MAGNETIC SUSCEPTIBILITY, BIOSTRATIGRAPHY, AND SEQUENCE STRATIGRAPHY: INSIGHTS INTO LATE DEVONIAN SEA LEVEL AND CLIMATE CHANGE, WESTERN CANADA

High-resolution magnetic susceptibility (MS) data from slope and basin deposits from the western Canadian Rocky Mountains (Alberta basin) provide insights into patterns of carbonate platform development and Middle-Late Devonian paleoclimatic change. Our MS data, combined with conodont/brachiopod biostratigraphy and sequence stratigraphy, provides temporal control on the relative timing of MS fluctuations/excursions and nine depositional sequences that coincide to Johnson et al.'s and Day et al.'s Transgressive-Regressive cycles IIa-2 to IIe. Sequence stratigraphic analysis supports earlier interpretations that the late Givetian through Frasnian stages of the Late Devonian are characterized by a second order sea level cycle (T-R cycle II). MS values track this second order event, with higher frequency fluctuations associated with shorter-term (3rd-4th order) sea level events. The MS signature is generally low in the late Givetian and early Frasnian (through Montagne Noire [MN] Zone 9) but displays a major bimodal MS increase in the middle to late Frasnian (MN zones 10-11). MS values return to generally lower levels during the late Frasnian (MN zones 12-13) and early Famennian. This general pattern of increasing followed by decreasing MS is interpreted to indicate variations in delivery of magnetically susceptible terrigenous material. The highest MS values directly correlate to the lithologic change associated with an influx of fine-grained siliciclastics in the Mount Hawk Formation. We interpret this rise in MS to be related to progressive climatic warming during the Frasnian, as evidenced by generally decreasing oxygen isotopic values, leading to higher rates of terrestrial weathering. Along with long-term trends that appear to be linked to climate change there are shorter term MS events that also appear to mirror trends in oxygen isotopic data and hence may also be climatically controlled. The late Frasnian is characterized by abrupt negative followed by positive oxygen isotopic excursions implying unstable climate and rapid sea surface temperature changes in the tropical ocean. The MS signature similarly displays rapid fluctuations at the same stratigraphic intervals and may indicate changes in sediment influx modulated by climate.