Origin and diagenesis of fine-grained slope sediments : Cow Head Group (Cambro-Ordovician), western Newfoundland

Abstract

The Cow Head Group is a base-of-slope apron deposit composed of 5 lithofacies: (1) conglomerate, (2) calcarenite, (3) siltstone, (4) shale, and (5) lime mudstone. Conglomerates were deposited by debris flows and the calcarenite grainstones by high-density turbidity currents. Siltstones and shales represent interbedded fine-grained turbidites and terrigenous hemipelagites. Rhythmically-bedded parted, ribbon, and nodular lime mudstones, composed mostly of microspar and pseudospar, resulted from the interplay of climatically-controlled carbonate-abundance cycles and event deposits. Peloids and intraclasts, derived mainly from the breakdown of the calcified algae Girvanella and Epiphyton, are the most important allochems in the fine-grained sediments. -- Burial compaction occurred mainly in argillaceous sediments. Limestones are uncompacted and demonstrate only minor pressure solution effects. Intrastratal deformation was due to submarine failure and layer-parallel compression during Taconic orogenesis. Subtly-expressed synsedimentary deformation fabrics indicate that slope failure was more common than is apparent based solely on the presence of intraformational truncation surfaces and slide masses. -- In conglomerates, clasts with diagenetic microfabrics identical to those of the thinly-bedded, fine-grained sediments indicate that lithification (calcite authigenesis, early dolomitization, and some silicification) occurred within several metres of the sediment-water interface. Carbon isotope analyses suggest that carbonate precipitation was driven by bacterial sulphate reduction and methane generation. Trace element (Mg, Fe, Mn, and Sr) and cathode luminescence patterns record calcite precipitation from progressively more reduced pore-waters. These patterns are indistinguishable from those generated during meteoric-water diagenesis. -- Cathode luminescence microfabrics of radiaxial fibrous calcite from shallow-water boulders in conglomerates and in situ displacive fibrous calcite indicate that these crystals are composite and grew both as spherocrystals and unit crystals. Aggrading neospar on the margins of mudstone beds and nodules resulted from decreased nucleation density and precipitation of progressively younger calcite, commonly as asymmetrical, irregular increments. -- Three types of dolomite are differentiated: (1) detrital, (2) early diagenetic, and (3) late diagenetic. Silt-size particles of detrital dolomite were derived from weathering of older carbonates or penecontemporaneous dolomite from the adjacent, shallow-water platform. Diagenetic dolomite occurs both as a replacement and a pore-filling phase. Early dolomitization was synchronous with or postdated the main phase of calcite authigenesis. Late dolomitization is related to tectonic faults and joints. Silicification also has a protracted diagenetic history, but its distribution is controlled primarily by the abundance of radiolaria and siliceous sponge spicules.