Marine low-magnesium calcite concretions are widespread in many siliciclastic and mixed carbonate–siliciclastic shelf and basinal settings. The process of concretion formation is generally well established and involves microbial influence (mostly sulfate reduction to oxidize organic material at or just below the seafloor). The microbes produce interstitial fluids that are conducive to abundant, and apparently rapid, precipitation of calcite cements. Pervasive cementation generates well-indurated beds or isolated flattened “pods” that are commonly confined to specific stratigraphic horizons. Stratabound concretions can be important as fluid-flow barriers during subsequent burial and compaction. Thin-section and scanning electron microscopy of Cenozoic and Mesozoic concretions has revealed a dense occurrence of small (mostly 2–10 μm), equant, mostly subhedral calcite crystals. The best resolution of both techniques is, however, unable to adequately characterize crystal boundaries, the distribution of clays or organic matter, or the nature of the pores within the calcite matrix. Here, we used scanning electron microscopy to examine ion-micromilled surfaces of concretions from Upper Miocene and Upper Jurassic strata. Results indicate that the dominant crystal size is 1 to 3 μm (mean 2.08 μm; standard deviation = 1.42 μm). Pores were formed at the intersections of calcite crystals by the constriction of the fluid-filled interstitial space, likely prior to dewatering and initial compaction. These (micro) pores are of the “type III, fitted fused” variety. Two-dimensional pore shapes analyzed on micromilled surfaces are near-equidimensional (length/width = ~1–1.5), oval (length/width = 1.5–5), and elongate (length/width = >5) forms. Equidimensional and oval pores occur at the intersections of calcite crystals (along with clay minerals and organic material). Elongate pores of uncertain origin are found at the boundaries between adjacent calcite crystals. The helium pycnometer porosity of the plugs associated with the Upper Jurassic micromilled sample is consistent with a relatively low total porosity, with values of 0.38, 0.58, and 0.82%. Micromilled surfaces improve our understanding of two-dimensional crystal structure and porosity within the matrix of marine concretions. The size and shape of cement crystals and pores suggest that relatively early, rapid, and pervasive precipitation produced a homogeneous mass of calcite and small isolated pores. The resultant low porosity and permeability formed a rock that was diagenetically stable and resistant to chemical and physical modification later during burial.