Sedimentology and diagenesis of middle and upper Cambrian platform carbonates and siliciclastics, Port au Port Peninsula, Western Newfoundland

Abstract

The Port au Port Group is an outer shelf, peritidal carbonate and siliciclastic platform deposit composed of six lithofacies: (1) parted limestone; (2) grey and variegated shale; (3) ooid calcarenite; (4) carbonate laminite; (5) stromatolite and thrombolite mound; and (6) mixed glauconitic sandstone and oolite. Ooid calcarenites, carbonate laminites, and variegated shales together were deposited as a series of carbonate sand shoals in which the shallow-subtidal and intertidal zones were each regimes of in situ ooid formation and accumulation. Parted limestones and grey shales were deposited on muddy tidal flats leeward of the carbonate shoals. Both muddy tidal flats and carbonate sand shoals were sites of stromatolite and thrombolite mound growth. Siliciclastic sands and silts represent eolian deposits and reworking of nearshore siliciclastics. -- The lithofacies occur in two types of meter-scale assemblages: (1) predictable meter-scale, shallowing-upward cycles of parted limestone and shale deposited in the muddy tidal flats, which were largely controlled by variable rates of sedimentation; and (2) unpredictable meter-scale assemblages of ooid calcarenite and carbonate laminite, which resulted from the sporadic but frequent migration of the carbonate shoals. These two assemblages together form Grand Cycles that reflect the complex interplay of sea-level eustasy and regional variations in sedimentation and environmental conditions. These Grand Cycles are distinct from western North American cycles because they are composed almost entirely of peritidal sediments; thinness of the cycles; and restricted areal occurrence. These differences may reflect narrower shelf widths; lower rates of sedimentation; and lower amplitudes of relative sea-level rise on the northeastern margin of the North American craton compared to the western margin. -- Concentric ooids in intertidal brown oolites were originally bimineralic; aragonitic laminae are now blocky calcite and laminae of fibrous and micritic Mg calcite have altered to calcite with well-preserved fabrics. Radial ooids in subtidal grey oolite were Mg calcite only and underwent progressive alteration during burial to give rise to a variety of cortical fabrics, ranging from radial to blocky calcite. The facies-specific nature of ooid mineralogy and morphology suggests that local environmental conditions strongly influenced ooid formation, particularly turbulence and sea floor topography. -- The spectrum of calcite cements, mudstones and dolomites represents a protracted history of diagenesis, which includes: (1) synsedimentary peritidal lithification, erosion and dolomitization; (2) possible shallow-burial meteoric lithification and alteration of metastable carbonates; (3) deep-burial lithification and dolomitization; and (4) cementation and dedolomitization related to uplift. Peritidal lithification and dolomitization, along with minor shallow-burial lithification, determined the final appearance of the rocks; deeper burial and re-exposure events resulted in only a diffuse overprinting of all lithofacies.