Sedimentology of coarse-grained alluvial fans in the Markham Valley, Papua New Guinea

In alluvial sediment sequences recognition of a hierarchy of bounding surfaces, and the shapes and associated lithofacies of the sediment bodies they define, provide an appropriate framework for understanding associations among depositional forms, the processes responsible for them, and their controls on system development. This methodology (architectural-element analysis) integrates principles from geomorphology and sedimentology (Miall, 1985, 1988). It is used here to analyse the evolution of the modern Umi Fan and the alluvial fan part of the Pleistocene Leron Formation in the Markham Valley, Papua New Guinea. A series of terraces has developed as the Umi River has incised into its fan. Detailed stratigraphic analysis of the lowest terrace, in exposures up to 25 m high and kilometres long, reveals that the fan is dominated by sheetflood deposits, with minimal preservation of either debris flow or hyperconcentrated flood flow sediments. Channel fill elements make up a larger proportion of exposures in the proximal-fan than elsewhere, while 95% of distal-fan exposures are composed of sheetflood sequences. These depositional features likely result from massive sediment dispersal associated with the rapidly uplifting upland terrain, abundant sediment availability and flashy discharge regime. Channel fill units, along with slope-related deposits from debris and hyperconcentrated flood flows, are only likely to be preserved in the trench backfilling phase following fan entrenchment. Reworking of deposits plays a dominant role in preservation of sheetflood deposits at the expense of slope-related deposits. An hierarchical framework of lithosomes is developed which characterizes various scales and bounding surfaces of depositional units which make up the Umi Fan (cf. Miall, 1988; DeCelles et al., 1991). Smaller elements, observed in terrace exposures, are described as first- to fourth-order lithosomes. They reflect geomorphic processes which are evident in and adjacent to the modern Umi River. Deposits which infill the trench are interpreted as a fifth-order lithosome constrained by trench geometry. Proximal-distal relationships are remarkably similar in the Umi Fan and Leron Formation fan sequences. In both instances debris flow deposits are seldom observed. The largest sixth- and seventh-order lithosomes, which were deposited within a time frame of approximately one hundred thousand years, are interpreted by analysis of the Leron Formation in relation to its tectonic setting. The Umi and Pleistocene Leron Formation fans exemplify fan development in a post-collisional molasse basin, under a tropical monsoonal climate. Carbonate concretions observed in distal-fan facies may provide a possible diagnostic feature of these tropical-savanna fans.