Temperature and basinal fluid controls on feldspar diagenesis, Lower Cretaceous sandstones, Scotian Basin, Canada

Abstract

The passive-margin Scotian Basin accumulated thick Upper Jurassic–Lower Cretaceous deltaic sandstones and shales, which were influenced by high heat flow in the mid-Cretaceous and deformed by salt tectonics. This study takes advantage of the complex thermal history and record of saline fluid flow in the basin to determine the relative importance of burial depth, temperature, salinity of basinal brines, overpressure, and sandstone permeability in controlling the dissolution of detrital feldspar and growth of authigenic K-feldspar and albite. Feldspar grains and volcanic lithic clasts from two transects, with differing thermal and salt tectonic histories, were studied by scanning electron microscope (SEM) backscattered electron images, energy-dispersive spectroscopy, SEM cathodoluminescence (CL) imaging, and hot-cathode CL microscopy. Published fluid inclusion measurements are integrated with burial history curves and timing of deformation on salt detachment surfaces to provides a new synthesis of the post-Jurassic thermal and saline fluid flow history of the basin. Surface-controlled dissolution of K-feldspar and growth of authigenic K-feldspar rims occur from 1.5 to 3.1 km. More destructive transport-controlled dissolution, down to 3.9 km, is more effective in thick-bedded permeable sandstones that acted as fluid pathways. Albitization of detrital K-feldspar and sodic plagioclase, and of feldspars in volcanic lithic clasts was facilitated by high Na+ content of formation waters migrating preferentially through such pathways at the time of silica and carbonate cementation, with strong albitization coinciding with transport-controlled dissolution of K-feldspar. Authigenic albite partially fills pores created by K-feldspar dissolution. Overpressured conditions enhance albite authigenesis, as a result of slower fluid advection. Episodic release of hot, saline, overpressured fluids in the zone of surface-controlled dissolution of K-feldspar favoured albitization but had no detectable effect on K-feldspar dissolution. Basin hydrology plays a definitive role in feldspar diagenesis, producing patterns that cannot be interpreted as a simple consequence of temperature and activity of Na+.