The formation and later filling of cavities (reticulate, isolated and caverns) and fractures (types A and B) in carbonate buildups of the Winnipegosis Formation was a critical phase in the porosity history of these carbonates. Reticulate cavities (up to 2 cm high and wider than the 3.5 in diameter core) with flat subhorizontal bases and irregular roofs formed beneath reoriented, cemented grainstone crusts. Isolated cavities (up to 6 cm high and wider than the core), which are irregular in shape, formed contemporaneous with deposition of peloidal packstones and wackestones and algal laminated bindstones. The presence of isopachous cements, radiaxial fibrous cements and peloidal sediments in many cavities suggests that they formed in open marine conditions. Caverns, up to 2.5 m high, probably formed by dissolution when the buildups were subaerially exposed. Type A fractures, which occur in the upper 10 m of the buildups on their windward margins, probably formed as a result of tectonic fracturing or oversteepening of a semilithified margin. Type B fractures, which occur as networks of thin veins, formed by compaction or by water escaping through semilithified sediment. Cavity and fracture systems are filled with micrite and silt that formed during subaerial exposure of the buildups.