Green Point Formation Research Articles

Paleoenvironmental variations during the Late Cambrian: Implications from Zn isotopes, I/(Ca+Mg) ratios, and other elemental proxies

The Upper Cambrian at the Martin Point section (∼110 m-thick) in western Newfoundland is a part of the Cow Head Group and partially spans the coeval Tuckers Cove and the Martin Point members of the Shallow Bay and Green Point formations, respectively. These units comprise alternating shale, minor conglomerate beds, and limestone rhythmites of a toe-of-slope apron. Earlier studies of the C-isotope profile, based on the lime mudstone interbeds, documented global negative δ13Ccarb shifts (from old to young: NL1, NL2, HERB) correlated with the lowermost Proconodontus posterocostatus, Proconodontus muelleri and base of Eoconodontus notchpeakensis conodont zones, respectively (post-Steptoean Positive Carbon Isotope Excursion NL1 and NL2 and Hellnmaria-Red Tops Boundary-HERB). Samples were extracted from the most preserved spots of micritic to near-micritic grain size (≤4 μm−10 μm). Fabric retention confirms petrographic preservation and insignificant correlations (R2 ≤ 0.1) of diagenetic proxies (e.g., Sr) with their environmental counterparts support the preservation of at least near-primary geochemical signatures. The paleoenvironmental proxies (TOC, Zn, P, Cu, Al/Ti, Mn, Fe, V, Mo) support dominant dysoxic conditions. The δ66Zn values (0.09–0.73 ‰ JMC Lyon) and (I/(Ca + Mg)) ratios (0.02–0.48 μmol/mol) vary consistently throughout the studied events. The minor decreases in δ66Zn values seem to denote inhibition of bioproductivity in a dysoxic water column, consistent with warm and humid climate during general transgressive settings and shoaling of organic-rich water into the shallow environment. The low I/(Ca + Mg) ratios (<0.5 μmol/mol) reinforce this interpretation, suggesting general dysoxic settings throughout the entire section.

Redox conditions across the Upper Cambrian of eastern Laurentia: Implications from geochemical proxies

The lower part of the Martin Point section (∼ 110 m - rhythmites) spans the uppermost Shallow Bay and lowermost Green Point formations (uppermost Franconian–lowermost Trempealeauan) of the Cow Head Group (western Newfoundland, Canada). It records two post-SPICE (Steptoean Positive Carbon Isotope Excursion) events marked by negative δ13Ccarb excursions (NL1 and NL2) that were also documented in South China. The petrographic and chemical screening of the investigated carbonate samples proves the preservation of at least near-primary geochemical signatures. The proxies of weathering (e.g., Mn, Fe, Al, and ∑REE), bioproductivity (e.g., δ13Corg, TOC, P, Ni, Zn, and Cu), and paleoredox (e.g., Cr, Th/U, Ce/Ce*) have been utilized to reconstruct the paleoenvironmental conditions during the post-SPICE events. The TOC, P, Ni, Cu, Cr, ∑REE, Mn, Fe, and U Ca-normalized profiles exhibit a positive pulse at the base of NL2 (upper negative δ13Ccarb excursion), followed by a drop. This might have resulted from shoaling organic-rich anoxic waters into an oxygenated shallow environment during transgression, which led to a negative shift in the δ13Ccarb and a decrease in TOC contents as well as an increase in the δ13Corg. The correlated enrichment in Zn and δ15Norg values and the Ce/Ce* around unity are consistent with the shoaling scenario. A similar scenario seems to be consistent with NL1 (lower negative δ13Ccarb excursion) although the lower resolution of correlated proxy variations makes the interpretations based on a few points and to be therefore taken with caution.

Variability of sedimentary pyrite δ34S records: A case study of slope shale of the Green Point Formation in western Newfoundland, Canada

The Green Point Formation in western Newfoundland, GSSP of the Cambrian-Ordovician (Є-O) boundary, is dominated by slope rhythmites of alternating lime mudstone and shale interbeds. This formation was deposited in a semi-restricted basin with varying connectivity to the open ocean. In the current study, we investigate textures and bulk δ34S signatures of pyrite (δ34Spy) in the shale to better understand factors influencing the sedimentary δ34Spy fluctuation. Petrographic and SEM examinations reveal two major types of pyrite: (1) framboidal pyrite and (2) anhedral to euhedral pyrite. The latter is further categorized into two subtypes: type 2a anhedral to subhedral pyrite characterized by relict framboidal textures and larger sizes (∼10 to 300 μm), and type 2b smaller (typically <10 μm) subhedral to euhedral pyrite. Type 1 pyrite was precipitated near the sediment-water interface (SWI), whereas type 2b pyrite was formed in sediments below the SWI with limited access to the overlying seawater sulfate. Type 2a pyrite was evolved from framboids during early and burial diagenesis. The bulk δ34Spy values, marked by a significant scatter (1σ =10.62‰), range broadly from −17.6 to +22.4‰ (VCDT) and exhibit a pronounced positive excursion of ∼20‰ near the Є-O boundary. The abundance of type 2b pyrite generally mimics the changes in δ34Spy, suggesting that the substantial δ34Spy dispersion could be partially attributed to differing proportions of type 2b pyrite within the samples. Moreover, notable negative correlations exist between the δ34Spy values and the abundances of Al, Th, ∑REE, and Fe, indicating that riverine fluxes might have influenced the Δ34Sseawater – pyrite by modulating the regional seawater sulfate and iron reservoir sizes. Therefore, rather than being indicative of oceanic redox oscillations, the positive δ34Spy excursion of ∼20‰ of this interval was probably driven by decreased sulfate and iron levels in the local waterbody. The decline in terrestrial input during this δ34Spy shift might have also contributed to a negative δ13Ccarb excursion by reducing nutrient supply and inhibiting primary productivity. Collectively, the bulk sedimentary δ34Spy variability recorded by the Green Point shale may be attributed to a combination of changes in regional terrigenous input and varying amounts of pyrite formed at different diagenetic stages within the samples. The general opposing trends between the δ34Spy signals and the abundances of Al, Th, ∑REE, and Fe, however, imply that fluctuations in riverine influxes might exert a stronger influence on the major δ34Spy trend. These findings suggest that bulk sedimentary δ34Spy variations may not be reliable evidence for perturbations of the global sulfur cycle.

Expanded marine anoxia at the Cambrian-Ordovician transition: Evidence from lime mudstone I/Ca and δ238U signatures of the GSSP in western Newfoundland, Canada

The early Paleozoic witnessed two spectacular radiations of marine organisms—the Cambrian (Є) Explosion and the Great Ordovician (O) Biodiversification Event. However, the period between them was characterized by a marine biodiversity plateau attributed to recurring biocrises. In the current study, we present the I/Ca records from slope lime mudstones of the Green Point Formation in western Newfoundland, along with a three-sink U-isotope mass balance model and previously reported limestone δ238Ucarb signals of the same interval, to further explore oceanic redox conditions at the Є-O boundary. The I/Ca ratios of the lime mudstones, deposited along the eastern Laurentian (western Iapetan) continental slope, exhibit a narrow range between 0.02 and 0.33 μmol/mol. Poor correlations between the I/Ca values and their [Sr], δ18O, Mn/Sr, Fe/Sr, Mg/Ca, and δ13Corg counterparts, together with near-micritic textures of the limestones, argue against significant influences of postdepositional alterations on the I/Ca signatures. The iodine-depleted lime mudstones, with I/Ca values well below the Proterozoic Eon baseline I/Ca ratios (∼0.5–1 μmol/mol), suggest the presence of shallow marine oxic-anoxic interfaces along the regional continental margin. Substantially low limestone I/Ca ratios (<0.5 μmol/mol) and dysoxic to anoxic depositional conditions have also been reported from several other age-equivalent sections deposited along the shelf and slope of ancient Iapetan and Laurentian continental margins. As a result, seawaters surrounding the Iapetan and Laurentian continental margins at the Є-O transition might have been commonly poorly oxygenated with shallow oxyclines or expanded oxygen minimum zones in the shelf and/or slope areas. This interpretation is further supported by our three-sink U-isotopemass balance modeling, which predicts widespread marine anoxia at the Є-O boundary with anoxic to euxinic water covering 1.0–21.1% of the ocean floor, significantly higher than the modern day (∼0.2%). Furthermore, the wide range of the oceanic δ238UOC (−1.22 to −0.25‰) values, estimated from the limestone δ238Ucarb signals, might reflect oceanic redox oscillations during this period. However, the δ238UOC fluctuations could also be attributed to variable accumulations of the 238U-enriched authigenic U phases during early diagenesis. Overall, evidence from the lime mudstone I/Ca ratios and the estimated extent of marine anoxia at the Є-O boundary in this study aligns with earlier viewpoints that the slowness of marine biodiversity accumulation during the late Cambrian and the Early Ordovician was linked to widespread oceanic anoxia.

The Mo- and U-isotope signatures in alternating shales and carbonate beds of rhythmites: A comparison and implications for redox conditions across the Cambrian-Ordovician boundary

The Green Point Formation (GPF) of the Cow Head Group in western Newfoundland (Canada) includes the Global Boundary Stratotype Section and Point (GSSP) for the Cambrian-Ordovician (Є-O) boundary on Laurentia. This formation is composed primarily of thick, deep subtidal to slope deposits (rhythmites) of alternating lime mudstones and shale interbeds. Here, we employ isotopic and elemental data of Mo and U of these shales and carbonates to better understand their depositional environments and the open ocean redox conditions at the Є-O transition. The variability of δ98Moauth (−0.40 to +3.16‰) indicates variable aqueous sulfide concentrations ([H2S]aq) in bottom waters or porewaters during deposition of the shales. A significant positive correlation of shale [Mo]auth with [U]auth coupled with low Mo concentrations compared to Phanerozoic euxinic shales further points to weakly reducing (~dysoxic/suboxic to anoxic) benthic conditions with H2Saq primarily confined to porewaters. For the lime mudstones, the δ98Mo variability (−0.30 to +2.34‰) suggests variable [H2S]aq in porewaters during early diagenesis. Low Mo concentrations ([Mo]carb = 0.25 ± 0.80 μg/g, 2σ) and a poor correlation of [Mo]carb with δ98Mocarb indicate weak sulfidic ([H2S]aq < 11 μM) porewater conditions and possibly dysoxic to suboxic bottom environments. Thus, the δ98Mocarb may not track the δ98Mo of basin waters because of low [H2S]aq in porewaters and insignificant accumulation of authigenic Mo during early diagenesis. This explanation is consistent with a poor correlation between δ98Mocarb and the δ238U of basin waters (estimated from δ238Ucarb). A cautious examination of MoEF/UEF, δ98Moauth, and δ238Uauth of the shales suggests that the GPF was likely deposited in a basin with topographic barriers. During deposition of the lower and middle part of the studied interval, the barriers in combination with sea-level fluctuations may have caused partial isolation of the local basin from the open ocean. Rapid sea-level rises in the earliest Ordovician greatly increased the connectivity between the local basin and the open ocean, resulting in remarkable increases in MoEF/UEF, [Mo]auth, and [U]auth of the shales. The δ238U of the earliest Ordovician open oceans, inferred from δ238Ucarb, exhibited perturbations and an average value of −0.78 ± 0.33‰ (2σ), reflecting oceanic redox oscillations and expanded ocean euxinia compared to the modern oceans.

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.

Rice florigen gene Hd3a has conserved functions in callus development

The transition of meristems is an important developmental process for crop plants. Florigen is considered to be produced in leaves, then moves into the shoot apical meristem (SAM), triggers the transition from the vegetative to the reproductive phase. However, little is known whether Florigen functions in callus development or not. By fused reporter gene β-glucuronidase (GUS) to 1.7 kb promoter of Heading date 3a (Hd3a), GUS signals were detected in the scutellum cells, as well as in green point of the putative transgenic calli. Quantitative RT-PCR results demonstrated that the expression level of Hd3a was increased gradually over time along with the transition from scutellum-deprived callus to shoot. As reported that ectopic expression of FT-like genes caused earlier flowering, we also found that 80% constitutive expression of Hd3a transgenic callus showed formation floral-like organ structures. However, Hd3a RNA interference (RNAi) transgenic calli did not show any obvious phenotype, although AP1 or AP1-like genes—OsMADS14, OsMADS15, and OsMADS18- expression level is decreased during callus development. Both in Hd3a and RFT1 overexpression transgenic calli, Hd3a also modulated AP1 or AP1-like genes, as well AEPALLATA (SEP)-like gene, OsMADS34 during green point formation. Meanwhile, transgenic calli of RFT1and OsMADS50, but not OsEhd1, shared similar results as Hd3a. All of these findings suggested that florigen genes Hd3a and RFT1 have partial conserved functions in the transition of meristems during callus development.

Carbon-isotope stratigraphy of the SPICE event (Upper Cambrian) in eastern Laurentia: implications for global correlation and a potential reference section

AbstractThe δ13C profile from the lower interval of the Martin Point section in western Newfoundland (Canada) spans the Upper Cambrian (uppermost Franconian – lowermost Trempealeauan). The investigated interval (∼110 m) is a part of the Green Point Formation of the Cow Head Group and consists of the upper part of the Tucker Cove Member (topmost part of the Shallow Bay Formation) and the lowermost part of the Martin Point Member (bottom of the Green Point Formation). It is formed of rhythmites of marine carbonates alternating with shales and minor conglomeratic interbeds. Multiscreening petrographic and geochemical techniques have been utilized to evaluate the preservation of the investigated lime mudstones. The δ13C and δ18O values of the sampled micrites (−4.8 ‰ to +1.0 ‰ VPDB and −8.2 ‰ to −5.3 ‰ VPDB, respectively) have insignificant correlation (R2= 0.01), as similarly do the δ13C values with their Sr counterparts (R2= 0.07), 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 excursion (∼4 ‰) that reaches its maximum at the bottom of the section and an upper narrow excursion (∼6 ‰) immediately above the boundary of the Tucker Cove/Martin Point members (Shallow Bay Formation – Green Point Formation boundary). The lower excursion may be correlated with the global SPICE event, whereas the upper excursion may match with a post-SPICE event that has been also recognized in profiles of equivalent sections on different palaeocontinents.

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.

Radiolarian diversity changes during the Late Cambrian–Early Ordovician transition as recorded in the Cow Head Group of Newfoundland (Canada)

Abstract Upper Cambrian to Lower Ordovician pelagic carbonate rocks of the Cow Head Group in western Newfoundland (Canada) were sampled for the study of their radiolarian fauna. Well-preserved and diverse faunal assemblages were obtained from a number of levels of the Shallow Bay and Green Point formations. Three different assemblages, including a total of five families, eight genera and 20 species, are recognized in three different sections situated on a palaeobathymetric profile. The various recognized assemblages are compared with previous studies conducted on the Cow Head Group; they improve considerably our understanding of the pattern of changes in radiolarian diversity during the Cambrian–Ordovician transition. The Radiolarian biotic record of western Newfoundland unveils a two step faunal change across the Cambrian–Ordovician transitional interval: one situated at the base of the uppermost Cambrian Stage 10 and the second at a poorly constrained interval situated towards the top of the same Stage 10. These biotic changes consist of genus-level selective extinctions, as well as within-clade species faunal turnover (i.e. Echidnina, Paleospiculum), decrease of species abundance, species extinction and within-clade episodes of species diversification (i.e. Protoentactinia, Parechidnina).

Middle Ordovician (late Dapingian–Darriwilian) conodonts from the Cow Head Group and Lower Head Formation, western Newfoundland, Canada1This article is one of a series of papers published inCJES Special Issue: In honour of Ward Nealeon the theme of Appalachian and Grenvillian geology.

Middle Ordovician (late Dapingian–Darriwilian) conodonts from the Shallow Bay and Green Point formations, Cow Head Group, and the Lower Head Formation are recorded from three sections in Gros Morne National Park. The collection was investigated to clarify local age relationships between the uppermost part of the Cow Head Group and the interbedded to overlying sediments of the Lower Head Formation. Conodonts from St. Pauls Inlet North section indicate a middle Dapingian age for the upper lower Bed 13, latest Dapingian to early Darriwilian age for the upper Bed 13, an early Darriwilian (Dw 1) age for the top beds or Bed 15 of the Shallow Bay Formation at Lower Head, and the Lower Head Formation is referred to the Darriwilian. The uppermost part of the lower Bed 13 contains Periodon hankensis n. sp., Gothodus sp. A , and Diaphorodus delicatus followed by Periodon macrodentatus , Ansella longicuspica , Erraticodon n. sp. A , and Spinodus wardi n. sp. in the lowermost part of upper Bed 13. The fauna with P. macrodentatus is referred to the newly established Periodon macrodentatus conodont (phylo-)Zone, which is used for global correlation. The uppermost fauna in the Cow Head Group, i.e., Bed 15, includes Histiodella holodentata , Nealeodus martinpointensis , Oistodella pulchra , Dzikodus peavyi , and Yangtzeplacognathus n. sp. A , which are included in the Histiodella holodentata conodont (Bio-)Subzone of the Periodon macrodentatus Zone. Nealeodus is a new genus introduced here; Periodon hankensis n. sp. and Spinodus wardi n. sp. are new species described from the beds 13 and 15, respectively, in the Cow Head Group; Drepanodus aff. D. giganteus , Drepanodus aff. D. robustus , Erraticodon n. sp. A, Protopanderodus cf. P. cooperi , P. cf. P. varicostatus , and Yangtzeplacognathus n. sp. A are taxa referred to in open nomenclature.

An exceptionally preserved parasitic arthropod, Heymonsicambria taylori n.sp. (Arthropoda incertae sedis: Pentastomida), from Cambrian – Ordovician boundary beds of Newfoundland, Canada

A three-dimensionally and completely preserved phosphatized microfossil has been found in Cambrian–Ordovician boundary bed limestones of the Green Point Formation at Green Point, western Newfoundland, Canada. It represents a new form of larvae, previously described by D. Walossek and K.J. Müller from Upper Cambrian anthraconitic limestones ("Orsten"). These authors identified the fossils as instars of marine stem-group representatives of the "tongue worms," Pentastomida. Pentastomida are parasites that today infest various land tetrapods and are presumed to represent one of the closest extant relatives of the Euarthropoda. This new fossil possesses remarkably well-preserved trunk limb vestigia and anal region. It is another example of exceptional three-dimensionally preserved, phosphatized fossils of the Orsten type that is no longer spatiotemporally restricted to the Upper Cambrian of Sweden.

Early development and flysch sedimentation in Ordovician Taconic foreland basin, west-central Newfoundland

During the Early to Late Ordovician the Taconic foredeep in west-central Newfoundland evolved from an underfilled to an overfilled state in response to cratonward advance, thickening, and erosion of the Taconic Orogen. Early orogen-derived sediment in the foreland basin consisted of middle(?) to lake Arenigian deep-water mudstones that accumulated on an inner (craton-facing) slope prism (uppermost parts of Shallow Bay and Green Point formations and correlative units). These deposits are interbedded with and overlie passive-margin slope sediments. In the middle Arenigian to early Llanvirnian, sand from the orogen formed several small, sand-rich submarine fans (Lower Head Formation and correlative units) on the lower reaches of the inner slope and basin plain. The fans may have been fed by closely spaced rivers draining the orogen, as presently occurs in western South America. Only proximal portions of these fans are now exposed. The flysch basin was narrow, constricted by the inner slope and the passive-margin slope, and located a short distance seaward from the buried hingeline of the proto-North American craton. As the orogen thickened sufficiently to override the crustal ramp, the carbonate shelf on the craton drowned, clastic depocentres migrated onto the foundered craton, and a thicker flysch (Mainland Sandstone) accumulated in Llanvirnian-Llandeilian time. In the Caradocian the foreland basin was overfilled with shallow-marine terrigenous sediments (Long Point Formation). Regional flysch dispersal was from a St. Lawrence promontory to a Quebec reentrant.

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.

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.