Sedimentary characteristics and genetic mechanism of mixed siliciclastic-carbonate rocks of the Bolila Formation in the eastern Qiangtang Basin

Mixed siliciclastic-carbonate sedimentation is a special phenomenon formed by the intermixture of carbonate and siliciclastic, which holds great significance for unconventional hydrocarbon exploration but also possesses considerable value in the study of sedimentary dynamics, and reconstruction of paleoenvironment-paleoclimate. The Bolila Formation drilled in well QZ-8, eastern part of the Qiangtang Basin, developed mixed siliciclastic-carbonate rocks and successions. However, their sedimentary characteristics and genetic mechanisms remain unclear. Based on well-logging data, core and microscopic observations, and zircon U‑Pb dating of mixed siliciclastic-carbonate rocks from the Bolila Formation in well QZ‑8, this study constrains the types, characteristics, and provenance of mixed siliciclastic-carbonate rocks. On this basis, an astronomical timescale for the Bolila Formation in well QZ‑8 was established, clarifying the controlling effects of tectonic evolution and astronomical orbital parameters on mixed siliciclastic-carbonate rocks. The Bolila Formation in well QZ-8 contains clastic rocks, carbonate rocks and mixed siliciclastic-carbonate rocks, with 18 lithofacies identified, including mudstone(F1), silty mudstone(F2), greywacke(F3), micrite/crystalline limestone(F4), intraclastic/ooid-peloid wackestone(F5&F6), floatstone(F7), calcareous/bioclastic mudstone(F8&F9), calcareous siltstone/sandstone(F10&F11), marlstone (F12), silty/mixed siliciclastic micrite(F13&F14), mixed siliciclastic ooid-peloid packstone(F15), mixed siliciclastic bioclastic wackestone(F16), mixed siliciclastic peloid wackestone/packstone(F17), and mixed siliciclastic sparry bioclastic-ooid grainstone(F18). It was deposited in a distally steepened carbonate ramp, which can be subdivided into inner ramp, middle ramp, distal slope, and outer ramp. The terrigenous clastic grains in the mixed siliciclastic-carbonate rocks are dominated by feldspar and lithic fragments, with poor sorting and roundness. Cathodoluminescence analysis of the quartz indicates magmatic origin. The detrital zircons are predominantly euhedral with distinct oscillatory zoning and high Th/U values, indicating the magmatic origin. The age spectrum shows a unimodal distribution, which correlates well with the reported U-Pb age spectrum of magmatic arcs in adjacent Tanggula Pass, Yanshiping, Geladandong, Zaduo and Shuanghu areas, indicating the provenance is mainly derived from the proximal magmatic arcs. Furthermore, the zircon U-Pb weighted mean age obtained from the tuffaceous mudstone sample at 286 m is 237.78±0.98 Ma,representing the depositional age of the strata. Taking it as the anchor point, combined with the filtering results of the long eccentricity(~405 kyr), a floating astronomical time scale for the Bolila Formation was established, constraining its age to 242.72–235.06 Ma with a duration of ~7.66 Ma, spanning the Middle Triassic Anisian to the Late Triassic Carnian. The formation of the mixed siliciclastic-carbonate rocks in the Bolila Formation is jointly controlled by tectonic evolution and the periodic changes of astronomical orbital parameters. During the Middle-Late Triassic, the Longmu Co-Shuanghu Paleo-Tethys underwent northward subduction, forming a multi-magmatic island arcs provenance supply system in the North Qiangtang Depression, providing sufficient material for mixed siliciclastic-carbonate rocks. Climate fluctuations driven by periodic variations of astronomical orbital parameters further regulated the formation of mixed siliciclastic-carbonate rocks. When long eccentricity and obliquity increased, monsoon intensity enhanced, which led to increased precipitation and intensified surface runoff, increasing terrigenous input and promoting the development of mixed siliciclastic-carbonate rocks; conversely, when long eccentricity and obliquity decreased, monsoon intensity weakened and precipitation reduced, resulting in the lower terrigenous input, facilitating the formation of carbonate rocks.