Abstract

Many calcrete profiles contain evidence of heterogeneous growth of displacive calcite which includes (1) cathode-luminescence-defined zones in crystals, indicating both linear unidirectional and polyhedral multidirectional growth vectors; (2) expanded micaceous grains; (3) broken quartz grains; and (4) the dispersal of clastic grains in a carbonate matrix. These features reflect the influence of the changing character of the growth environment consequent upon changes in the sediment itself. In the vadose zone of arid areas, rapid evaporation leads to supersaturation and thus crystallization. During drying, water withdraws from grain surfaces to capillary pores within micaceous grains and at grain-to-grain contacts. These form the primary sites for precipitation of calcite. Kinetic barriers to nucleation ensure that growth is initially restricted to a few stressed sites, and, since no other nucleation sites are available, continued growth of crystals is displacive. With the retention of greater volumes of pore water, there is a change from linear growth of a few faces to a polyhedral expansion of packed groups of crystals. The resulting dilation creates additional stresses within the sediment which are transmitted between grains and which, focussed by smaller areas of point contacts, break quartz grains. As permeability is reduced, larger volumes of saturated water are retained within the sediment. More nuclei form in pore throats, and subsequent growth promotes further expansion and dispersal of framework grains. This is primarily by growth of individual displacive crystals but may include the deposition of some micrite reflecting high nucleation and low growth rates. Later, oversized pores formed by expansion are filled by unstressed grains growing as a coarse cement. Continued post-burial diagenesis is reflected in neomorphism of displacive crystals, of micrite, and of cements, indicating a general trend towards chemical homogeneity and structural stability.

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