Llandoverian graptolite zonation in the northern Canadian Cordillera

A number of well exposed and structurally simple, graptolite sections, ranging in age from latest Ordovician through Wenlockian strata are present in the northern Canadian Cordillera. Recent more detailed sampling permits much finer zonation, than previously possible. The following lower and middle Llandoverian zones (following the zonal scheme of Hutt 1975) are recognized or tentatively recognized: acuminatus Zone, atavus Zone, acinaces Zone, gregarius Zone, triangulatus Zone, magnus Zone?, argenteus Zone, convotutus Zone, sedgwicki Zone, and turricutatus Zone. The crispus, griestoniensis and crenulata Zones are only tentatively and indirectly recognizable; instead, expanded and stratigraphically thick spiralis and Cyrtograptus sakmaricus - C. laqueus Zones appear to represent biofacies equivalents. The latter zone is correlated with the grandis and lapworthi Zone of some writers. The earliest Wenlockian centrifugus Zone may be present in a single section. The lowest Llandoverian persculptus Zone is only tentatively recognized, but there appears to be no evidence for a stratigraphic break between the Ordovician and Silurian. Lower and middle Llandoverian zones are thin, whereas those of the upper Llandoverian are considerably thicker, a fact probably related to the temporal duration of the zones.