Tectonics of the Marion Plateau
The eastern Coral Sea has been affected by two distinct tectonic events. The earlier event, late Jurassic-Early Cretaceous in age, was responsible for the formation of the Queensland and Townsville Basins, which underlie the present-day bathymetric features of the Queensland and Townsville Troughs (Fig. 6). These basins formed because of oblique extension along pre-existing Paleozoic structural trends (Struckmeyer and Symonds, 1997). The Queensland and Townsville Basins do not appear to have been affected by the later tectonism responsible for seafloor spreading in the Tasman and Coral Sea Basins (Struckmeyer and Symonds, 1997).
In the Late Cretaceous, rifting in the Coral Sea Basin created numerous continental fragments, which are now capped by carbonate platforms, such as the Marion and Queensland Plateaus (Fig. 6). Rifting in the Coral Sea was an extension of Late Cretaceous (80 Ma) seafloor spreading in the Tasman Basin, which extended to the north to form the Cato Trough and the Coral Sea Basin by 65 Ma (Fig. 6; Weissel and Hayes, 1971; Hayes and Ringis, 1973; Shaw, 1978). Spreading is believed to have ceased along the length of this system by the earliest Eocene (52 Ma; Gaina et al., 1999). Thus, the main physical elements of the western Coral Sea were in place by the early Tertiary (Davies et al., 1989). Although the exact structural style and development history of the rift system is still not completely understood, it is clear that the late Jurassic-Early Cretaceous rifting event controlled the gross architecture of the margin in addition to the form of the high-standing structural elements on which the carbonate platforms in the area are located.
The Marion Plateau is a largely undeformed basement block with faults occurring only on its margins. Basement along the northern margin consists of gently dipping ramps that gradually deepen toward the Townsville Trough until a fault is encountered. Normal extensional faults along this northern margin are restricted to the edge of the plateau and include both down-to-basin faults with dips to the north-northwest and normal faults of opposite polarity that dip beneath the plateau (Symonds, et al., 1988). The eastern margin of the plateau is free of major structural offsets (Mutter and Karner, 1980), and the slope is apparently simple and continuous. Faults along this margin are steeply dipping to vertical and the margin of the plateau downfaults into the Cato Trough. The southern part of the plateau is formed by a southeasterly plunging, gently arched basement high. The top of the arch is unstructured, and faults are confined to the flanks of the arch and appear on conventional seismic data to be high-angle, down-to-basin normal faults (Pigram et al., 1993).
The basement of the Marion Plateau is likely to be similar to that of the Queensland Plateau to the north. This basement was cored during Leg 133 (Sites 824 and 825; Fig. 1) and consists of fine grained, dark gray, poorly foliated, well-lithified quartz-feldspar-mafic metasediment or metavolcanic rocks. These rocks are similar to those found in the onshore Queensland Hodgkinson Province (Ordovician-Devonian), which outcrops as part of the northern Tasman Fold Belt (Feary et al., 1993). Planation of the surface occurred during subaerial exposure in the Mesozoic and Paleogene prior to the deposition of Megasequence A.
As stated previously, no direct sampling of the basement under the Marion Plateau has occurred, but seismic data and a recently developed plate model indicate that basement crustal blocks of the Queensland and Marion Plateaus had roughly similar tectonic histories in regard to rifting and extension (Gaina et al., 1999; Struckmeyer and Symonds, 1997). The presence of shallow-water (~20 m) carbonate sediments directly overlying basement at Sites 824 and 825 on the Queensland Plateau indicates that the planated basement surface of the Queensland Plateau was at or near sea level immediately prior to the onset of sedimentation. Using this information, we can estimate the thickness of the crust under the Queensland Plateau. Assuming average crustal density, the upper surface of a 30-km-thick crust would exist at sea level. Tectonic subsidence models show almost no change in the depth of the Queensland Plateau surface between 25-10 Ma (Fig. 7). Thus, we can conclude that the early Tertiary opening of the Coral Sea resulted in little crustal thinning on the Queensland Plateau. Otherwise, we would not observe thermal subsidence rates equal or close to zero in the early Tertiary. These results also show that thermal subsidence from the earlier Late Jurassic-Early Cretaceous event could no longer be detected on the Queensland Plateau during the Neogene.
Subsidence History of the Marion Plateau
The tectonic histories of the Marion and Queensland Plateaus are well constrained by Leg 133 sites and extensive multichannel seismic data. Subsidence curves for these plateaus have been produced using both benthic foraminifera (Fig. 8; Katz and Miller, 1993) and geohistory modeling (Fig. 7; Müller et al., 2000). Geohistory models were calculated using integrated geophysical logs, biostratigraphic/lithologic information, and seismic reflection data (Müller et al., 2000). These models predict post-9-Ma subsidence of 1300 ± 200 m in the Queensland Trough and 650 ± 200 m on the western margin of the Queensland Plateau and post-5-Ma subsidence of 500 ± 30 m on the southern margin of the Queensland Plateau and 660 ± 50 m on the northern margin of the Marion Plateau (Fig. 7; Müller et al., 2000). Although the Marion and Queensland Plateaus are located on a passive margin ~1000 km south of the Pacific-Australian plate boundary, geohistory models predict a greater amount of post-9-Ma subsidence than simple elastic models do. This subsidence occurred in pulses between 9 and 5 Ma on both plateaus. It is difficult to account for this observed subsidence, either by means of thrust loading in Papua New Guinea or by a combination of such thrust loading and in-plane stresses originating from collision along the Australian-Pacific plate boundary (Müller et al., 2000).
Müller et al. (2000) suggest that the observed post-9-Ma tectonic subsidence of the Queensland and Marion Plateaus and Queensland Trough is largely caused by dynamic surface topography resulting from Australia's northeastern margin overriding a slab burial ground and modulated by flexural deformation resulting from collision tectonics north of Australia. This conclusion is supported by shear-wave tomography data (Zhang and Tanimoto, 1993) that shows a north-northwest to south southeast trending band of anomalously high velocities in the upper mantle at depths between 300 and 650 km. Although unproven, this explanation appears to be the most reasonable for all available data.
Post-9-Ma subsidence rates on the Marion Plateau are much lower than those of third-order sea-level changes and can thus be differentiated from glacial eustasy. In addition, any unaccounted subsidence will be the same for all sites along the transect as they were selected along lines of equal subsidence. Although we will attempt to quantify in detail the additional water depth added to all sites as a result of tectonic subsidence, this increase is likely to be less than 10 m between N12-N14 and thus will not greatly affect our attempts to quantify eustatic sea-level variations.
Stratigraphy of the Marion Plateau: Evidence from Prior Drilling
Stratigraphies for the Marion Plateau were obtained during ODP Leg 133 (Fig. 9), and these data supplement previously acquired extensive seismic surveys over the plateau (Fig. 10). Both of these data sets have enabled a description of the Marion Plateau depositional history. Initiation of shallow marine carbonate sedimentation on the Marion Plateau began during the latest Paleogene as the sea transgressed across the metasedimentary basement of the plateau (Davies, McKenzie, Palmer Julson, et al., 1991). We have no direct age controls on the sediments that form Megasequence A as they have not yet been sampled. Their age is inferred by their stratigraphic position under Megasequence B (Pigram et al., 1993; Figs. 3 and 10). These first sediments over basement are believed to be primarily siliciclastics, with temperate water carbonates occurring in the eastern part of the sequence.
Sedimentary facies recovered during Leg 133 and their correlation to seismic profiles indicate that tropical reef development was initiated on the Marion Plateau in the early Miocene, and by the middle Miocene there was extensive reef growth on the plateau (Davies, McKenzie, Palmer-Julson et al., 1991). These reefal sediments are part of Megasequence B and include the aggrading and prograding MP2 carbonate platform (Figs. 3, 10).
In the late middle Miocene, carbonate bank productivity rapidly diminished on the Marion Plateau, as shown by a reduced fine-grained bank-derived component in slope sediments. This decline was primarily the result of subaerial exposure resulting from a sea-level regression that caused the demise of the MP2 platform. During the low sea-level interval between 11 and 7 Ma, the MP3 platform was initiated on the eastern side of the Marion Plateau. Despite the fact that MP3 developed during a lowstand, the platform continued its development during subsequent highstand intervals. MP2, on the other hand, did not reinitiate even after being reflooded during the subsequent sea-level increases. During the development of MP3 the western two-thirds of the Marion Plateau was exposed, forming a broad, low-relief karstic surface. Unlike MP2, MP3 has not completely drowned but is now restricted to the area of Saumarez Reef. The limited sampling of MP3 inhibits speculation on the cause for partial drowning, although some likely factors are reduced sea-surface temperatures (Isern et al., 1996) and increased terrigenous inputs causing increased water-column turbidity (Pigram et al., 1993).
Carbonate production from the Pliocene to Holocene never again achieved the areal extent that existed in the Neogene. Instead, hemipelagic drift sediments dominated sedimentation on the Marion Plateau.
Scientific Objectives | Table of Contents