International Geologiical Congress - Oslo 2008

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HPS-05 Recent developments in the Geologic Timescale

 

Magnetostratigraphy of the Santonian-Campanian lea park formation, southern Alberta and Saskatchewan, western Canada sedimentary basin

 

Andrew Mumpy, University of Alberta (Canada)
Octavian Catuneanu, University of Alberta (Canada)
John Lerbekmo, University of Alberta (Canada)
 

 

The Lea Park Formation of the Western Canada Sedimentary Basin (WCSB) is a regionally extensive succession of marine sediments deposited in the Western Interior Seaway between upper Santonian and middle Campanian time. The Lea Park is thought to represent a mostly complete/continuous interval of sedimentation, especially in distal regions of the WCSB. Numerous biostratigraphic studies, which include rigorously defined zonation schemes for ammonites, planktonic foraminifera, and palynomorph assemblages, along with radiometric dates, constrain the timing of Lea Park deposition to within magnetic polarity chrons 33n, 33r, and the uppermost part of chron 34n. Despite this large database of available chronostratigraphic data, the detailed stratigraphy and associated time relationships internal to the Lea Park Formation are fraught with complexities and remain poorly understood. The Lea Park and time-equivalent units in the WCSB have historically been plagued with uncertainties surrounding the ages and regional extent of major unconformities. To help establish a high resolution time framework for the Lea Park Formation, magnetostratigraphic samples were taken from the Amoco B-1 Youngstown core, a classic reference core for the WCSB which continuously penetrates much of the foreland basin fill. The palynological zonation of the Youngstown core is previously published, and can be used for age calibration. 207 samples were taken from the core (average sample spacing ∼1.2 m) starting at the base of the Foremost Formation, which overlies the Lea Park, and continuing down to 75 m below the base of the Lea Park interval.

Based on a previously published sedimentation rate for the Campanian of southern Alberta of ∼60 m/m.y., the sampling interval ensures the recognition of polarity reversals with durations as short as ∼20,000 years. Though the core is unoriented with respect to declination, inclination values provide sufficient evidence for the evaluation of polarity reversals at the relatively high latitude of the study area. Samples were measured on a cryogenic magnetometer and 7 steps of AF demagnetization were applied in 5 mT increments. Preliminary analysis of inclination values from the Youngstown core indicates that numerous additional reversals (subchrons) are present within magnetozones 33n and 33r, which ostensibly constitute intervals of normal and reversed polarity, respectively. The presence of frequent polarity reversals, which may be used to establish time lines within the Lea Park Formation, has major implications for the application of genetic stratigraphy within this interval, and will be used to facilitate accurate regional correlations as reversal patterns are established for additional cores currently being incorporated into the study. This work may also contribute toward refinement of the geomagnetic polarity time scale for the Santonian and Campanian stages.

 

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