Cambrian, Ordovician, and Silurian strata of Manitoba, Saskatchewan, northern North Dakota, and northeastern Montana comprise two vertically separated parts: a lower sequence of non-carbonate clastic beds ranging from Cambrian to Upper Ordovician, and an upper sequence of dolomitic limestones and dolomites ranging from Upper Ordovician to Silurian. The rocks floor all the northern part of the Williston Basin, reach more than 3,000 ft. in thickness, and are unconformably overlain by Middle Devonian or younger beds. The formations cropping out along the north-eastern side of the basin can be traced (in the form of para-time-rock units) by means of subsurface sections across the entire area. The full sequence in upward order is: Deadwood formation, Winnipeg formation; Red River, Stony Mountain, and Stone- wall formations (Bighorn group); and Interlake group. The clastic Deadwood and Winnipeg formations are separated by an unconformity of probable post-earliest Ordovician pre-Middle Ordovician age. The carbonate succession above records numerous small hiati, but no major breaks of sedimentation. Toward the end of Red River carbonate deposition marked cyclic alternation between thin evaporitic anhydrites and fossiliferous-fragmental limestones occurred. The evaporites extend through a wide arc on the northern and western sides of the basin. Lateral (updip) changes from limestones to dense dolomites cause divergence between the marker bed and facies boundaries: lithologic features typical of the upper part of the Bighorn group appear to descend into the Red River formation toward the basin margin. Shales at the base of the overlying Stony Mountain formation seem to be the result of slight tectonic adjustment on the southeastern side of the basin, but the carbonate-evaporite cycles returned during later Stony Mountain and Stonewall deposition. Interlake group deposits are characterized by dense dolomites with pseudo-ooelitic and fragmental beds. Sedimentation was interrupted by several non-sequential beds of highly rounded quartz-sand grains and silts. These horizons are utilized as para-time-rock markers. Isopach maps reveal a progressive movement of the axis of the basin from a nearly E.-W. trend during Red River deposition to a NNW-SSE trend during Interlake deposition. Petroleum accumulations (either "shows" or producible quantities) occur at many porous levels in the succession, particularly in stratigraphic traps developed by the upper Red River and Stony Mountain-Stonewall evaporite cycles on the western side of the basin (S.-central Saskatchewan, western North Dakota, and northeastern Montana), and in paleotopographic or tectonic structures over truncated Interlake porosities at the pre-Middle Devonian unconformity. Both kinds of trap are analogous to Mississippian reservoirs and cap rocks in the north-eastern part of the basin. The areal distribution of presently known oil occurrences is directly related to the distribution of highly saline formation waters.