Keywords

Introduction

The Green Point section of the Cow Head Group in western Newfoundland, Canada (Fig. 1) is composed of a nearly complete succession of fine-grained marine clastic and carbonate sediments that were deposited during the late Cambrian and Early Ordovician. The section serves as the GSSP for the global Cambrian–Ordovician system boundary (Cooper et al. 2001) and has been biostratigraphically well studied (Barnes 1988; Erdtmann 1988; Cooper et al. 2001). Recently, the exact stratigraphic level of the boundary, based on conodont biozonation, has been an issue of debate (Terfelt et al. 2012).

Fig. 1
figure 1

Location map showing the study area

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A δ13Ccarb curve from the Green Point section has not been published, because the data were not considered reliable (Cooper et al. 2001). Hence, the main purpose of this contribution is to present a high-resolution δ13Ccarb isotope curve for the Cambrian–Ordovician boundary interval sediments exposed at Green Point, Newfoundland, and to relate the new isotope curve to existing and important global biostratigraphic horizons coeval with the Green Point GSSP.

Geological Setting: Stratigraphy and Depositional Facies

The mid-Cambrian to Middle Ordovician strata of the Cow Head Group (up to 500 m thick) are deep-water deposits that accumulated along the Laurentian margin. The sediments are composed of interbedded shale and limestone, but significant conglomerate beds containing blocks of shallow-water carbonates were transported into the deep-water facies.

The marine distal slope Green Point Formation of the Cow Head Group consists of the Martin Point (lower), Broom Point (middle), and St. Pauls (upper) members. Shale and minor mud limestone dominate the Martin Point Member, the Broom Point Member is a ribbon to parted lime mudstone with beds of grainstone, conglomerate, and siltstone, and the overlying St. Pauls Member includes red and green shales and minor carbonates (James and Stevens 1986). The two lower members of the Green Point Formation are exposed at Green Point (Fig. 2) and the succession represents the base of a slope depositional environment (James and Stevens 1986).

Fig. 2
figure 2

Diagram showing the δ13C profile and associated geochemical variations in P, Al, Si, and ∑REE across the Cambrian–Ordovician GSSP in western Newfoundland, Canada

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Biostratigraphy

The succession at Green Point was deposited during the Eoconodontus notchpeakensis to Cordylodusintermedius conodont biozones (Furongian Series, uppermost Cambrian) and the C. lindstromis.l. and Cordylodus angulatus conodont Biozone (Tremadocian Stage, Lower Ordovician) (Barnes 1988; Terfelt et al. 2012). The GSSP horizon with the golden spike in bed 23 is placed in the C. hintzei Subzone of the Cordylodus intermedius Zone (Fig. 2; Terfelt et al. 2012). The first planktonic graptolites (i.e., Rhabdinopora praeparabola and Staurograptus dichotomus) occur in bed 25 (Erdtmann 1988; Cooper et al. 2001), which is about 5 m above the global Cambrian–Ordovician boundary horizon in the Green Point section (Fig. 2).

Materials and Methods

Eighty-four samples composed of lime mudstone were collected from and logged into the Green Point section starting at the top of bed 17 (Fig. 2). The degree of preservation of the studied micritic carbonates of the Green Point Formation was evaluated by petrographic and geochemical multiscreens following the protocol described in Azmy et al. (2010). The total rare earth element (∑REE) contents of carbonates are enriched by diagenesis (Azmy et al. 2011) and are, along with their Mn and Sr counterparts, a powerful tool in recognizing alteration in extreme deep-time carbonates.

Results

The δ13Ccarb and δ18O values of well-preserved micrite microsamples range from −4.7 to \( +1.7 \) ‰ (VPDB) and from −8.7 to −5.5 ‰ (VPDB), respectively; they exhibit very insignificant correlation (R2 = 0.00002) despite the considerable correlation between δ18O and Mn/Sr (R2 = 0.35). Similarly, the δ13C and Mn/Sr values show almost no correlation (R2 = 0.00004). Despite the strong correlation between the ∑REE and δ18O values (R2 = 0.36), they exhibit insignificant diagenetic alteration with their δ13C counterparts (R2 = 0.05). This suggests that the analysed carbonates with their near-primary δ13C signatures can be utilized to reconstruct a reliable C isotope profile at the Green Point GSSP for global high-resolution chemostratigraphic correlations.

Chemostratigraphic Pattern

The δ13Ccarb profile of the studied boundary section reveals a composite of three peaks with a negative shift of ~6.0 ‰, which starts at the base of the Broom Point Member. The negative δ13Ccarb shift is preceded by a significant input of P (3000–9000 ppm) and mild increases of Al (1100–1800 ppm) and Si (2200–4000 ppm) that reflect a significant eustatic drop in sea level. This is consistent with the high total REE contents (∑REE = 150–270 ppm) observed immediately before the C-shift, and with the change from relatively shale-dominated lithology of the Martin Point Member to a more carbonate-dominated in the Broom Point Member. Th/U ratios are invariably constant at about <2 (suggesting reducing conditions) and thus indicate no significant change in redox conditions throughout the formation.

Conclusions

The carbon isotope curve constrains the position of the golden spike at the Cambrian–Ordovician System boundary at the GSSP section, with a position just ahead of a prominent positive excursion within the Cordylodus intermedius conodont Zone. The detailed δ13Ccarb carbon isotope stratigraphic profile from the uppermost Cambrian to lowermost Ordovician strata in the Green Point section also permits precise matching to global sections with δ13Ccarb isotope information covering the same stratigraphic interval.