[N] 2006 Shishogou formation revealed
Eberth. D., Clark, J., Machlus, M. & Hemming, S. (2006) The dinosaur-bearing Shishogou formation (Jurassic-Nortwest China) revealed. JVP 26(3) Abstracts pp.58
The Shishugou Formation is exposed in northeastern Junggar Basin, Xinjiang, China, and is famous for fossils of vertebrates and wood. At Wucaiwan, an intensively studied area, the formation is 378 meter-thick, rests sharply on the Xishanyao Fm, and is unconformably overlain by sediments of suspected Cretaceous age. At Jiangjunmiao the formation is only partially preserved.
We recognize a lower, red, upward-fining interval; a thick, middle interval consisting of red-orange, upward-coarsening/fining successions; and an upper, orange-tan, upwardcoarsening interval. Alluvial fan/plain facies in the lower half of the formation give way to caliche- and tuff-rich alluvial and paludal deposits in the upper half, recording an increase in volcanism, seasonal aridity, and a lowering of depositional slope. In the uppermost interval, caliches are rare, indicating less aridity, an increase in sediment supply, or both.
The previously proposed Wucaiwan Formation is broadly equivalent to portions of the lower and middle intervals, but is not adequately defined.
Cm-scale bentonites occur through most of the formation, whereas m-scale, white-tan zeolitic tuffs are limited to the upper half. 40Ar/39Ar dating of sanidines yields an age range of 161-159 Ma for the upper half of the formation, and suggests that the Shishugou likely spans the M-U Jurassic boundary. The stratigraphic distribution of vertebrates indicates a faunal transition up through the middle of the formation, corresponding roughly to the inferred M-U Jurassic transition.
Vertebrates are preserved in a variety of taphonomic modes and underscore stratigraphic changes in sedimentology. Preservation quality is poor in the lower Shishugou, reflecting frequent exposure and reworking in paleochannels. In the middle, fossils are better preserved and occur often in massive mudstones, caliches and splay deposits, indicating a bias for preservation in overbank settings and ephemeral water bodies. In the upper Shishugou, excellently preserved medium/small vertebrates, especially non-avian theropod dinosaurs, occur in massive, thick paludal deposits, suggesting rapid burial and limited reworking in wetland settings.