Ordovician and Silurian strata in southern Manitoba are composed of shelf-type sediments that comprise in ascending order the Winnipeg (shale and sand), Red River, and Stony Mountain formations and Interlake group (carbonates), Paleozoic sedimentation commenced in Late or possibly Middle Ordovician time with deposition of quartzose sand and green shale on a peneplaned surface in Precambrian (and local Cambrian) rocks in southern Manitoba. Initially, considerable sand and interbedded shale were laid down in the western areas, whereas mainly green shales accumulated at the E. Thence, during upper Winnipeg time, a large E-trending sand body (more than 125 mi. long, 20-40 mi. wide, and up to 90 ft. thick) was deposited in the Carman area, and it now extends eastward from Pelican Lake to the subcrop SE of the city of Winnipeg. It is postulated that approximately in middle Winnipeg time differential tectonic movements in the eastern area caused an E-trending break in slope, S of which quartz sands were laid down in shallow water, forming the Carman sand body, whereas nondeposition and slight erosion may have occurred N of this line. This was followed by uniform subsidence and dominantly shale deposition in all areas to the end of Winnipeg time. The lower Red River bioclastic limestones (containing basal transitional argillaceous beds) that accumulated next were affected by penecontemporaneous or diagenetic dolomitization. Where the latter process was incomplete, irregular areas of the matrix of calcarenites were most readily dolomitized to produce characteristic mottling. Of the fossils, crinoid ossicle were most resistant to dolomitization. Secondary chert nodules formed partly after dolomitization. Extensive shoals developed during the latter part of Red River deposition, and an evaporitic environment prevailed, especially in the western part of southern Manitoba. Here, cream-colored and extremely fine-textured (microcrystalline and cryptocrystalline) dolomites that are associated with anhydrite are interpreted to be both secondary and primary in origin. Eastward, increasing amounts of very fine crystalline dolomites occur. Relict pseudo-ooelitic and bioclastic textures in secondary dolomite suggest that there was considerable differentiation of shoal and intershoal environments during late Red River sedimentation. Lowering of the depositional profile of equilibrium largely removed the restriction imposed by carbonate shoals, and gray bioclastic, fossiliferous limestones (lower Stony Mountain) containing argillaceous material were deposited. In part of the western area, the limestones were subject to penecontemporaneous dolomitization. Extremely fine-textured dolomite, exhibiting relict pseudo-ooelites and bioclastic grains, in the upper Stony Mountain (Gunton member) indicate a return to bank-type deposition. Arenaceous, silty, and argillaceous material also was transported into southern Manitoba, particularly toward the termination of Stony Mountain time. Quartz sands were concentrated in interpreted shoal areas E of Daly field and in the southeastern part of the map area. Fine-textured cream dolomites also occur in the approximate lower 150 ft. of the Interlake group within which the Ordovician-Silurian time boundary is placed. Above, very fine to medium crystalline dolomites are characteristic. Thin biostromes and small patch reefs (coral, algal, or stromatoporoidal) containing brachiopods, similar to those described in outcrops, are probably present in the subsurface. Presence of relict ooelitic, pseudo-ooelitic or pelletoid, and bioclastic textures in secondary dolomite indicates the diversity of carbonate types that formed on this ancient bank. The Ordovician and Silurian sequences contain potential petroleum reservoirs, including porous quartzose sands (Winnipeg) and fine to medium crystalline porous dolomites (Red River, Stony Mountain, Interlake). Petroleum accumulations may occur in porous rocks that change facies to nonpermeable rocks up regional dip as well as laterally along strike where anticlinal folds are not present. Porous Interlake dolomites may be sealed by underlying and overlying nonpermeable strata at the pre-Middle Devonian unconformity where the latter truncates the Silurian rocks up regional dip (eastward) and is overlain by Ashern redbeds.