Peter T. Harris

A preliminary study of sedimentation in the tidally dominated Fly River Delta, Gulf of Papua

High resolution seismic profiles, surface sediment samples and cores document sedimentary facies of the tidally dominated Fly River Delta. Tidal currents dominate in the transport of sandy sediments throughout the Fly Estuary. On the delta front (5–17 m water depth) surface waves generated by southeasterly trade winds rework the muds and sands to winnow out the fine grained sediments, from March to November. During the northwest monsoon (December–March), minimal surface wave activity results in the deposition of a mud drape, resulting in seasonal sand-mud interbeds (varves). Prodelta deposits extend below this depth from 17 to 45 m and contain massively bedded muds accreting at a mean rate 4 cm year−1. A facies model for the deltaic sequence is presented and employed to derive a preliminary sediment budget for the delta. The budget demonstrates that of the 85 million tonnes year−1 of sediment discharged by the Fly, about 47 million tonnes year−1 is deposited in the delta area adjacent to the months of the Fly River and less than 2% is deposited in Torres Strait to the south. Suspension load transport removes sediments northeastwards into the Gulf of Papua and westwards along the coast, whereas fluid muds may remove sediments eastwards onto the shelf and, possibly, to the adjacent Coral Sea Basin. The budget also demonstrates that a ∼38 m vertical section, encompassing distal delta, prodelta and delta front facies, is deposited in about 1100 to 1600 years. The application of the facies model in interpreting other tidally dominated deltaic deposits is discussed.

Large-scale bedforms as indicators of mutually evasive sand transport and the sequential infilling of wide-mouthed estuaries

Abstract Large-scale bedforms (2–10 m in vertical and 102–103 m in horizontal dimensions) found in wide-mouthed estuaries are described. Different bedform types occur depending upon the local availability of sand. With an increasing sand supply, sand ribbons grade into elongate trains of sand waves and then form sandwave fields. Inshore, headland-associated sand banks are formed which multiply into en-echelon sand banks. Based upon a review of data on directions of sand transport from the Bristol Channel and Thames Estuary, U.K., and from Moreton Bay, Australia, charts of ebb- and flood-dominant transport zones are constructed for lower estuarine environments which have undergone different degrees of infilling. Linear sand banks are seen to delimit partially the boundaries between opposing sand transport zones. Transport paths demonstrate how sediments derived from outside of the estuary are dispersed through ebb and flood transport zones, to supply other areas of net deposition. A comparison between different estuaries reveals that variations in the compexity of ebb- and flood-dominant transport zones and the morphologies of large-scale bedforms are coupled with apparent changes in the relative amounts of sand available to each system. A model for the sequential infilling of estuaries and the evolution of large-scale bedforms is presented and applied to the interpretation of present day examples. Vertical sequences predicted to be generated by such bedform evolution are described and discussed, in terms of their preservation in the geological record.

Late Quaternary Deltaic and Carbonate Sedimentation in the Gulf of Papua Foreland Basin: Response to Sea-Level Change

ABSTRACT The rivers that drain the wet, mountainous island of New Guinea discharge about 1.5 billion tonnes/yr of sediments into the adjacent seas, including the foreland basin between New Guinea and Australia. Despite this huge sediment input, there appears to have been only limited deposition in the Gulf of Papua during the (Holocene) postglacial rise in sea level. Seismic and core data indicate that the transgressive systems tract in the Gulf of Papua is thin and patchy. It is confined to regions within and north of an incised, east-west-trending shelf-valley system. Of the possible explanations for the absence of a significant transgressive systems tract, inland storage and along- and off-shelf transport of the sediment are of greatest significance. Reef growth up to the latitude of the east-west-trending incised-valley system in the southern Gulf of Papua is considered to have been facilitated by a northward-flowing coastal boundary current, the Coral Sea Coastal Current. This current now sweeps turbid, brackish waters and terrigenous sediments discharged by the rivers northwards away from the reefs. An observed northward offset in transgressive sediments in relation to the axis of the shelf valleys suggests that such a northward.flowing shelf current operated during the late Pleistocene and early Holocene. The northern limit of the Great Barrier Reef could thus be controlled by the balance between fluvial sediment supply and northward advection of suspended sediment by the Coral Sea Coastal Current. This current may also be imp rtant in maintaining a supply of clear water to the eastern Gulf of Papua, thus enabling photosynthesis and the flourishing of calcareous-algae (Halimeda) bioherms or biostromes at depths of up to 100 m over much of the middle and outer shelf, directly offshore of the modern Fly Delta. These carbonate sediments represent the exposed maximum flooding surface and condensed section. Modern highstand delta deposits have begun to prograde over this layer on the inner shelf.

Late Quaternary sedimentation on the Great Barrier Reef continental shelf and slope east of Townsville, Australia

Cores collected in a shelf to basin transect from northeastern Australia document the stratigraphic development of late Quaternary deposits. Low sea-level (to − 120 m) slope sedimentation is dominated by prograding fluvial deposits with wave-dominated deltas forming on the shelf edge and upper slope. The onset of sea-level transgression into the excavated shelf river channels on the outer shelf resulted in estuarine sedimentation along an indented coastline (15,000–11,000 yrs B.P.), reducing terrigenous sediment supplies to the slope. On the outer shelf, Halimeda muddy sandy gravels were deposited over a Pleistocene limestone erosional surface from about 10,500 yrs B.P. Transgression submerged the limestone outcrops (and estuaries) of the middle shelf where coral reef complexes formed; the outer and middle parts of the shelf thus became dominated by carbonate sedimentation while the inner shelf received fluvial input mixed with locally derived carbonates. The sequence is bounded by an unconformity, clearly marked by erosional down-cutting. The results of the study are combined with those of previous workers to develop transgressive and continued high sea-level system tract models. The latter (continued high sea level) model is used to extrapolate present high sea-level sedimentation patterns into the future, providing a model for comparison with ancient sequences which may also have formed in such a depositional environment.

Sediments, bedforms and bedload transport pathways on the continental shelf adjacent to Torres Strait, Australia—Papua New Guinea

Abstract Tide-generated bedforms were mapped using air photographs and correlated with sediment sampling and side-scan sonar data over the Torres Shelf, a rimmed continental shelf environment of mixed carbonate and siliciclastic sedimentation. Based upon the morphology of bedforms net bedload transport pathways were determined. Pathways tend to diverge from the constricted passes formed between coral reefs and islands reflecting the “bedload parting zone” model derived from west European tidal seas. The sequence of bedform morphologies changes down drift (in the direction of weakening current) from scoured channels to sand ribbons and finally to sandwave fields as predicted by the European model. Lateral mixing of sediment appears to be restricted on the Torres Shelf; areas of quarztose and carbonate sand bedforms occur on opposite sides of a bedload parting zone. Three areas of sediment deposition are identified and possible sources are discussed for each. In one depocentre where samples were collected, terrigenous and carbonate muds are the major sediment components. Sea grass communities occur in association with depositional areas; these represent a potential hydrocarbon source proximal to coral reef and coarse-grained deposit (bedform) potential reservoirs.

Evolution of subtidal sandbanks in Moreton Bay, eastern Australia

Abstract Vibrocores, current meter data, boomer seismic profiling and sidescan sonar data provide an insight into the processes governing the formation of subtidal linear sandbanks in Moreton Bay, Australia. Bedforms observed on sidescan sonar images are predominantly 10–20 m wavelength dunes with flood-oriented slip faces within the section of the sandbank field studied. The sandbanks separate mutually evasive ebb- and flood-dominant sand transport pathways. Rates of sand transport, calculated from the current meter data, are greatest during flood tides, flowing between the interval low-low to high-high water (mixed tidal regime). Subordinate, ebb-directed transport is insignificant by comparison, which facilitates the preservation of flood-oriented foreset beds in high energy depositional settings (i.e. sandbank crests). Sediment peels obtained from cores indicate that foreset beds are preserved only in deposits of the sandbank crests; trough deposits show bioturbation. 14C dating establishes that sandbanks located further away from Moreton Island were formed prior to 3 k.y. B.P. and that the banks have not been rebuilt since this time. In contrast, material obtained up to 380 cm beneath the surface in sandbanks located inshore are of modern age, showing that these banks undergo regular rebuilding. The formation of linear sandbanks in Moreton Bay is mainly a consequence of the infilling of the bay mouth with sand delivered via littoral drift along the adjacent coastline.

Sandwave movement under tidal and wind-driven currents in a shallow marine environment: Adolphus Channel, northeastern Australia

Abstract Synoptic bathymetric surveys and current meter data collected over a sandwave field in Adolphus Channel (20 m water depth), Australia, yield average estimated celebrities of 0.75 and 0.25 m day−1, respectively. The sandwaves average 3.9 m in height, 102 m in wavelength and are comprised of up to 96% carbonate, consisting primarily of intact and fragmented calcareous alga Halimeda, benthic foraminifers, bryozoans and molluscs. The sand has a modal grain size of 0.8 mm. Current speeds measured 1 m above the bed averaged 0.42 m−1 and reached a peak of 1.36 m−1. Surveys carried out in September and February show that the sandwaves reversed their asymmetric orientation over this time interval, which is attributed to a change in the direction of the wind-driven currents during the monsoon season. The reversal of asymmetry was accompanied by a statistically significant change in the degree of sandwave asymmetry (ratio of stoss and lee slope lengths) whereas no change in mean wavelength was detected. The reversal is estimated to have required 47 days to occur based upon estimates of average sandwave cross-sectional area and bedload transport rates predicted from the current meter data.

Reversal of subtidal dune asymmetries caused by seasonally reversing wind-driven currents in Torres Strait, northeastern Australia

Abstract Large subtidal sand dunes (sandwaves) located in Adolphus Channel, Torres Strait, have been observed to reverse their asymmetric orientation between September-February. This has been attributed to a reversal in wind-driven currents, which flow westward during the SE trade season (April-November) and eastwards during the NW monsoon season [December-March: Harris (1989) Continental Shelf Research, 9, 981–1002]. Observations in September 1988 and February 1989 from another area of dunes in Torres Strait corroborate this asymmetry reversal pattern. The results indicate that such reversals may be common in Torres Strait and in other areas where subtidal bedforms are subject to modification by superimposed, seasonally reversing, wind-driven currents.

Estimation of annual bedload flux in a macrotidal estuary: Bristol Channel, U.K.

Sedimentological data from the Bristol Channel indicate that “mutually evasive” ebb- and flood-dominant zones of net bedload transport are present. To test this hypothesis, long-term observations of currents from the channel are combined with empirical formulae to estimate annual bedload transport rates under the combined influence of tidal currents and surface wind-waves. Extrapolating these values over the ebb- and flood-dominant zones, sand flux to the upper part of the channel is estimated at about 64 × 105 t yr?1 with supply and removal being of nearly equal magnitude. Such patterns of sand and water movement may be typical of tidally dominated estuaries and embayments.

Comparison of tropical, carbonate and temperate, siliciclastic tidally dominated sedimentary deposits: Examples from the Australian continental shelf

Surficial deposits of the tidally influenced Australian shelf seas exhibit a variation in fades related to energy gradient. These deposits comprise a high energy gravelly facies, a mobile sand sheet facies and a low energy muddy sand facies. Such a facies distribution conforms generally with the existing model of continental shelf tidal sedimentation, derived for the west European tidal seas. However, the carbonate rich and mainly warm water deposits of the Australian shelf differ from the mainly quartzose and temperate cold‐water deposits of the European type case in terms of: (i) the role of seagrasses in trapping fine‐grained sediment; and (ii) the relative importance of the production of carbonate mud by mechanical erosion of carbonate grains. Seagrasses in Spencer Gulf, Gulf of St Vincent and Torres Strait are located in regions of strong tidal currents, associated with bedforms and gravel lag deposits. Thus, in the case of tropical carbonate shelves, seagrass deposits containing fine‐grained and poo...