paper the dynamics

Journal of Sedimentary Research, 2013, v. 83, 503–521

Research Article

DOI: 10.2110/jsr.2013.42

SEDIMENT DYNAMICS AND DEPOSITIONAL SYSTEMS OF THE MAHAKAM DELTA, INDONESIA:ONGOING DELTA ABANDONMENT ON A TIDE-DOMINATED COAST

SALAHUDDIN1AND JOSEPH J. LAMBIASE2

1Department of Geological Engineering, Gadjah Mada University, Yogyakarta 55281, Indonesia2Petroleum Geoscience Program, Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

e-mail: [email protected]

ABSTRACT: A quantitative analysis of the depositional processes on the Mahakam Delta indicates that it is presently subsidingand is, in essence, a drowned delta that is being transgressed and modified by marine processes. Calculations of sediment transportrate indicate that most, if not all, fluvially derived sand is being stored onshore in the distributaries, whilst finer-grained sedimentmoves offshore. A fining-upward and increasingly marine-upward succession is being deposited in the distributaries, which isanalogous to nearby outcropping and subsurface successions that have previously been interpreted as progradational.

The mixed fluvial and tide-dominant shoreline morphology is not solely a product of the deltas present-day processes. Thefluvial component is a relict feature from a phase of progradation that preceded the ongoing transgression and is now beingmodified by tidal processes. Facies distribution is a much better indicator of modern depositional processes than deltamorphology on the Mahakam Delta, suggesting that facies-based delta classifications are more accurate than morphology-based classifications. All the apparently anomalous components of the sedimentology and morphology are reconciled by atransgressive interpretation, including the overly deep distributaries, gently dipping subaqueous delta plain, penetration bybenthic marine organisms far into the distributaries, widespread Nypa palm on the lower delta plain, and the long, mud-filledgap between sand in the distributaries and on offshore bars.

INTRODUCTION

The Mahakam Delta has long been viewed as one of the world’s bestexamples of a mixed river-dominated and tide-dominated delta based on itsmorphology (Galloway 1975). Located in Indonesia on the east coast ofBorneo (Fig. 1), the delta has been the subject of several sedimentologicalstudies that interpreted the channel morphology and sedimentary facies onthe delta as the product of the interplay of fluvial and tidal processes andtheir balance as important in the channel abandonment process (Allen et al.1976). Gastaldo et al. (1995) suggested that the relative influence of fluvialand tidal processes varies geographically by attributing the distribution ofsand to fluvial processes and sand–mud couplets to tidal influence.

Channel morphology also played a key role in previous interpretations;straight channels connected to the Mahakam River were identified asdistributaries and interpreted as dominated by fluvial processes, sinuous andflaring channels were presumed to be tide-dominated and those with straightupper reaches and flared lower reaches were interpreted to be distributariesin the process of abandonment (Allen and Chambers 1998). Based on thosecriteria, Allen and Chambers (1998) interpreted the eastern area of the deltaas abandoned because it has sinuous, flared channels where the bottomsediments are mainly silty mud to silty sand. They also speculated that thesouthern area is more fluvially active because channels are straighter, sandoccurs farther seaward, and there are several sand bars in the distal channelreaches; the northern area was assumed to be in the process of abandonmentbecause it has mostly straight channels with flared mouths (Fig. 1).

It is important to recognize that all the previous interpretations werebased solely on qualitative analysis of delta and channel morphology plusthe sediment distribution in a few selected areas; there was no previous

systematic mapping of the sediment distribution. There was no attempt toquantify sediment dynamics, the present-day depositional systems, or theinterplay between fluvial and tidal processes and their role in channelabandonment. A pervasive underlying assumption is that the delta ispresently prograding (Allen et al. 1976), although it has been speculatedrecently that the present-day fluvial sediment discharge is much smallerthan in the past (Storms et al. 2005).

The present study investigated the sediment dynamics and depositionalsystems of the modern Mahakam Delta with quantitative data. A high-resolution bathymetric survey was carried out to map delta morphologyin detail in the distributaries, in the estuaries, and on the subaqueous deltaplain. A comprehensive sedimentary facies map was generated frombottom sediment samples that were systematically collected with a grabsampler and shallow cores. Vertical current velocity, salinity and turbidityprofiles, plus water depth, were recorded in the thalwegs of the variouschannels. The hydrodynamic data were used to identify water-massmovement and calculate sediment transport rates and directions acrossthe delta. These were integrated with the data on sediment distributionand morphology to determine sediment dynamics and depositionalsystems, investigate the process of channel abandonment, and interpretthe recent depositional history of the delta.

PHYSIOGRAPHY

Morphologic Elements

The modern Mahakam Delta developed after the Holocene transgres-sion about 5000 years ago and has deposited a 50-m-thick succession that

Published Online: June 2013

Copyright E 2013, SEPM (Society for Sedimentary Geology) 1527-1404/13/083-503/$03.00

is generally aggradational, though punctuated by rapid progradationsand transgressions related to high-frequency eustatic variations (Caratiniand Tissot 1988; Mora et al. 2001). The 2800 km2 delta plain has a lobate,fan-shaped morphology with an extremely gentle slope of about 0.06 m/km and is dissected by numerous channels; about 60% is subaerial and40% subaqueous (Fig. 2). The upper delta plain (i.e., the area without anymarine influence) is relatively small, inasmuch as a large portion of thesubaerial delta plain is covered by Nypa fruticans and mangrove,indicating marine influence. The gentle slope persists offshore acrossthe subaqueous delta plain, which extends from the shoreline to 5 m waterdepth and has several subtidal channels that are offshore extensions ofdistributaries (Fig. 2). An abrupt break in slope at 5 m marks the top ofthe delta front, and a thin prodelta sheet extends seaward from the base ofthe delta front, which is in 25 m of water, to a depth of about 50 m(Roberts and Sydow 2003).

Channel Geometry

A high-resolution bathymetric map of the channels and subaqueousdelta plain was compiled by integrating the existing offshore bathymetricmaps of Jawatan Hidro-Oseanografi (1982), Division of Hydro-Ocean-ography (1989, 1990), Bakosurtanal (1998), and TotalFinaElf E&PIndonesie (2002) with a new bathymetric survey in all the onshorechannels comprising 381 transverse profiles at a line spacing of about1 km. The complete set of profiles is presented in Husein (2008).

It has been long recognized that two types of channels can bedistinguished based on their morphology (e.g., Allen and Chambers1998). Distributaries (i.e., seaward-branching pattern) are connected tothe Mahakam River at the delta apex and are relatively straight with lowsinuosity, although some have distinctly flared mouths (Fig. 2). Theirwidth tends to be relatively constant and ranges from 0.5 to 1 km, whilst

FIG. 1.—The shoreline morphology of theMahakam Delta and its location on the eastcoast of Borneo. Hydrodynamic measurementstations are shown as circled numbers, shallowcore locations as small filled squares, andlocations of bottom sediment samples as smallfilled circles.

504 SALAHUDDIN AND J.J. LAMBIASE J S R

depth varies from 5 to 20 m. Distributaries have asymmetric transverseprofiles, and the thalweg meanders occasionally between banks, creatingpoorly defined lateral bars (Fig. 3A). The lower reaches and the mouthsof distributaries generally are shallower, and elongate bars are oftenpresent that generally range from 2 to 5 km in length and from 0.3 to 1 kmin width. The offshore extensions of distributaries generally are 7–10 mdeep, gradually shallow across the subaqueous delta plain to 2–5 m, andterminate at the slope break (Fig. 2).

Estuaries are sinuous and flared channels that form a contributive (i.e.,seaward-converging) pattern. They are very sinuous in their upper reacheswith symmetrical cross sections, becoming less sinuous and moreasymmetric in their lower reaches with flared mouths (Fig. 3B). Estuariesare wider than distributaries but have similar water depths; they are notconnected to the Mahakam River, although they connect to distributariesthrough narrow and sinuous tidal channels at their onshore ends. Theiroffshore extensions shallow rapidly seaward and die out well before theslope break (Fig. 2). The southern part of the subaerial delta plain isdominated by distributaries, whilst the central area has only estuaries andno distributaries and the northern area has an equal number of each.

Sediment Distribution

Previous studies of the sedimentary facies were limited to thedistributaries of the southern area (Allen et al. 1976; Gastaldo et al.1995; Allen and Chambers 1998) and its intertidal bars (Allen andMercier 1994) plus subaerial delta-plain deposits (Roberts and Sydow2003; Storms et al. 2005). The present study focused on the distribution ofsedimentary facies in the channels and subaqueous delta plain, where atotal of 398 bottom samples were collected with a grab sampler thatpenetrated 0.05–0.10 m. The vertical sedimentary succession on intertidalbars was determined by trenching and collecting five shallow cores up to1 m in length.

Generally, the distributary floors are covered by sand that gradually finesseaward but does not extend to the channel mouths, whilst the estuaries andsubaqueous delta plain are dominated by mud (Fig. 4). In distributaries, theseaward limit of sand approximately matches the seaward limit of the mixedfresh-water hardwood and palm forest on the subaerial delta plain; seaward,sand is gradually replaced by mud and Nypa palms become the dominantvegetation on the adjacent delta plain (Figs. 2, 4).

The sedimentary facies range from medium sand to mud, with themedium sand facies confined to the upper reaches of distributaries(Fig. 4); thin mud drapes, mud clasts, and plant debris are abundant inthis facies, which has a median grain size of 0.25 mm and contains 10–20% mud. Muddy fine sand occurs in the distributaries immediatelyseaward of the medium sand facies, where it occupies the middle reachesof distributaries in the south and floors the upper to middle reaches ofdistributaries in the north (Fig. 4). There are a few mud clasts andabundant mud lenses in this facies, which has a median grain size of0.212 mm and contains 20–40% mud.

Muddy fine sand also occurs on intertidal flats and bars on thesubaqueous delta plain, in the lower to distal reaches of distributaries onelongated intertidal sand bars that are both perpendicular and parallel tothe shoreline and have dunes, ripples, and plane beds on their surfaces,and as isolated and attached sand bars perpendicular to the shoreline(Fig. 4). However, in these areas the sand is generally finer (median grainsize 0.125 mm) and muddier than in the middle reaches of distributaries;mud lenses and mud clasts are abundant. It also is highly bioturbatedand contains abundant shell fragments, indicating significant marineinfluence.

Sandy mud occurs in intertidal areas near channel mouths, and in thelower reaches of distributaries as a transitional facies between muddy finesand and mud, but it is rarely present in the middle reaches ofdistributaries (Fig. 4). The sand fraction constitutes 10–40% of each

sample and is very fine, with a median grain size of 0.073 mm. The sandymud facies is intensely bioturbated, mostly by vertical to subverticalburrows of the Skolithos and Psilonichnus ichnogenera, and occasionallyincludes abundant shell fragments.

Mud with less than 10% sand is the dominant facies in estuaries, thedistal reaches of distributaries, on intertidal flats and bars in the northernand central areas, and on the subaqueous delta plain (Fig. 4). The mud isgenerally soft and dark brown with common shell fragments and is highlybioturbated, mainly by organisms of the Skolithos, Psilonichnus, andArenicolites ichnogenera. Detrital organic debris is widely distributedalong the high-tide shoreline and forms peat beaches, especially in thesouth (Fig. 4).

Benthic Organisms

Benthic marine organisms, including ostracods, pelecypods, andechinoderms, were recovered in the grab samples up to 20 km landwardfrom the coastline (Fig. 4). The onshore limit of benthic marineorganisms corresponds to a transition from muddy fine sand tobioturbated muddy fine sand in some distributaries. Burrowing organ-isms that generate trace fossils of the Skolithos and Cruziana ichnofaciesoccur virtually everywhere seaward of the onshore limit of benthicorganisms.

Polychaete worms are the most abundant group of burrowingorganisms, especially in muddy subtidal environments within estuariesand the lower reaches of distributaries. Onuphid are the most commontype, along with Capitellid polychaetes. Burrowing shrimp are commonon the muddy intertidal to subtidal flats and Spionid worms occur in thelower reaches of distributaries. Suspension feeders (Onuphid polychaetes,Spionid worms, and burrowing shrimp) prefer the high-turbidityenvironments such as distributary or estuary mouths, while depositfeeders (Capitellid polychaetes) favor nutrient-rich substrates in quiet andoxygen-depleted environments such as estuaries.

HYDRODYNAMICS

Fluvial Discharge and Sediment Supply

The Mahakam River drainage basin has an area of 77,400 km2 (Dahuri1992). Based on available rainfall data and the size of the drainage basin,Allen et al. (1976) estimated mean river discharge to be 1000–3000 m3/s. Arecalculation using a runoff coefficient of 21% for dense tropical rainforest with undulating terrain (Dierks 1992) yielded a mean discharge ofabout 2300 m3/s. However, flow velocities measured by Dutrieux (1991)indicate a mean discharge of about 1500 m3/s, and hydrodynamicmeasurements for the present study suggest a mean river discharge ofabout 1750 m3/s.

There are large seasonal variations in river discharge, with peak flow inthe months of April to May (SE monsoon) and December to January(NW monsoon) when the inland rainfall is high. However, variations influvial discharge have virtually no impact on the present study, becausefloods apparently never occur on the delta, even at peak fluvial discharge,because the large Kutai Lake system and surrounding lowland landwardof the delta absorb any excess flow (Allen and Chambers 1998).

Waves

Wave energy is low due to limited fetch in the Makassar Strait (Fig. 1).Fourteen kilometers offshore, average wave height is 0.3 m with a periodof 6 seconds and maximum wave height is 0.6 m, with the largest wavesapproaching from the southeast (Total 1986). Although littoral drift isminimal (Allen and Chambers 1998), spit growth north of the deltasuggests longshore transport to the south. The delta rarely experiencesstorms because of its equatorial location (Roberts and Sydow 2003).

DEPOSITIONAL SYSTEMS OF THE MAHAKAM DELTA: ONGOING DELTA ABANDONMENT 505J S R

Tides

Tides in Makassar Strait are semidiurnal with a marked diurnalinequality and a pronounced fortnightly neap–spring cycle. Tidal rangevaries from less than 0.5 m during neap tides to 2.5 m during springtides. Tides affect almost the entire delta plain; the mean daily tidal

range of 1.2 m is enough to inundate the delta plain up to 20 km inlandfrom the coastline, and they normally influence the lower 140 km of theMahakam River, with tidal fluctuations observed as far as 360 kmupriver during extremely dry periods (Schuettrumpf 1986). The tidalprism on the Mahakam Delta was calculated from the cross-sectionalarea at the mouth of each inlet, the duration of flood tidal flow, and

r

FIG. 2.—Physiographic elements of the Mahakam Delta, with isobaths in m and the locations of Figures 3, 13, and 15. Circled numbers refer to the river names asfollows: 1, Badak River; 2, Berau River; 3, Kaeli River; 4, Ilu River; 5, Pantuan River; 6, Tambora River; 7, Bayor River; 8, Pamakaran River; 9, Terusan PamakaranRiver; 10, Buyit River; 11, Bekapai River; 12, Ulu River; 13, Mati River; 14, Jawa River; 15, Dondang River.

FIG. 3.—A) Bathymetric map and cross sec-tion of a typical distributary, and B) bathymetricmap and cross section of a typical estuary, withthe cross-section locations indicated by a solidblack line on the corresponding bathymetricmap. The bathymetric map locations are shownin Figure 2.


current speed measurements for complete flood tidal phases made forthe present study. The tidal prism is 190.2 3 106 m3, which is equivalentto a mean tidal discharge of 8453 m3/s or nearly five times the meanfluvial discharge.

Salinity

Measurements of salinity, turbidity, and current were taken at 22stations in the distributaries and estuaries (Fig. 5). Vertical profiles wererecorded at 0.5 m water depth intervals every 30 minutes for completespring and neap tidal cycles at each station. The complete hydrodynamicdataset is presented in Husein (2008).

Salinity generally increases seaward, although saline water movesdynamically in the distributaries and estuaries mostly during spring tides(Fig. 5). At high tide, brackish water with 10% salinity intrudes up to30 km landward from the coastline in the north and up to 10 kmlandward in the south, whilst the 30% isohaline reaches up to 15 kmlandward in the north but lies near the distributary mouths in the south(Fig. 5A). During peak ebb current flow, the 10% isohaline is pushedslightly seaward and the 30% isohaline is moved offshore of thedistributary mouths. By low tide, the isohalines are moved slightlyfarther seaward (Fig. 5B); during peak flood current flow, they arealready close to their positions during the previous high tide. Estuarieswith negligible fluvial discharge have stable salinities throughout the tidalcycle (Fig. 5). The isohalines are less dynamic near neap tide. Saline waterintrudes the distributaries, but the isohalines move , 10 km during atidal cycle, whilst estuaries have stable salinities throughout the tidalcycle.

Turbidity

Turbidity is generally high in the upper and lower reaches ofdistributaries and low in the middle reaches (Fig. 6). High turbidity isassociated with strong seaward movement of a low-salinity water mass

during ebb tide in the upper reaches, whilst strong landward movement ofa high-salinity water mass in the lower reaches during flood tide producesa second period of maximum turbidity. During spring tides, bottomturbidity exceeds 100 NTU (NTU 5 nephelometric turbidity units, ameasure of light scattering by particles suspended in a fluid; NTUincreases as turbidity increases), and can exceed 200 NTU in manydistributaries (Fig. 6A), with the turbid water reaching the coastlineduring peak ebb flow. Zones of high turbidity occur in the distributariesat neap tide, but they do not reach the coastline and are restricted to thelower reaches (Fig. 6B). Conversely, estuaries are marked by generallylower turbidity; high-turbidity zones are restricted to their upper reachesduring maximum flood flow (Fig. 6A).

Current Velocity

Current velocities are highly variable during a tidal cycle; maximumvalues occur approximately midway between high and low tide at eachstation (Fig. 7). There also is significant variation in maximum velocityfrom neap to spring tide at the same station. Maximum seaward (ebb)velocities exceed maximum landward (flood) velocities in distributaries,although the difference is less pronounced at spring tide, especially at themore seaward stations (Figs. 1, 7, Table 1), whilst maximum flood andebb current velocities tend to be more equitable in estuaries (Fig. 1,Table 1). Maximum ebb velocities also generally decrease seaward asrelative tidal influence increases and fluvial discharge is divided intoincreasingly more distributaries (Fig. 1, Table 1).

Water-Mass Movement

Tidal processes dominate the hydrodynamics at spring tide; ebbcurrents move seaward a high-salinity and low-turbidity water mass thatwas emplaced by the previous flood tide (Figs. 1, 7, Table 1). Salinitydecreases as ebb current speeds increase, whilst turbidity increases andreaches a maximum at peak ebb discharge and then decreases again near

FIG. 4.—Sedimentary facies and physiographic subdivisions of the Mahakam Delta, plus a depositional model derived from calculations of quantitative sedimenttransport rate (see the text for explanations). The landward limit of marine organisms and marginal marine vegetation marks the boundary between the upper and lowerdelta plains.


FIG. 5.—Bottom salinity distribution at A) spring high tide and B) spring lowtide. Contours are parts per thousand.

FIG. 6.—Bottom turbidity on the Mahakam Delta during maximum ebb flow.A) At spring tide, and B) at neap tide in nephelometric turbidity units (NTU).


low tide because of settling. Flood currents begin after the subsequent lowtide and move the now low-salinity and low-turbidity water masslandward. Bottom turbidity increases as bottom flood current speedsincrease. Salinity increases, and bottom turbidity decreases, after floodcurrents decelerate toward high tide.

At neap tide, tidal influence is less important and is frequently offset byfluvial processes. Water masses often move in two different directions,particularly during flood tides, as strong ebb currents move a high-turbidity and low-salinity water mass seaward at the surface and weakflood currents move a low-turbidity and high-salinity water masslandward near the bottom (Fig. 8).

There also is a significant areal variation in the relative importance oftides and fluvial processes. About 79% of the fluvial discharge of theMahakam River enters the southern distributaries and 21% flows into thenorthern distributaries, whilst fluvial discharge into estuaries is negligible(Fig. 9). Consequently, fluvial processes are most important, relative totides, in those distributaries with the highest fluvial discharges, which iscompatible with the turbidity and salinity distributions (Figs. 5, 6). Tidalinfluence increases as fluvial discharge decreases, so that the estuaries arecompletely tide-dominant.

SEDIMENT TRANSPORT

Bedload and suspended-sediment transport rates were calculated fromthe data on current velocity, temperature, salinity, and turbidity collectedat each of the 22 hydrodynamic stations, plus the grain-size distribution ofa bottom sample collected at each station using the equations of Van Rijn(1984a, 1984b, 1984c). The algebraic expression of Soulsby and White-house (1997) was used to determine initiation of movement. The weightpercent of mud and sand was determined for each sample, and the sandfraction was sieved at quarter-phi intervals. Twenty-four grain-size classeswere defined, ranging from coarse sand to very fine silt, and the transportrate for each class was calculated separately (Soulsby 1997). Sedimenttransport rates were calculated for each grain at 30 minute time intervalsduring a complete tidal cycle at each hydrodynamic station; the results wereintegrated to yield net flood and ebb transport rates per tidal cycle. Thetotal rate of sediment transport on the Mahakam Delta is estimated to be6.35 3 105 m3/yr based on bedload and suspended-load transport ratesper meter width calculated from hydrodynamic measurements and thewidth of the river; approximately 96% of that sediment is mud.

FIG. 7.—Mean current velocity for a complete spring and neap tidal cycle athydrodynamic stations 14 and16. See Figure 1 for the station locations.

TABLE 1.— Maximum vertically averaged spring tide seaward (ebb) andlandward (flood) current velocities at the 22 hydrodynamic stations. See

Figure 1 for the station locations. Asterisks indicate incomplete spring-tidecurrent-velocity data.

Station Umax Ebb (m/s) Umax Flood (m/s)

1 0.81 0.632 0.34 0.203 0.31 0.354 0.65 0.415 * *6 0.38 0.367 0.32 0.288 0.22 0.379 0.55 0.40

10 0.34 0.3611 0.33 0.3712 0.32 0.4113 0.26 0.2714 0.65 0.2715 0.41 0.2916 0.35 0.4017 0.62 0.3518 0.33 0.2519 0.30 0.3520 0.83 0.4121 0.36 0.3322 0.37 0.30

FIG. 8.—Profiles of vertical current speed, salinity, and turbidity 0.5 hourbefore neap low tide at hydrodynamic station 14. See Figure 1 for thestation location.


Bedload

The annual bedload transport rate in the Mahakam River is estimatedto be 28.1 3 103 m3/yr (Table 2). The bedload is dispersed into thevarious distributaries; estimated transport rates suggest that the bedloaddistribution is approximately proportional to the fluvial discharge in each

FIG. 9.—Fluvial discharges (m3/s) in the distributaries and estuaries of theMahakam Delta. Discharges were calculated by integrating current speedsmeasured for a complete tidal cycle and cross-sectional area.

TABLE 2.—Net bedload and suspended-load sediment transport rates plus fluvial discharge rates at the 22 hydrodynamic stations; italics indicates landwardnet transport (see Fig. 5 for station locations).

Hydrodynamic Station Number Median Grain Size (mm) Bedload (m3/yr) Suspended Load (m3/yr) Fluvial Discharge (m3/s)

1 0.250 28,118 606,567 17502 0.125 - 34,611 1123 0.073 2.56 3 1023 116,209 1074 0.212 1628 187,608 3675 0.125 - 115,471 -6 0.125 1.30 3 1024 531,362 917 (mud) 0 211,129 918 0.106 5.11 3 1025 25,888 19 0.125 301 29,635 163

10 (mud) 3.81 3 1025 201,191 15811 (mud) 7.61 3 1025 64,005 15612 (mud) 1.52 3 1022 26,909 813 (mud) 0 15,875 014 0.250 2014 592,364 48315 0.212 4.61 3 1021 53,244 24116 0.090 1.67 3 1023 284,405 16217 0.250 885 237,488 24518 (mud) - 141,367 23519 (mud) 1.35 3 1024 153,056 18920 0.250 9189 590,507 89321 0.150 - 111,152 23722 (mud) - 280,335 149

FIG. 10.—Sediment transport through a tidal cycle at A) spring tide and B) neaptide at hydrodynamic station 14. See Figure 1 for the station location.


distributary (Table 2). Virtually all bedload transport occurs only duringspring tides and is restricted to a few hours per tidal cycle (Fig. 10).

Seaward net bedload transport dominates the delta apex and the upperto middle reaches of distributaries, although there is a significant seawarddecrease in rate and duration (Table 2; Fig. 11A). Bedload transportpersists for 4 hours per tidal cycle at the delta apex, for about 3 hours inthe upper reaches of distributaries, and for about 1 hour in the middlereaches of distributaries. The lower reaches of distributaries and estuariesare dominated by shoreward net bedload transport, despite a duration ofonly , 1 hour (Fig. 11A). A short-lived subordinate-phase landwardbedload transport also occurs in the middle to upper reaches of somedistributaries (Fig. 11A). The bedload transport rates indicate thatMahakam River-derived bedload is being deposited in the distributarieswell landward of the coastline and suggest that a minor amount ofoffshore-derived sediment enters the estuaries and lower reaches of thedistributaries as bedload (Fig. 11A). This sediment probably is derived bytidal-current reworking of intertidal bars and mouth bars.

Suspended Load

The suspended load in the Mahakam Delta system occurs during bothspring and neap tide and has two components. Mud that is suspended bycurrents is the major component, and its contribution was estimated fromcurrent measurements; it has a fluvial and/or marine source and occurs inhigh concentrations during peak tidal-current discharge and lowconcentrations during slack water. Residual suspended sediment in thewater column is less important. Its volume was estimated from turbiditymeasurements in NTU by converting them to suspended-sedimentconcentrations in mg/l using a correlation factor of 0.79, which wasderived empirically in a previous study (Research Center for Marine andCoastal Resources 2002).

The suspended load has a net seaward transport in the distributariesand a net shoreward transport in the estuaries during spring tides(Fig. 11B). Generally, the suspended-sediment transport rates decreasesignificantly seaward from the delta apex and upper reaches ofdistributaries, where they are about four times larger than in the lowerreaches (Table 2). Seaward and landward transport of suspendedsediment occur for nearly equal durations in estuaries, althoughshoreward transport tends to be dominant (Table 2; Fig. 11B).

During neap tide, mud that is locally resuspended in the distributariesby tidal currents has a dominantly seaward transport, although ratesdecrease slightly seaward. Weak flood tidal currents are not able tosuspend distributary mud, and there is no suspended load transport in theestuaries. Neap-tide tidal currents maintain the residual suspendedsediment, which has a net seaward transport in the distributaries and anet shoreward transport in the estuaries (Fig. 11C). The residualsuspended-sediment transport rates are an order of magnitude lowerthan the rates during spring tide.

The suspended load on the Mahakam Delta has a fluvial origin; theMahakam River transports suspended sediment seaward at an estimatedrate of 6.1 3 105 m3/yr based on suspended-load transport rates permeter width calculated from hydrodynamic measurements and the widthof the river, whilst distributaries have an estimated total suspended-loadtransport rate of 1.22 3 106 m3/yr using the same calculation method.The significantly higher rate in the distributaries is caused by resuspen-sion of mud by tidal currents and it is because there are multiple

r

FIG. 11.—A) Bedload transport, B) spring-tide suspended-load transport, andC) neap-tide suspended-load transport. Arrow length is approximately propor-tional to the magnitude of transport in the direction indicated. See Figure 1 forstation numbers.


distributaries that it offsets the overall seaward decrease in suspended-sediment transport rates.

Turbidity maxima trap most of the suspended sediment in theMahakam Delta system. They are located near the distributary mouthsand in their lower reaches. Estuaries have a net landward suspended-loadtransport rate of at least 26.9 3 103 m3/yr and have turbidity maxima in

their upper reaches of the estuaries. Locations of turbidity maxima in thedistributaries and estuaries match the maps of bottom sedimentdistribution and turbidity (Figs. 4, 6). Suspended sediment is depositedby the turbidity maxima, causing a rapid decrease in suspended-sedimentconcentrations from the distal reaches of the distributaries to less than1.0 mg/l in the adjacent offshore (Eisma et al. 1989). Much of the

FIG. 12.—Schematic diagram of the faciesdistribution and morphology of the MahakamDelta at A) the progradational stage immediatelypreceding the ongoing transgression, B) anintermediate stage, and C) the present day. PartsA and B are schematic representations of theproduct of multiple delta lobes that were not allactive simultaneously. The dashed line representsthe approximate landward limit of dominantmarine sedimentary processes.


suspended load that reaches the subaqueous delta plain is againtransported shoreward to the lower reaches of distributaries and theestuaries, and a small percentage is transported offshore.

DEPOSITIONAL MODEL

Depositional Processes

The sediment transport patterns and facies distribution indicate thattidal currents are the dominant sedimentary process on the MahakamDelta. Tidal processes control sediment transport over the entire delta, asindicated by bedload transport that occurs only during spring tides andby suspension transport that is an order of magnitude higher at springtide than at neap tide. Fluvial influence decreases seaward, as evidencedby the decrease in the seaward magnitude of the net bedload transportfrom the delta apex accompanied by seaward fining of the bottomsediments (Figs. 4, 11). Increasingly more abundant mud drapes andprogressively more abundant brackish water to marine organismsseaward plus landward net bedload transport in the estuaries and lowerreaches of distributaries indicates a corresponding increase in tidaldominance. Waves influence only the southern coastline, where theywinnow mud and slightly modify the geometry of intertidal sand bars.Wave energy is mostly attenuated on the broad subaqueous delta plain,although waves concentrate detrital organic debris along muddyshorelines to form peat beaches and ridges.

The sediment transport patterns and facies distribution suggest that theMahakam Delta is presently subsiding and that it is, in essence, a drowneddelta that is being transgressed and modified by marine processes. Thebedload transport patterns indicate that most, if not all, fluvially derivedsand is being stored onshore in the distributaries. Other indicators oftransgression are the dominant marine processes that rework relict sandbodies on the subaqueous delta plain and build detrital peat beaches (Allenand Chambers 1998), plus the presence of benthic marine organisms up to20 km landward in the distributaries (Carbonel and Moyes 1987).

Some of the sediment cores previously collected on the Mahakam Deltaexhibit clear evidence of reworking based on 14C dating (Gastaldo andHuc 1992), which is consistent with tidal scouring during transgression.However, the age gap can be interpreted as a disconformity thatcorresponds to transgressive deposits overlying the youngest prograda-tional strata. The disconformity surface represents the time required fortransgressive depositional processes that initially influenced only thepaleo-shoreline to shift landward and affect the relevant location. It isuncertain when the ongoing transgression began, but it probably started2000–3000 years BP, based on 14C dating of lower-delta-plain deposits(Storms et al. 2005), and by analogy with other SE Asian deltas where anongoing transgression began , 2000 BP (e.g., the Baram Delta; Calineand Huong 1992; Lambiase et al. 2002).

The subaqueous morphology suggests that approximately 5 m ofsubsidence shifted the shoreline landward from its former position nearthe offshore break in slope at the delta front and submerged part of thelower delta plain and the distal reaches of the distributaries, some ofwhich remain as topographic features on the seafloor (Fig. 2; Scruton1960; Coleman and Roberts 1991). As subsidence and transgressionproceed, distributaries become overly deep relative to fluvial discharge,fully marine waters and their associated fauna penetrate farther into thedistributaries, and the boundary between fluvial and tidal dominancemoves progressively landward, causing the seaward limit of fluviallyderived sand to move landward as well. As the shoreline retreats,mangrove and Nypa are drowned and mud and soil are removed bymarine processes, leaving intertidal sands that are ultimately buried bysubtidal marine mud (Fig. 12). At present, the entire lower delta plain isessentially abandoned, in that it is not accumulating fluvially suppliedsediment. Marine processes have reworked all the facies and aremodifying the distributary and interdistributary morphology to reflectthe dominant tidal processes. As transgression proceeds, abandonmentprocesses are expected to continue until the lower delta plain issubmerged and the topography is further subdued as it is buried bymud and becomes part of the low-relief subaqueous delta plain.

FIG. 13.—Near-seafloor seismic line with progradational clinoforms and onlapping reflections above a flooding surface. Clinoform thickness corresponds to the 20 mheight of the delta front, and nearby sea-floor sampling indicates that the onlapping strata are mud. See Figure 1 for the line location. All the interpreted surfaces, exceptfor the flooding surface, are from Cibaj (2007); the interpreted flooding surface and the written interpretations are from the present study.


A transgressive interpretation for the subaqueous delta plain issupported by near-seafloor seismic data where progradational clino-forms are onlapped by parallel reflections at a flooding surface (Fig. 13).The approximately 20-m-thick clinoforms correspond almost exactly tothe height of the delta front (Fig. 2), suggesting that present-daymorphology reflects the most recent progradational phase, whichdeposited approximately 20 m of sediment. The onlapping reflections

probably comprise mud, based on their parallel, nearly horizontalgeometry and that they occur seaward of sampled sea-bottom mud(Figs. 1, 2, 4, 13).

Predicted Stratigraphic Succession

The sediment transport patterns strongly suggest that distributaries arethe principal areas of sedimentation on the Mahakam Delta and that theyare being retrogradationally backfilled. The facies succession will be acomposite of the various present-day areas; the backfilling process isexpected to produce a stratigraphic succession that becomes progressivelyfiner-grained and more marine upward (Fig. 14).

The predicted succession includes a basal unit that consists ofdistributary-channel sands that are several meters thick and increasinglytide-influenced upward. The lowermost of these sands were depositedduring the progradational phase that immediately preceded the ongoingtransgression and are expected to be relatively thin and relatively coarse,cross-bedded sands with erosional bases and channel lags (Fig. 14). Oncethe current transgressive phase started, stacked cross-bedded fluvial sandsbegan to accumulate by backfilling (Lambiase 2011). Later, initial tidalinfluence generated cross-bedded medium sands with mud drapes andplant debris equivalent to those presently being deposited in the proximalreaches of distributaries (Fig. 14). As tidal influence increases with time,there is a gradual transition into fine sand with increasingly more muddrapes, flasers, and a few Skolithos burrows. Bioturbation intensityincreases upward into the overlying bioturbated, muddy fine sand thatcaps the distributary-sandstone unit.

The distributary sand is overlain by a few meters of estuarine mud withabundant lenticular beds and organic matter similar to that presentlybeing deposited in the lower reaches of the distributaries (Fig. 14).Bioturbation is pervasive and dominated by Chondrites. The totaluncompacted thickness of the distributary sand and estuarine mud isapproximately the depth of the distributaries, which is 15–20 m.

As the transgression continues and the shoreline approaches, intertidalsand bars are reworked into a relatively thin, bioturbated muddy finesand with a sharp base, like those presently being deposited in the distalreaches of distributaries (Fig. 14). Flaser beds and mud drapes are

FIG. 14.—Composite predicted stratigraphic succession for the Mahakam Deltacompared with the Miocene Attaka Field of the Mahakam Delta Province, whichis approximately 20 km north of the Mahakam Delta (see Fig. 1). Depositionalenvironments in the Attaka Field have been reinterpreted from Trevena et al.(2003), and the grain size scale for the Mahakam Delta succession is schematic; seetext for explanations.

FIG. 15.—Schematic diagram of progressive channel abandonment on theKaeli, Ilu, and Pantuan Rivers; see text for explanation. The locations of the riversare shown in Figure 2.


common, as is plant debris that forms peat layers locally. Heavilybioturbated offshore mud caps the stratigraphic succession.

Shoreline Morphology

Delta morphology, especially shoreline morphology, has long beenaccepted as an indicator of dominant hydrodynamic process (e.g.,Galloway 1975), but recent studies suggest that, on some deltas (e.g.,the Sao Francisco Delta, Dominguez et al. 1992; the Baram Delta,Lambiase et al. 2002), a significant component of the modernmorphology is inherited from a pre-modern system, so that present-daymorphology is not necessarily a reliable indicator of hydrodynamicdominance. The Mahakam Delta has a lobate geometry, which generallycharacterizes river-dominated deltas prograding into shallow water(Fig. 1; Fisher et al. 1969), although it has been described as a mixedfluvial and tidal morphology by several authors based on shoreline andchannel morphology (e.g., Galloway 1975; Allen et al. 1976). Lobatemorphology built from multiple distributaries generally indicates a highfluvial discharge distributing the sediment load with a short recurrenceinterval of bifurcation and avulsion (Olariu and Bhattacharya 2006).

However, the present-day Mahakam Delta is tide-dominant and notstrongly fluvially-influenced. The modern depositional processes and itsrecent depositional history indicate that its fluvial-dominated morphol-ogy is not a product of present-day processes, but reflects a phase offluvially-dominant progradation that occurred prior to the present-daysubsidence and transgression. The relict morphology is preserved becauseit cannot be reworked at the advancing transgressive shoreline by thesmall, fetch-limited waves whose energy is further dissipated across thebroad subaqueous delta plain.

Channel Morphology

Landward sediment transport prevails in the lower reaches of all thechannels on the Mahakam Delta, indicating that they all are estuaries withrespect to sedimentation (Dalrymple et al. 1992). However, they do notstrictly fit the Dalrymple (2006) estuary classification because the deltaforms a protruding coastline rather than a drowned embayment. Inaddition, there is some sedimentation offshore; very fine-grained suspendedsediment is deposited on the subaqueous delta plain, creating a southwardbulge in the prodelta geometry (Fig. 2). Therefore, the Mahakam Delta isbest regarded as a transgressed delta rather than an estuary or series ofestuaries, although it may eventually evolve into the latter.

The distributaries and estuaries of the Mahakam Delta are distinctlydifferent with respect to their hydrodynamics; distributaries havesignificant fluvial discharge whilst estuaries have little or none, suggestingthat estuaries are abandoned distributaries. Essentially, the funnel-shapedestuaries and flared distributary mouths represent ongoing tidalmodifications to pre-existing distributary morphology.

Morphological evidence suggests that there are three processesconverting distributaries to estuaries on the modern Mahakam Delta.Regional transgression is converting all the distributaries to estuaries,although at a different rate for each distributary. The central part of thedelta, which is too broad to have always been an interdistributary areaand whose subaqueous extension projects as far seaward as on other partsof the delta (Fig. 2; Allen et al. 1976), has become abandoned through aprocess apparently akin to lobe switching. Most fluvial discharge now

flows through the southern distributaries and the remainder through thenorthern distributaries (Fig. 9), indicating that the locus of fluvialdischarge shifts with time even though the delta is almost certainly notcurrently prograding.

At a smaller scale, individual channels are progressively abandoned.This is illustrated by the hydrodynamics, sediment transport, sedimentaryfacies, and morphology in the Pantuan, Kaeli, and Ilu rivers (Figs. 2, 15).The Kaeli River was the main sediment conduit, but fluvial sand is nolonger transported into its lower reaches; bedload is now delivered intothe Pantuan River, which has become the active channel whilst the KaeliRiver is now partially abandoned. Tidal processes have increased in theKaeli River, and tidal scouring has started to widen its mouth. The IluRiver, which is located between the Pantuan and Kaeli rivers, is totallyabandoned, and has a net landward suspended-load transport and aflared mouth (Fig. 15).

DISCUSSION

Sedimentation on the Mahakam Delta

The transgressive depositional model reconciles the various morpho-logical and sedimentological features of the Mahakam Delta whichtraditionally have been viewed by previous authors as unusual or unique(e.g., Allen et al. 1976). Recognizing that the offshore intertidal sand barsare remnants of subsiding interdistributary areas, rather than mouth bars,explains the up to 20 km mud-filled gap between them and sand in thedistributaries, without resorting to unconventional hydraulics (Fig. 4).Similarly, ongoing subsidence accounts for the gently sloping subaqueousdelta plain with offshore extensions of distributaries that reach theoffshore break in slope at the delta front.

The distributaries and estuaries are overly deep relative to the present-day hydrodynamics because they were downcut during a precedingprogradational phase. Also, marine organisms penetrate up to 20 km intothe lower reaches of the distributaries because the fresh Mahakam Riverwater flows on top of marine waters with nearly normal marine salinities(Fig. 4). The lower delta plain is extensively covered by Nypa, rather thanthe more typical and more salinity-tolerant mangrove, because theflooding tidal prism lifts the river water out of the subsided, non-equilibrium distributaries and onto the lower delta plain, keepingsalinities low enough for Nypa growth and depositing post-disconformitysediment. The close conjunction of the seaward limit of bedload transportand the landward limits of benthic marine organisms and marginalmarine vegetation approximates the landward limit of purely marinedepositional processes. Mixed fluvial and tidal sedimentation extendsfarther landward well beyond the delta apex (Fig. 4).

The ‘‘lateral bars’’ in the upper reaches of the distributaries that Allenet al. (1976) and Allen and Mercier (1994) described are interpreted as aproduct of mixed fluvial and tidal processes. Their distribution isrestricted to the part of the delta where distributary morphology isexpected to be partially modified by tidal processes, prompting Husein(2008) to interpret them as partially reworked fluvial point bars. Withcontinued transgression and subsequent backfilling of the distributaries,the tidal-channel bases may be preserved as tidal ravinement surfaces, acommon feature in transgressive deltaic successions (e.g., Ponten andPlink-Bjorklund 2009).

r

FIG. 16.—Backfilled middle Miocene distributary in the Mahakam Delta province with increasing marine influence upward. A) The erosionally based succession inoutcrop, B) its stratigraphic column with the locations of photographs C–I, C) basal, cross-bedded coarse fluvial sandstone, D) cross-bedded medium sandstone with organic-matter flasers, E) muddy fine sandstone with Ophiomorpha burrows and organic-matter drapes, F) flaser-bedded and mud-draped fine sandstone, G) burrowed sandymudstone, H) burrowed wave- and current-rippled thin-bedded fine sandstone, and I) marginal marine coal and coaly mudstone overlying thin-bedded fine sandstone (afterNirsal 2010).


Relevance to the Subsurface Succession

It is generally accepted that the transgressive phases of deltas have lowpreservation potential (Reading and Collinson 1996), implying that sedimentsdeposited during the present-day transgressional phase of the MahakamDelta are not likely to be preserved (Verdier et al. 1980). However,transgressive successions are becoming more widely recognized (e.g., Plink-Bjorklund 2005; Sixsmith et al. 2008; Ponten and Plink-Bjorklund 2009) andhave been documented in other late Tertiary Borneo deltaic systems(Lambiase et al. 2003). Integrated seismic, core, and high-resolutionbiostratigraphic data suggest that as much as half of the mid-Miocene toRecent Mahakam Delta succession was deposited during transgressiveconditions (Lambiase et al. 2010).

The limited reworking of delta morphology during transgression causesMahakam Delta transgressive successions to include two distinct phases,each of which has a very different proportion of transgressive versusregressive facies. The initial phase backfills the inherited progradationalmorphology, generating successions that can be nearly as thick as theprogradational wedge (Figs. 4, 16). However, these successions havelimited geographic extent with respect to the whole delta, in that they areconfined initially to distributaries that constitute approximately 20% ofthe delta’s surface area (Fig. 12A). The proportion of transgressive facieswill increase somewhat with time as some distributaries evolve intoestuaries (Fig. 12B, C). This suggests that the facies ultimately preservedwithin a progradational lobe will include a comparable proportion oftransgressive facies, almost all of which will occupy the sites of formerdistributaries and estuaries.

The second phase of transgressive sedimentation consists of aggrada-tional units that extend laterally across the entire delta once the inheritedprogradational topography has been buried, and separate differentprogradational delta lobes in the subsurface stratigraphy (Lambiaseet al. 2010). The seaward end of the modern subaqueous delta plain, withits nearly filled distributaries, is just reaching the transition from the firstto second phase of transgressive sedimentation (Fig. 12C).

One of the intervals that Lambiase et al. (2010) interpreted astransgressive includes the Miocene Attaka Field succession in theMahakam Delta Province, interpreted by Trevena et al. (2003) as tidallyinfluenced distributary deposits. The Attaka succession is very similar tothe modern, transgressive succession with respect to bed thickness,lithology, and sedimentary structures. However, the Trevena et al. (2003)interpretation invokes a transgressive–regressive cycle based on aprograding-delta model, with the upper sandstones designated as regressivemouth-bar deposits (Fig. 13). The succession is better interpreted as atransgressive, back-filled distributary where estuarine mud is smoothlycapped by an intertidal sand bar rather than by a mouth bar.

Nirsal (2010) identified a back-filled distributary in a middle Miocenesuccession that crops out in the Mahakam Delta province. Twelve-meter-thick stacked sandstones that become finer-grained and have increasingtidal influence upward are overlain by shoreline and shallow marine mudsand thin-bedded sandstones, indicating that the succession is transgressive(Fig. 16). Also, a seismic attribute analysis of the Nilam Field in theMahakam Delta province indicates a number of elongate, middle Miocenesand bodies up to 1 km wide, 100 m thick, and tens of kilometers long withorientations that are similar to the present-day distributaries (Prasetyo2003). The sand bodies are interpreted as stacked distributary sandsgenerated by retrogradational stacking during transgression.

The Miocene analogues strongly suggest that retrogradational succes-sions are commonly preserved on the Mahakam Delta. This differsmarkedly from conventional transgressive models, e.g., the MississippiDelta (Elliot 1986; Penland et al. 1988), where Gulf of Mexico wavesrework abandoned river-dominated lobes into wave-dominant barrierislands that are eventually drowned and overstepped by shelfal muds.Lambiase et al. (2010) attribute the preservation of the transgressive

FIG. 17.—A) Sedimentary facies and shoreline morphology plotted on theclassification schemes of Galloway (1975) and B) sedimentary facies plotted on theOrton and Reading (1993) diagram for basinal processes in fine-grained sediment,which is pervasive in the marine-dominated lower delta plain of theMahakam Delta.


strata of the Mahakam Delta to high sedimentation rates coupled withongoing subsidence and low wave energy that cannot rework the quicklyaccumulating and subsiding strata.

Implications for Delta Models

Interactions among fluvial, tidal, and wave processes are generallyacknowledged as the principal controls on deltaic sedimentation and theirrelative importance forms the basis for many widely used deltaclassification schemes (e.g., Galloway 1975; Orton and Reading 1993).Those classifications implicitly assume that the three hydrodynamicprocesses act simultaneously yet at a variable proportion, and theirproducts are represented directly by shoreline morphology. Based onthose assumptions, the Mahakam Delta was previously classified as amixed fluvial and tidal delta (Galloway 1975). However, the depositionalprocesses and sedimentary facies indicate that it currently is strongly tide-dominant and that its morphology is not in equilibrium with the present-day hydrodynamics but instead represents a previous, fluvial-dominantphase that has been partially modified by tidal processes.

Facies distribution is a much better indicator of modern depositionalprocesses than morphology on the Mahakam Delta. This is illustrated bya comparative plot of facies and morphology as proxies for depositionalprocesses (Fig. 17A), in which there is a substantial distance between thepoints representing facies and morphology, which demonstrates theinadequacy of morphology-based classification, as recognized elsewherefrom internal facies architecture in outcropping successions (e.g., Ganiand Bhattacharya 2007; Helland-Hansen 2010). Orton and Reading(1993) used grain-size as a proxy for sediment supply to represent fluvialinput and superimposed their grain size distribution on the tidal rangeversus wave height classification of Hayes (1979). The Mahakam Deltalies well within the tide-dominated field of this scheme (Fig. 17B), againsuggesting that facies-based classifications more accurately reflectdominant and subordinate depositional processes than morphology-based classifications.

The relationship between hydrodynamics, facies, and morphology onthe Mahakam Delta indicates that deltaic sedimentation generallyproceeds as fluvial sediment supply followed by marine redistribution,rather than as the simultaneous interaction of fluvial and marineprocesses. Sedimentary facies on the Mahakam Delta are redistributive,whilst its morphology is partially relict and partially redistributivebecause facies respond much more rapidly to marine redistributiveprocesses than does morphology, simply as a function of the much largervolume of sediment that must be transported to modify morphology.

The Mahakam Delta also suggests that it is the relative rates of sedimentsupply and redistribution that determines the fluvial versus marinecharacter of a delta, whilst the relative strength of tidal versus waveprocesses determines the nature of the marine redistribution of sediment.Those two independent interactions control lateral facies distribution,vertical facies succession, and sand-body geometry. The relative impor-tance of the hydrodynamic processes varies significantly across the surfaceof a delta as a function of distance from a distributary, water depth, andexposure to wave approach. This implies that interpretation of thehydrodynamic setting from one vertical facies succession is unlikely to berepresentative of the whole delta. Furthermore, the relatively rapidresponse of sedimentary facies to redistribution by marine processes meansthat morphology, thus sand body geometry, and preserved facies are notcoupled. This exposes another limitation to delta classification schemesthat assume delta morphology, sedimentary facies, and preservedstratigraphy are all linked to a single, dominant hydrodynamic settingand indicates that the stratigraphic record of a delta may not correspondclosely to the succession predicted by interpreting dominant hydrodynamicprocesses primarily from morphology.

CONCLUSIONS

A quantitative analysis of the sediment dynamics and depositionalsystems of the Mahakam Delta indicates that:

1. Sand floors the distributaries and gradually fines seaward from thedelta apex. It is replaced by mud well landward of the shoreline;mud extends seaward across the subaqueous delta plain.

2. Benthic marine organisms inhabit the distributaries up to 20 kmlandward from the shoreline.

3. Saline water moves dynamically in the distributaries up to 10 kmupriver during spring tides, but saline tidal waters remain in thelower reaches of the distributaries during neap tides.

4. High bottom turbidity occupies the entire length of the distribu-taries at peak ebb discharge during spring tides. High turbidity isrestricted to the lower reaches at spring tide peak flood dischargeand peak neap tide ebb discharge.

5. Tides are by far the most important sedimentary process on theMahakam Delta, where fluvial processes play a subordinate roleand waves have a minor influence.

6. Most bedload sediment transport occurs during spring tides andtransitional tides when the tidal range is $ 1.5 m.

7. There is a significant seaward decrease in the duration and rate ofbedload transport as well as a change in the direction of net transport.Seaward bedload transport extends from the delta apex through theupper and middle reaches of distributaries, whilst net landwardbedload transport prevails in the lower reaches of distributaries and inestuaries. The bedload transport pattern causes most, if not all,fluvially derived sand to be deposited in the distributaries.

8. Suspended sediment is derived from the Mahakam River and istrapped in turbidity maxima. Some of the suspended load thatreaches the subaqueous delta plain is transported back landwardinto the lower reaches of distributaries and estuaries.

9. Distributaries generally have net seaward suspended-load transport,and estuaries have net landward suspended load transport.

10. The distributaries are the principal areas of sedimentation and are beingretrogradationally backfilled to produce a stratigraphic succession thatbecomes progressively finer-grained and more marine upward.

11. The delta is presently subsiding and is, in essence, a drowned deltathat is being transgressed and modified by marine processes. Hence,the river-dominant delta morphology is not a product of present-day processes, but relict from a progradational phase that precededthe ongoing subsidence and transgression.

12. The Mahakam Delta suggests that facies distribution probably is abetter indicator of depositional processes than morphology, andthat delta classifications based on facies distribution may be moreaccurate than morphology-based classifications.

13. Miocene subsurface and outcrop analogues from the Mahakam DeltaProvince strongly suggest that retrogradational deltaic successions arecommonly preserved in low-energy marine environments.

ACKNOWLEDGMENTS

Salahuddin extends his gratitude to Total E&P Borneo BV and Total E&PIndonesie for sponsoring his Ph.D. Scholarship and field work, respectively,and all the Total E&P Indonesie staff technical staff and boat crews whoassisted with the field work logistics and data-gathering. Janok Bhattacharyaread an earlier version of the manuscript and made several very helpfulsuggestions. We also wish to thank journal reviewers M. Royhan Gani andRobert Dalrymple for their constructive comments and suggestions.

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Received 8 April 2012; accepted 26 February 2013.


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