Abstract: Paleoclimatologic Analysis of an Upper Jurassic Petrified Forest, Ulgay Khid, Southeastern Mongolia

Al ysa M. Keller

A well preserved Upper Jurassic petrified forest located near the Ulgay Khid monastery, southeastern Mongolia (43° N; 108° E), crops out in a belt approximately 100 m wide and 720 m long, and has been mapped in detail at a scale of 1:400 as a result of cooperative studies between Stanford University and the Mongolian Academy of Sciences. The forest is represented by more than 84 fossil trees, including in-situ stumps and preferentially oriented logs, all encased in volcanic tuff. The largest stump is 1.75 m in diameter and the largest log is 13 m in length. Cell structure of most trees is excellently preserved and is the focus of detailed paleoclimatologic tree-ring analysis. The forest is characterized by very little evidence of transport of logs (e.g., fluvial reworking), ommon alignment of logs, and excellent preservation of bark on most trees. These characteristics suggest that the forest may be a death assemblage, killed by the volcanic tuff which encases the logs. In addition to dating the tuff via ⁴⁰M/³⁹Ar, detailed tree-ring analysis and statistical techniques are used to test whether the trees died instantaneously. Placed in regional context, the paleoclimatologic interpretations resulting from this study help test recently proposed models of regional Middle to Late Jurassic changes in atmospheric circulation resulting from the breakup of the Pangean landmass and the cessation of Late Triassic to Middle Jurassic monsoonal circulation over much of central Asia. Given the regionally extensive deposition of Triassic to Upper Jura sic petroleum source rocks in central Asian basins west of the study area (Tarim, Junggar, and Turpan basins) and the development of regional anoxic lakes east of the study area in the Late Jurassic/Early Cretaceous (Hailar, Erlian, and Songliao basins), paleoclimatologic analysis of the Ulgay Khid forest may yield insights into conditions necessary for source rock development during this period of climatic transition.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994