Early Diagenesis and Lithification in Carbonate Sediments

Abstract

ABSTRACT The dominant carbonate minerals in carbonate depositional terrains are aragonite and high-magnesian calcite. These undergo mineralogical and textural changes on subaerial exposure leading to lithification. If these minerals after deposition continue to be exposed to marine water, they retain their original mineralogical composition and textural characteristics and resist lithification. Under subaerial conditions during lithification, mineralogical and textural changes occur concurrently. In carbonate sediments devoid of micrite or with only a minor micrite content which are exposed to subaerial processes, low-magnesian calcite is precipitated as an interparticle cement to bind the grains together and at an early stage only partially occludes pore space. The calcium carbonate necessary to provide this cement is derived from the dissolution of aragonite. Magnesium is removed from high-magnesian calcite to yield low-magnesian calcite or high-magnesian calcite is removed and low-magnesian calcite formed on a micro scale by solution-deposition without a textural change of the grains affected. By contrast, aragonite grains are leached out to form moldic porosity, and the molds are infilled by a drusy low-magnesian calcite mosaic. Additional low-magnesian calcite, derived from the dissolution of aragonite, is precipitated as a drusy mosaic in the interparticle pore space. pH and salinity of the waters control the changes involved in lithification. In the presence of waters of requisite pH and salinity, lithification can probably occur in the subsurface. Paramorphic replaceme t of aragonite grains and micrite by calcite without an intervening stage of mold formation is common. Water probably takes part in this reaction with solution-deposition on a scale so small that cavities of molecular dimension are involved in this change. Thus small textures are fully preserved. Aragonite grains under subaerial conditions have a metastability sequence which in order of increasing stability is skeletal grains<ooids<pellets and cryptocrystalline grains. Pellets, cryptocrystalline grains and, to some degree, oooids tend to resist mold formation under subaerial conditions. During diagenesis they prefer to undergo paramorphic replacement by calcite. Dolomite was found to form under three depositional-diagenetic conditions: as a presumably penecontemporaneous sediment in the intertidal zone of pellet muds among algal mats in the Bahamas, under deep water marine conditions below the water-sediment interface on the Bermuda Apron, and by replacement of calciteinfilled molds of originally aragonitic grains. These three modes of formation during early diagenesis emphasize the diversity of conditions under which dolomite is apt to form.