Magnetic Susceptibility, Biostratigraphy, and Sequence Stratigraphy: Insights into Devonian Carbonate Platform Development and Basin Infilling, Western Alberta, Canada
Published:January 01, 2008
Michael T. Whalen, James E. (Jed) Day, 2008. "Magnetic Susceptibility, Biostratigraphy, and Sequence Stratigraphy: Insights into Devonian Carbonate Platform Development and Basin Infilling, Western Alberta, Canada", Controls on Carbonate Platform and Reef Development, Jeff Lukasik, J.A. (Toni) Simo
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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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Controls on Carbonate Platform and Reef Development
Carbonate platforms and reefs emerge, grow and die in response to intrinsic and extrinsic mechanisms forced primarily by tectonics, oceanography, climate, ecology and eustasy. These mechanisms, or controls, create the physical, biological and chemical signals accountable for the myriad of carbonate depositional responses that, together, form the complex depositional systems present in the modern and ancient settings. If we are to fully comprehend these systems, it is critical to ascertain which controls ultimately govern the “life cycle” of carbonate platforms and reefs and understand how these signals are recorded and preserved. Deciphering which signals produce a dominant sedimentological response from the plethora of physical and biological information generated from superimposed regional to global-scale controls is critical to achieving this goal. With this understanding, it may be possible to extract common time- and space-independent depositional responses to specific mechanisms that may, ultimately, be used in a productive sense. Extensive research on a wide variety of carbonate platform and reefal systems in the past few decades has provided the foundation and understanding necessary to take carbonate research to a new level. With assistance from rapidly advancing computer software and an increasing use of cross-disciplinary integration, carbonate research is shifting from description and morphological analysis towards a science that is more focused on the assessment of process and genetic relationships. The aim of this special publication is to present a cross section of recent research that shows this evolution from a variety of perspectives and scales using examples distributed throughout the Phanerozoic.