THE NATURE of HIGHLAND VALLEYS, CENTRAL PAPUA NEW GUINEA with 2 Figuresanc^ 2 Photos
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212Erdkunde Band 30/1976 THE NATURE OF HIGHLAND VALLEYS, CENTRAL PAPUA NEW GUINEA With 2 figuresanc^ 2 photos R. J. Blong and C. F. Pain Zusammen assung: Die der Hochlandtaler over a area now j Morphogenese wider (Fig. 1) allow re-examination von Central Neu-Guinea. Papua, of the nature of Papua New Guinea highland valleys. Bik andeutete, dafi die des Obgleich (1967) Bergtaler In order to test the hypothesis that the superficial Papua Neu Guinea-Hochlandes oberflachlich den von Louis resemblance between the Papua New Guinea highland beschriebenen ?Flachmuldentalern mit Rah (1957, 1964) and Louis' Flachmuldentdler mit Rahmenhd menhohen" sind erst valleys gleichen, eingehende Untersuchungen hen ismore to kurzlich unternommen worden. than justmorphological, it is necessary the nature and of on the Die Hangfufibereiche in einer Anzahl von Talern erge study age deposits valley ben sich aus dem fortwahrenden Zuwachs von tonreichen slopes and floors; in particular it is necessary to show whether or not the floors are erosio Sedimenten oder aus der spatpleistozanen kolluvialen Ab valley and slopes lagerung. In nahezu jedem untersuchten Tal haben vulkani nal features. zu sche Ablagerungen bis mehreren hundert Metern Dicke die vorher V-formigen Talboden aufgefiillt. In einigen Ta lern setzt sich die Auffiillung mit lakustrischen, organischen Valley footslopes und fluviatilen Sedimenten, die vor mehr als 50 000 Jahren With reference to the Andabare Plain, Bik writes: begann, bis zum heutigen Tage fort. Die machtige Auf . where the often abut the schuttung und fortlaufende Ablagerung erlaubt die Zuriick footslopes against stream the former are of weisung von Bik's Vorstellung, dafi die Talboden und channels, certainly slopes aus of waste. Hangfufibereiche Erosionsflachen sein konnten, ererbt transport weathering However, sheetwash, as der pleistozanen Absenkung morphoklimatischer Zonen. postulated by Louis the transporting agent, does not on area operate the well vegetated slopes in the In a 1967 with zo essay dealing morphoclimatic of study. In fact the mechanism of waste transport nation of landforms in the central of highlands Papua across gentle footslopes at these altitudes (c. 2525 m) New M. Bik out that the intra Guinea, J. J. pointed is not at all clear, and the subject deserves detailed montane of the and Wasu to plains Andabare, Kandep, research, especially determine whether transport resemble the 'hill-bor ma-Kagua valleys superficially occurs at all at present. If it does occur, as it is dered which Louis saucer-shaped valleys' (1964) known to do above 2500 m, these footslopes are Flachmuldentdler mit Rahmenhdhen designated (Bik, active surfaces of 'ramp-slope' nature. If transport 1967, p. 44). Bik noted that the Papua New Guinea does not occur below this level, could such surfaces are characterized wide central alluvial flats valleys by be inherited features resulting from Pleistocene low concave of l?-3? with rather short footslopes abutting ering of themorphoclimatic zones?' at of 25?-35?. against steep valley sides rising angles (Bik, 1967, p. 45) Implicit in theFlachmulden concept is the notion of long continued erosion under conditions of tectonic stability, rapid weathering and effective subaerial At Kuk Tea Research Station, on the southern foot denudation. The landscape is essentially erosional, the slopes of Ep Ridge, in the centre of the upper Wahgi res valley floor and the ramp slopes (Rampenhang) Valley (Fig. 1), a series of shallow trenches250 m long pectively exhibiting a superficial veneer of transported reveals the footslope stratigraphy. Ep Ridge itself is sediment and a deep residual soil (Bik, 1967; Louis, an outlier of pre-Permian metagreywacke and phyllite 1957; 1964). and Jurassic tuffaceous sandstones and siltstones (Bain On the other hand, A. Guilcher (1970) believed et al.9 1970). These materials are deeply weathered, were the morphological development of the Lake Iviva and mantled during the Late Pleistocene by centres. (Sirunki) depression resulted from continued Quater tephras from Mount Hagen and other eruptive nary tectonism and infillingof the basin with swamp Much of the tephra mantle has subsequently been sediments.Guilcher also postulates that ponding and stripped from the ridge. on fluvial deposition of theAndabare, Kandep and Mari Some exposures the footslopes reveal mudflow ent basins result from tectonism. sediments composed of slightlyweathered angular to comments clasts in a matrix. Although Bik and Guilcher restrict their subangular clayey, tephra-derived, which is sel on basin morphology to only a few valleys, their Buried soils underlie themudflow debris, to dom more than 1 m thick. most descriptions of surface form are equally applicable However, exposures on reveal unbedded de other highland valleys including the upper Wahgi low the footslopes only clay at some lenses of re (particularly theGumants), the upper Kaugel, and the posits, separated sites by thin units The lowerNembi valley around Poroma (Fig. 1 and Photo worked but identifiable tephra (Fig. 2). the of numer 1).Recent detailed stratigraphie studies of theGumants stratigraphy is complicated by presence observations ous drains and other evidence of and Kaugel Valleys, and reconnaissance prehistoric early Hi -^XV-r \>v\VOwmountainnrangesandpeaks J\.K SolomonInfilledX:.-..highlandSchematic valley^k%representationof^ J-1-1-1S' : ' T I*River Sepik To ( g\X/X^^^~^ToRiverPurari>A >(-^v _x-n?c-r?^ W^^^^^M\\LW*l^S??'* PKarekore- infilledFig.showvalleysMap1:theandPapuaNewGuineaHighlandstoofvolcanicmajorcentres ^^^^^^^V^v y ^^TTN^:.PurariRiverMot 5I| PorfM?resby^^/X^;RiversSettlements \-^-r"-. ^JKikoriToRiverI__I,44oE ,I, ,_ k c-{L^r- \kX la"'U*3465m3 FlyRiverTo\ l44?E>^ i ^?VlvTi'nd, ~J \# >^ ^Rwmb^fc* ? ^"""^ 214 30/1976 _Erdkunde_Band \Grev**K X I-1-1-1-I ' 20 15 10 5 0 l X n^ wV\ Scale In ; >v metres ! J\^%Ox 1.0 \ = white; ^ X V. E. 10 ,=. V Crumb v. ^Base of^ black X_ excavation %x^ws- EpVv'*%*~ ^5^^^i:;:%^^structure ^.^^^^^ white' clay iT i^^^f^^ ^_ ^ excavation ^^^^^^ ;?#>v?S?j I .."1 Clayey black & clayey black equivalent Scattered nodules of tephra-c. 11,"000 yrs. B.R [ .' .1 Mixed layer of sandy clay laminations Z - L-1 Lens of 250 B.P-in channel fill and clay and organic materials tephra year llllllillllBlack organic clay \^ Macro crumby black structure R Lens of tephra-c. 4400 years B.P A Angular rock fragments .-. Q White? sandy clay laminations W Schematic representation of barets filled with tephra-c. 1100 years BP -0-5; Unidentified tephra in organic layers surrounded by light-greybrown clays a Kuk Tea Research Station Fig. 2: Cross-section through the lower portion of footslope, Ep Ridge, stream agriculture (Golson, 1974). In the upper part of the provided by incision reveal bedded weathered to to of at least 6 m. trench the clay-rich sediments extend the surface. subrounded gravels depths Again, As some drains have been filled to the surface with the landforms as well as the deposits are characteristic occur on the northern clay-rich sediments, and the drains are known by of alluvial fans. Similar deposits the lower tephrochronology to date from about 250 years BP, margin of the Gumants Basin and flanking In a it is evident that clay sedimentation has continued to portions of the Sepik-Wahgi Divide. borehole a on 14.5 m the present time. through footslope (D 12 Fig. 1) of grav on a at Other thin tephra units, ranging in age from c. 1,100 elly fan deposits sit peat dated 13,000 years to more than 11,000 years BP and parallelling the BP. can at a number In and present footslope surface, be identified the upper Kaugel Valley (Fig. 1) footslopes a narrow of sites down the slope (Fig. 2), though the tephra fans with slope angles up to 8? occupy strip remnants are not always in situ. As the tephra unit between the valley floor and the commanding slopes are two to note these more than 11,000 years old occurs at a depth of less (Photo2). There points about slo are at and than 1.4 m, and as clay-rich and organic sediments pes. Firstly, they nearly all incised present, are more than 3.5 m thick, accretion spans at least degradation is confined to the floors of channelswhich ex traverse it has been demonstrated post-glacial time. It is evident that the footslopes the slopes. Secondly, owe amined have been accreting slowly for a lengthyperi that both the footslopes and the fans their origin in od of time, and deposition is not balanced by erosion. to colluvial deposition (Pain, 1973; 1975). Thus, Thus, the southern side of Ep Ridge is flanked by a common with the footslopes of Ep Ridge and the are not ero series of slowly-accumulating, coalescing alluvial (and Gumants Basin, the Kaugel Valley forms landforms are sional in places colluvial) fans. Such not'ramp 'ramp-slopes'. slopes' in the sense of Louis' (1964) Rampenhang. In the Yumbis-Karekare Basin (c. 2,500 m, upper - one a On the northern side of Ep Ridge footslopes of Wage Valley Fig. 1) at least exposure through m l?-3? drain to theGumants Basin, Various exposures trenched footslope indicates that 0.5 of fine sediment R. J. Blong and C. F. Pain: The Nature of Highland Valleys, Central Papua New Guinea 215 Photo 1: The infilled valley of the Gumants River. The Gumants and its tributaries have well-developed levees with are extensive accreting backswamps. Low-angle footslopes and alluvial fans still accumulating sediment. Photo 2: An infilled basin of the Upper Kaugel Valley. Low-angle footslopes result from colluvial deposition. has accumulated since the deposition of a thin tephra Blong, in press) indicate that the uppermost tephra in unit only 250 years old. theKandep area is theTomba Tephra of Pain (1973), Bik notes that the gentle footslopes in theKandep erupted fromMount Hagen, and now believed to be Plain are mantled by tephra which makes it difficult more than 50,000 years BP.