Subaqueous prograding hyperpycnal fan deltas and fluid-mud flow in a rift lake: a case study from the Bantou Formation in Fujian Province, southeast China

ABSTRACT

Hyperpycnal fan deltas (HFDs) represent a distinct deltaic model where subaqueous progradation is governed by sustained, flood-generated hyperpycnal flows, which differ significantly from subaerial deltas, Gilbert-type deltas, or wave-, and 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 HFDs that seem to be a compound delta. The subaerial clinoform (delta) has deposits of terrestrial debris flows and braided channels, reflecting a conventional fan-delta setting, while sandy–gravelly hyperpycnites dominate the delta-front part 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 they sometimes develop laminae enriched in plant debris (“lofting rhythmites”). Fluid-mud deposits refer to nonbioturbated mudstones. Inferred muddy to sandy hyperpycnites and fluid-mud deposits are stacked to form coarsening-upwards cycles representing delta 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 because of basinal shear sorting in transitional flow with collapse of a lofting plume or because of suspension settling 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 into explaining lacustrine and deltaic facies and its source-to-sink systems.