Abstract:

The Devonian-Carboniferous boundary is an extensive unconformity. In the eastern Qaidam Basin, typical sedimentary diversity developed in the Upper Devonian-Lower Carboniferous sediments, representing a large-scale sedimentary environmental change during the Late Devonian-Carboniferous period. To investigate the differences and inherent relations between the Upper Devonian and Lower Carboniferous, we compared the rock assemblage, distribution and lithofacies between the two, using petrologic methods, and described the spatial distribution characteristics of the Late Devonian-Early Carboniferous sediments. By combining the above results with geological background, we discussed the inherent factors for the sedimentary distribution difference. The results demonstrate not only obvious sedimentary differences but also inherent correlation between the Upper Devonian and Lower Carboniferous strata in the eastern Qaidam Basin according to material characteristics and paleogeomorphologic features. Although there is a depositional discontinuity between the two periods, the early-stage Early Carboniferous sediments display inherited paleogeomorphologic features of the Upper Devonian. Under the Late Caledonian NS-trending stress, a NS-differentiated geomorphologic framework started to emerge in the Late Devonian, and the area developed sedimentary systems featuring alluvial fan-volcano. In the early-stage Early Carboniferous paleogeomorphologic framework showing more obvious inheritance, the basic sedimentary composition was similar to that of the Late Devonian period. However, deplanation became dominant as orogeny weakened, and humid climate further altered sedimentary morphology. The rock assemblage evolved from the bottom oxide color upward to gray and grayish-black, and the lithofacies changed from discerning river to mixed tidal flat, accompanied by small-scale interbedding of transgressive and biological marls. The late-stage Early Carboniferous exhibited greater variety than inheritance until deplanation ended. The warming of climate caused glacier melting, and the changing sea level led to global large-scaled transgression. Thus, with the expansion of the Paleo-Asian and Paleo-Tethys oceans, seawater flooded northward across the study area. In addition, due to its low altitude location during that period, the Qaidam Basin experienced marine bloom under warm climate that eventually promoted sedimentary deposition of large-scale epicontinental marine carbonate rocks.

Key words: Qaidam Basin, Upper Devonian-Lower Carboniferous, sedimentary differentiation, controlling factor, inherited deposition