An investigation of depositional mechanisms of pyroclastic density currents using anisotropy of magnetic susceptibility (AMS) and a detailed stratigraphic study of the La Caleta Formation, Tenerife
thesisposted on 05.02.2013, 11:20 by Clare Maher
This thesis describes La Caleta Formation and its eruptive history interpreted using sedimentary logs, isopach and isopleth data, granulometry and geochemical studies. The formation has been subdivided into six members, which were emplaced during five eruptive phases. The eruption commenced with fallout from a Plinian eruption column that collapsed, generating a rapid succession of short-lived pyroclastic density currents that deposited extensive ignimbrites and co-ignimbrite ash layers, culminating in a more sustained pyroclastic current that deposited a thick, predominantly massive ignimbrite, capped with a lithic breccia interpreted to represent caldera collapse. Fine ash layers can be deposited by ashfall or pyroclastic density currents and their fine grain size coupled with poor exposure, can hinder interpretations of their depositional origin, which is vital if using ash layers for hazard assessment. A new method of discriminating between these two types of deposit is presented in this thesis. Samples of unequivocal depositional origin were collected from formations within the Bandas del Sur Group and their fabrics, analysed using Anisotropy of Magnetic Susceptibility (AMS). This revealed differences in the distribution of susceptibility axes, the imbrication of the magnetic foliation and the strength and shape of the anisotropy between the two types of deposit. However the discrimination using the traditional AMS plots was found to be equivocal in some instance, so a new discriminant plot has been devised, which can successfully discriminate between the two types of ash layer. AMS has been used to categorise cross-stratified, stratified, diffusely bedded and massive lithofacies, and to interpret flow-boundary conditions and depositional mechanisms. Samples were collected from ignimbrites and ash layers from the Bandas del Sur Group and their AMS fabrics compared. The distribution of the magnetic susceptibility axes on the stereonets has been used to infer flow-boundary conditions; girdle distributions represent more tractional processes, whereas well-grouped distributions represent more granular processes and random distributions represent fluid escape dominated flow-boundary zones. The most significant outcome was the recognition of different types of massive deposit, based on variations in the distribution of the susceptibility axes, which are interpreted to have been deposited at a fluid-escape dominated flow-boundary influenced by other processes (traction and/or granular shear). This indicates that not all massive deposits are emplaced in the same way.