We report a structure that is characterized by a localized increase in porosity or dilation within a tabular band of on average 1--2 mm thickness in porous, poorly consolidated sand. These dilation bands formed in predominantly opening-mode and provide a sharp contrast to planar opening-mode fractures or joints with two discrete surfaces in consolidated, low porosity brittle rock. Our observation of dilation bands, together with previously recognized shear and compaction bands, complete the spectrum of localized deformation modes along tabular bands in poorly consolidated sand and sandstone consistent with recent theoretical predictions of deformation localization in granular media.