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Abstract

This study applies high-resolution sequence stratigraphy, biostratigraphy, and magnetic susceptibility (MS) stratigraphy to better constrain correlation of upper Middle and Upper Devonian strata and geologic events in western Alberta, Canada. We also explore the potential of MS stratigraphy as a long-range correlation tool and paleoclimatic or oxygen isotope proxy. High-resolution MS data from slope and basin deposits near the isolated Miette and Ancient Wall platforms provide insight into patterns of carbonate- platform development and infilling of the Devonian Alberta basin. Our MS data, combined with conodont and brachiopod biostrati-graphic data and sequence stratigraphy, provides additional control on the relative timing of five major and fifteen higher-frequency MS excursions and nine depositional sequences. Sea-level events that initiated deposition of seven of nine late Givetian-early Famennian third order depositional sequences in western Alberta coincide with Devonian transgressive-regressive (T-R) cycles IIa-2 to IIe. Eight of these form the main sequence stratigraphic architectural units of the isolated Miette and Ancient Wall platforms. Sea- level events were identified based on significant sequence stratigraphic horizons, including exposure and marine flooding surfaces, and were biochronologically calibrated using combined conodont and brachiopod biostratigraphy. Identification of sequence boundaries and differentiation of highstand and lowstand slope and basinal deposits was based on the geometry, mineralogy, and clast content of redeposited carbonate units. The magnetic-susceptibility signature of slope and basin facies is also shown to vary systematically within the sequence stratigraphic framework. Spikes in the MS record coincide with events associated with lowstand or initial transgression. The MS stratigraphy displays a consistent pattern across the Alberta basin, with generally higher MS values toward the east. The MS signature is generally low in the late Givetian and early Frasnian (through 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 correlate directly to the lithologic change associated with an influx of fine-grained siliciclastics in the Mount Hawk Formation. The generally consistent pattern of MS change across the Alberta basin points toward the utility of MS stratigraphy as a regional correlation tool.

Several other positive MS excursions documented here are also associated with increased detrital input and are coeval with decreasing or low oxygen isotope values (increasing or high paleotemperatures) reported from both Laurasia and Gondwana. This relationship implies a paleoclimatic linkage with increasing temperatures and weathering rates resulting in higher detrital input and higher MS values. Published oxygen isotope data are too coarse to conduct high-resolution comparison with our MS data, but the parallel trends noted here suggest that further research on the use of MS as an oxygen isotope or paleoclimate proxy is warranted. The MS signature of coeval Devonian rocks from highly condensed sections in Morocco displays a shape structure similar to our data and reinforces arguments that MS stratigraphy has potential as a long-range correlation tool.

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