DEPOSITIONAL DYNAMICS AND PRESERVATION POTENTIAL IN A PROGRADATIONAL LACUSTRINE FLUVIO-DELTAIC SETTING: IMPLICATIONS FOR HIGH-RESOLUTION SEQUENCE STRATIGRAPHY (UPPER TRIASSIC, NORTHWESTERN CHINA)

摘要

Lacustrine deltaic systems differ in several ways from marine ones, and classic sequence stratigraphical concepts need to be modified. "This study uses a quantitative outcrop analogue approach with an exceptionally high-resolution record of a lacustrine delta complex (centimeter to decimeter) of the Upper Triassic at the southern margin of the Junggar Basin (Xinjiang, northwestern China). Sedimentological logging of lithofacies and architectural elements were combined with gamma-ray measurements and two-dimensional maps of outcrop faces. The data are analyzed and interpreted in terms of depositional dynamics, cyclicity, stacking pattern, accommodation vs. sediment supply, and preservation potential. The sedimentary inventory comprises various types of gravelly channel bodies, sheetlike sandy and clayey units, as well as ferrocrete horizons and coal seams organized in four depositional environments: Delta slope, delta front, delta top and distal alluvial plain. In contrast to existing fan-delta models, weakly incised, coarse-grained channel fills reaching far beyond the delta front are intercalated into fine-grained delta-slope deposits. Thus, we propose a new type of a mixed-load fan delta, which might be common in other very large lake systems. A fourfold cycle hierarchy with systematic superposition of cycles was identified. It shows a closely linked control of sediment transport, depositional processes, and accommodation space typical for lake systems. According to preservation of cycles and regional geodynamic data, tectonic rates did not change markedly in the Late Triassic. However, a decrease of bisaccate pollen and biodiversity but an increase of spores suggests an increase in humidity, but also in climate "extremes" towards the Jurassic-Triassic boundary. Preservation potential and cycle symmetry are related to the accommodation space by the sediment supply (A/S) ratio, which shows an overall decrease during Norian times with a subsequent increase during the Rhaetian. Gamma-ray measurements (GR) also show a distinct cyclicity, however frequently independent of grainsizes. Channelized coarse-grained sediments show increased natural gamma-ray emission (total counts) due to ~(40)K in claystone clasts. The U/Th ratio is often higher in the delta-front environment due to interaction of fresh river water and lake water.