Abstract
Summary. The St Bees Sandstone is a reddened lower Triassic sandy alluvial sequence which has suffered a complex history of iron oxide diagenesis. Detrital clay-oxides have been post-depositionally reddened and particulate grain coatings of haematite precipitated. Syntaxial overgrowths of haematite and titaniferous haematite have been precipitated on coarse detrital oxides (specularites). In situ alteration of specularite (principally the martitization of detrital titanomagnetite and titanomaghaemite) has taken place. Also ferromagnesian phyllosilicates (principally biotite) have suffered partial to complete pseudomorphism by tine grained haematite. These diagenetic changes took place during contemporary geomagnetic field variations and each diagenetic phase carried a magnetic component recording the direction (and hence date) of its formation. In general magnetizations can be described as: pigment-dominant, specularite-dominant or mixed pigment-specularite. Using a combination of chemical and thermal demagnetization the individual magnetic components can be isolated using Zijderveld vector diagrams. Four examples are illustrated: (a) A specularite-dominant magnetization in which a reversed component carried by coarse detrital grains had a superimposed normal component carried by authigenic overgrowths of haematite. Both components are considered to be Triassic in age. (b) A pigment-dominant magnetization carried primarily by clay-oxide pellicles formed during intrastratal alteration. The main isolated component has an ‘anomalous’ direction and its precise time of formation cannot be determined. (c) A mixed specularite-pigment magnetization in which a ‘normal’ specularite component has a superimposed ‘normal’ pigment component. These normal components are considered to be of different Triassic ages. (d) A mixed specularite-pigment magnetization in which a near-horizontal specularite component, possibly due to inclination error, has a superimposed
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