Estimating the amplitude and timing of past eustatic sea level changes is essential both for the establishment of an accurate Phanerozoic sea level curve and for the interpretation of sediment sequences on continental margins. The magnitude and timing of sea level changes can be estimated in tectonically stable environments by correlating seismic data with lithologic facies, biofacies (for paleowater depths), and chronostratigraphy.
During Leg 194, a series of eight sites were drilled through Oligocene-Holocene mixed carbonate and siliciclastic sediments that record the depositional history and past sea level variations of the Marion Plateau, northeast Australia. Seismic sequence stratigraphy provided the geometric framework to locate the drill sites for optimal sampling of highstand and lowstand sequences with geometries that enable quantification of Miocene relative sea level variations. At each site, ~500-700 m of sediment was drilled, and five sites penetrated to acoustic basement. During this leg, ~2 km of sediment was recovered from a total cored interval of 5 km. Water depths of the drill sites ranged from 304 to 419 m.
In addition to determining sea level magnitudes, Leg 194 drilling also addressed the following scientific themes: (1) the development of carbonate platforms in a current-dominated environment, (2) the development of subtropical carbonate platforms, (3) facies changes and the development of sequence stratigraphic units controlled by sea level changes in a mixed carbonate and siliciclastic sediment system, (4) the record of Oligocene-Pliocene third-order sea level fluctuations, (5) the mechanisms and causes of fluid flow within pure carbonate and mixed siliciclastic/carbonate depositional environments, and (6) the role of climatic and paleoceanographic change in the subtropical South Pacific and its influence on carbonate platform development.
The original hypothesis for Leg 194 drilling postulated that the magnitude of the middle Miocene (Zones N12-N14; 12.5-11.4 Ma) sea level fall could be estimated from seismic sequences and facies relationships between the top of the northern Marion Plateau 2 (MP2) deposited during the highstand and the base of the southern Marion Plateau 3 (MP3) platform initiated during the lowstand. Drilling results showed, however, that the southern platform is not solely composed of MP3 age-equivalent sediments. Instead, ~120-160 m of MP3 overlie a thick middle to early Miocene platform (MP2). The magnitude of the middle Miocene sea level fall was therefore estimated by reconstructing the paleobathymetry of the MP2 platform with respect to a late middle Miocene lowstand ramp immediately adjacent to its margin. The base of the lowstand unit lies 145 m below the top of the MP2 platform today. Paleowater depths estimates are 10-50 m for the top of the MP2 platform at Site 1193 and 30-50 m for the lowstand unit at Site 1194. Physical properties information was used to remove the compaction effects imparted by the overlying sediments at both sites. Combining the decompaction with the paleowater depth estimates, a late middle Miocene sea level drop between 55 and 115 m is required to produce today's sediment geometries at Sites 1193 and 1194. These values assume infinite crustal strength between the sites. If zero flexural strength is assumed for the crust, the local isostatic compensation would further reduce the required magnitude of the sea level drop to 10-50 m. The undisturbed and consistently dipping sediments, as well as the horizontal basement geometry between the two sites, favor the infinite flexural strength model over local isostacy.
Lithostratigraphic and biostratigraphic data obtained from the Neogene carbonate platforms of the Marion Plateau during Leg 194 reveal that platform architecture was controlled by a series of complexly related factors including sea level change, bottom-current action, and biological assemblages. Two platform to slope transects were drilled across the Miocene carbonate platforms of the Marion Plateau. An important finding is that the oldest platform phase of the southern carbonate platform developed in a topographic depression as opposed to the more common condition of nucleation on a topographic high. Furthermore, the steep-sided geometry of both Marion Plateau carbonate platforms is typical of tropical to subtropical carbonate platforms. Despite this, cores retrieved during Leg 194 document a cool subtropical faunal assemblage consisting primarily of red algae, bryozoans, and larger foraminifers. These calcite-dominated biogenic sediments have a lower diagenetic potential than their aragonite-dominated counterparts in the tropical realm. They can therefore be reworked more easily as they undergo less cementation. In addition, the fragmentation of these sediments leads to the formation of silt- to fine sand-sized particles rather than carbonate mud. Another important finding of Leg 194 is that unlike other carbonate systems, the morphologies of which are predominantly controlled by wind direction, the carbonate platform architecture observed on the Marion Plateau was strongly influenced by high-energy currents near the seafloor, similar to those that exist on the modern Marion Plateau. These currents inhibit sedimentation in the upcurrent position and form wide low-angle clinoforms in the downcurrent position, resulting in an asymmetric platform shape.
Pore waters from Leg 194 sediments provide clear evidence that seawater is circulating through the proximal sediments on the Marion Plateau. The extensive dolomitization found in both platforms is itself indirect evidence for past fluid circulation, as dolomite formation on a large scale requires fluid exchange to deliver magnesium to the precursor calcium carbonate sediments. Although sampling of pore waters from within the MP2/MP3 platform was not possible because of low recovery, samples taken from the adjacent periplatform sites are characterized by seawater values for most elements. These samples provide evidence that seawater is circulating through these sediments even though they are overlain by ~200 m of hemipelagic deposits. By inference, seawater is probably also circulating through the MP2/MP3 platform. Similarly, pore water samples from directly above and below the carbonate platform facies of MP2 at Site 1193 show elemental concentrations close to seawater in composition, suggesting seawater circulation.
Acoustic basement was penetrated at five sites during Leg 194, and the basement rocks collected differ greatly from those drilled on the Queensland Plateau. Rather than metasedimentary rocks, highly altered volcanic flows and volcaniclastics were recovered. The lack of deformation in hand samples and thin sections suggests that these volcanics may have formed during the Late Cretaceous-Paleocene rifting along northeastern Australia from the Papuan Plateau and the Lord Howe Rise in the south. High-quality paleomagnetic data collected from these basement volcanics, when compared with the Australian apparent polar wander path, may provide age estimates for both the emplacement of the basalts and the timing of their low-temperature alteration.
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