Sandstone blocks in an ancient mud diapir in the Tertiary Shimanto belt record isotropic dilation in the form of a three-dimensional (3-D) disaggregation texture that is similar to textures documented in ancient and active mud diapirs in many locations worldwide. We attribute this unusual texture to the combination of porosity loss in the margins of the sandstone block due to diagenetic reactions, and decompression as the diapir was emplaced. The expansion and degassing of pore-filling fluids, coupled with the low porosity, resulted in the isotropic dilation of the blocks. Similar porosity-reducing diagenetic reactions may seal off permeable wall-rock units penetrated by mud diapirs and thus aid in the generation and maintenance of overpressures. In addition, the textures we document may be useful in recognizing mud diapirs in the rock record.