GEOCHRONOLOGY AND ASTROCHRONOLOGY OF MID- TO HIGH-LATITUDE MESOZOIC CONTINENTAL SYSTEMS OF CHINA AND LOW-LATITUDE NORTH AMERICA

We are conducting cyclostratigraphic, astrochronologic, and zircon U-Pb geochronologic analyses of lacustrine and fluvio-lacustrine formations in northern China that span significant parts of the Late Triassic though median Cretaceous. These include: 1) the fluvio-lacustrine, coal-bearing Late Triassic-Early Jurassic Haojiagou Formation and Badaowan Formation (paleo ~60° N), for which we have completed a preliminary astrochronology (1); 2) the Middle Jurassic Tiaojishan Fm. (and related units: paleo ~45° N) (2); the lacustrine Early Cretaceous Yixian Fm. (paleo ~45° N) (3); and the fluvio-lacustrine, coal-bearing “median” Cretaceous Jixi Group (Chengzihe and Muling fms., paleo ~45° N). The Tiaojishan and Yixian fms. host the Daohugou and Jehol biotas, famous for their feathered non-avian dinosaurs, birds, and mammals, and we have found that the Jixi group produces dinosaur footprints. We are testing our astrochronologies, which are thus far “floating”, by recovering zircons from multiple levels, primarily from air-fall ashes. We are surveying them using U-Pb LA-ICP-MS and suitable samples will ultimately be dated by U-Pb CA-TIMS. Thus far we have been very successful with the Jixi Group and Yixian Fm., the latter of which we have 3 new cores with many ashes, collected by NIGPAS, on which to base our very high-resolution stratigraphy. Preliminary results suggest a very important role for obliquity forcing compared to low latitude sites. This is a contribution to IGCP 632 and which we hope to ultimately bring some clarity to a relatively poorly calibrated portion of the Mesozoic timescale as well as associated macro-evolutionary and climatic events.

1) Sha J, et al. 2015. PNAS 112.12:3624-3629; 2) Sullivan C, et al., 2014. JVP 34.2:243-280; 3) Zhou Z-H, et al. 2003. Nature 421:807–814.