Sequence architecture and sedimentary characteristics of a Middle Jurassic incised valley,western Sichuan depression,China

The Middle Jurassic Shaximiao Formation encompasses tens of meters of thick lowstand meandering valley （LMV） strata in the western Sichuan foreland basin. Ancient LMVs newly discovered in this area were further studied based on sequence stratigraphy and seismic sedimentology. The aim of the present study was to investigate the sedimentary characteristics, sequence architecture, and the controls on LMV deposition in this tectonically active basin using field survey data, seismic sections, seismic amplitude imaging, core description, and comprehensive application of drilling data. The results show the following： （1） Three regional sequence boundaries and two flooding surfaces were recognized, and the Shaximiao For- mation was divided into two-third-order sequences and four systems tracts. （2） Three sedimentary facies associations were identified： incised valley-fill, tributary channel, and overbank facies. Incised valleys are 5-17 km wide, 20-60 m deep and traceable for 120 km along their axes. （3） In the downstream segment, the role of tectonism gradually diminishes, and periodic base-level changes control the form and evolution of the incised valleys. Three types of LMVs--AI, A2, and A3--developed with changes in base level （lake level）; of these types, the base level of the A3 LMV was likely the lowest.

Middle Jurassic climate oscillations from paleosol records of the Sichuan Basin,SW China

The Middle Jurassic is characterized by major plate tectonic changes,variable atmospheric CO_(2)concentrations(pCO_(2)),and climate oscillations in both marine and terrestrial realms.However,the limited evidence from the terrestrial sedimentary records hampers a comprehensive understanding of global climate state in this period.Well-exposed strata of the Shaximiao Formation in the Sichuan Basin,Southwest China,preserve successions of paleosols that offer the possibility of evaluating the terrestrial paleoclimatic changes during the Middle Jurassic(Bajocian to Callovian).A total of 151 paleosol profiles belonging to four paleosol types(Entisol-like,Inceptisol-like,Aridisol-like and argillic Inceptisol-like paleosols)were identified and characterized.The quantitative paleoclimate reconstructions based on the bulk geochemistry of paleosol horizons and depths of carbonate nodules indicate a generally semiarid-subhumid climate alternating with arid-humid and cool/warm-temperate climates during this time interval.This dynamic climate terminated in a dry,subhumid,humid/perhumid and superhumid moisture regime characterized by steppe/wet or rain forest floral provinces.We suggest that these climatic fluctuations may have been related to global geodynamic(e.g.,the fragmentation of Pangea,“megamonsoonal”circulation,and bolide impacts)and regional tectonic uplift(e.g.,the uplift of the Longmenshan Mountains in the South China Block)during the Middle Jurassic.The pCO_(2)estimates suggest relatively low pCO_(2)levels within the range of~100 ppm-~890 ppm during the Bajocian to Callovian.Our results coincide with the threshold for the initiation of glaciation and may help to explain the occurrence of cool intervals in the generally warm Jurassic.These pCO_(2)fluctuations,which coincide with paleotemperature variations in the low paleolatitudes of the Northern Hemisphere,are possibly attributable to global geological events(e.g.,wildfire events,eustatic sea-level oscillations,and the transient development of glaciations)in the Middle Jurassic.

Late Triassic sauropodomorph and Middle Jurassic theropod tracks from the Xichang Basin,Sichuan Province,southwestern China:First report of the ichnogenus Carmelopodus

Upper Triassic and Middle Jurassic strata of the Xichang Basin in Sichuan Province, southwestern China, yielded important dinosaur ichnofossils. From the Xujiahe Formation of the Yiguojiao tracksite, we report a Late Triassic footprint assemblage in China and the first discovery of diagnostic Triassic sauropodomorph tracks in this region. The tracks share a number of features in common with the ichnogenera Eosauropus(Late Triassic) and Liujianpus(Early Jurassic). The neighboring Bingtu tracksite is stratigraphically younger(Shaximiao Formation, Middle Jurassic) and preserves small tridactyl theropod tracks that represent the first occurrence of the ichnotaxon Carmelopodus in China and Asia. While these tracks are morphologically comparable to those from the Middle Jurassic type locality in North America, the specimens from China show the proximal margin of the digit IV impression in a more cranial position, which may indicate a trackmaker with a relatively short metatarsal IV. In addition to the skeletal record, the Carmelopodus footprints document the presence of small theropods in the dinosaur fauna of the Middle Jurassic Shaximiao Formation.