Abstract

Nodules of silicified oomicrite (chert) occur in the upper part of the shallow-water Precambrian Bhander Limestone of central India. The nodules characteristically are surrounded by aureoles, elongated parallel to bedding, of plastically deformed ooids. Both the intensity of deformation and packing density of the ooids in the limestone decrease away from the rigid nodules until a point is reached where there is neither detectable deformation nor condensation (intensification of “closeness” of grains). Lack of pervasive deformation in the limestone is interpreted to suggest that although in places compaction-deformation began early in diagenesis, cementation also began almost simultaneously, preventing the process from affecting the sediment beyond the limits of the aureoles.

Deformation of allochems, particularly nonductile ones, appears to be possible only under unusually intense overburden pressure (or shear stress). Anomalously high stress around rigid nodules of precompaction origin may lead, however, to the deformation of allochems at much lower overburden pressures, and therefore earlier than normally possible. Whereas the presence of deformed allochems proves compaction, the converse may not be true. Sediments do compact, but overburden pressure probably does not normally exceed the load-bearing capacity of the allochems; in such cases, allochems would not be visibly deformed. Within the normal range of lithostatic pressure likely in compaction, sutured micrite fabric and condensation of allochems are therefore more likely than deformed allochems. Cementation may commonly intervene before sufficient overburden pressure is built up to deform allochems. Overemphasis on the presence of deformed allochems as a sign of compaction appears to have overshadowed the importance of compaction in the diagenetic evolution of limestones.

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