Characterizing the occurrence, mineralogy, geochemistry, and geochronology of an ash bed in the Verulam Formation (Upper Ordovician), Ottawa Embayment, eastern Ontario

Two thin (∼4.5 and 2.5 cm), very friable biotite-bearing shale beds separated by a 1-cm thick crinoidal-brachiopod floatstone can be traced in two cores through the Upper Ordovician Verulam Formation (Ottawa Group) along a 24-km transect in the western portion of the Ottawa Embayment, eastern Ontario. A further 3 km northwest of this transect, in both core and outcrop, the beds are not present at the equivalent stratigraphic position, but trace biotite occurs in shale typical of the Verulam Formation. The friable shale beds contain a predominant (60-65%) crystalline clay matrix of illite and minor kaolinite within which are the following components: abraded fragments (<10 %) of partially chlorite- and illite-altered biotite; elongate to rounded clay-lithic fragments (containing angular quartz and potassic feldspar and framobidal pyrite); aggregates of framboidal pyrite; rare skeletal carbonate; and, accessory mineralogy of phenocrystic apatite (defined by XRD and SEM), gypsum and feldspar (recognized by XRD only), and magnetite (black in hand specimen, defined by XRD). In the lower bed, biotite grains decrease abruptly to the NW along the 24-km transect in both abundance (<10% to trace amount) and grain size (300-400 mum to 50 to 75 mum). In the upper bed, there is a similar grain size and abundance to the southeast, and only trace amount of the silt-size biotite with increased admixture of carbonate particles and mud matrix to the northwest.;The presence of phenocrysts of biotite and rare apatite, and soft friable nature of these shales, is compatible with documented characteristics of several Upper Ordovician and Lower Silurian altered ash beds in continental shelf deposits of the peri-Iapetan region. Geophysical correlation of the core transect reveals continuity of this bentonite interval over 1000 km along a SW-NE transsect from the Michigan Basin to southern Quebec. This continuity is likely a strike-parallel orientation given the observed NW-directed decrease in grain size. Thus, the magmatic source may lie to the ESE or SE, and geochemical proxies identify a trachyandesitic to rhyodacitic/dacite parent magmatic source within a collisional plate boundary setting, as was developing along eastern Laurentia in the Late Ordovician. Calculated Mg...