Lacustrine oncoids and thrombolitic clots mixed with terrigenous clasts in the Lower Cretaceous Junggar Basin, Central Asia

Oncoids and thrombolites are microbial carbonates and important indicators for paleogeography. We firstly found lacustrine oncoids and thrombolitic clots mixed with terrigenous clasts in the Lower Cretaceous in the Junggar Basin in Central Asia and aimed at their mixing process and the paleogeographic significance. We conducted a comprehensive petrological study including optical, cathodoluminescence, fluorescence, quantitative evaluation of minerals by scanning electron microscopy, Raman spectroscopy, carbon and oxygen isotopes. Intraclasts, calcite spherulites, botryoidal grains, cortoids, and ooids also mixed with the terrigenous clasts. The cortices of oncoids and ooids are composed of layers of organic carbon-rich micrites, sparry calcites, and apatites. Some oncoids exhibit alternating dark and light laminae with unequal thicknesses and binding structures of microbiota. Thrombolitic clots grew with silt-sized terrigenous clasts and ooids. Grains were cemented by fibrous, dog-tooth, and interstitial granular calcite cement, and some grains are enveloped by micrites. Compared to published data in the Tianshan Region and the Junggar Basin, positive δ¹³C of the mixed siliciclastic-carbonate sediments may indicate the ¹²CO2 absorption of microbiota. Based on the geomorphology and sedimentary distribution, we concluded that mixed siliciclastic-carbonate sediments were developed on a gentle slope with a relatively inadequate sediment supply during highstand periods. The sparry calcite-cemented mixed siliciclastic-carbonate sediments experienced rapid in-situ mixing process in an agitated lakeshore environment. Sparry carbonate cortices peeled off from the oncoids were densely packed in a shallow lake environment. These mixed siliciclastic-carbonate sediments were sedimentary response to extensive lake transgression and humidification in the Early Cretaceous following coarse-grained alluvial sediments and aridification in the Late Jurassic.