Lacustrine mixed siliciclastic-carbonate sedimentary rocks in the Paleogene Funing Formation of the Subei Basin, eastern China: Characteristics and origin

Abstract

The study of sedimentary characteristics and genetic mechanism of mixed siliciclastic-carbonate sediments (MSCSs) is not only the focus of sedimentary geology, but it is also an important issue in petroleum exploration and development. We have discovered the lacustrine MSCSs, the interstratified siliciclastic-carbonate sediments and the textural mixture of siliciclastic and carbonate materials (s-MSCSs) in the second member of the Paleogene Funing Formation ([Formula: see text]) in the Gaoyou and Jinhu sags, Subei Basin, eastern China. We use core, mud-log and thin-section data to clarify the [Formula: see text] s-MSCSs into five types, and we also use geochemical data of the [Formula: see text] mudstone including X-ray diffraction (XRD) whole rock, XRD clay, and trace elements to quantitatively analyze lake paleosalinity. The [Formula: see text] s-MSCSs mainly consist of siliciclasts, ooids, intraclasts, and mud, and they could be siliciclast-dominated, allochem-dominated, or without dominant components. We analyze that mixed sediments occur under either traction flow or gravity flow. The s-MSCSs of the traction flow origin develop small-scale, wavy cross-bedding and consist of siliciclasts and ooids of similar size; the s-MSCSs of the gravity flow origin develop massive bedding and graded bedding with complex lithology, and they mainly consist of siliciclasts and allochems of different size. We hypothesize that the increase of lake paleosalinity controlled by the climate and the moderate siliciclast influx under stable tectonic setting provide the material source for s-MSCSs in the lacustrine basin. Multidirectional flow is common during the deposition of s-MSCSs of traction flow origin, and seasonal flooding produces s-MSCSs of the gravity flow origin. Lacustrine s-MSCSs in the Gaoyou and Jinhu sags have significantly smaller scale and quantity, and ooids rather than bioclasts occupy most of the carbonate components.