REEVALUATION OF PERMIAN GLACIATION IN SIBERIA DURING THE LATE PALEOZOIC ICE AGE: PRELIMINARY ANALYSES ON THE ORIGIN OF CAPITANIAN DIAMICTITES IN THE ATKAN FORMATION, OKHOTSK REGION, RUSSIA

Despite a lack of detailed sedimentologic analyses, diamictites in the Middle Permian Atkan Formation have been interpreted as glacially influenced marine deposits. This interpretation has allowed this unit to play a prominent role in paleo-environmental, paleoclimatic, and biogeographical reconstructions and modeling associated with presumed bipolar glaciation during the late Paleozoic ice age. Here we present preliminary results of a study that challenges the glacial interpretation for these strata.

The 400-1200-m-thick Atkan Formation was deposited in a back-arc basin associated with the development of the Okhotsk-Taigonos volcanic arc located along the leading edge of Pangea as it drifted across the North Polar Circle. The presence of tuffs, an abundance of volcanic clasts of Guadalupian age (> 90% of clasts), and an abundance of glass shards in the diamictite matrix indicate that the sediment was derived from the nearby arc. Cooling and solidification of some clasts during sedimentation is suggested by the occurrence of the following features: 1) clasts with embayments and/or finger-like protrusions that extend into the surrounding matrix, 2) clasts with columnar jointing, 3) clasts with textural zoning, and 4) chilled borders/alteration rims in the matrix surrounding some clasts. ID-TIMS dating of zircons within the clasts, matrix, and the tuffs indicate a Wordian-Capitanian age, which is consistent with fossils contained within the strata. No striated clasts were observed, and outsized clasts piercing bedding are rare.

Thin- to thick-bedded diamictites deposited by debris flows dominate the strata. These units are tens of m thick and occur as downlapping packages that thicken then thin upward. The strata contain channel and slump structures. Mudrocks and graded sandstone beds deposited by turbidites also occur. Strata above and below the Atkan Fm. contain abundant graded beds and deep-water trace fossils. The occurrence of bedded diamictites, channels, slumps, graded beds, volcanic grains (clasts, glass, and tuffs), and an absence of glacial indicators suggest deep-water deposition for the Atkan Fm. along the slope and basin floor of a back-arc basin that resulted from activity associated with the development of the Okhotsk-Taigonos volcanic arc rather than deposition due to glacial activity.