with a ‘double switch-back’ about the boundary point and then continues on to the highest value in the uppermost part of Bed 23. In addition, the Ce*/Ce values approximately between 0.8 and 1.0, through the GSSP suggest generally dysoxic waters before, during, and immediately after the boundary. The high-resolution chemostratigraphic results, curve, and trends covering the GSSP should facilitate correlation of the Cambrian–Ordovician boundary sections/sequences from other locations.

뉴파운드랜드 Cow Head Group에서 발견한 후기 Tremadocian 방산충 군집과 생층서대 재정립에 관한 연구

지금까지 보고된 하부 오르도비스기의 Tremadocian과 Arenigian에 속하는 방산충 군집들 간에는 유사성이나 연관성이 거의 보이지 않았다. Tremadocian에서 Arenigian의 방산충 군집의 진화경향을 알 수 없었던 이유는 후기 Tremadocian 방산충 군집에 대해 알려진 바가 없었기 때문이다. 캐나다 뉴파운드랜드의 Cow Head Group에서 후기 Tremadocian에 속하는 방산충 화석 군집들이 처음으로 발견되었다. 이 새로운 방산충 군집들은 모두 6과, 11속, 26종에 속한다. 이들 가운데 6속 17종이 새로이 발견된 것들이다. Arenigian에서만 보고되었던 Proventocitidae에 속하는 종들이 다양하게 산출되었다. 초기 Tremadocian에 번성했던 Protoentactiniidae와 Echidniniidae에 속하는 것들은 감소하고 Aspiculumidae에 속하는 것들은 더욱 분화되어 다양하고 풍부하게 산출되었다. 방산충과 함께 출현하는 코노돈트 연구로 Low Diversity Interval에서 M. dianae Zone 그리고 하부 P. proteus Zone에 속하는 후기 Tremadocian 방산충 군집들로 밝혀졌다. 또한 R. manitouensis Zone에 속하는 코노돈트 화석군집이 발견되어 중기와 후기 Tremadocian에 속하는 Cow Head Group이 분포하는 여러 지역 간의 (Martin Point, Western Brook Pond, Broom Point, 그리고 St. Paul's Inlet) 층서대비를 보다 정확하게 할 수 있었고, 코노돈트 생층서에서는 R. manitouensis Zone, Low Diversity Interval, M. dianae Zone, 그리고 하부 P. proteus Zone이 식별되었다. Late Tremadocian radiolarian faunas were first recovered from the Cow Head Group, Newfoundland, Canada. Three faunal assemblages were recognized, one from Martin Point and two from Western Brook Pond in Gros Morne National Park. These radiolarian faunas include six families, 11 genera, and 26 species. In these faunas, six genera (Archeoproventocitum, Cowheadia, Neopalaeospiculum, Protospongentactinia, Protoproventocitum and Westernbrookia) and 17 species (Archeoproventocitum nudiformum, A. retiformum, Aspiculum densum, A. jamesi, A. multistratum, A. gigantium, Cowheadia duplextesta, Neopalaeospiculum densum, N. laxum, N. transformum, Pararcheoentactinia stilla, Protospongentactinia spongiosa, Protoproventocium nazarovii, P. aitchisoni, Westernbrookia cancella, W. diversa, and W. ovata) are new. Late Tremadocian faunas are characterized by the appearance of proventocitiids and diversification of aspiculumids and reduction of protoentactiniids and echidniniids that had flourished in the early Tremadocian faunas. An examination of the biostratigraphic range of co-occurring conodonts indicates that the radiolarian faunas described here belong to the late Tremadocian, from the Lower Diversity Interval through the M. dianae Zone to the lower P. proteus Zone. With the recovery of conodonts of the R. manitouensis Zone from other localities in the study area, the correlation among Martin Point, Western Brook Pond, Broom Point, and St. Paul's Inlet strata has became more precise. Also, the middle and late Tremadocian Cow Head Group can be subdivided into the R. manitouensis Zone, the Low Diversity Interval, and the M. dianae Zone of North America. The lower P. proteus Zone of the latest Tremadocian for northern Europe is recognized in the Western Brook Pond South section.

EARLIEST ORDOVICIAN (EARLY TO MIDDLE TREMADOCIAN) RADIOLARIAN FAUNAS OF THE COW HEAD GROUP, WESTERN NEWFOUNDLAND

Well-preserved earliest Ordovician (early to middle Tremadocian) radiolarian faunas were recovered from carbonate rocks of the Cow Head Group of the Great Northern Peninsula of the island of Newfoundland, Canada. The earliest Ordovician faunal assemblages are from Green Point, Martin Point, Broom Point North and South, and St. Paul's Inlet in Gros Morne National Park. Latest Cambrian faunas were also recovered from Green Point and St. Paul's Inlet, but are extremely low in both abundance and diversity. The radiolarian faunas include five families, 10 genera, and 24 species. Of these, one family Aspiculumidae, one genus, and 19 species are new. The new family and new genus are Aspiculumidae and Aspiculum, respectively. The new species are Pararcheoentactinia? cowheadensis, Aspiculum eccentricum, Aspiculum? angulatum, Parechidnina delicata, P. variospina, Curvechidnina multiramosa, Echidnina conexa, E. laxa, E. semiconexa, E. severedeformis, Echidnina? immanis, Palaeospiculum curvum, P. multifurcatum, P. neofurcatum, P. tetractium, Protoentactinia deformis, P. kozuriana, P. primigena, and P. transformis. The Aspiculumidae is established on the basis of the new genus Aspiculum and on Parechidnina, whose family-level assignments were previously indeterminate. The new family Aspiculumidae is distinguished from the other four families by the absence of the spicule system.All genera of the earliest Ordovician radiolarian faunas can be placed in the families Aspiculumidae, Archeoentactiniidae, Echidninidae, Palaeospiculumidae, and Protoentactiniidae, as can the genera of the Cambrian radiolarian faunas. However, echidninids from Cambrian faunas are generally characterized by interlocked or fused spicules whose original structure is recognizable, while those from the earliest Ordovician are commonly characterized by fused and/or modified spicules. Also, the very rare protoentactinids of the Late Cambrian are extremely abundant and diverse in the earliest Ordovician faunas described herein. Specimens of the families Palaeospiculumidae and Archaeoentactinidae are less diverse and/or less plentiful in the earliest Ordovician compared to those in Cambrian. The genus Parechidnina, which now belongs to Aspiculumidae, is more plentiful and very diverse in the earliest Ordovician, and, at the same time, lineages of the new genus Aspiculum and a related not-yet-named genus began to evolve.The detailed biostratigraphic ages of the earliest Ordovician radiolarian faunas were determined mainly by the co-occurring conodonts. The age range of the earliest Ordovician faunas represented extends from the Cordylodus lindstromi Zone through the C. angulatus Zone to the Rossodus manitouensis Zone.

Late Cambrian radiolarian faunas and biostratigraphy of the Cow Head Group, western Newfoundland

Well-preserved Late Cambrian radiolarian faunas were recovered from carbonate rocks of the Cow Head Group of the Great Northern Peninsula of the island of Newfoundland, Canada. Several different faunal assemblages were recognized from three strata at Green Point, one from a stratum at Martin Point, and three from strata at Broom Point South in Gros Morne National Park. The faunas contain nine genera, five of which are new, and 33 species, all but two of which are new. The five new genera areCurvechidnina, Grosmorneus, Pararcheoentactinia, Ramuspiculum,andSubechidnina.Most of the genera belong to the families Archeoentactinidae, Echidninidae, Palaeospiculumidae, and Protoentactiniidae; a few taxa have an uncertain taxonomic position.Echidnina,whose taxonomic identity was uncertain, is shown to be a radiolarian.The diversity of each of these faunas is similar, but the difference in faunal compositions among the faunas examined is distinct. From this analysis, the Late Cambrian strata between Martin Point and Green Point can be more precisely correlated, and the previous correlation has been revised. With the recovery of the conodonts in this study, the provisionally placed boundary between the Franconian and Trempealeauan at Martin Point by earlier researchers is revised and positioned between unit 30 and unit 31.The biostratigraphic range of the co-occurring conodonts indicates that the youngest fauna described in this paper belongs to theEoconodontus notchpeakensisZone of late Trempealeauan age and that the other faunas (from Martin Point and Green Point) are of Franconian age.

LATE CAMBRIAN RADIOLARIAN FAUNAS AND BIOSTRATIGRAPHY OF THE COW HEAD GROUP, WESTERN NEWFOUNDLAND

Abstract Well-preserved Late Cambrian radiolarian faunas were recovered from carbonate rocks of the Cow Head Group of the Great Northern Peninsula of the island of Newfoundland, Canada. Several different faunal assemblages were recognized from three strata at Green Point, one from a stratum at Martin Point, and three from strata at Broom Point South in Gros Morne National Park. The faunas contain nine genera, five of which are new, and 33 species, all but two of which are new. The five new genera are Curvechidnina, Grosmorneus, Pararcheoentactinia, Ramuspiculum, and Subechidnina. Most of the genera belong to the families Archeoentactinidae, Echidninidae, Palaeospiculumidae, and Protoentactiniidae; a few taxa have an uncertain taxonomic position. Echidnina, whose taxonomic identity was uncertain, is shown to be a radiolarian. The diversity of each of these faunas is similar, but the difference in faunal compositions among the faunas examined is distinct. From this analysis, the Late Cambrian strata between...

The proposed Cambrian–Ordovician global Boundary stratotype and point (GSSP) in Western Newfoundland, Canada

AbstractSections exposing the Cambrian–Ordovician Boundary interval at Broom Point in western Newfoundland have been proposed earlier for a global systemic boundary stratotype. These lie within the Cow Head Group, a late Middle Cambrian to early Middle Ordovician allochthonous unit of limestone, shale, and conglomerate deposited at the toe of the ancient continental slope and on the adjacent continental rise. Several recent studies have further investigated the stratigraphy, sedimentology, and palaeontology of the Cow Head Group and others are under way on magnetostratigraphy and chemostratigraphy. These aspects are reviewed for six key boundary sections representing proximal to distal facies: Cow Head Ledge, Broom Point South, Broom Point North, St Pauls Inlet Quarry, Martin Point, and Green Point. In particular, new data are presented from 260 conodont samples that yielded 15500 conodonts. This intense sampling has allowed the discrimination of minor hiatuses in the proximal to intermediate facies where conglomerates have eroded and cannibalized underlying strata. New conodont data from Broom Point North have lowered the base of the C. lindstromi Zone into unit 74 conglomerates, thereby making this section unsuitable as a boundary stratotype. New collections from Green Point have yielded abundant conodonts and over 9400 conodonts have been recovered from 77 samples.The conodont, graptolite, and trilobite biostratigraphy through the boundary interval is documented allowing accurate correlation between sections and more precisely revealing small hiatuses in the proximal and intermediate facies. The sequence of conodont zones is: Eoconodontus notchpeakensis, Cordylodus proavus, C. caboti, C. intermedius, C. lindstromi and C. angulatus. These can be correlated with trilobite zones established from both in situ and clast faunas from the proximal to intermediate facies and with graptolite assemblages (of Cooper 1979) especially in the intermediate to distal facies. Three new species of Cordylodus are described (C. andresi, C. hastatus and C. tortus) and the full apparatus of Iapetognathus preaengensis is illustrated.The criteria for selecting a global boundary stratotype and point (GSSP) are reviewed in terms of the Cow Head sections. The Green Point section is shown to meet, and largely surpass, the prerequisites required of a stratotype. The Green Point section is proposed to be the global boundary stratotype with the base of the Ordovician System defined at the base of unit 23, which is the base of the Broom Point Member, Green Point Formation, at a level coincident with the base of the Cordylodus lindstromi Zone. In addition to an abundant and superbly preserved conodont fauna, this section preserves the best sequence of earliest planktic graptolites through a 40 m interval; the first nematophorous graptolites (of Assemblage 1) occur in unit 25, 6.9 m above the base of the C. lindstromi Zone. This level can be readily correlated into the proximal facies where both deep and shallow water trilobites (in situ and in clasts, respectively) show the base of the C. lindstromi Zone to lie within the Symphysurina brevispicata trilobite Subzone.

The earliest Ordovician nematophorid graptolites: taxonomy and correlation

AbstractThe earliest nematophorid (planktic) dendroid graptolites (Family Anisograptidae) are described and revised from several heterofacial facies sections of the Broom Point Formation, Cow Head Group, western Newfoundland (Broom Point North and St Paul's represent medial to upper continental slope, Martin Point and Green Point distal slope environments). Noting potential conodont recycling in the palaeoslope environments of the proximal Cow Head Group sections, graptolites are the only group of fossils that could not have been subject to post-burial transport. The graptolites belong to an informal group of quadriradiate form-species principally representing two originally related but then diverging phyletic groups: pendent Rhabdinopora Eichwald, 1855 and horizontal to sub-horizontal Staurograptus Emmons, 1855; both being stratigraphically within Assemblage Zone I (emended after Cooper, 1979) of earliest Tremadoc or early Ibexian age (top of Cordylodus intermedius to base of C. lindstromi conodont zones). Assemblage I graptolites offer a higher resolution than coeval conodonts and are a better tool for inter-facies correlations both within the Broom Point Formation and with other graptolitic sequences on a global scale. Form-species of quadriradiate Rhabdinopora rustica (such as ?R. praeparabola and R. parabola) and of Staurograptus dichotomus (e.g. ‘Radiograptus’ or ‘Heterograptus’ flexibilis or ‘Aletograptus’) are interpreted as astogenetic morphs, or as phylogenetically descendant variants of these forms. Upon evidence of late astogenetic insertion of dissepiments in Staurograptus and Anisograptus the names ‘Heterograptus’ and ‘Radiograptus’ are placed into junior synonymy, and ‘Aletograptus’ is regarded a junior synonym of Staurograptus because its post-primary dichotomies are merely delayed distally (or omitted) to be observed exclusively in very mature specimens. The Cow Head Group fauna is compared with earliest nematophorids from Oslo, Norway (Naersnes) and Dayangcha, Jilin, China (Xiaoyangqiao), and a new comprehensive ecostratigraphic zonation for early Tremadoc graptolites is proposed.

Introduction to papers on the Cambrian–Ordovician Boundary

AbstractThe Working Group has the responsibility of recommending a specific level within a suitable stratigraphic section to serve as the global stratotype for the Cambrian–Ordovician Boundary. Commencing in 1974, comprehensive studies culminated in a plenary session in Calgary in 1985 resulting in decisions on the ‘golden-spike’ principle of selecting the boundary at a horizon just below the first influx of nematophorous graptolites. Conodonts are to be used as the primary guide for the selection of the specific horizon and the global stratotype section must have potential for studies using paleomagnetism, geochemistry and other non-biological correlation tools. The specific horizon will correspond approximately to the base of the Tremadoc Series of Wales and slightly higher than the base of Canadian Series as used in western North America.The Calgary meeting considered the Broom Point and Green Point sections in Newfoundland, Canada, and the Xiaoyangqiao section at Dayangcha, China, as prime candidates to serve as the global stratotype. A delegation from the Working Group inspected the Dayangcha sections in 1986. For this visit, comprehensive documentation and description of the sequence were prepared and published and are briefly summarized in the present issue.Specific uncertainties remain to be resolved for the Dayangcha and Newfoundland stratigraphic sections before the Working Group will be ready to recommend selection of a global stratotype.

Conodonts as biostratigraphic tools for redefinition and correlation of the Cambrian–Ordovician Boundary

AbstractThe Cambrian–Ordovician Boundary is recognized at different horizons on various continents by utilizing several fossil groups. Conodonts are abundant, diverse, and less provincial and facies-controlled than other fossils in this interval; many species are widespread and some are cosmopolitan. Strata representing the Cambrian–Ordovician Boundary interval from Asia, Australia, and North America can be correlated easily when those strata are from cratonal or shelf environments. Strata from slope facies have fewer conodonts, fewer taxa, and taxa may have different ranges compared with cratonal and shelf facies. It may be easier to correlate intercontinentally within cratonal/shelf facies than to correlate intracontinentally from cratonal/shelf facies to slope facies.A new hierarchical arrangement of conodont interval zones and subzones is proposed for western North America. These include theProconodontus tenuiserratusZone, theP. posterocostatusZone (new), theP. muelleriZone (new), theEoconodontusZone (new, withEoconodontus notchpeakensisandCambrooistodus minutusSubzones), theCordylodus proavusZone (emended, withHirsutodontus hirsutus, Fryxellodontus inornatus, andClavohamulus elongatusSubzones), theCordylodus intermediusZone (new, withHirsutodontus simplexandClavohamulus hintzeiSubzones), theCordylodus lindstromiZone (new), andCordylodus angulatusZone (new).The boundary point for the base of the Ordovician System will be chosen so as to coincide with a correlatable conodont zonal boundary; other fossil groups will support correlation of this boundary point. Three alternative horizons are being considered. The base of theCordylodus proavusZone is the most distinctive but is regarded by some as older than is appropriate for the base of the Ordovician. The base of theCordylodus intermediusZone is recognizable by faunal changes in several evolutionary lineages at a level that is slightly younger than the presently recognized boundary in Australia, North America, and parts of Asia but slightly older than the base of the Tremadoc Series of Europe. The base of theC. lindstromiZone can be recognized in most areas by the lowest occurrence of the nominate species, which is of questioned taxonomic validity and uncertain biostratigraphic utility; this horizon is closest to the base of the Tremadoc Series of Europe.Choice of a stratotype section has been narrowed to the Cow Head Group in Newfoundland, eastern Canada; and to the Fengshan and Yehli formations in Jilin Province, northeastern China. The Cow Head Group was deposited on and near the base of the continental slope. Erosion by debrisslide breccias resulted in a significant hiatus near the boundary interval in the Broom Point sections, a condition contrary to guidelines for choice of boundary sections. This may be less of a problem in other sections, such as Green Point. The Dayangcha section in China was deposited on the outer part of a continental shelf and has acritarchs, conodonts, graptolites and trilobites. Additional study of the China section is needed to document fully the ranges of critical conodont taxa.

Source rocks of western Newfoundland

Studies on the organic geochemistry of natural oil seeps, although not always associated with major economic deposits, offer opportunity to compare chemical characteristics of an oil to a sedimentary depositional environment. One such seep occurs in the geologically well-documented Parsons Pond region of Western Newfoundland. A number of potential sources for the oil, which has been observed for 170 years, have been proposed including shales from the Shallow Bay and Green Point Formations of the Cow Head Group, the Table Head Group and similar sediments from the Curling Group, all which represent deposition along a continental margin in the Cambro-Ordovician. Total organic carbon in the shales ranges from quite low (0.6%) to moderate (5.2%). Gas chromatography-mass spectrometry shows evidence that suggest weathering. Odd predominance prior to C 21 is seen in these oils and is characteristic of most Ordovician oils. The oils contain a slight even predominance in the C 25+ alkanes. Pristane and phytane were both observed to be present yielding a ratio of 1.6. Isotopic compositions from the Green Point Formation extracts are closest to that of the oils (−28.5%). Similar variations in isotope values from one fraction to another are observed in both the oils and Green Point extracts. Nitrogen isotope studies show a large range in the extracts with the best comparison between the Broom Point (0.5%ℵ) and seep oils (0.6%ℵ). The composition of the organic matter from the Broom Point and Martin Point Members of the Cow Head Group reflect the hydrocarbon compositions of the oil The effect of thermal maturation eastward across the Parsons Pond region is observed in increased bitumen contents and heavier carbon isotope compositions.

Correlation of Cambrian–Ordovician boundary between Europe and North America: new data from western Newfoundland

The Cow Head Group in western Newfoundland includes a record of continuous graptolite evolution across the Cambrian–Ordovician boundary. In sections at Broom Point, interbedded breccias contain blocks with rich platform trilobite faunas which give a complete succession of North American zones. The stratigraphic relationship between graptolites and trilobites clearly establishes that the base of the type Tremadoc Series lies above that of the Missisquoia Zone. The sequence of earliest planktonic graptolites is the most complete known and serves to remove objections to the selection of Dictyonema flabelliforme, or one of its subspecies, as an index for the base of the Ordovician System.

STRATIGRAPHY OF THE COW HEAD REGION, WESTERN NEWFOUNDLAND

Seashore exposures in the Cow Head area display a succession of limestone conglomerates interbedded with shales and limestones. Conglomerate layers range in thickness from 1 foot to more than 200 feet. The material consists of flat angular chips or moderately rounded boulders of fine-grained gray or white limestone. Fossiliferous boulders from any one layer yield trilobites of the same limited age. Boulders from the lowest fossiliferous conglomerate at Broom Point contain Kootenia, Zacanthoides, Orriella, Agraulos, and Peronopsis. Succeeding bedded limestone yields Meneviella and Tomagnostus. A boulder in the overlying conglomerate also contained Meneviella and Tomagnostus. Cambrian conglomerates at Cow Head yield younger trilobites in successive layers: Tricrepicephalus appears in the lower, then Taenicephalus, followed by Ctenopyge, Rasettia, and Keithiella. Above are beds with Staurograptus and Dictyonema, overlain by conglomerates with early Ordovician trilobites. Successively higher black shales contain graptolites of the Levis Shale zones, and interbedded conglomerates yield successively younger Ordovician trilobites. Shale with Trigonograptus and Isograptus underlies the highest conglomerate which yields Nileus, Bathyurellus, Remopleurides, and Ectenonotus. The thick limestone conglomerate at Lower Head includes enormous boulders, one of which contains many of the trilobites described by Billings (1861–1865) as from Cow Head. This layer is underlain by black shale with Isograptus. The Cow Head Group, therefore, is not a single formation but a succession of conglomerates and intervening beds ranging in age from Middle Cambrian to Middle Ordovician. With few exceptions the boulders of any conglomerate are approximately the same age as the immediately underlying strata. The limestones of the fossiliferous boulders formed in shallow water; the beds intervening between the conglomerate layers formed in deeper water and consequently are almost devoid of fossils other than graptolites.