Abstract

A new type of ooid has been found in southeastern Tunisia in Holocene sands cemented in a vadose environment. The sands are associated with serpulid and stromatolite bioherms and correspond to undisturbed relict lagoonal beaches. They contain ooids that have cortical spines located near the points of contact between grains. These coated grains resemble ooids that have been deformed by burial compaction, although the geologic context and 14C dates clearly indicate that they have never been subjected to lithostatic pressure. The location of the spines and the evidence of vadose diagenesis point to postdepositional deformation and suggest that residual capillary waters control this phenomenon.

The spines may be related to the compressive effect of aragonite crystal growth, which remained active in the external cortices after ooid deposition. Deformation due to burial compaction and deformation related to early diagenetic processes can be differentiated on the basis of the specific location of the compressive deformations. Such a distinction is important when interpreting the diagenetic history and porosity evolution of oolitic sands.

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