Diagenetic history of the Lower Port au Port Group carbonates in Western Newfoundland

Shembilu, Noel Clement (2021) Diagenetic history of the Lower Port au Port Group carbonates in Western Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The investigated Middle–Upper Cambrian carbonates span, from bottom to top, the uppermost Hawke Bay (40 m), the March Point (˜83 m thick), and the lowermost Petit Jardin (˜47 m) formations of the Port au Port Group (western Newfoundland, Canada). The deposits consist of dolomitized shallow-water carbonates ranging from microbial mudstones to oolitic grainstones. Petrographic examination of the March Point and Petit Jardin carbonates reveals three generations of dolomite: (i) micritic to near-micritic (D1, 4 μm −35 μm) fabric-retentive dolomite, (ii) coarse crystalline sucrosic (D2, 60 μm−150 μm) fabric-destructive dolomite (sub- to euhedral zoned rhombs with clear rim and cloudy core), and (iii) coarse anhedral (D3, >0.5 mm) fracture-filling saddle dolomite. The microthermometric measurements of primary two-phase fluid inclusions retained in D2, and D3 samples yielded homogenization temperatures (Tₕ) showing a general increase with burial. The mean values of Tₕ and salinity are 89.1 ± 6.4 ﾟC vs 21.2 ± 2.2 eq. wt% NaCl, and 110 ± 8.2 ﾟC vs 21.1 ± 4.7 eq. wt% NaCl, respectively. The dolomites show a decrease of Sr from D1 (90 ± 44 ppm) to D3 (47 ± 15 ppm), which reflects the influence of progressive burial. The estimated δ¹⁸O values of the D1 parent fluid (−5 to −10‰ VSMOW) are bracketed by those suggested for the Middle Cambrian meteoric and seawaters. The fluid-inclusion gas analyses provided CO₂/CH₄ and N₂/Ar ratios coherent with the conclusions from the petrographic examination and geochemical compositions. II A multi-technique evaluation of the preservation of carbonates was applied by using several petrographic and chemical screening tools. The δ¹³C and δ¹⁸O values of dolomicrites (−1.2 ± 1.1 ‰ VPDB and −6.7 ± 1.3 ‰ VPDB, respectively) exhibit an insignificant correlation (R² = 0.16) and similarly, the δ¹³3C values with their Mn/Sr ratio (R² = 0.02). This supports the preservation of at least near-primary δ¹³C signatures that can be utilized to construct a high-resolution δ¹³C profile for reliable global correlations. The δ¹³C profile exhibits two major negative excursions, the lower excursion (˜4 ‰) immediately below the lower boundary of the March Point Formation (Hawke Bay/March Point boundary) and the upper excursion (˜2 ‰) almost at the lower boundary of the Petit Jardin Formation where these excursions are likely associated with short-term sea level changes. The trilobite biozonation scheme suggests the correlation of the lower excursion with the DICE (Drumian Carbon Isotope Excursion) event and the upper excursion with the base of the SPICE (Steptoean Positive Carbon Isotope Excursion) counterpart and both events have been globally documented on different paleocontinents. The variations in several proxies of bioproductivity (e.g., P, Ni, and Cu), input of weathering products (Mn, Fe, Al, and ΣREE), and paleoredox (e.g., Th/U) have been utilized to study the paleoenvironmental conditions during the investigated time interval. Distinct positive shifts in the profiles of Al, ΣREE, Mn, and Fe have been associated with the negative shifts recorded by the δ¹³3C profile and correlated with the DICE (Middle Cambrian) and base of the SPICE (lowermost Upper Cambrian) events. III They reflect contributions from detrital weathered material during sea level falls. Similar correlated positive shifts are also documented by the P, Ni, and Cu profiles, thus suggesting an increase in the riverine inputs of nutrients associated with the drop of sea level and increase of weathering activities. In addition, the changes in the bioproductivity and weathering proxies were associated with relative rising in the Th/U ratios (0.1 to 8.3), which reflects variations in the redox state towards relatively more oxidizing conditions.

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