TAPHONOMIC AND PALEOECOLOGICAL ASPECTS OF A SILICIFIED STAND OF THE GYMNOSPERM PROTOPHYLLOCLADOXYLON FROM THE WUTONGGUO LOWER ORDER CYCLE, BOGDA MOUNTAINS, XINJIANG PROVINCE, WESTERN CHINA

Fossil plants in the latest Permian and earliest Triassic transitional succession of western China provide a means by which to assess the impact of the end-Paleozoic extinction in the mid-latitudes of NE Pangea. The southern foothills of the Bogda Mountains, Xinjiang Province, expose sediments deposited in a half-graben that overlie uppermost Carboniferous basement. Strata range from Carboniferous to the Lower Triassic, are constrained by a number of geochronometric ages (ID-TIMS), and the succession has been subdivided into cyclostratigraphic units referred to as Low-Order Cycles (LC) subdivided into two hierarchies of higher order (HO) cyclicity. The Wutonggou LC records the transition from the late Permian (253.11 Ma) into the earliest Triassic (250.31 Ma), with a total of 128 HO cycles identified. Beds of Cycle 14 dip at an angle of 44° and preserve a horizon of upright and rooted, silicified gymnosperm trunks assigned to Protophyllocladoxylon that can be traced along strike for 265 meters.

Silicified trunks are buried by sandy coarse siltstone overlying a rooted paleosol also comprised of a sandy coarse mudrock. Roots may be coalified or preserved as elliptical casts surrounded by a coalified rind with dispersed, decayed aerial axes scattered in the paleosol. The entombing sediment was emplaced by a high discharge flooding event, evidenced by cm-scale fining upward trough crossbeds which asymmetrically onlap the upstream side of trunks. Downstream deposits show evidence of scour and are of a different orientation. Bedding planes are covered in comminuted plant debris. Above a sharp bounding surface are steep planar cross beds which are overlain by coarse siltstone wherein the trees are truncated. These can be seen in plan view on one dip slope through which they project.

Nine inclined trunks occur over an area of 128 m², range in diameter from 6-15 cm, and are variously preserved. All trunks are inclined relative to vertical, with a prevailing (present day) northeasterly direction. Cellular preservation, in general, is very good with at least one trunk exhibiting interior rot prior to silicification and compaction, resulting in the distortion of the basal part of this tree. There is no evidence for the presence of growth rings in any trees. Hence, this latest Changshingian or earliest Induan assemblage is interpreted as an uneven-aged stand of Protophyllocladoxylon, a wood taxon ranging from Carboniferous to Eocene, that grew in this mid-latitudinal zone under a seasonally equitable climate.