GLACIAL-EUSTATIC CYCLICITY WITHIN THE LOWER PERMIAN JOHNSTON CANYON FORMATION, SOUTHWEST ALBERTA AND SOUTHEAST BRITISH COLUMBIA

The Lower Permian Johnston Canyon Formation has visible cyclicity alternating between silty shale and phosphatic dolostone, but the origin of these cycles is uncertain. Facies and time series analyses help to test whether this obvious cyclicity can be attributed to glacial eustacy driven by Milankovitch scale cycles. These cyclic sediments were deposited in an interpreted restricted portion of the Ishbel Trough along the margin of the Western Canada Sedimentary Basin bounded on the west by the Kootenay Terrane that was generated by Antler and post-Antler tectonism and on the east by the North American craton. Siliciclastic input was probably restricted to an aeolian source.

A sedimentation rate was estimated by integrating conodont biostratigraphy and geochronologic ages for zoned boundaries at potential stratotype sections in Russia; this allowed for the determination of cycle duration. Key biostratigraphic boundaries signify the local first occurrences of Sweetognathus merrilli (advanced form) and S. whitei, which represent indices for the base-Sakmarian and base-Artinskian stages. A time series analysis of detailed gamma-ray values collected at 10 cm intervals was integrated with an assumed constant sedimentation rate of 226.1 kyr/m. This revealed cycles of 458.7 kyr and 112.4 kyr, which, given the uncertainties of the method, reasonably approximate Milankovitch eccentricity cycles of 400 kyr and 100 kyr.

Facies analysis was used to determine that the sea level magnitude of cyclicity did not exceed 30 m. This was determined based on the interpretation that most sedimentation was below storm wave base, though occasional reworked phosphate lags indicate that the sediments must have been influenced by episodic large storms. Phosphate in the form of nodules is found in all parts of the cycles indicating slow rates of deposition in a cool, nutrient-rich setting. The deepest facies has abundant pyrite and scarce bioturbation, indicating deposition at or below a pycnocline.

The magnitude of sea-level change seen in these glacially influenced cycles is significantly less than those of Late Pennsylvanian and earliest Permian age. The Johnston Canyon Formation data supports previous research indicating that glacial cyclicity during the Sakmarian and Artinskian was driven by substantially reduced ice-volume.