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Sedimentary Geology, 86 (1993) 297-324 297 Elsevier Science Publishers B.V., Amsterdam
Sedimentology of coarse-grained alluvial fans in the Markham Valley, Papua New Guinea
Gary J. Brierley a, Keyu Liu b and Keith A.W. Crook b,. a Department of Biogeography and Geomorphology, Research School of Pacific Studies, The Australian National University,
GPO Box 4, Canberra, ACT 2601, Australia b Department of Geology, The Australian National University, GPO Box 4, Canberra, ACT 2601, Australia
Received March 19, 1992; revised version accepted December 2, 1992
ABSTRACT
Brierley, G.J., Liu, K. and Crook, K.A.W., 1993. Sedimentology of coarse-grained alluvial fans in the Markham Valley, Papua New Guinea. Sediment. Geol., 86: 297-324.
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-ele- ment 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 (of. 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.
Introduction
Remarkably few studies have described the sedimentology of tropical alluvial fan depositional
Correspondence to: G.J. Brierley, School of Earth Sciences, Macquarie University, North Ryde, NSW 2109, Australia (present address). * Present address: Hawaii Undersea Research Laboratory,
1000 Pope Road, Honolulu, HI 96822, Hawaii, USA.
environments (Darby et al., 1990), yet such de- posits have frequently been inferred in analysis of ancient alluvial deposits (Nilsen, 1969; McGowen and Groat, 1971; Vos, 1975; McLean, 1977; Heward, 1978). The Umi Fan, in the Markham Valley, Papua New Guinea (Figs. 1 and 2), pro- vides an excellent opportunity to study the sedi- mentology of a tropical alluvial fan, as recent tectonic uplift of the Finisterre and Sarawaget Ranges has resulted in massive sediment supply
0037-0738/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
298 G . J . B R I E R L E Y E T A L
I I l I ! I
142 o E MANUS W
~.:.~-" .. Ireland BASIN
. . . . ' ~ . . S o u t h B i s m a r c k -4 °s \~,.,~ "'"".i:.:. S e a P l a t e
w1 ~ ~Wd ~ : :l
kk\~ ~ . -~ . . . . . . . . . . . . . . PAPUA , ...:..~: -e NEW .~.~ ~ . H u ~ ' t a i ~ : _ . GUINEA \,, ~ P e n i n s u l a ~ ~
~,~ ~'~_ ~,**~.******~, ~ S o l o m o n I L ~ ~ _: . : : .~ ..... Study ~.:.mJ Sea Plate
~.rea Port :--.-. : ~ - M o r e s b y W O O D L N ~ . - :
_1o 0 -. . ~2 AUSTRALIAN PLATE "." • .. -.
Fig. 1. Tectonic setting of the Markham Valley, Papua New Guinea (modified from Silver et al., 1991). N B T = New Britain Trench, TT = Trobriand Trough, RMFZ = Ramu-Markham Fault Zone, TJ = Triple Junction.
Fig. 2. Generalized geological map of the Markham Valley (after Robinson, 1974; Tingey and Grainger, 1976) showing lithostratigraphic units and major structures of the Finisterre Terrane. Note the extensive fans extending from the Finisterre and
Sarawaget Ranges. These have pushed the Markham River to the southern limit of the Markham Valley.
SEDIMENTOLOGY OF COARSE-GRAINED ALLUVIAL FANS IN THE MARKHAM VALLEY 299
and fan development as rivers enter the Markham Valley. Subsequent river incision into fan de- posits has resulted in a series of five or six ter- races which extend virtually from the outlet at the mountain front to the junction with the Markham River, a distance of about 10 km (Figs. 3a and 4). Dense grassland vegetation precludes analysis of upper terrace sequences; however, the lowest, river-marginal terrace has exceptional exposures up to 25 m high, and kilometres long.
The extensive nature of exposures on the Umi Fan enables detailed analysis and interpretation of depositional sequences at a range of spatial scales. Consequently, a hierarchy of lithosomes and their associated bounding surfaces can be recognized (Miall, 1985, 1988; DeCelles et al., 1991). Unravelling the origins of the different bounding surfaces leads to reliable interpretation of past depositional history. When lithosomes of various orders are analysed within a geomorpho-
Fig. 3. (a) Terraces in the mid-fan section of the Umi Fan. The sediment inventory in this study is based on the lowest terrace,
adjacent to the contemporary Umi River. In this photo the lowest terrace is about 12 m high. (b) The Leron River with the Leron
Fan in the distance. The Leron Formation is exposed as dipping strata in several terrace exposures.
300 G.J . B R I E R L E Y E T A L
logic framework it is possible to evaluate associa- tions among depositional forms, processes re- sponsible for them, and their spatial association. These principles form the basis of architectural- element analysis (Miall, 1985, 1988) which, to date, has largely been restricted to interpretation of sandy fluvial systems. However, this approach has been applied to Permian and Triassic low-
sinuosity and meandering gravel-based streams in Spain (Ramos and Sopena, 1983, and more re- cently Munoz et al., 1992), while Smith (1990) differentiated among three accretionary styles of fluvial conglomerates and pebbly sandstones in the Lower Triassic Budleigh Salterton Pebble Beds in southwest England. Except for the recent work by DeCelles et al. (1991) on the Palaeocene
j •
,0' /
I \ I \
f _
/ I
/ /
/ /
UMI FAN SURFACE
\ \
\
\, \
\ \
\ \
\
~-- 400 ~
Mountainous area
Margins of Umi ~ m fan surface
River ( ~ terrace
NORTH
0 1 2 3 i i i - J
Kilometres
/ \ ~Oo
\
\ chaotic channl on fan surface
\ /
Maniang fan s'~urface
Fig. 4. Terrace sequences of the Umi Fan. Note the parallel distribution of up to six terraces, and the braided configuration of the contemporary Umi River. Terrace sequences become markedly less well-defined down-fan.
S E D I M E N T O L O G Y OF COARSE-GRAINED ALLUVIAL FANS IN T H E MARKHAM VALLEY 301
Beartooth Conglomerate in Wyoming and Mon- tana, an element-style approach has yet to be applied to coarse-grained alluvial fans.
In this study, element analysis is used to inter- pret the sedimentology and geomorphic evolution of tropical alluvial fan sequences in the Markham Valley, Papua New Guinea. Down-fan sedimen- tologic variations are explained by comparison of the character and spatial association of elements making up terrace sequences with morphostrati- graphic units described for contemporary Umi River bars. These insights are then used to inter- pret mechanisms of fan development.
Element-scale analysis of Umi Fan deposits is then compared with a Pleistocene analogue for these sequences, the Leron Formation (Figs. 2 and 3b). Alluvial fan sequences of the Leron Formation are located in the lower foothills of the Finisterre and Sarawaget Ranges in the Markham Valley. The sedimentology of the up- per, fan part of the Leron Formation was anal- ysed by field mapping and stratigraphic correla- tion of well-exposed outcrops using both conven- tional two-dimensional vertical profiles and three-dimensional architectural elements. Depo- sitional units which comprise the upper part of the Leron Formation are related to their corre- sponding architectural element as observed in the Umi Fan terrace sequences. These insights are then used to construct a seven-order lithosome and bounding surface hierarchy for fan deposits in this tropical post-collisional molasse basin.
The primary goals of this paper are: (1) to demonstrate the application of three-dimensional architectural-element analysis techniques to coarse-grained alluvial fans; (2) to relate contem- porary processes of the Umi River and element units observed in adjacent terraces to fan de- posits of the upper Leron Formation; and (3) to outline sedimentologic characteristics which will be preserved in such tectonically active tropical- savanna settings that may prove useful in identifi- cation of such deposits in the ancient rock record.
Alluvial fans in the Markham Valley
The Umi River is the primary headwater tribu- tary of the Markham River, approximately 100
A
~-. ~ ~ / ~ " x .
3
II \\ -\,. ='~ ~ Y4,.~,,/ ~,..soo
B 600
Madang
~ " 5 0 0 - - Erap - - ~
- - - Lae ~ _ _ _ / " \ - , 400 ...... Kaiapit - J \ \
..... : "
200
0 h i J i i i 1 i i i i , J F M A M J J A S O N D
Mon th
Fig. 5. (A) Mean monthly precipitation at various locations in the Markham and Ramu Valleys. (B) Spatial distribution of
mean annual rainfall, Huon Peninsula, PNG.
km upstream of Lae in Morobe Province, Papua New Guinea (Figs. 2 and 4). The Markham River drains a catchment area of about 12,000 km 2. The Umi River itself drains a catchment area of about 700 km 2 in the Finisterre Ranges. Beyond the mountain front, the Umi Fan surface covers an area of 80 km 2.
Two atmospheric circulation patterns prevail in the Markham Valley area. December to March is the monsoon season, in which low-pressure vortical systems associated with the Inter Tropi- cal Convergence Zone dominate the circulation. From May to October the 'southeast' trade winds are dominant, resulting in pronounced precipita- tion in the coastal ranges, but limited rainfall up-valley. Accordingly, the degree of seasonality is reduced towards the coast (Fig. 5B). Lae re- ceives over 4400 mm/yr precipitation, while Erap, just 40 km to the west, receives annual precipita- tion of about 1250 mm. Kaiapit and Wantoat at the edge of the mountains both have annual
302 G.J. B R I E R L E Y ET AL.
precipitations of about 2400 mm (Fig. 5A). Dis- charge of the Umi River exhibits pronounced seasonality. The annual average discharge is about 40 m 3 s -1, but between December and March discharge averages over 150 m 3 s-1 (Holloway et al., 1973). Research on tropical rivers elsewhere in Papua New Guinea suggests that flood vari- ability in the tropics is less than in other climatic regimes (Pickup, 1984).
Mean daily temperature range in the Markham Valley exceeds the range of mean monthly tem- peratures. This effect is especially pronounced up-valley, away from the maritime effect, as the mean daily temperature range at Erap is 11.3°C, whereas the annual range of mean monthly max- ima is just 3.6°C. Mean annual temperature ranges at Lae and Erap are 24.7-27.4°C and 25.8-28.7°C, respectively. Inland areas of the Markham Valley experience pronounced water budget deficits, with estimated excess potential evapotranspiration over annual rainfall of around 500 mm. The Umi Fan surface reflects these savanna conditions, and is covered by lowland grassland and scrub. Headwater regions are still native forest. Soils are base saturated and moderately alkaline, with min- imum pedogenic development (Holloway et al., 1973). Upper Markham grasslands are sparsely populated, although there have been recent in- fluxes of people onto the upper Umi Fan follow- ing large landslides in upland catchments.
Tectonically the Finisterre and Sarawaget Ranges are on the Cainozoic Finisterre Volcanic Arc Terrane which accreted onto the northern Australian continental margin in the Pliocene as a result of the oblique collision between the Aus- tralian and the South Bismarck plates (Fig. 1; Pigram and Davies, 1987). The collision is still occurring in the Western Solomon Sea as a triple junction between the Australian, South Bismarck and Solomon Sea plates, located at 148°E 8°S, migrating eastwards along the New Britain Trench (NBT). The Ramu-Markham Fault Zone (RMFZ) represents the sutured sector of the collision zone (Crook, 1989a; Fig. 1) west of the triple junction. Post-collisional convergence con- tinues to occur along the RMFZ.
The Markham Valley, which follows the on- shore part of the RMFZ, is a post-collisional
molasse basin which has been infilled in places by up to 1000 m of coarse-grained alluvial sediments in the latest Pleistocene and Holocene (Pettifer, 1974). The Finisterre and Sarawaget Ranges northeast of the RMFZ comprise an Oligocene to Early Miocene volcanic sequence (the Finisterre Volcanics and the Mebu Beds; Fig. 2) overlain by Middle Miocene through Pliocene limestones on the northern flanks (Robinson, 1974; Tingey and Grainger, 1976). On the southern flanks of the Ranges the volcanic sequence is structurally em- placed over a Neogene and Quaternary accre- tionary prism. The Leron Formation, which is the youngest rock unit of the prism, forms the lower foothills of the Finisterre and Sarawaget Ranges in the Markham Valley. It was deposited during the Pleistocene as alluvial fans and fan-deltas. The latter formed in a shallow lacustrine environ- ment under distal floodplain conditions. These units were deposited between two promontories on the Australian plate (Liu and Crook, 1991).
Structurally the southern flanks of the Finis- terre and Sarawaget Ranges are dominated by high-angle thrust faults largely parallel to the RMFZ (Fig. 2). The Ramu-Markham Fault (RMF) itself is an active single thrust separating the Australian continent from the Finisterre Ter- rane. SedimenSs northeast of the RMF have been and are being elevated, faulted and deformed. The nature and styles of deformation are mainly high-angle thrusting associated with fault-propa- gation folding, which have been particularly well documented in the Leron Formation (Abbott and Silver, 1991; Crook and Liu, in prep.).
The Finisterre and Sarawaget Ranges north- east of the RMFZ have experienced continuous uplift since the Pliocene and reach elevations beyond 4000 m. Rapid uplift on the southern flanks of the Finisterre and Sarawaget Ranges is indicated by elevated river terraces and faulting in the foothills. Recent study of Holocene syn-de- positional tectonic movements in the Lae urban area has determined three superimposed styles of uplift over the past 10,000 years: (i) 1 m/ka steady creep, (ii) small-scale, short-term stepwise vertical jumps of the order of 2-3 m every 250 years, and (iii) relatively long-term tectonic tilting of up to 10-15 ° every 8-10,000 years as a result
SED1MENTOLOGY OF COARSE-GRAINED ALLUVIAL FANS IN THE MARKHAM VALLEY 303
of fault-propagation folding (Crook, 1989b; Crook and Liu, in prep.). Given the diachronous dock- ing nature of the Markham Suture, which is pro- gressively younger to the southeast, the tectonic setting of the Leron Formation in the studied area was probably similar in the Late Pleistocene to that in the present-day Lae urban area. In fact, Silver et al. (1991) estimate that the triple junc- tion has migrated along the Markham Suture about 200 km over the past million years.
Tectonic uplift of the Finisterre Ranges, along with contemporaneous fan development, has con- fined the Markham River to the southern side of the valley. The steep relief, along with coarse particle size, vast sediment inputs and variable discharge regime, have imposed a braided config- uration on the Markham River along its 140 km course from the Ramu divide until it discharges into Huon Gulf just South of Lae.
Fans to the north of the Markham River coa- lesce to form sloping piedmonts, although the individual fans of the larger systems--the Erap, Rumu, Leron and Umi Fans (Fig. 2)--retain their distinctly broad, cone-like fan surface. Given the progressive suturing from NW to SE along the RMFZ, the Umi Fan is the more deeply incised, with up to 30-35 m incision at the fan head. In
contrast, the Leron Fan is only partially incised, and the Erap Fan is barely incised at all. Terraces on the Umi Fan are remarkably straight, planar surfaces which tend to be discontinuous in plan view. Both paired and unpaired terraces are ob- served. Terrace sequences observed on the Umi Fan have their counterpart in the upper Ufim Valley (Fig. 4), where perched gravel remnants are found many tens of metres above the position of the contemporary channel. The post-1943 air- photo record indicates significant channel changes on the Erap, Leron and Rumu Fan systems, with crevassing and channel re-occupation (Holloway et al., 1973). Such adjustments are restricted to the distal end of the Umi Fan.
Although laterally constrained, the contempo- rary Umi River has a braided channel configura- tion. Total channel width varies between 50 and 200 m. The 'trench' which the river has incised broadens down-fan from about 150 m wide at the fan head to about 400-550 m wide down-fan (Fig. 4). In the proximal-fan area, extending 2 km from the mountain front, within-channel sedimentation is characterized by small diagonal bars up to 200 m in length. For the next 5 km down-fan, mid- channel and diagonal bars are up to 600 m long, while the distal-fan reach is characterized by dis-
TABLE 1
Morphostratigraphic units on a mid-channel compound bar adjacent to exposure A
Morphostratigraphic unit Sediment character
Bar framework
Bar margin
Bar tail
Chute channel
Side channel
Very poorly sorted, clast- or matrix-supported gravels, up to 80 cm in diameter. Boulders are typically subrounded but some angular clasts are observed. Deposits frequently have a sand cap atop.
Imbricated, moderately to poorly sorted gravels, typically 30-80 cm in diameter. These are well-rounded gravels, from which the matrix has commonly been winnowed.
Downstream accretionary unit of highly variable character. Deposits range from imbricated, well-sorted, planar or horizontally bedded gravels (up to 30 cm in diameter), to sand sheets and silt drapes.
Clast-supported, imbricated gravels, up to 50 cm in diameter. These have a planar-bedded, medium sand drape. At the downstream end of the channel this drape is composed of rippled and horizontally bedded sands. Several logs are found within the channel.
Erosional channel at the margin of the contemporary bar and adjacent terrace. This has clast-supported gravels at its base, with horizontally bedded sands atop. Isolated boulders, > 3 m in diameter, are interpreted as remnants from debris flow events. These have subsequently been reworked, and the boulders have distinct coarse gravel clusters at their lee.
304 G.J. BR1ERLEY ET AL.
sected, mid-channel compound bars. At its con- fluence with the Yati River, the Umi River is a 1 km wide braidplain, with barely incised terraces. Beyond the Yat i -Umi confluence, the river is called the Markham.
Maximum clast size in the active channel of the Umi diminishes relatively little down-fan. Over a river distance of about 13 km, maximum clast sizes diminish from around 80 cm at the fan head to about 55 cm in the distal-fan.
Sedimentologic character of contemporary Umi River deposits
Various morphostratigraphic units make up the contemporary bars of the Umi River (Table 1; Fig. 6). The spatial organization of these units is determined primarily by chute channel reworking of bar deposits. In the example presented (Fig. 6), the bar framework is composed of poorly sorted, subrounded to well-rounded boulders with a sand veneer. Gravels are clast-supported, with clasts up to 80 cm in diameter. Some grasses and pio- neer trees are found on the bar surface. Deposits at the bar tail differ from the bar framework as they represent gradual accretion of well-sorted, 10 cm or smaller gravels, with horizontally bed- ded or rippled sands atop. Bar margin deposits
are similar to bar framework gravels, but tend to be more winnowed. Chute channels are typically aligned down-bar. They are infilled primarily by planar-bedded gravels, with sand drapes at their downstream end. Many logs are found within chute channels and at bar margins. Side channels, located between bars and terrace risers, are in- filled by matrix-supported and planar-bedded gravels, and commonly grade down-channel to various sand facies. In several instances large boulders, up to several metres wide, are observed in side channels. These commonly have coarse gravel clusters in their lee. These boulders are interpreted as remnants from reworked debris- flow deposits.
Elemental sedimentology of the Umi Fan
Seventeen exposures, extending over a total distance of 13.5 river kilometres, were studied on the lower terrace of the Umi Fan (Table 2; Fig. 7). The largest exposure analysed was more than 350 m long, and over 20 m high. The sediment inventory was obtained by detailed analyses of exposures both in the field and from pho- tographs. Analyses were undertaken at both bed- form-facies and element scales, wherein each ele- ment comprises a specific association of sedimen-
Fig. 6. Schematic plan showing the spatial distribution of morphostratigraphic units on a mid-channel compound bar in the proximal-fan reach. The bar is located adjacent to exposure A (Fig. 7).
SED1MENTOLOGY OF COARSE-GRAINED ALLUVIAL FANS IN THE MARKHAM VALLEY 305
tary facies and dist inct geometry (Brierley, 1991a,
b). Bedform-scale facies types used in this study
accord closely with those of Miall (1977, 1978)
and are p resen ted in Tab le 3. E l e m e n t morpholo-
gies have b e e n recons t ruc ted from photographs
of sed iment exposures, based on wel l -def ined
b o u n d i n g surfaces. Bedform-scale facies associa-
tions, sect ion m e a s u r e m e n t s and assessments of
clast size were ident i f ied on field inspect ion.
Most e l ements which make up the U m i F a n
have a sheet- l ike appearance . O n the basis of
their sed iment character , these are d i f ferent ia ted
into stratif ied sheets, diamict sheets and unsor ted
c las t -supported sheets. C h a n n e l fill uni ts were
also identif ied. E l e m e n t character is summar ized
in Tab le 4.
Sheet element (i): stratified sheet deposits
Sed iment sequences on the lower terrace of
the U mi River F a n are domina t ed by crudely
horizontal sheets of stratified deposits. These
have fiat, conformable basal contacts, a l though
contacts are locally scoured and i r regular in out-
line. Stratif ied sheet deposits are composed pri-
marily of horizontal ly bedded , s u b r o u n d e d grav-
els and sands (facies Gh and Sh). Beds are char-
acteristically 30 to 50 cm thick (Fig. 8). The
textural and s tructural composi t ion of strat if ied
sheet deposits is more variable down-fan, with
greater p ropor t ions of t rough and p lanar cross-
bedded sands and gravels, a long with horizontal ly
bedded sands and gravels. The cross-bedded lay-
TABLE 2
Exposure summary
Exposure Distance Exposure code downstream a dimensions b
E l e m e n t abundance in percentage terms c
sheet sheet sheet (i) (ii) (iii)
Channel fill
A 1230- 1270 42 x 7.2 47.9 0.0 0.0 41.4 B 1470- 1480 13 x 6.0 24.2 60.6 0.0 15.2 C 1755- 1845 90 x 8.8 46.4 0.0 0.0 53.6 D1 2245- 2255 10 x 3.6 0.0 0.0 54.5 45.5 D2 2300- 2310 10 × 3.6 20.0 0.0 28.0 52.0 D3 2365- 2370 6 x 3.2 30.8 0.0 46.2 23.1 D4 2415- 2420 5 x 3.4 0.0 0.0 70.0 30.0 D5 2445- 2450 6 x 3.6 0.0 0.0 100.0 0.0 D6 2505- 2510 7 x 3.6 39.0 0.0 39.0 23.0 D7 2585- 2590 7 × 3.2 0.0 0.0 0.0 100.0 D8 2790- 2795 7 x 5.6 8.3 0.0 74.2 12.5 E 2470- 2530 62 × 5.0 34.8 0.0 0.0 65.2 F 2960- 3075 116 x 7.0 52.1 0.0 0.0 47.9 G 3780- 3870 92 × 13.6 80.3 0.0 0.0 19.7 H 4410- 4450 39 x 11.8 45.5 0.0 0.0 54.5 I 5210- 5245 37 x 11.4 100.0 0.0 0.0 0.0 J 5400- 5420 22 × 11.2 70.7 0.0 0.0 29.2 K 5725- 5735 12 × 10.8 73.0 0.0 0.0 28.0 L 7050- 7205 156 x 21.0 80.0 0.0 0.0 19.6 M 9100- 9300 199 X 14.6 76.6 0.0 0.0 23.2 N 9450- 9835 86 x 9.6 100.0 0.0 0.0 0.0 O 10810-11125 315 x 16.6 96.1 0.0 0.0 9.9 P 11950-12065 115 x 17.2 100.0 0.0 0.0 0.0 Q 13070-13120 48 x 9.8 90.4 0.0 0.0 9.6
a Distance measured along the course of the Umi River from its junction with the Ufim (in metres). b Longitudinal length X height (in metres). c Sheet units are differentiated as follows: sheet (i) = stratified deposits (i.e. sheetflood sequences); sheet (ii) = diamict deposits
(i.e. debris flow units); sheet (iii) ffi unsorted, clast-supported gravels (i.e. hyperconcentrated flood flow deposits).
A~
B
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D ~
mm
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E ~
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,I
Ele
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nt
Co
de
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et f
lood
D
Cha
nnel
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Deb
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low
m
Hyp
erco
ncen
trat
ed f
lood
flo
w
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Dra
pe o
ver
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sure
---~
Flow
dire
ctio
n
16
12
me
tre
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~ ....
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!,.
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~
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--
-.,.
, ..
~
, i
i
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,
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. 7.
Ele
men
tal
sed
imen
tolo
gy o
f th
e U
mi
Fan
. E
xpos
ure
s ar
e lo
cate
d i
n th
e in
set
figu
re.
Exp
osu
res
A
to H
con
stit
ute
th
e p
roxi
mal
-fan
, ex
pos
ure
s I
to M
are
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the
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-fan
,~
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nd
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osu
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o Q
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al-f
an.
>
SEDIMENTOLOGY OF COARSE-GRAINED ALLUVIAL FANS IN THE MARKHAM VALLEY 307
e r s t e n d to o c c u r w i t h i n w e l l - d e f i n e d u n i t s u p t o 5
m t h i c k (Fig . 9). G r a v e l p a r t i c l e s ize in t h e s e
s t r a t i f i e d s h e e t d e p o s i t s c h a n g e s l i t t l e d o w n - f a n ,
w i t h bma x c o n s i s t e n t l y a b o u t 30 cm. S o r t i n g is
h i g h l y i r r e g u l a r f r o m e x p o s u r e to e x p o s u r e . T h e s e
u n i t s a r e i n t e r p r e t e d t o r e p r e s e n t s h e e t f l o o d
e v e n t s in w h i c h o v e r b a n k - s t y l e d e p o s i t s s p r e a d
o v e r a f a n s u r f a c e w i t h p o o r l y d e f i n e d c h a n n e l s .
TABLE 3
Facies coding scheme used in the study
D o w n - f a n v a r i a b i l i t y is c o n s i d e r e d to r e f l e c t t h e
g r e a t e r e a s e w i t h w h i c h s h e e t f l o o d s r e w o r k d e -
p o s i t s o v e r t h e f a n s u r f a c e .
Sheet element (ii): diamict sheet deposits
D i a m i c t s h e e t g r a v e l s a r e o b s e r v e d so le ly in
o n e e x p o s u r e ( e x p o s u r e B, 1.5 k m d o w n s t r e a m
Facies Lithofacies character Mechanism interpretation
Gc Clast-supported gravels, with Bma x up to 50 cm. Often imbri- Traction deposits or deposits from cated. Generally poorly sorted, with subrounded gravels. Occa- hyperconcentrated flood flows. sionally over-loose. Observed up to 4 m thick.
Massive or crudely bedded matrix-supported gravels. Typically about 30 cm thick, with average Bma x of 50 cm, but maximum observed Bma x in excess of 200 cm. Very poorly sorted, with angular to subrounded gravels.
Horizontally bedded gravels, with Bma x up to 30 cm. Typically in beds 20-50 cm thick.
Low-angle inclined gravel sheets. Subrounded gravels with Bma x up to 30 cm.
Planar cross-bedded gravels. Subrounded gravels, with Bma x up to 50 cm. Observed at inclinations up to 30 ° downstream.
Trough cross-bedded gravels. Observed in units up to 10 m wide, in beds up to 4 m thick. Often internally graded units, with Bma x up to 40 cm. Composed primarily of subrounded gravels.
Horizontally bedded fine to coarse sand. Often internally graded, in units 20-40 cm thick.
Gm
Gh
GI
Gp
Gt
Sh
St
Sp
Se
Sm
Fm
C
Medium to coarse sand, occasionally pebbly cross-bedded troughs. Internally graded units, generally > 40 cm thick. Occur as cross-bed sets up to 5 m thick.
Medium to coarse sand, occasionally pebbly planar to tabular cross-bedded units. Internally graded units, generally > 40 cm thick.
Crudely cross-bedded erosionally scours with intraclasts.
Massive, fine to medium sands, typically 10-30 cm thick.
Laminated to massive silts and muds.
Irregular-shaped carbonaceous nodules/concretions typically ob- served in 2-10 cm thick bioturbated horizons.
Pedogenic carbonate concretion with tubules.
Bedload deposit or debris flow unit. May also represent bar framework deposits.
Sheet flood deposit, or upper flow regime plane bed deposits.
Diffuse gravel sheets, possibly laid down as a traction carpet.
Probably represents channel fill or possibly bar tail deposits.
Channel fill sequences.
Upper flow regime plane bed (or lower flow regime for sands ~< 0.6 mm).
Dune migration of lower flow regime.
Foresets from avalanche faces of advancing sand sheets.
Scour fill.
Overbank deposiis, typically in crevasse splay forms.
Overbank sequences, deposited from suspension.
Chemical precipitate in very shal- low water.
Caliche formation in palaeosols.
TA
BL
E 4
Ele
men
t su
mm
ary
Ele
men
t M
orph
olog
y, s
cale
and
A
bu
nd
ance
and
pos
itio
n F
acie
s co
mpo
siti
on a
nd c
hara
cter
In
terp
re-
type
ba
sal
cont
act
in s
eque
nce
tati
on
She
et
Hor
izon
tal
shee
ts,
whi
ch
typi
call
y ha
ve
Do
min
ant
feat
ure
of
Do
min
ated
by
faci
es G
h a
nd S
h. P
ropo
rtio
n of
fac
ies
Sh
incr
ease
s do
wn-
fan,
S
heet
floo
d
(i),
co
nfor
mab
le,
flat
ba
sal
cont
acts
, al
- vi
rtua
lly
all
expo
sure
s,
whe
re t
here
are
als
o fr
eque
nt b
ands
up
to 5
m
thic
k of
fac
ies
Gt/
St
and
/or
depo
sits
stra
ti-
thou
gh
thes
e ar
e oc
casi
onal
ly
irre
gula
r al
thou
gh t
he p
ropo
rtio
n fa
cies
Gp
/Sp
. W
ell-
bedd
ed,
subr
ound
ed s
ands
and
gra
vels
, w
ith
b ax
es u
p to
fled
an
d sc
oure
d.
Uni
ts
exte
nd
dow
n-ex
- te
nds
to i
ncre
ase
dow
n-
30 c
m l
ong.
Sor
ting
var
ies
grea
tly
from
exp
osur
e to
exp
osur
e. B
eds
are
typi
call
y
posu
re (
obse
rved
up
to 3
50 m
lon
g, 2
5 m
fa
n.
up t
o 50
cm
thi
ck,
but
gene
rall
y th
in (
to 1
0-20
cm
) do
wn-
fan.
th
ick)
.
She
et
Unc
lear
mor
phol
ogy.
Uni
ts a
re u
p to
6 m
O
nly
obse
rved
in
on
e C
ompo
sed
sole
ly o
f m
atri
x-su
ppor
ted
grav
els
(fac
ies
Gm
). A
ver
y po
orly
sor
ted,
D
ebri
s fl
ow
(ii)
, th
ick
and
exte
nd
dow
n-ex
posu
re.
Bas
al
expo
sure
in
th
e pr
oxi-
w
ell-
cem
ente
d un
it
of
suba
ngul
ar
grav
els
in
a pr
edom
inan
tly
sand
m
atri
x,
depo
sits
.
diam
ict
cont
acts
are
fia
t an
d co
nfor
mab
le,
mal
fa
n, w
ith
stra
tifi
ed
Coa
rses
t cl
asts
ext
end
up t
o 20
0 cm
in
thic
knes
s bu
t ra
nge
wid
ely
in s
ize.
The
re
shee
t de
posi
ts a
top.
is
no
evid
ent
stru
ctur
e, a
ltho
ugh
coar
se g
rave
ls t
end
to o
ccur
in
clus
ters
. B
asal
shee
tflo
od d
epos
its
may
rep
rese
nt i
nver
se g
radi
ng.
She
et
(iii)
,
unso
rted
, cl
ast-
sup-
port
ed
Cha
nnel
fill
unit
s
She
et-l
ike
unit
s w
hich
are
>
4 m
thi
ck i
n
one
expo
sure
, bu
t ar
e ge
nera
lly
2.5-
3 m
thic
k. C
onti
nuou
s th
roug
hout
eig
ht s
mal
l,
disc
onti
nuou
s ex
posu
res
(> 5
50
m
long
in t
otal
). B
asal
con
tact
s ar
e ir
regu
lar
and
scou
red.
Sha
rply
def
ined
, as
ymm
etri
cal
infi
ll u
nits
,
wit
h sc
oure
d an
d cu
rved
bas
al c
onta
cts.
Pro
xim
al a
nd m
id-f
an c
hann
els
exte
nd u
p
to
5 m
in
th
ickn
ess,
whi
le
the
deep
est
obse
rved
ch
anne
l in
th
e di
stal
fa
n w
as
only
3 m
de
ep.
Cha
nnel
s av
erag
e ab
out
25 m
lon
g in
obs
erve
d ex
posu
res,
ind
icat
-
ing
that
w
idth
-dep
th
rati
os
incr
ease
dow
n-fa
n.
Onl
y ob
serv
ed
in
a se
-
ries
of
eigh
t sm
all,
dis
-
con
tin
uo
us
exp
osu
res
that
fo
rm
an
inse
t te
r-
race
in
th
e pr
oxim
al
area
of
th
e fa
n.
Typ
i-
call
y ha
s ba
r fe
atur
es
bene
ath,
and
thi
n st
rati
-
fied
she
et u
nits
ato
p.
Pro
port
ion
dim
inis
hes
dow
n-fa
n fr
om
> 40
%
of
prox
imal
-fan
ex
po-
sure
s to
<
10
%
of d
is-
tal-
fan
expo
sure
s. C
har-
acte
rist
ical
ly
obse
rved
wit
h st
rati
fied
sh
eet
unit
s.
Alm
ost
enti
rely
co
mpo
sed
of c
last
-sup
port
ed
grav
els
(fac
ies
Gc)
. E
xtre
mel
y
poor
ly s
orte
d, s
ubro
unde
d gr
avel
s, w
ith
b ax
es u
p to
120
cm
. T
hin
( <
20 c
m)
sand
she
ets
wit
hin
may
ind
icat
e ph
ases
or
puls
es i
n on
e ev
ent.
A w
ood
sam
ple
from
wit
hin
this
uni
t ga
ve a
rad
ioca
rbon
dat
e of
100
+ 0
.9%
a (
AN
U-7
812)
.
Hig
hly
vari
able
pr
opor
tion
s of
fa
cies
G
p,
Gt,
G
m,
Gh,
G
I an
d th
eir
sand
equi
vale
nts.
Ind
ivid
ual
chan
nel
fill
s te
nd t
o ha
ve m
ore
vari
able
fac
ies
com
posi
-
tion
s in
di
stal
ex
posu
res.
M
oder
ate
to
poor
ly
sort
ed,
subr
ound
ed
to
plat
ey
grav
els,
whi
ch d
ecre
ase
in s
ize
dow
n-fa
n fr
om a
bout
50
cm t
o 25
cm
, w
hile
the
prop
orti
on
of
sand
un
its
also
in
crea
ses
in
that
di
rect
ion.
In
fill
s ar
e hi
ghly
vari
able
in
char
acte
r re
flec
ting
loc
al c
ircu
mst
ance
. D
ista
l-fa
n un
its
freq
uent
ly
exhi
bit
upw
ard-
fini
ng
part
icle
si
ze
tren
ds.
The
de
gree
of
so
rtin
g te
nds
to
incr
ease
dow
n-fa
n as
the
pro
port
ion
of s
and
unit
s in
crea
ses.
Hyp
erco
n-
cent
rate
d fl
ood
flow
depo
sits
Cha
nnel
fill
depo
sits
.
SEDIMENTOLOGY OF COARSE-GRAINED ALLUVIAL FANS IN THE MARKHAM VALLEY 309
from the Umi-Uf im junction, Fig. 4). This di- amict sheet rests atop clearly bedded fine gravels and a horizontally bedded, upward-coarsening sand unit (Fig. 10a). Contacts between these units are flat and conformable. The upper unit, up to 6 m thick, is a very poorly sorted unit of subangular gravels in a predominantly sand matrix. Some clasts reach 200 cm in width, but sizes range widely. Clasts tend to occur in clusters within this well-cemented unit, but there is no evidence of sedimentary structures. This is interpreted as a debris flow unit, the sediments at the base of which may reflect inverse grading. Extremely coarse boulders, up to several metres in diameter, observed both on terrace surfaces and in side- channels adjacent to the contemporary Umi River, may reflect reworked debris flow deposits.
Sheet element (iii): unsorted, clast-supported sheet units
Between 2.2 and 2.8 km downstream from the Umi-Uf im junction is a 4 m high inset terrace
composed of unsorted, clast-supported gravels. This terrace is notably less thick than all other analysed exposures and was analysed in a series of eight small exposures in a 600 m section (ex- posure D; Fig. 7). The inset terrace comprises a sheet-like form which rests with an irregular, scoured contact above moderately sorted, clast- supported gravels, with thin stratified sheet de- posits atop. These subrounded, clast-supported gravels (facies Gc) are extremely poorly sorted (Fig. 10b), with an observed bma x axis of 120 crn. They are interpreted as deposits from a hyper- concentrated flood flow event, reflecting sedi- mentation from high-discharge flows intermedi- ate in sediment /water ratio between stream flow and debris flow (Smith, 1986). In this interpreta- tion, extremely large volumes of sediment, with a wide range of particle sizes, would have been moved and deposited relatively rapidly from hy- perconcentrated dispersion. The mixed sand and gravel matrix, along with some discontinuous bands of horizontally bedded sands, may reflect pulses in deposition of this unit. A radiocarbon
G l ~ h o 7 ° - - ~ ° ~
• - s h . . . . - . - - ~ ' - ~ ' _ ' . " G h T. ° • ." ~ " * " - c ~ " ~ " ~ . . - - - - 1 • ° . • n t o - . . o , o . . " 4
o ~ ° o . " . : . * " o " " o ~ - o ,
o - . ~ " ~ i o ~ , s , ; ' - . - . . . ° . \ - o ~ ~ - ~ . o . . . ' . " .~..°. . _ _ .
° ' " " " o s ° ~ . . . ~ \ ~ o C , " ° ° o ~ . ~ ; o , ~ ° " J O h / ~ a . . . ~ ~ _ _ _ ~ a : " o . - - o • _ _ - - -
O ~ ~ ~ " e ¢ ~0 ~ c~ " . o _ ~ " ~ e ~ i - ~ - . . . . ~ o ~ _ e a
Fig. 8. Element- and facies-scale sedimentoiogy of exposure A (located in Fig. 7). Stratified sheet deposits (facies Oh and Sh) are observed in two distinct bands down the entire exposure. These are interpreted as sheetflood units. The basal clast-supported gravel unit (facies Gc), along with the mixed facies sequences in mid-exposure, observed within discrete scour features, are channel
fill elements.
310 G.J . B R I E R L E Y E T AL.
sample from a piece of wood taken from this unit (exposure D5; Fig. 10b) gave an age of 100 years + 0.9% (i.e. 1840-1860 A.D.; A N U 7812).
Channel fill elements
Channel fill units are characteristically ob-
served within stratified sheet units (e.g. Fig. 8). The abundance of channel fills is highly variable, although their relative proportion diminishes down-fan (Table 2). Channel fill units are charac- teristically 3-5 m deep, and up to 25 m wide,
although wid th-depth ratios tend to increase down-fan. In most instances channel fills are asymmetrical units, although channel fills ob-
served in the downstream half of the fan tend to be more saucer-shaped in outline. Basal contacts are sharply defined and erosive or scoured. Bed-
form-scale facies compositions are highly variable from channel fill to channel fill, with differing proportions of facies Gc, Gp, Gt, Gm, Gh and G1 and their sand equivalents. While channel fills in
the proximal-fan are composed largely of facies Gc, channel fills in distal-fan exposures tend to have more variable facies compositions. Moving down-fan, maximum clast size in channel fills
diminishes from about 50 cm to about 25 cm, channel fills tend to become better sorted, and the proportion of sand units increases. Indeed,
channel units in the distal fan frequently demon- strate upward-fining particle size trends, with fa-
cies sequences indicative of upward reduction in depositional energy. In all instances channel fill units are moderately to poorly sorted, and are composed of subrounded to platey gravels.
Summary elemental composition of the Umi Fan
Observed sediment sequences in the lower ter- race of the Umi Fan are dominated by stratified sheetflood units (Table 2). These make up ap- proximately 40% of exposures between 0 and 4.5 km downstream from the U m i - U f i m confluence (Fig. 4), 70-80% of exposures between 4.5 and
~ . ~ : : • I, s, ~ - ~ J ~ - ~ . = - .~ -~ .~ , , . . . . . a , . , . . . . . .
G l ~ r G ' . L " _ _ , , " _
Fig. 9. Element- and facies-scale sedimentology of exposure O (located in Fig. 7). The exposure is dominated by stratified sheet deposits, with minor channel scour features at the surface and base of the exposure. Note the greater diversity of facies types
observed in the sheetflood deposits in this distal-fan exposure when compared with sheetflood units in the proximal fan (Fig. 8).
S E D I M E N T O L O G Y OF COARSE-GRAINED ALLUVIAL FANS IN T H E MARKHAM VALLEY 311
9.4 km d o w n s t r e a m f rom the U m i - U f i m conf lu-
ence, and > 90% of exposures b e y o n d 9.4 k m
f rom the U m i - U f i m conf luence (Tab le 2). This
down- fan inc rease in s t ra t i f ied shee t f lood ele-
men t s can be used to d i f f e ren t i a t e p rox imal - ( 0 -
4.5 km f rom the conf luence of the U m i and Uf im
River) , mid- (4 .5-9 .4 km) and d is ta l - fan r eaches
( > 9.4 km). Debr i s flow and h y p e r c o n c e n t r a t e d
f lood flow e l emen t s a re obse rved solely in proxi-
mal - fan exposures . The l a rge r p r o p o r t i o n of
channe l fill, debr i s flow and h y p e r c o n c e n t r a t e d
f lood flow sed imen t s in the p rox imal fan may
s imply ref lec t the g r e a t e r po ten t i a l for fluvial
rework ing o f depos i t s down- fan or the fanning
ou t of non- f lood channe ls into smal le r and more
e p h e m e r a l d is t r ibutar ies . Summa ry sed imen to -
Fig. 10. (a) Element-scale sedimentology of exposure B (located in Fig. 7). The upper part of this exposure is composed of coarse angular blocks (up to 2 m in diameter) in a fine-grained matrix. This is interpreted as a debris flow unit, which rests atop well-graded gravel and sand units. (b) Element-scale sedimentology of exposure D5 (located in Fig. 7). Note the extremely broad range of clast sizes within an equally variable matrix. This very dense unit of subrounded gravels is interpreted as deposits from an hyperconcentrated flood flow event. The asterisk marks the position from which a wood fragment was taken for radiocarbon dating
and yielded a date of 100 ± 0.9% Modem (ANU 7812).
312
TABLE 5
Sedimentologic character of proximal-, mid-, and distal-fan zones of the Umi Fan
G.J. BRIERLEY ET AL
Proximal fan Mid fan Distal fan
Element abundance: Sheetflood (%) 40 Channel /bar units (%) 30-40 Hyperconcentrated flood
flow (%) < 15 Debris flow (%) < 15
Element character:
Sheetflood (a) Morphology (b) Scale
(c) Facies composition
(d) Maximum particle size
(e) Bedding thickness (f) Other properties
Channel fill units (a) Morphology
(b) Scale (c) Facies composition
Horizontal sheet Up to 8 m thick, but
typically about 2 m thick 80% G h / S h 20% G t / S t / G p / S p
30 cm Typically 10-30 cm thick Moderately to poorly sorted,
subrounded gravels
Asymmetrical
Up to 5 m thick Highly variable from unit
to unit (d) Maximum particle
size 50 cm (e) Bedding thickness Up to 30 cm (f) Other properties Extremely variable
Hyperconcentrated flood flow (a) Morphology (b) Scale (c) Facies composition (d) Maximum particle
size (e) Bedding thickness (f) Other properties
Debris flow (a) Morphology (b) Scale (c) Facies composition (d) Maximum particle
size (e) Bedding thickness (f) Other properties
Shee t - l ike Up to 4 m thick 95% Gc
120 cm N / A Extremely poorly sorted,
rounded to subrounded clasts in a mixed sand and gravel matrix
Irregular unit Up to 6 m thick 100% Gm
200 cm N / A Extremely poorly sorted,
sub-angular boulders in a fine-grained matrix
70-80 95 20-30 5
Not observed Not observed Not observed Not observed
Horizontal sheet Up to 22 m thick
Highly variable, with % G h / S h ranging from 20 to 80%
30 cm Typically 10-50 cm thick Highly variable character,
associated with differing facies compositions
Asymmetrical
Up to 6 m thick Highly variable from unit
to unit
40 cm
Extremely variable; several units are upward-fining
Not observed
Not observed
Horizontal sheet > 10 m thick in all instances
50% G h / S h 50% G t / S t / G p / S p
25-30 cm Generally thin ( < 20 cm) Moderately to poorly sorted,
subrounded gravels
Asymmetrical-to-saucer- shaped
Up to 3 m thick Composed primarily of facies
Gt, Gp, Gc and Gm units
25-30 cm
Moderately to poorly sorted platey to subrounded gravels
Not observed
Not observed
A 40
-60
lli=
,i
Gm
I S00
Gm
600
Gm
Gh
4OO
Gh
250
C~
300
Gt
800
Gh
P Ss
Sm
F
m
300
Gh
Gh
P Fm
S=
200
Gh
B 4,
,o,,
-pe
Gm
Gm
Fm
S
s G
m
P
Ss
Gh
Sm
G
h
Gh
Sm
G
h
Fm
Gh
O
40
~ II
II
II
Gm
Gh
Gh
Gh
Gt
Sm
Gm
Gm
Gh
P C,p
Gh
E
4 o
-60
illl
ll
16o
~,
120
Gilt
Ss
Gh
Fm
9O
Gt
Ss
C,p
F
m
60
Gh
St
C
9O
Gh
P Gt
Fm
C
F
m
Ss
C~
C
Ss
Sm
Sm
Gh
Fm
12
0 G
h C
Gh
90
Fm
Gm
F 4
0 -6
(~
I I
]I
I I
, ~
~ P
leis
toce
ne
'
,~
Le
ron
Fro
. fa
ns
~ M
od
ern
allu
vial
fa
ns
E,
Lo
cati
on
of
I\L
~ ',
\~\~
-,k
5ecl
ions
LEG
EN
D
BIB
M
ud
sto
ne
Sa
nd
sto
ne
Co
ng
lom
era
te
m
We
ll d
efi
ne
d
ho
rizo
nta
l st
rati
fica
tio
n
Ho
rizo
nta
l st
rati
fica
tio
n
Pla
na
r/ta
bu
lar
cro
ss s
tra
tifi
cati
on
Tro
ug
h c
ross
str
ati
fica
tio
n
Rip
ple
la
min
ati
on
Gm
F
aci
es
ced
e (
see
Ta
ble
3)
3oo
Ma
xim
um
cl
ast
siz
e i
n m
m
_q S C)
0 0 > > r" 7 > t"
Fig.
11.
Six
mea
sure
d se
ctio
ns o
f th
e up
per
part
of
the
Ler
on F
orm
atio
n in
the
Mar
kh
am V
alle
y be
twee
n th
e ri
vers
Ler
on a
nd E
rap.
Ver
tica
l pr
ofil
es A
, B
and
C a
re p
roxi
mal
-fan
sequ
ence
s, w
hile
pr
ofil
es
D,
E
and
F ar
e di
stal
fa
n se
quen
ces.
Not
e ho
w
the
prop
orti
on
of f
acie
s di
ffer
s be
twee
n th
ese
two
sets
. T
he
inse
t sh
ows
the
loca
liti
es
and
the
pala
eoge
ogra
phy
of t
he a
lluv
ial
fans
.
k~
D'
o
4 0
J6~
4 0
-6 O
D
' ,,
~]l
,
3 \
4
, g
3 LE
GE
ND
I~
Mud
ston
e
4
Wel
l o
ef~l
ho
rizon
tal
4 L~
=JL~
Ho~
zont
al
3
K~
4 5
Con
giom
~ate
we. ~
ed
t
horiz
onta
l stra
tific
alzo
n /I
Hor
izon
tal S
tratif
icat
ion
Pla
na#t
abul
ar c
ross
sl
ratif
icaf
ion
Trou
gh c
ross
stta
tifca
lion
Rip
ple
lam
inat
ion
Hie
r ach
y of
bou
ndin
g s4
Jrfa
c.,e
NO
RTH
",5"
....
....
C
liff/T
erra
ce
• Th
rust
--~
Ant
idin
e
-< 4
s B
eddi
ng
D '
' M
ea
sure
d s
ectio
n
----
4--
--
Hie
rarc
hy o
f bo
undi
ng s
urfa
ce
met
res
0 3o
o
\ \
\ \
'-\
\\
~\
\\
\\
\ \
\ -,
\
, X
's-
\
\ \
\ '\
\ \
\~
\ "x
\
\ • \
\ \
\ \
\ \
X
\ \
\ \
\ \\
\
\ \
\ \
\,
\ \,
\,
\ \
/ \
\ \
/ \
\ ~
/
Fig.
12.
Thr
ee c
lose
ly s
pace
d m
easu
red
sect
ions
, D
, D
' an
d D
" us
ed t
o de
fine
arc
hite
ctur
al-e
lem
ents
in
the
uppe
r pa
rt o
f th
e L
eron
For
mat
ion.
The
rel
atio
nshi
ps b
etw
een
the
thre
e m
easu
red
sect
ions
are
sho
wn
in p
lan
view
, in
dica
ting
the
bou
ndin
g su
rfac
e hi
erar
chy
and
lith
osom
e ge
omet
ry.
SED1MENTOLOGY OF COARSE-GRAINED ALLUVIAL FANS IN T H E MARKHAM VALLEY 315
. . . . . . . . . . . . . . . . . . . . • - . . . . ~ o . . . . . . . l . . . . . . . . . . . . . . . . . . . .
Fig. 13. Architectural element analysis of the Leron Formation outcrops at D and D', showing the bounding surface hierarchy. Points X and Y refer to overlap/correlation points between photos (a) and (c) and (b) and (c), respectively. Photographs are taken looking eastwards. Photo (a) shows the spatial relationship between sections D, D ' and D", and the fifth-order bounding surface. The lowest terrace is 15 m high, and the package in between the two fifth-order bounding surfaces at D" is about 100 m thick. Photo (b) shows third- and fourth-order bounding surfaces. Photo (c) shows first- and second-order bounding surfaces and facies
codes.
316 G.J . B R I E R L E Y E T AL.
logic characteristics of these three reaches of the Umi Fan are presented in Table 5.
Laterally unconfined sheetflood sequences which make up the lowest terrace of the Umi Fan contrast starkly with deposits associated with the present braided channel of the Umi River. Con- temporary bars are made up of imbricated clast- and matrix-supported gravels, along with some planar and trough cross-bedded sequences (Table 1, Fig. 6). These deposits are equivalent to chan- nel fill units observed in terrace exposures. Such channel fill deposits are continuous down-fan in the contemporary channel, but are only found in any abundance in proximal- and mid-fan terrace exposures (Fig. 7). Down-fan variation in the abundance and character of channel fill deposits accounts for the pattern of downstream changes in gravel clast sizes. Clasts in the contemporary channel of the Umi River beyond the bridge on the Highlands Highway have b axes up to 60 cm, compared with 30 cm in deposits in the adjacent fan terraces. Schumm et al. (1987) describe a similar pattern of down-fan complexity in particle size distribution associated with trenching at the fan head and reworking of coarser deposits. Downstream variation in clast size reflects dimin- ished channel definition, and hence reduced flow competence, down-fan. The laterally confined modern channel is able to transport coarser clasts further down-fan than were the sheetfloods re- sponsible for distal-fan deposits.
Sedimentology of the upper fan part of the Pleis- tocene Leron Formation and comparison with Umi Fan deposits
The upper part of the Late Pleistocene Leron Formation, which forms the spectacular Sawtooth Range in the Markham Valley between the rivers Leron and Erap (Fig. 2), comprises an alluvial fan sequence over 500 m thick. This sequence devel- oped as the subaerial part of an alluvial fan-delta sequence in the Late Pleistocene (Liu and Crook, 1991). It was probably deposited in a very similar tectonic setting and climatic regime to the Umi Fan, and therefore provides an excellent 'ancient' example of a tropical-savanna alluvial fan.
Although the Leron Formation in this area has
been disrupted by folding and thrusting, detailed field mapping and sedimentary facies studies have enabled its palaeogeography to be reconstructed. The upper part of the Leron Formation com- prises several (at least four) coalesced alluvial fans covering an area of approximately 40 km by 10 km (Fig. 11). The six measured sections on these fan sequences are simplified from the origi- nal detailed logs (1:50 scale) to highlight the characteristics of the conglomerate facies (Fig. 11). Among the measured sections, three of them (A, B and C) are from the proximal parts of the alluvial fans, and the other three (D, E and F) are from the relatively distal parts.
Outcrops suitable for three-dimensional analy- sis are rare in the studied area because of fault- ing, folding and vegetation growth. At the best locality (point D in Fig. 12), about 1 km down- stream from the Leron Bridge, the Leron Forma- tion is exposed along 15 m cliffs on either side of the river (profiles D and D'), and is also well exposed roughly perpendicular to these exposures along Wanch Creek, roughly 1 km downstream of this point east of the Highlands Highway (profile D"; Fig. 13). Stratigraphic correlations between these three exposures are well controlled in the field by detailed structural and lithofacies map- ping using an enlarged 1:100,000 scale air-photo.
The proximal parts of fans in the upper part of the Leron Formation are dominated by conglom- erates which occupy more than 60% of the sec- tions (Fig. 11, sections A, B and C). Massive to crudely stratified, clast-supported conglomerate (facies G c / G m ) constitutes about 35% of the total conglomerate and occurs mainly in the up- permost section, whereas horizontally stratified conglomerates (facies Gh) account for about 50% of the total conglomerates and are dominant in the middle and lower parts of the sections stud- ied. Minor p lanar / tabular to trough cross-strati- fied conglomerates (facies Gp and Gt, respec- tively) are intercalated with facies Gm and Gh as thin beds. Individual conglomerate beds of facies Gm and Gh are commonly > 5 m thick and sometimes extend up to 20 m in thickness. Maxi- mum clast size in the conglomerates averages 20 to 40 cm, is occasionally up to 150 cm, and is rarely smaller than 10 cm.
SEDIMENTOLOGY OF COARSE-GRAINED ALLUVIAL FANS IN THE MARKHAM VALLEY 317
Sandstones are characteristically massive peb- bly sandstones (facies Sm), with occasional scour- and-fill sandstone lenses (facies Ss). Mudstones and very fine sandstones (facies Fm) are seldom observed in proximal-fan deposits, and constitute < 10% of observed sections. The latter are fre- quently observed in association with carbonate concretions and pedogenic carbonate (facies C and P, respectively). Facies Sm, Ss and Fm are usually organized in upward-fining cycles.
In contrast to these proximal-fan exposures, mudstones and very fine sandstones (facies Fm) constitute up to 50% of sections examined in the distal parts of observed Leron Formation fans (sections D, E and F in Fig. 11). These fine- grained sequences are typically observed in asso- ciation with abundant carbonate concretions (facies C) and minor pedogenic carbonate (facies P). Horizontally stratified conglomerates (facies Gh) make up 30% of distal-fan sections. Planar/tabular to trough cross-stratified con- glomerates (facies Gp and Gt, respectively) make up about 10% of these exposures. The remaining 10% of strata are composed of minor massive sandstone (facies Sm), scour and fill pebbly sand- stone (facies Ss), and ripple laminated sandstone (facies Sr). Maximum clast size in the conglomer- ates is usually between 10 to '20 cm, and rarely exceeds 25 cm. Conglomerate facies (Gh, Gm, Gp and Gt) vary from 2 to 10 m in thickness, and are composed of relatively well-sorted, moderately well-rounded to subrounded pebbles to cobbles. Irregular erosional bases and channel fills are common.
The fine sediments in the distal parts of the alluvial fans (facies Fm, Ss, Sr, C and P) are massive, grey to yellow-brown units. These consist of sandy to pebbly mudstones interbedded with sandy units in upward-fining cycles that range from 20 to 200 cm in thickness. Irregularly shaped, reddish to white carbonate nodules (facies C) are very abundant, and typically form horizons 5 to 20 cm thick overlying facies Fm units. Bioturba- tion is common in the fine sediments, and is especially prevalent in facies C. Most of the bur- rows in facies C have a simple, cylindrical, non- branching and unlined form, and are similar to the fluvial ichnofacies Scoyenia described by
Seilacher (1967). The high proportion of mud- stones and abundant carbonate nodules are inter- preted to have been deposited in a water- saturated floodplain environment at the distal end of Leron Formation fans (Liu and Crook, 1991).
Facies associations described for six vertical profiles in the upper Leron Formation show very distinct proximal-distal relationships in fan de- posits (Fig. 11). These closely resemble trends described for the modern Umi Fan. This similar- ity is particularly well reflected in the relative proportions of conglomerates and sandstones to mudstones, and in maximum clast sizes. Hyper- concentrated flood flow and debris flow conglom- erates are seldom observed in either instance. These occur only in the uppermost part of proxi- mal-fan sections of the Leron Formation, reflect- ing poor preservation elsewhere. They are too thin to show on the vertical profile logs, and are represented as facies Gm units in Fig. 11. Thin planar/tabular to trough cross-stratified con- glomerates (facies Gp and Gt) are well developed in the middle part of proximal sections and in the upper parts of the distal sections of the upper Leron Formation. Horizontally stratified con- glomerates (facies Gh) occur throughout all sec- tions, but they are particularly well developed in the middle to lower parts of the proximal sections and the upper parts of the distal sections. These trends reflect the increased complexity of facies associations described down-fan for sheetflood sequences on the Umi Fan.
The only notable difference between Umi Fan deposits and the upper part of the Leron Forma- tion is the presence of muds with carbonate nod- ules and carbonate horizons in the Leron Forma- tion. This is relatively easily explained by the lack of a lacustrine depositional environment in the distal Umi Fan. Even during flood stages, flow in the Umi is confined within the channel and flushes fine sediment into the Markham River and hence into the Huon Gulf. In contrast, dur- ing the deposition of the upper part of the Leron Formation, there was a shallow lake at the distal end of the alluvial fans. This lake served as an accumulation zone for fine-grained sediments (Liu and Crook, 1991).
TA
BL
E 6
Lit
hoso
me
and
boun
ding
sur
face
hie
rarc
hy o
f M
ark
ham
Val
ley
fans
(m
odif
ied
afte
r M
iall
, 19
88 a
nd D
eCel
les
et a
l.,
1991
) O
¢
Lit
ho
som
e/
Bou
ndin
g su
rfac
e ch
arac
teri
stic
s
boun
ding
an
d fa
cies
surf
ace
orde
r
Sca
le
Inte
rpre
tati
on a
nd e
xam
ple
Pos
tula
ted
tim
efra
me
(in
year
s *)
Fir
st
Sec
ond
Thi
rd
Fou
rth
Fif
th
Cro
ss-b
ed
set
boun
ding
su
rfac
es.
Lit
tle
or n
o in
tern
al e
rosi
on a
ppar
ent.
Sim
ple
cose
t bo
undi
ng s
urfa
ces.
The
sur
-
face
is
typi
call
y no
t m
arke
d by
sig
nifi
cant
bedd
ing
trun
cati
ons
or o
ther
evi
denc
e of
eros
ion.
Cro
ss-c
utti
ng e
rosi
on s
urfa
ces
that
dip
at
a lo
w a
ngle
(ty
pica
lly
< 15
°) a
nd t
runc
ate
unde
rlyi
ng b
eddi
ng s
urfa
ces
(oth
er
than
firs
t- a
nd s
econ
d-or
der
surf
aces
).
Fla
t to
co
nvex
-upw
ard
surf
aces
w
hich
trun
cate
un
derl
ying
fir
st-
to
thir
d-or
der
surf
aces
at
a lo
w a
ngle
. E
rosi
onal
ba
ses
of l
arge
len
ticu
lar
lith
osom
es.
Sur
face
s w
hich
bo
und
sand
an
d gr
avel
shee
ts,
incl
udin
g ch
anne
l fi
ll co
mpl
exes
.
Few
cen
tim
etre
s to
a
few
met
res
long
. E
nclo
sed
lith
osom
e is
a f
ew
cent
imet
res
to
a fe
w
deci
met
res
thic
k.
May
ext
end
late
rall
y fo
r up
to
20
m,
wit
h an
en
clos
ed
lith
osom
e a
few
met
res
thic
k.
Up
to
tens
of
met
res
long
, w
ith
an
encl
osed
li
thos
ome
up
to
10
m
thic
k.
Ext
ensi
ve,
flat
to
cu
rved
er
osio
n
surf
aces
. M
ay e
xten
d la
tera
lly
for
> 10
0 m
w
ith
an
encl
osed
li
tho-
som
e up
to
10 m
thi
ck.
Ext
ensi
ve
surf
aces
w
ith
flat
to
slig
htly
co
ncav
e-up
war
ds
outl
ine.
May
ex
tend
la
tera
lly
> 10
00
m
wit
h an
enc
lose
d li
thos
ome
up t
o
100
m t
hick
.
The
se s
urfa
ces
repr
esen
t vi
rtua
lly
cont
inuo
us s
edim
enta
tion
of
bedf
orm
s an
d as
soci
ated
be
ddin
g pl
anes
of
in
divi
dual
depo
siti
onal
eve
nts.
Sty
les
of e
vent
inc
lude
dun
efie
ld m
igra
-
tion
, de
bris
flo
w,
or h
yper
conc
entr
ated
flo
od f
low
eve
nts.
Ero
sion
su
rfac
e re
pres
ents
up
war
d tr
ansi
tion
in
fa
cies
typ
e
indi
cati
ng c
hang
e in
fl
ow
cond
itio
ns
or
flow
di
rect
ion
(e.g
.
from
fac
ies
Gh
to G
m o
r fr
om f
acie
s G
h t
o G
h).
Sur
face
s in
dica
te
stag
e ch
ange
s,
or
chan
ges
in
bedf
orm
orie
ntat
ion,
su
ch
as
late
ral
and
dow
nstr
eam
m
igra
tion
of
grav
elly
m
acro
form
s (f
acie
s G
p,
Gt)
in
sh
allo
w
brai
ded
chan
nels
. T
hese
com
mon
ly h
ave
thin
fin
e-gr
aine
d un
its
(fac
ies
Sm
, S
s an
d F
m)
atop
. T
his
repr
esen
ts t
he s
cale
of
mor
phos
-
trat
igra
phic
uni
ts o
n ba
r su
rfac
es.
Sur
face
s re
pres
ent
the
uppe
r bo
undi
ng
surf
aces
of
mac
ro-
form
s su
ch a
s ch
anne
l fi
lls.
Typ
ical
ly c
ompo
sed
of f
acie
s G
m
and
Gh
int
erca
late
d w
ith
faci
es G
p, G
t, S
s, S
m a
nd F
m.
Thi
s
lith
osom
e is
nor
mal
ly c
appe
d by
fac
ies
Sm
, Ss
, S
e an
d F
m i
n
upw
ard-
fini
ng c
ycle
s.
Sur
face
is
mar
ked
by p
ossi
ble
intr
afor
mat
iona
l an
gula
r un
con-
form
itie
s in
the
Ler
on F
orm
atio
n, a
nd r
efle
cts
the
scal
e of
the
fill
ed-i
n tr
ench
of
the
Um
i F
an.
Six
th
Sur
face
s w
hich
de
fine
m
appa
ble
stra
ti-
Lit
hoso
me
may
be
h
un
dre
ds
of
Lit
hoso
me
repr
esen
ts t
he s
cale
of
the
fan
itse
lf,
and
is u
sual
ly
10 5
grap
hic
subd
ivis
ion
of a
fan
uni
t, c
apab
le
met
res
thic
k an
d se
vera
l ki
lom
e-
trun
cate
d by
fau
lts
tow
ards
the
sou
rce
area
.
of d
efin
itio
n as
mem
bers
or
subm
embe
rs,
tres
wid
e.
Sev
enth
E
ithe
r an
ang
ular
unc
onfo
rmit
y or
lat
eral
L
itho
som
e m
ay b
e ki
lom
etre
s th
ick
Fan
com
plex
, th
e pi
edm
ont
wit
hin
the
enti
re m
olas
se b
asin
, 10
5
boun
dary
be
twee
n ou
ter-
fan
faci
es
and
and
man
y te
ns o
f ki
lom
etre
s w
ide.
th
e fa
n pa
rt o
f th
e L
eron
For
mat
ion
basi
nal
fluv
ial
faci
es.
10
i to
101
101
to 1
02
10 2
to
10 3
103
to 1
04
10
4
to
10
5
* A
ge s
cale
s ar
e ba
sed
on:
(a)
firs
t- a
nd s
econ
d-or
der
lith
osom
e ac
cum
ulat
ion
rate
s ar
e de
term
ined
fro
m k
now
n de
posi
tion
rat
es (
arou
nd
6-1
0 m
/ka)
; (b
) th
ird-
fou
rth-
and
fift
h-or
der
lith
osom
e ac
cum
ulat
ion
rate
s ar
e in
ferr
ed f
rom
rat
es o
f ne
otec
toni
c m
ovem
ent
at L
ae (
Cro
ok a
nd L
iu,
in p
rep.
); (
c) s
ixth
- an
d se
vent
h-or
der
lith
osom
e ac
cum
ulat
ion
>
rate
s ar
e in
ferr
ed f
rom
TL
dat
ing
of t
he a
lluv
ial
fan
part
of
the
Up
per
Ler
on F
orm
atio
n: a
TL
sam
ple
from
a v
olca
nic
ash
bed,
att
aine
d ad
jace
nt t
o th
e m
od
ern
Ru
mu
Riv
er i
n th
e
Saw
toot
h R
ange
s (F
ig.
2)yi
elde
d an
age
est
imat
e of
117
_+
18 k
a (W
1316
).
S E D I M E N T O L O G Y OF COARSE-GRAINED ALLUVIAL FANS IN T H E MARKHAM VALLEY 319
Geomorphic evolution of Markham Valley fans and their sedimentologic implications
Evolution of the Umi Fan can be interpreted from the character and distribution of elements making up its lowest terrace. The preponderance of stratified sheet deposits indicates that massive sediment dispersion is the most significant phase in the build up of the alluvial fan. The potential for rapid sediment dispersal has probably been available in the Umi-Erap area since the Late Pleistocene. Pervasive deformed mudstones and sandstones of the Oligocene to Miocene Mebu Beds and Pliocene thinly bedded, weakly in- durated turbidites (Abbott and Silver, 1991), have been thrust over the Leron Formation, forming steep, extremely unstable mountain slopes. Field analyses indicate that more than 70% of clasts at the head of the Umi Fan, and in the upper part of the Leron Formation, are derived from these sediments. While depositional sequences in the lowest terrace of the Umi indicate the dominance of sheetflood deposition in the past, such mecha- nisms are no longer prevalent in the main Umi channel itself. Sheetflood events are prevalent on the upper fan surface, however, as evidenced by the recent Kaiapit landslide (Dreschler et al., 1989).
The presence of the extensive trench along the course of the Umi River infers either adjustment to the sedimentation regime of the river or re- gional uplift. Regardless of cause, incision of the river into its fan has resulted in laterally con- strained flow between a series of terraces. As a consequence of this, the proportion of sheetflood deposits has diminished, while channel fills have become more prevalent. Within-trench reworking of deposits determines that only those deposits pushed beyond the former fan limits by longitudi- nal fan extension have a high likelihood of preservation. Side-channels which develop at the base of terrace risers typically infill with much coarser deposits than the sheetflood deposits into which they have incised (e.g. Fig. 7, exposure L). Such side-channel fills also have high preserva- tion potential. Conversely, high-magnitude depo- sitional events that produce inset features as they are funnelled between the lowest terrace se-
quences have limited preservation potential, as they are readily reworked. Infrequent debris flow events which overtop the lower terrace have a better chance of being preserved, however, as their deposits on upper terrace surfaces are be- yond the limits of channel reworking. Given the constrained nature of the channel during this phase, clasts transported in the channel are coarser down-fan than clasts transported during non-incised sheetflood phases. Debris flow, hy- perconcentrated flood flow, and channel fill de- posits are only likely to be preserved within the incised trench itself.
The limited preservation potential of deposits resulting from high-magnitude, slope-related events, is confirmed by analysis of the 1988 Ka- iapit Landslide, in an adjacent catchment to the Umi (Fig. 2; Dreschler et al., 1989). Deposits from this landslide infilled the river valley by up to 100 m at its proximal end, and resulted in sheetflood conglomerates at least 4 m thick ex- tending 10 km downstream from the landslide site on the Yafatz and Maniang Fans. Field ob- servations indicate that by 1990 the sheetflood conglomerate on the Yafatz and Maniang Fans had been incised, and formed a 2 m high terrace extending a few kilometres downstream. While sediment reworking continues within the braided channel, small terraces remain intact. It is quite likely that these various stages in fan develop- ment--build up, incision and reworking, and backfilling--may be occurring simultaneously on differing parts of Markham Valley fans.
Interpretation of lithosome hierarchy
Depositional elements described for the Umi Fan represent first- to fourth-order lithosomes in the hierarchical ordering scheme of Miall (1988) and DeCelles et al. (1991), as described in Table 6. Sheet-like elements are composed either of deposits from individual events, or stacked se- quences of deposits of similar style. These first-, second-, and third-order lithosomes represent mi- gration of gravel dunes and/or waning stage flood deposits, erosional or non-depositional surfaces during changes in flow stage, and lateral and/or down-fan migration of gravelly sheets or bars in
320 G.J. BRIERLEY ET AL,
channels, respectively. Channel fills, which result from accretion and burial of large macroforms in channels, represent fourth-order lithosomes. Fourth- and lower-order bounding surfaces and their enclosed lithosomes are controlled by intrin- sic factors during fan development, associated with geomorphic processes such as channel shift- ing and fan aggradation, incision and backfilling. Since deposition rates for the Leron Formation are roughly 6-10 m/ka , the suggested timeframe for accumulation of first- to fourth-order litho-
somes is of the order of a few years to a few hundred years.
The character and stacking arrangement of first- to fourth-order lithosomes as a fifth-order package in the upper Leron Formation (Figs. 12 and 13) is consistent with that shown for the Umi Fan. Amalgamated conglomerate units, measured up to 40 m thick in an alluvial channel, are equivalent to a fourth-order lithosome (Wanch Creek, section D" in Fig. 12). Fan trenches repre- sent a fifth-order lithosome, which fill with first-
North
Thrust km Y
Subduction MR UF Leron Fm \ 1. I I F - - - ~ Finisterre arc and
-...~ I ~ l ~ _ ~ , , ' ~ ' ~ , ' ~ lore- Pleistocene ® o , .,onoo o,,x
I i
Fig. 14. Schematic diagrams depicting the fifth-, sixth- and seventh-order lithosomes in Markham Valley alluvial fan sedimentary
sequences. The upper figure shows the tectonic setting of the Leron Formation and Umi Fan ( U F ) at the margin of the Australian
plate and the Finisterre Arc and pre-Pleistocene subduction complex ( M R = Markham River). The lower figure outlines the
tectonic evolution of the Leron Formation fans showing syn-depositionat tectonic movement and its influence on fan development.
S E D I M E N T O L O G Y OF COARSE-GRAINED ALLUVIAL FANS IN T H E MARKHAM VALLEY 321
to fourth-order lithosomes of varying composi- tion. Fifth-order bounding surfaces are induced by extrinsic factors such as climatic variation, tectonic movement or catastrophic events.
When long-term tectonic influences in the col- lision zone are added to this five-order lithosome hierarchy, sixth- and seventh-order lithosomes can be defined (Fig. 14). Each fifth-order lithosome in the Leron Formation is associated with step- wise tectonic tilting caused by fault-propagation folding. Inner parts of the fan are eventually detached and back-rotated by stepping out of the deformation front. If neotectonic movement data from the Lae area are consistent with the Pleis- tocene record in the upper Markham Valley, as suggested earlier, these 60-100 m thick fifth-order lithosome packages are deposited over a period of about 10,000 years. Each fifth-order bounding surface represents a n intraformational uncon- formity. Field mapping suggests that tilting is normally 10-15 ° . Downstream propagation of the alluvial fans may be temporally interrupted by tectonic tilting and folding, but subsequent up- stream build-up of sediments of the next fifth- order lithosome immediately following tilting will eventually offset the topography. The sixth-order lithosome is represented by individual alluvial fans which have been internally bound by fifth- order bounding surfaces. A thermoluminescence dating sample from a volcanic ash bed in the upper Leron Formation, attained from the Saw- tooth Ranges adjacent to the modern Rumu River, gave an age estimate of 117+ 18 ka (W1316). As such, this lithosome represents a period of about 100,000 years. The seventh-order lithosome is the alluvial fan complex itself within the entire molasse basin. As the fault-propa- gation folding evolved further, the entire alluvial fan system was finally elevated above the deposi- tional surface. This phase is probably contempo- raneous with initiation of new fans at the front of the foothills, as represented by the present Umi Fan.
Discussion and closing remarks
The conceptual framework used to generate the sediment inventory of both the Umi Fan and the fan part of the upper Leron Formation is very
similar to that described by DeCelles et al. (1991). Based on architectural-element analysis (Miall, 1985, 1988), sedimentary sequences have been analysed as an hierarchy of lithosome orders con- tained within differing orders of bounding sur- faces (Table 6). This framework serves to explain the operation of different depositional processes at differing scales. For example, deposits from debris flow and hyperconcentrated flood flow events are first-order lithosomes, stacked sheet- flood elements are second-order lithosomes, fa- cies assemblages associated with morphostrati- graphic units on bar surfaces are third-order lithosomes, channel-scale features are fourth- order lithosomes, and the trench fill, defined by a series of terraces, represents a fifth-order litho- some. Not all of these features can be recognized directly in the Leron Formation, but sediment sequences described for the fan part of this for- mation essentially represent a fifth-order package made up of an assemblage of first- to- fourth- order lithosomes. The latter are stacked and ar- ranged in a similar manner to element associa- tions described for the Umi Fan. The tectonically active post-collisional convergent setting is then used to characterize sixth- and seventh-order lithosomes at the scale of individual fans and the coalesced fan piedmonts, respectively.
Controls on fan sedimentology vary widely over a broad range of spatial scales. The first four orders of the lithosome hierarchy are controlled primarily by high-frequency, short-period intrinsic factors, whereas fifth-stage lithosomes and be- yond are caused primarily by relatively long-term tectonic movements (Table 6, Fig. 14). An addi- tional complicating factor affecting fifth-order lithosome character is climate change during the Quaternary (e.g. Sah and Srivastava, 1992). Such signals are superimposed on tectonic controls, and are difficult to isolate in coarse-grained trop- ical alluvial fans.
Much has been written on the presumed geo- morphic distinctiveness of tropical landscapes but, as clearly outlined by Kesel (1985), unless this can be related specifically to climatically induced pro- cesses, the chances of resulting depositional suites being categorically defineable as tropical in origin is remote. Sheetflood sequences dominate the
322 C i J B R 1 E R L E Y E T AL.
Umi Fan and the upper part of the I~ron Forma- tion; debris flow deposits are notable by their absence. Neither of these characteristics can be seen to be peculiar to tropical fans. The spectrum of sedimentologic diversity for fans in differing climatic regimes is not straightforward. Sediment sequences described in this study differ signifi- cantly from those summarized in Kochel and Johnson (1984) and recently reported by Darby et al. (1990). The sedimentologic differences may be explained, in part, by differences in climatic set- tings within the tropics themselves, as fans docu- mented in this study are located in a tropical- savanna climatic regime. In their study of Pleis- tocene humid-tropical coarse-grained alluvial fan sediments in Colombia, Darby et al. (1990) deter- mined that diagnostic features for recognition of such fans included: (a) lack of debris-flow de- posits, (b) very coarse-gravel sequences, (c) deep and rapid weathering of gravels, (d) occurrence of thick weathering profiles and/or palaeosols, and (e) abundant plant remains such as large logs and leaf mats in sandstone and mudstone beds. The last three features are totally lacking in the Umi Fan and in the upper part of the Leron Forma- tion. Markham Valley fans are certainly com- posed of coarse gravels, however, and their depo- sitional suite is dominated by coarse-grained sheetflood sequences.
One unusual feature of fans studied in the Leron Formation is the abundance of carbonate nodules or concretions (facies C). These are espe- cially abundant in association with facies Fm units in distal-fan sequences where they vary from 5 to 20 cm in thickness (Liu and Crook, 1992). Such unusually abundant carbonate concretions have not been reported elsewhere for tropical alluvial fan sediments. These features may be better as- cribed to tropical-savanna fans. This climatic as- sociation is supported by the contemporary cli- mate regime in the Markham Valley, and it is quite possible that similar conditions prevailed in the Late Pleistocene when the upper part of the Leron Formation was deposited. The presence of abundant carbonate concretions can be attributed to (1) the dominant marine strata provenance, and (2) the seasonal climatic variations, with high evaporation rates during the dry season and an-
nual water deficiency. Such alternating wet and dry conditions are conducive to the formation of carbonate concretions. Bioturbation during peri- ods of non-deposition (dry seasons) may also pro- mote the formation of nodules by mixing the calcareous mud with underlying sediments.
The Umi Fan and the upper part of the Pleis- tocene Leron Formation represent alluvial fan deposition in a post-collision and suturing conver- gent margin. In comparison with the other eleven tectonic settings outlined for alluvial fan deposi- tion by Miall (1981), such detailed site descrip- tions for fan sedimentation are rare. The closest analogues are probably the alluvial sediments of the Alpine molasse (Van Houten, 1974, 1981) and the Indo-Gangetic Trough (Datta and Shastri, 1977). In both cases the alluvial fans were studied only at the broad scale in the context of regional tectonic history. The fifth- to seventh-order litho- somes outlined in this paper for Markham Valley fans are strongly influenced by collisional tectonic activity during fan growth. Comparisons are war- ranted with tropical fans in extensional and strike-slip basins, in order to establish the ele- ment patterns that characterize these different tectonic settings.
In addition to these observations on tropical fan development, this study has also outlined some methodologic issues concerning sediment inventory and its interpretation. Depositional en- vironments inferred solely from conventional two-dimensional vertical profiles can sometimes be misinterpreted when studying laterally vari- able alluvial sedimentary sequences. This is clearly shown in Fig. 12. If sections D, D' and D" had been studied solely as vertical profiles, they may well have been interpreted as the result of three different depositional regimes. It is now evident from the three-dimensional analysis that these sediment sequences actually represent differing phases and styles of alluviation within the same depositional system. In fact, part of sections D and F (Fig. 11) were interpreted as deep-marine non-fan turbidites on the basis of vertical profiles and sparse reworked marine fossils (Crook, 1989a, figs. 25, 26 and 32). These have now been reinter- preted as distal-fan deposits using three-dimen- sional architectural-element analysis.
SEDIMENTOLOGY OF COARSE-GRAINED ALLUVIAL FANS IN THE MARKHAM VALLEY 323
In practice, the generally poor quality of out- crops of coarse-grained alluvial fans dictates that three-dimensional approaches cannot yet replace conventional two-dimensional vertical-profile methods. However, a combination of the two procedures can be both useful and enlightening, and in this case it proved possible to gain a three-dimensional elemental perspective using closely spaced, stratigraphically well-correlated vertical-profiles (Fig. 13). Equivalent examples from other three-dimensional element-based studies will likely simplify evolutionary interpreta- tions in some circumstances, but will more likely result in better refined, more complex interpreta- tions in other situations.
Acknowledgements
Funding for this project was provided through the Research School of Pacific Studies at the Australian National University and an ANU Scholarship awarded to Keyu Liu. Colin Camp- bell provided admirable field assistance and was in charge of the Hasselblad. On-site field assis- tance was provided by Burar and Jonah from Yanuf Village, while accommodation and hospi- tality were provided by Karl Samasan and his family at Leron Plains station. We are grateful to the Morobe Provincial Research Committee, Benson Nablu, Veronica Tolube and Benson Suwang for providing access to field sites. We thank Nigel Duffey in the Cartographic Depart- ment in the Research School of Pacific Studies for his many hours of labour in preparation of the figures, and Keith Fitchett in the Department of Biogeography and Geomorphology for his draft- ing of Fig. 3. Final production of figures was completed in the Drafting Office at the School of Earth Sciences, Macquarie University. Referee comments by Dennis Darby clarified many points in the text.
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