Introduction - the Markham river system supplying sediment to the Huon Gulf, Papua New Guinea, offers a very special opportunity to examine active high energy sediment transport processes from mountain source areas all the way to distal deposition in a deep sea trench. There is an ideal combination of geologic and geomorphologic factors and conditions. Large volumes of fluvial sediment are supplied from high rainfall, landslide – prone high mountains, and carried through a highly energetic flood plain to the coastline. Sediment continues to move down steep underwater slopes, through a sea floor canyon to a trench in more than 6000 m water depths. The entire system is affected by long term tectonically induced changes in relief and slope gradients, with mountain uplift contrasting with sea floor subsidence across a major plate boundary. There are frequent earthquakes, which trigger landslides, both in the mountains and on the sea floor. Very heavy rainstorms cause floods and debris torrents in the mountain catchments. Combinations of landslide damming of rivers, floods and dam breaching bring huge volumes of sediment to fan deltas on the coast.

Tectonic setting - the Markham drainage system, the Huon peninsula, and inner portion of Huon Gulf lie within an active seismic zone, between the northerly - moving India-Australian macro plate, and the westerly moving Pacific plate. The India-Australia plate moves north northeastwards at 3.3 cm/year and the Pacific plate moves west southwest at 10 cm/year. Sandwiched between these two major plates are the smaller Solomon and Bismarck micro plates giving a complex pattern of movement, (Ripper and Letz, 1991). The interaction between the plates affects regional uplift and subsidence patterns, and is the source of earthquake activity. Huon Gulf is believed to be the location of a major thrust front or transcurrent, left-lateral shear, representing the offshore extension of the Markham / Ramu lineament, (von der Borch, 1972), also referred to by Davies et al., (1987) as the Ramu / Markham fault. Across the fault there is uplift to the north, and subsidence to the south. The Markham flood plain follows the line of the Markham / Ramu fault.

On the northern side of the Huon Gulf the 4000 m relief Finisterre mountains, show uplift rates on the order of 3.5 mm/year, identified from dating of raised marine terraces. For the Lae area at the mouth of the Markham river, Crook, (1989), estimates uplift rates between 2 – 7.6 mm/year for the last 20 -30,000 years, based on inferred water depths for particular marine facies.. For eastern Lae , Crook and Liu, (1993), report uplift and sedimentation rates of 6 mm/year, and believe that for the central business district rates of 8 mm/year are likely. Geodetic measurements across the Markham valley indicate significant crustal shortening during the Finisterre earthquake series in 1993, and no doubt an accompanying uplift component.

Abers and Mc Caffrey, (1994), and Galewsky et al., (1996) consider the Markham valley and its offshore extension into Huon Gulf to be the foredeep associated with the Finisterres. Galewsky et al., (1996) also report isotopic measurements from drowned sea floor coral on the southern side of the Gulf indicating subsidence of 2000 m within the last 348 thousand years at an average rate of 5.7 mm/year. The combined uplift / subsidence of 12 - 13 mm/year across the faulted axis of the Gulf represents an important long term influence on sea floor sediment stability, through long term slope tilting in a generally north to south direction.

The tectonic setting of the Huon peninsula and Gulf also means that large earthquakes are common. While no major earthquakes have been centered on Lae since 1900, the surrounding region has experienced magnitude 7 earthquakes, such as the most recent 1993 event which caused widespread landsliding in the Finisterre mountains, (Tutton and Browne, 1994). The expected return period of magnitude 7 earthquakes within a radius of 50 km radius of Lae is 50 years, and the return periods of maximum ground accelerations of 0.1g, 0.2g, and 0.5g are 3.3, 20, and 232 years respectively, (Ripper and Anton, 1995). Mountain slopes and sea floor sediments and underwater slopes are thus subjected to significant, medium – term, earthquake - induced stresses.

Sediment supply - very large volumes of sediment move through the Markham drainage system, and the Huon Gulf receives very large water and sediment discharges from the Markham, and the neighboring Butibum, and Busu rivers. River mouth sediments are mainly sandy gravel, gravelly sand, and boulders up to 50 cm diameter, (Deacon, 1993; Chaimanee, 1998)). The Markham river drains an area of about 13000 sq. km with high rainfall and high sediment supply rates, sometimes related to catastrophic mountain landslides. One landslide in the Markham catchment at Kaiapit mobilized 1.8 cubic km of rock with sufficient velocities and energy for air fluidization, (Peart, 1991). Subsequently the landslide debris has become an additional sediment supply to the Markham river.

It is estimated that since 1988 more than 3 cubic km of landslide debris has been moved by landsliding in the catchment, (Tutton and Kuna, 1995), and geophysical surveys suggest that the Markham valley is floored by more than 1000 m of unconsolidated Quaternary sediments. Chaimanee, (1998), identifies major landslide- derived terraces along the flanks of the Markham floodplain, which supply coarse gravel and cobbles to the river. Nedeco- Haskoning and Maunsell, (1980) estimate that about 150 – 160 tons of sediment per square km is moved annually by the Markham river with an average annual bedload of about 2 million tons. Not surprisingly, the river has high bedload, and high water flow velocities at its mouth. Von der Borch (1972), reports the following…"sand, gravel, and pebbles are being transported seaward by a river current of the order of 3 - 4 knots, and are being moved directly into the submarine canyon." Another estimate of river velocity at the mouth is 1.5 m per second, sufficient to cause standing waves 0.5 m high, (Nedeco-Haskoning and Maunsell,1980). At periods of low sediment discharge large sand bars are temporarily deposited at the Markham river mouth, but are swept away at maximum flow periods.

Similarly, the neighboring Butibum and Busu rivers deliver very large sediment loads of sand gravel and boulders to the Huon Gulf, especially when rainstorms generate floods supercharged with landslide debris. Landslide dam bursts are particularly high energy sediment supply events. The Busu river provides an example of this process. A landslide dam formed in 1993 as a result of slope failure accompanying a major earthquake, burst in 1996 sending catastrophic floods and debris torrents down the river valley to the sea. Since October 1993 four major highways have been destroyed in the Lae coastal region by dam bursts and peak river discharges have been estimated at 25000 cubic m/second.

The rivers entering the north side of Huon Gulf have constructed large fan deltas, (Crook and Liu, (1993); Liu et al., (1995). Fan delta construction is believed to have been occurring along the northern coast of Huon Gulf for at least 40,000 years, with both long and short term changes in the locations of the active river mouths. The fan delta deposits overlie the Pleistocene Leron formation, comprising well – laminated mudstones, siltstones, pebbly sandstone and sandy conglomerates, (Crook and Liu, 1993).

The Busu river subaerial fan covers about 60 square km, with an average gradient of 0.46 degrees, (Liu, et al., 1995). The subaerial fan sediments include the remnants of large debris flows, alternating with channels and there are abundant coarse gravels and boulders, up to 20 - 30 cm in diameter. The river mouth channel bar is composed of cobbles (up to 10 cm diameter), abundant pebbles, ( 2 – 4 cm), and coarse sand, (Chaimanee, 1998). The Butibum fan is also composed of very coarse sediments, but large boulders are more rare. Grab samples from surface sediments on the floor of the Gulf, to a maximum water depth of about 250 m clearly reflect the generally high energy supply regimes, and high energy sea floor transport processes.

Overall there is a fairly widespread distribution of silts and clays, especially on the Markham delta front. Clays and silts are introduced by the rivers as suspended load and distributed widely offshore as surface and near surface suspension plumes. These plumes on satellite imagery, show the dominant plumes are from the Markham river. Two different plume patterns are recognized from repeated Landsat TM 321satellite imagery. There are often small plumes extending 2 – 3 km offshore, on both sides of the river mouth, generally close to the Gulf shores. These coastal plumes apparently are due to the effects of currents and waves, forcing the plumes in locally specific directions. Also there can be a single major plume, which extends up to 12 km from the river mouth in a general southeastwards direction, tapering from about 2 km wide close to the shoreline to about 1 km wide over much of its length. The clays and silts on the sea floor off the Markham river appear to be the expected delta front progradational units deposited from suspension. However, immediately, southeast of the river mouth there is coarse sediment, (gravels, sands and fine sands) on the sea floor, occupying a zone about 500 m wide, to maximum observed water depths of 300 – 350 m.

Sea floor processes - various studies have suggested that submarine landslide and turbidity current processes are active in the Huon Gulf and neighboring area. For example, movement of large amounts of sediment from the mouth of the Markham river at the head of Huon Gulf, apparently causes turbidity currents which damage SEACOM submarine cables crossing the New Britain Trench in water depths greater than 6000 m. Krause et al., (1970) suggested that average velocities for these turbidity currents could be between 50 – 30 km/hour. It was speculated that the turbidity currents were due to slope failure of Markham river mouth sediment and subsequent flows down the deeply incised submarine Markham canyon. Describing the head of the Markham canyon, von der Borch (1972) suggested that repeated failure of mud deposits from the Markham river caused highly irregular bottom topography, seen on echo-sounder records near Lae harbor. The Huon Gulf east of Lae also contains examples of very large, open-sea fan delta systems, transporting coarse-grained sediments to water depths of 1400 m distances of 13 km from the river mouths (Liu et al., 1995). Imagery from the Hawaii MR1 side scan sonar system provides perspectives on the fan architecture for the Buso, Buhem and Mongi fan systems and for a portion of the Markham canyon. There are also eye witness accounts of coastal and offshore slope failure, "at the end of August about 5 acres of the shore simply slid into the sea carrying with it the wharf, two or three sheds, a large steam crane and about 100 yards of railway," (Pacific Monthly, 1932). Everingham, (1973) identifies submarine landslide events in the coastal area near the city of Lae in 1932, 1969, 1971, and in August 1972 there were tsunami waves with 2.4 meter amplitudes lasting several minutes associated with submarine slides.

There are several factors in the general characteristics and regional setting of Huon Gulf that, individually and in combination, are conducive to episodic movements of large volumes of sediment away from the river mouth coastal sources towards deep water.

• A dominant characteristic of the Huon Gulf and neighboring New Britian Trench region is the presence of deep water relatively close to the coast, high local submarine relief, and associated steep slope gradients. Regionally the head of the Trench, at a depth of 6000 m, lies approximately 140 km southeast from Lae, while the 2000 m isobath is only 30 km away. The Gulf is the source area for three major canyons, the Markham, Francisco and Mongi systems, (Davies et al., 1987). The larger Markham canyon was identified, as a sinuous feature east to the New Britain Trench. Its overall thalweg gradient is about 1.5 degrees, but in the inner Gulf the canyon floor reaches a water depth of 700 m only 8 km from the river mouth – an average slope of 5 degrees.

• In the inner Gulf, immediately seawards of the Markham river mouth and its delta there is a sea floor gradient of 8 degrees, between 30 – 300 m water depths. The sea floor slope immediately south of the port of Lae reaches 200m water depth over a distance of 750 m - a gradient of 14.9 degrees. Similarly, off the mouth of the Busu river, east of Lae, the bottom slopes achieve a maximum gradient of 14 degrees between 50 m and 300 m water depths, eventually reaching 550 m over a distance of 2400 m – a slope of 11.8 degrees. Locally there are even steeper gradients. For example, along the western side of the Markham canyon there are slopes in excess of 45 degrees, and in some places the incised canyon walls appear almost vertical on bathymetric profiles.

Where the Markham river discharges into the inner Huon Gulf from a broad, braided flood plain there is a very dramatic, and distinct sea floor channel which plunges down the submarine delta front. This is the head of the Markham canyon and the sea floor feature is a direct continuation of the subaerial system, with a water depth of 15 m in the channel, only 150 m from the shoreline. The river mouth lacks a pronounced subaqueous mouth bar, with very straight bathymetric contours off the river mouth between 15 - 150 m water depths. The sea floor gradient between 15 and 100 m water depths within the channel is 13 degrees, declining to 8 degrees between 100 – 200 m. The channel on the sea floor, is clearly the headward end of the Markham canyon. Von der Borch,(1972), observed that the Markham river drains directly into the head of the canyon, "intercepting most of the bedload, as well as a proportion of the flocculated sediment." The Markham canyon is known to experience turbidity currents, and the direct connection between the river and the submarine channel suggests that the Markham river is a source of bottom –following, high energy flows. The channel morphology and sediment distribution suggest that the Markham river has sufficient current velocity, bedload, and possibly suspended sediment load for high density sediment water mixtures to underflow the sea water at the river mouth, and continue energetically downslope into deep water. These bottom – following hyperpycnal flows, (exceeding the density of sea water), are in contrast to the near – surface suspended sediment (hypopycnal) plumes which distribute sediment across the inner gulf, driven by winds, waves and tidal currents.

The apparently high frequency sediment transport events, which occur in the Huon Gulf, offer opportunities for monitoring experiments. Direct measurements of active high energy sediment transport processes such as turbidity currents, submarine slope failure, and gully erosion, remain generally elusive. However, the Markham river and its offshore channel / canyon could be instrumented for measurement of turbidity flow frequency and velocity, following the methods used by Prior et al., (1987), and Ren et al., (1996), expanded for example to acquire data on sediment suspension and transport rates. Similarly, expanded measurements of sediment geotechnical properties, from offshore borings, should be complemented by sophisticated stability analyses. In situ measurements of pore water pressure, potentially artesian flows on the delta front, and bottom sediment movement are possible in such a dynamic environment. An array of sea floor seismic recorders would determine frequency, magnitude and distribution of earthquake stresses. Empirical data for all these parameters would greatly expand the understanding of the Markham river / Huon Gulf system, and would contribute substantially to the general understanding of high energy sediment transport processes.

The recognition of the various components of the Markham / Huon Gulf / New Britain sediment transport system will also be helpful in the interpretation of sea floor processes and bottom features elsewhere. For example, some of the apparent process interactions will be relevant to sediment transport and deposition during periods of lower relative sea level, both on active and passive margins. Low stand, high rates of sediment delivery to high relief shelf margins and steep upper continental slopes are analogous to the Markham / Huon Gulf situation.