Abstract

Examples from the Ordovician of Western Canada and the Permian beneath the North Sea indicate that ancient desert sandstones of aeolian dune and wadi origin go through a definite early diagenetic history. This paragenesis consists of various combinations of early cementation (gypsum for some dunes, calcite for wadis), infiltration and deposition by clay-rich surface waters, and dissolution of unstable clasts (usually ferro-magnesian grains) by ground-water. Subsurface water chemistry changes induced by these early diagenetic events lead to alteration of infiltrated clay to haematite, accretion of feldspar and quartz overgrowths, and formation of authigenic clays. Late diagenetic events resulting from deep burial and tectonism may destroy original clasts and early diagenetic features, replacing them with authigenic feldspar, quartz and/or clay and calcite, anhydrite or other cements.

The early post-depositional features discussed are diagnostic of desert climatic conditions during deposition and early diagenesis. Preservation of these features precludes later severe deep burial diagenesis.

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