Stratigraphy and sedimentology of the Pleistocene Maobitou Limestone in the Western Hengchun Tableland, southern Taiwan

The Hengchun Peninsula is the southernmost part of Taiwan Island, and marks the youngest portion of this active orogenic belt. At the southwestern corner of the peninsula, the Western Hengchun Tableland is underlain by the Pleistocene Hengchun Limestone, one of the youngest stratigraphic units of the Hengchun Peninsula. The Maobitou Limestone, which only crops out at the southern tip of the tableland, is a distinctive part within the Hengchun Limestone. Its ubiquitous large-scale cross beds have sparked controversy in its sedimentary environments. The location of this unit makes it a crucial stratigraphic marker for further understanding the growth processes of the peninsula, and even the orogen itself. However, the Maobitou Limestone has not been studied as much as the other parts of the Hengchun Limestone. Therefore, we investigated 15 outcrops of the Maobitou Limestone along the coastline for their detailed sedimentological and stratigraphical characteristics. We interpreted the limestone as a deposit under storm-influenced environment, due to the observations of (i) amalgamated beds with hummocky cross-stratification (HCS); (ii) graded grains observed within polished slabs; (iii) shelly beds rich in broken barnacles; (iv) various taphonomic conditions of fossils; (v) micrite found in grainstones; and (vi) slightly bioturbated or burrowed beds at bottom of units. Three lithofacies groups were established from these observations: (a) bioclastic packstone facies; (b) bioclastic packstone-grainstone facies, and (c) bioclastic grainstone facies. Variations within these groups were also identified through further subdivision with sedimentary structures and abundance of lithoclasts. We can further correlate these facies between stratigraphic columns using an irregular scouring surface at a similar horizon, and with a 70-m long borehole core. Based on biostratigraphy, the depositional age of the Maobitou Limestone likely ranges between 0.61 and 1.70 Ma, and appears to be different from the other parts of Hengchun Limestone. The source of winnowed skeletal materials may be a topographic high that is likely a coral reef area, supporting our interpretation that the Maobitou Limestone is deposited from high energy storm waves, above the storm wave base off the ramp. As the orogen grows, the depositional setting evolved from a storm-influenced middle ramp and shallowed upward into the inner ramp.