Abstract Stratigraphic sections in the Monitor Range and Roberts Mountains provide both shelf and basin depositional records for two prominent stratigraphic intervals in the Ordovician System. The Whiterock Narrows section in the Monitor Range is proposed as a candidate global stratotype section for the base of the Middle Ordovician Series. The boundary interval in the upper Ninemile Formation and lower Antelope Valley Limestone, deposited on the carbonate platform, contains excellent biostratigraphic records of conodonts, used to define the boundary, trilobites and brachiopods. The boundary level can be correlated into a deep basin facies at the Red Canyon section in the Roberts Mountains because conodonts that define the boundary co-occur with graptolites in calcareous sandstones of the Lower Member of the Vinini Formation . The Vinini Creek section in the Roberts Mountains provides continuous exposure of the stratigraphic interval in the Upper Member of the Vinini Formation that includes the Late Ordovician mass extinction and was deposited in a deep basinal setting. The decline in diversity of the graptolite fauna coincides with the initiation of sea-level fall. However, conodont and chitinozoan faunas were unaffected. The Copenhagen Canyon section in the Hanson Creek Formation in the Monitor Range records the Late Ordovician events in a platform margin setting. There, faunas were affected by the sea-level fall much earlier than in the deep basinal setting, and the extinction of conodonts and chitinozoans occurred substantially later than that of graptolites .