Compositional variation of siderite and its significance in deciphering the diagenetic evolution of the Lower Cretaceous sedimentary system of the Scotian Basin

Abstract

Diagenetic siderite is widespread in the Scotian Basin with age ranging from sea-floor diagenetic to among of the last diagenetic products. Through the use of minor element chemistry five types of siderite have been identified. The relative ages of these five types of siderite have been established using their textural relationships in back-scattered electron (BSE) images, and through the use of petrographic microscopy. The studied siderites show extensive substitution of Mn and Mg and to a lesser extent of Ca for Fe, with the substitution of Mn for Fe being the most common in certain wells. Factors that may influence the chemistry of the diagenetic siderite include: chemistry of the circulating fluids, lithofacies, pre-existing detrital and diagenetic minerals, and availability of permeability. All these factors seem to have played a role in the chemistry of the studied siderites. The two competing mechanisms to produce the observed siderite chemical variability are regional basin hydrology and bio-mineralisation. Recrystallisation and re-precipitation of siderite are ubiquitous in most studied samples. They seem to be closely related with various dissolution events, syn-sedimentary deformation, and late regional salt tectonics. Of the dissolution events, late dissolution is especially enhanced in certain samples, and suggests the presence of very corrosive pore fluids. The best explanation for our data sets is the involvement of a variety of hydrogeological events. Factors that might have influenced the onset of these events include: regional tectonics, and in particular salt tectonics, transport of hydrocarbons, and overpressure, in addition to sea-level changes. Chemical evidence from siderite and associated minerals suggests that the circulating fluids that have influenced the precipitation of particular types of siderite must have changed drastically through time both in their chemistry and their temperature.