Millimetre-Scale Localisation of Strain and Dissolution in Oolitic Grainstone

Abstract

Millimetre-scale localisation of strain and dissolution in oolitic grainstone ii KeywordsCarbonate, clay dissolution, cement, CPO, ductile, deformation mechanisms, EBSD, grain boundary sliding, geological history, initial interfaces, microfabric, ooid, oolite, porosity, pressure solution, SPO, strain localisation, stylolite, viscosity, XFMMillimetre-scale localisation of strain and dissolution in oolitic grainstone iii AbstractLocalisation of finite strain in rocks is commonly explained by variations in loading conditions, rock mechanical properties and rock fabric. In a geological sequence of layered rocks where loading conditions (such as temperature and far-field stress) are similar, strain variations are commonly constrained to layers with differing material properties. The examined Gudman Oolite presents a unique and interesting example of how variable finite strain is influenced by minor variations in material properties.The Carboniferous Gudman Oolite features alternating millimetre-to centimetre-thick layers of coarse ooids (mean diameter ~1-3mm) and fine ooids (mean diameter ~0.5-1mm) within a sparitic cement (30-300µm grains). Each layer is predominantly (>95%) calcite, formed in the same depositional environment and experienced the same tectonic history. However, the coarse-ooid layers show high non-coaxial deformation and extensive stylolite textures. In contrast, the fine-ooid layers appear nearly undeformed, and are almost devoid of dissolution textures.To shed light on the reasons for this marked localisation of deformation and dissolution, the rock mechanical properties and rock fabric were measured using a combination of X-ray Fluorescence Microscopy (XFM), Electron Backscatter Diffraction (EBSD), X-ray Diffraction (XRD), palaeotemperature analysis and petrographic methods including Scanning Electron Microscopy (SEM), Cathodoluminescence (CL) and transmitted light microscopy. Examination of overprinting relationships in the coarse-ooid layer shows that localised dissolution (D1) occurred prior to localised non-coaxial strain (D2). Primitive stylolite textures suggest that dissolution seams formed in concentric mica-rich layers within ooids which occasionally linked to surrounding ooids to form laterally extensive stylolite seams. I argue that the relatively larger ooids in the coarse-ooid layer contain more extensive micaceous layers which promoted linking and dissolution to form laterally continuous stylolite seams.I conclude that D1 dissolution destroyed the C3 cement framework and reduced the strength of the coarse-ooid layer relative to the largely preserved fine-ooid layer. The later D2 shear deformation event localised strain in the weakened coarse-ooid layer.