Dolomite within the St. George Group (lower Ordovician), western Newfoundland

Haywick, Douglas Wayne (1984) Dolomite within the St. George Group (lower Ordovician), western Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Seven varieties of dolomite and dolostone, the products of four stages of dolomitization, are recognized within the St. George Group (Lower Ordovician) of western Newfoundland. -- Dololaminites are syngenetic, formed in a tidal flat environment, characterized by prominent shallow water sedimentary structures, local bioturbation, δ ¹⁸O values of -4 to -8 o/oo and composed of anhedral, very finely crystalline, uniformly luminescent dolomite rhombs. Siliciclastic minerals are subordinate. -- Early-diagenetic (eogenetic) dolomitization, possibly initiated by the presence of mucopolysaccharides and controlled spatially by permeability, has resulted in three varieties of dolomite and dolostone. δ ¹⁸O values range between -4 and -10 o/oo. Mottle dolomite selectively replaces body and trace fossils and is localized along pressure solution seams. Rhombs of matrix dolomite are evenly distributed in mudstones and wackestones and range in abundance from trace quantities to 80 percent. Both varieties are characterized by finely crystalline, well zoned, idiotopic to xenotopic dolomite. Though initially nucleated during early- diagenesis, they have undergone a prolonged period of growth continuing at least until the onset of pressure solution. -- Pervasive A dolostones are mottled rocks characterized by bimodal crystallinity; finely crystalline dolomite in mottles, medium crystalline dolomite between mottles. Both are xenotopic and uniformly luminescent to moderately zoned. These rocks are coincident with early phases of mottle/matrix dolomitization and may have developed due to the mixing of meteoric and marine waters. They have not been subjected to late-diagenetic periods of growth. -- Hydrothermal alteration, probably related to tectonics during initial phases of the Taconic Orogeny (Middle Ordovician), is a late-diagenetic (mesogenetic) event and in the northern portion of the study area (Great Northern Peninsula), has developed two extensive field varieties. Pervasive B dolostones are bimodal rocks resulting from overprinting of a dolomite-mottled limestone. Saddle dolomite is a void and fracture filling cement and is associated with sphalerite mineralization near Daniel's Harbour, Newfoundland. Both varieties are composed of coarsely crystalline, uniformly luminescent and strained dolomite rhombs (commonly with curved crystal outlines), and are characterized by δ ¹⁸O values ranging from -8 to -12 o/oo. -- Hydrothermal dolomitization in the southern portion of the study area (Port au Port Peninsula) is rare. This variety of matrix dolomite is restricted to the intergranular (matrix) areas of wackestones and packstones. It is similar both petrographically and isotopically to saddle dolomite. -- Cavity-filling dolostone has filled dissolution voids in pre-existing dolostones formed during periods of subaerial exposure and is characterized by δ¹⁸O values ranging from -6 to -9 o/oo. The dolomite is very finely crystalline, uniformly luminescent and annedral. Accessory minerals are diverse and abundant. -- KEYWORDS: St. George Group; Lower Ordovician; western Newfoundland; dolomite; dolomitization; diagenesis; isotope geochemistry; cathodoluminescence; petrography.

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