Fluvial gravel facies architecture of late Pleistocene age at Po Chue Tam, Lantau Island, Hong Kong: tectonic versus climatic control?

A cross-section of fluvial gravel deposits of late Pleistocene age exposed at Po Chue Tam, Lantau Island, Hong Kong contains two facies: a lower facies of planar cross-bedded gravel (Gp) and an overlying facies of clast-supported, massive gravels (Gcm). The Gp gravels include five gravel couplets. Each couplet consists of a clast-supported, coarse gravel-dominated bed and an overlying clast-supported, fine gravel-dominated bed with a discrete bounding surface. Tectonic uplift predating the last interglacial transgression produced a large amount of detritus in the source area. Excessive peak rainfall intensity resulting from enhanced seasonality of monsoonal precipitation in the following glacial period triggered catastrophic floods, which transported mature detritus in large quantities into a fault-controlled piedmont basin. The Gp gravels were deposited by pulsating flood flows. In relation to kinematic waves of particles, bedload sediment was longitudinally sorted and segregated into a train of gravel sheets. They draped over each other and accreted laterally due to expansion of flow, producing planar cross stratifications that are characteristic of recurrent, couplet-style coarse/fine cross beds. In contrast, Gcm gravels were laid down as a single, nearly horizontal bed by a catastrophic flood that was not subject to flow pulsation.