Earth Sciences Publications

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Proceedings of the Geological Association

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Determining the process of silicification in silcretes is essential to understanding their environmental significance. For the late Paleocene silcretes of the Anglo-Paris basin this is of particular interest due to their association with the PETM (Paleocene-Eocene Thermal Maximum). Here puddingstone samples from Hertfordshire and Buckinghamshire have been examined by optical, BSEM and CL petrography, X-ray diffraction and oxygen isotope analysis. The range of quartz sand luminescence colours indicates a diverse provenance. Flint pebbles show little variability, consistent with a single source. The oxygen isotope compositional range of the flint pebbles is consistent with chemical sedimentation at normal temperatures from Cretaceous seawater, and with the pebbles being derived from the Chalk Group (Upper Cretaceous). The majority of pebbles are well rounded and have an outer zone that is either iron-stained or weathered. Fracturing of pebbles typically post-dates the weathering/oxidation and pre-dates deposition of the pebble bed. A small proportion of the pebbles have been fractured in situ; this fracturing post-dates deposition and pre-dates silicification. Matrix fabrics are diverse, ranging from very fine sand with a cryptocrystalline quartz cement to medium sand with macro-quartz cement with luminescence zoning. Minor formation of authigenic Ti oxides has occurred and locally authigenic Fe oxides are abundant. Ti oxides formed during, or immediately before, silcrete formation, while the Fe oxides are possibly associated with more recent weathering. The oxygen isotope data for the silica cement are consistent with silcrete formation from highlands-derived, low-18O meteoric groundwater at warm near-surface temperatures.

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