Drowned carbonate platforms in the Huon Gulf, Papua New Guinea

Abstract

[1] The western Huon Gulf, Papua New Guinea, is an actively subsiding foreland basin dominated by drowned carbonate platforms. We investigated these platforms using new high-resolution multibeam, sidescan sonar and seismic data, combined with submersible observations and previously published radiometric and sedimentary facies data. The data reveal 14 distinct drowned carbonate platforms and numerous pinnacles/banks that increase in age ($20-450 kyr) and depth (0.1-2.5 km) NE toward the Ramu-Markham Trench. Superimposed on this overall downward flexing of the platforms toward the trench is a systematic tilting of the deep platforms 15 m/km toward the NW and the shallow platforms 2 m/km toward the SE. This may reflect the encroaching thrust load from the NW (Finisterre Range) and spatial variations in the flexural rigidity of the underlying basement. The drowned platforms form a complex system of promontories and reentrants, with abundant pinnacles and banks preserved at similar depths seaward of the main platforms. This configuration closely mimics the present-day Huon coastline and its seaward islands fringed by modern coral reefs. The platforms retain structural, morphologic, and sedimentary facies evidence of primary platform growth, drowning, and subsequent backstepping, despite some lateral erosion of the platform margins (<100 m slope defacement) by mass wasting. Both platforms and pinnacles are composite features containing multiple terrace levels and notches, corresponding to multiple phases of growth, emergence, and drowning in response to rapid climatic and sea level changes during the evolution of each structure. On the basis of all observational and numerical modeling data, we propose a chronology for the initiation, growth, and drowning of the 14 platforms. Over shorter timescales ( 100 kyr) the rate and amplitude of eustatic sea level changes are critical in controlling initiation, growth, drowning or subaerial exposure, subsequent reinitiation, and final drowning of the platforms. However, G 3

Coral reef evolution on rapidly subsiding margins

“…As a result of plate flexure, the rate of subsidence varied through time and space as the subsidence rates are highest closest to the subduction load. Superimposed on this general northeast downward flexing towards the trench, Webster et al (2004b) found that the deep (N−1000 m) reefs have also been systematically tilted ∼15 m/km towards the northwest, and the shallow reefs (b−300 m) tilted ∼2 m/km towards the southwest. They concluded that post-depositional reef tilting is related to the location of the centroid of the encroaching thrust mass (the Finisterre Range) to the NW ( Fig.…”

“…1A). A classic fore-deep has developed on the lower plate of the convergence zone in the Huon Gulf (Galewsky et al, 1996) with a complex series of 14 reefs, pinnacles, and banks now preserved 100 to 2500 m below sea-level (Webster et al, 2004b). U/Th dating of corals (details in Section 5) from the drowned reef at −2000 m (348 ka) and −250 m (60 k) constrain the rate of subsidence between 2 and 6 m/ka (Galewsky et al, 1996;Webster et al, 2004b) (Fig.…”

Drowned carbonate platforms in the Bismarck Sea, Papua New Guinea

“…Such platforms have been observed around the Hawaiian Islands (Moore and Campbell 1987;Grigg et al 2002), in the Solomon Sea, Papua New Guinea (Davies et al 1987a;Galewsky et al 1996;Webster et al 2004), in New Zealand (Stern and Holt 1994), and elsewhere. Subsidence in Hawaii was caused primarily by flexural volcanic loading (Campbell 1984), with the platforms tilting landward towards the locus of loading (Moore and Campbell 1987).…”

“…1). Davies et al (1987a), Galewsky et al (1996), and Webster et al (2004) described a series of submerged pinnacle carbonate platforms and drowned shallow-water reefs on the under-riding plate in the Huon Gulf. In MR1 sidescan imagery, the platforms appear as a sinuous line of high backscatter, which corresponds to the platform rim (Fig.…”

“…Subsidence in Hawaii was caused primarily by flexural volcanic loading (Campbell 1984), with the platforms tilting landward towards the locus of loading (Moore and Campbell 1987). In the Solomon Sea, however, subsidence was due to flexural loading of the subducting Australian plate by the overriding South Bismarck Plate, resulting in seaward tilting (Davies et al 1987b;Abbott et al 1994;Galewsky et al 1996;Galewsky 1998;Wallace et al 2004;Webster et al 2004). …”