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2007: The influence of denitrifying seawater on graptolite extinction and diversification during the Hirnantian (latest Ordovician) mass extinction event. Lethaia , Vol. 40, A continuous trench exposure within the uppermost type Vinini Formation at Vinini Creek, Roberts Mountains, Nevada, provides an unparalleled opportunity to examine the fate of graptolites, prominent Paleozoic zooplankton, during most of the Hirnantian mass extinction event. On the basis of a detailed biostratigraphic and sedimentological dataset, the relatively complete extinction record is examined in the context of ecological constraints, and it is found to reflect an ecological collapse driven by glacioeustatic sea-level fall and associated changes in oceanic circulation. Diverse graptolite populations of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) fauna, which flourished in denitrifying waters within the oceanic oxygen-minimum zone (OMZ) during sea-level highstand, largely vanished with the loss of these conditions during glacio-eustatic sea-level fall. However, populations of one clade, the normalograptids, which inhabited the oxygenated waters of the photic zone, not only survived but diversified. These survivors gave rise to rapid recolonization and diversification with re-establishment of the oxygen-minimum and denitrifying conditions during post-Hirnantian sea-level rise. This ecological model also applies globally to other welldocumented coeval stratigraphic intervals, representing both oceanic and platform sea settings.

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Abundant and regionally unique dolostone lithoclast breccias occur throughout the shallow‐marine, Lower to Middle Ordovician Pogonip Group in the Nopah Range and adjacent ranges in eastern California and southern Nevada. Breccia bodies display sharply cross‐cutting relationships with host dolostone bedrock stratigraphy. They also show stratigraphic variability in size, shape and dolostone clast composition, but similarity in breccia matrix composition and framework texture and fabric. These characteristics are consistent with a palaeokarst origin. Upsection changes in breccia clast lithology as well as multiple occurrences of associated quartz sand‐filled grikes (solution‐widened fissures) indicate multiple episodes of carbonate platform exposure and karstification. Repeated karstification is also indicated by stratiform bodies of quartz sand and thin terra rossa palaeosols that locally truncate breccias and grike systems, thus bracketing karstified exposure surfaces. Facies successions and stacking patterns between recognized exposure surfaces are developed as transgressive–regressive cycles and thus show depositional sequence architecture. Hence, these breccias and other associated palaeokarst features are related to a succession of disconformities that provide a sequence‐stratigraphic framework for assessing Ordovician relative sea‐level history of the south‐western Cordilleran margin of Laurentia.