A Cretaceous Hyperpycnal Fan Delta in a Arid Rift Lake: Facies and Geochemistry of the Bantou Formation in Fujian Provience, Southeast China

Hyperpycnal fan delta (HFD) represents a distinct deltaic model where subaqueous progradation is governed by sustained, flood-generated hyperpycnal flows, differing significantly from subaerial deltas, Gilbert-type deltas, or wave-/tide-dominated deltas. Using the Lower Cretaceous Bantou Formation (SE China) as a rift-lake case study, this paper documents the diagnostic facies architecture and flow evolution of HFD which seems to be a compound delta. The subaerial clinoform (delta) contains deposits of terrestrial debris flows and braided channels, reflecting a conventional fan delta setting, while sandy/gravelly hyperpycnites dominate the delta front portion of the subaerial clinoform. In contrast, the subaqueous clinoform (delta) is composed of muddy hyperpycnites and fluid-mud deposits. Inferred hyperpycnites form amalgamated composite beds with basal inverse grading, and sometimes develop laminas enriched in plant debris (“lofting rhythmites”). Fluid mud deposits refer to unbioturbated mudstones. Inferred muddy to sandy hyperpycnites and fluid mud deposits are stacked to form coarsening-upwards cycles representing deltaic progradation. Vertical facies successions reflect downstream evolution of hyperpycnal flows, with a trend from sediment-laden turbulent flow to transitional flow. Clay minerals were transported as clay flocs. In distal settings, hyperpycnal flow and hypopycnal flow may entrain these flocs and promote generation of fluid mud flow. Clay flocs and plant debris can be deposited due to basinal shear sorting in transitional flow with collapse of a lofting plume, or to suspension setting of fluid mud flow. Therefore, such flow evolution produces pronounced sedimentary differentiation in the HFD, and its sedimentation rates could be like that of other deltas. HFDs are likely under-recognized in the geological record, particularly in shallow-shelf and lacustrine basins with high suspended-sediment loads. This study emphasizes the critical role of hyperpycnal flows and associated fluid mud flow in deltaic progradation, and provides insights in explaining lacustrine and deltaic facies and its source-to-sink systems. Geochemical analyses indicate coupled tectonic-climatic control mechanism on the development of hyperpycnal fan deltas has been clarified. It provenance should be acidic volcanic rocks in an active continental margin setting, with an overall arid paleoclimate. The study demonstrates that extensional tectonics (providing provenance, steep slopes, and deep-water accommodation) established the material and spatial foundation for the system, while the seasonal arid climate (regulating weathering and episodic flooding) provided the high-frequency, event-driven trigger. Their synergistic control resulted in a “supply-dominated” high-sedimentation-rate filling pattern in the study area.