Two transects were drilled across the Miocene carbonate platforms of the Marion Plateau during ODP Leg 194. The northern transect includes the proximal slope (Site 1194), distal slope (Sites 1192 and 1195), and platform (Site 1193) sites of a mixed carbonate-siliciclastic depositional system adjacent to the Australian continent. The northern MP2 platform drilled at Site 1193 represents the second phase of carbonate production that occurred during the middle Miocene on the Marion Plateau (Pigram et al., 1992; Pigram, 1993). The southern transect includes the platform (Sites 1196 and 1199), the leeward upcurrent slope (Site 1198), and the windward downcurrent slope (Site 1197) sites of an isolated carbonate bank ~65 km to the southeast of MP2 (Fig. F2). This platform was labeled MP3 by previous investigators (Pigram et al., 1992; Pigram, 1993) because it was thought to be entirely composed of the third phase of carbonate sedimentation occurring on the Marion Plateau in the late Miocene. A primary discovery made during ODP Leg 194 is that this southern platform is a compound edifice that has undergone three main phases of carbonate accumulation.
The following sections reconstruct the evolution of these two platforms on the basis of lithologies recovered during ODP Leg 194.Northern Transect (Sites 1192-1195)
Sedimentation on the Marion Plateau basement initiated on a topographically irregular basement consisting of volcanic and volcaniclastic rocks (Fig. F14A). The oldest sediments at Site 1195 include pieces of inner platform carbonates of possible Eocene age that could be transported clasts (not represented in Fig. F14A). During the early Miocene (23.8-16.4 Ma) siliciclastic estuarine sediments containing large oyster shells and larger benthic foraminifers accumulated at Site 1193. A subsequent sea level rise resulted in the deposition of a thick succession of mixed carbonate/siliciclastic material bearing glauconite and phosphate, which was also recovered at Site 1195. Between these two locations, a basement high at Site 1194 apparently formed an island in this time interval. Around 18 Ma (late early Miocene), silt-sized skeletal packstones/grainstones with quartz and terrigenous clays were deposited throughout the northern transect. They likely represent the mid- to distal slope sediments of a platform that was located to the north or north-northwest of the study area. This sedimentary package formed a gentle, eastward-facing ramp (Fig. F14B). Shallow-water facies, including skeletal rudstones/floatstones with bryozoans and larger benthic foraminifers first appeared in the northern transect in the early middle Miocene (~16 Ma) (Fig. F14C). Silt-sized skeletal wackestones/grainstones with clay and planktonic foraminifers accumulated downslope from these facies at Sites 1194, 1192, and 1195. At this time, this carbonate system still showed a homoclinal ramp geometry.
During the middle Miocene (16.4-11.2 Ma), a thick succession of shallow-water grainstones/floatstones with bryozoan fragments and larger benthic foraminifers accumulated as part of the MP2 platform (Site 1193; Subunit IIIA; Fig. F14D). Sediment shedding was directed eastward, as shown by seismic data and the occurrence of upper-slope skeletal packstones at Site 1194 (Unit IIIB). Distal slope sediments were retrieved from Site 1195. However, these sediments may not originate from the MP2 platform because the corresponding lithologic horizon at Site 1192 (Subunit III) consists of hemipelagic packstones with glauconite, clay, and no neritic components. During this time interval, the MP2 platform exhibited a flat-top platform morphology with a steep upper-slope between Site 1193 and 1194 (Fig. F14D).
An important sea level fall, possibly related to a major ice-building phase in Antarctica, occurred near the middle/late Miocene boundary. The Marion Plateau carbonate platforms were exposed and karstified (Figs. F14E, F14C). A 30-m-thick succession of skeletal packstones/floatstones dominated by bryozoans (Subunit IIIA; Site 1194) was deposited seaward of MP2 between ~13 and ~11 Ma, likely corresponding to a lowstand platform. Paleowater depth estimates of these sediments were used to calculate the amplitude of this major sea level change (see "Sea Level Variability").
A significant hardground with phosphate was formed on the top of the previously deposited lowstand platform sediments at Site 1194. During the ensuing late Miocene sea level rise, the MP2 platform was drowned and carbonate production ceased. Platform production did not reinitiate, as was observed for the southern platform. Lastly, from the latest Miocene to the Pleistocene (5.6 to 0.7 Ma), a hemipelagic sediment drift consisting of greenish gray planktonic foraminiferal mudstone/wackestone with clay onlapped on the drowned MP2 platform (Fig. F14G).Southern Transect (Sites 1196-1199)
In this area, the irregular basement topography likely includes numerous basalt lava flows that formed an escarpment in the vicinity of Site 1198. The earliest deposits encountered are phosphate sands mixed with siliciclastic material of latest Oligocene age (24.6-24.2 Ma) recovered from Site 1196 (Fig. F15A). It is not clear whether these sediments were produced in situ, transported laterally, or reworked from older rocks. The presence of oysters in these beds indicates a water depth <30 m. During the early Miocene (23.8-16.4 Ma) a carbonate platform developed in a topographic low at Site 1196 (Fig. F15A), building a thick series of aggrading rhodalgal carbonates. Contrary to earlier hypotheses (Pigram et al., 1992; Pigram, 1993), this platform did not nucleate on the slope sediments from the MP2 platform, but clearly predates them and thus likely corresponds to the MP1 phase of carbonate production seismically imaged along the eastern margin of the Marion Plateau (Pigram, 1993). Facies homogeneity suggests that accommodation space remained more or less constant during this time interval. Numerous exposure surfaces within Subunit IIIC at Site 1196 (variegated dolostone) indicate that water depth was less than the amplitude of the early Miocene sea level fluctuations. Off-platform shedding was restricted, but included a thin package of coarse skeletal grainstone with benthic foraminifers at Site 1197 and stable deep-euphotic accumulations of rhodalgal-foraminiferal floatstone at Site 1198. MP1 platform growth ceased near the early/middle Miocene boundary, corresponding to the first occurrence of shallow-water carbonate production in the northern transect.
MP2 carbonate production phase initiated in the early middle Miocene (16.4 Ma) (Fig. F15B). Red algal-dominated and carbonate reefs formed during the renewed transgression, exporting detritus to the slopes of both Sites 1197 and 1198. Occurrence of reefs and possibly shoals or islands on the platform top is inferred from the variable depth in the sedimentary section, at which these facies were recovered at Sites 1196 and 1199 (Figs. F9B, F15B). Shedding of silt-sized skeletal carbonate slope deposits was asymmetric, with greater accumulation at Site 1197, probably due to prevailing current directions. Between 15.2 and 13.3 Ma, MP2 growth occurred in a shallow-water setting dominated by fine grainstone rich in coralline algae, miliolid foraminifers, and seagrass remains. This facies filled the previously created irregular topography on the platform top (Fig. F15B). Reef growth was locally maintained, particularly on the northeast edge of the platform, where a buildup apparently existed as indicated by multichannel seismic data. At this stage, the platform was asymmetric with a flat-topped, rimmed western margin and an eastern margin showing a distally steepened ramp geometry (Fig. F15B). Platform shedding predominantly occurred in a downcurrent, downramp direction toward Site 1197 at the southeast of the section. Sediment shedding may also have taken place on the northwestern slope. However, the material forming Subunit IIC at Site 1198 more likely originated from a northwestern source, or a source located outside of the plane of the figure. The final pulse of the MP2 growth phase in this area was characterized by an increase in accommodation space expressed by the facies change from shallow (<20 m) seagrass-rich beds to rhodalgal floatstone (Subunit ID; Sites 1196 and 1199) (Fig. F15C) that suggest 60- to 100-m water depths. Reef growth speculatively continued on the northwest side of the platform, where it acted as a marginal barrier system. Sediment shedding at this time was exclusively toward Site 1197 in the southeast.
Lithologic data from Site 1199 suggest that sea level retreat at the onset of the late Miocene was rapid because no evidence of shallowing could be observed in the depositional environment (60-100 m) of the rhodalgal floatstone forming Subunit ID, which occurs directly below an exposure horizon (Fig. F15C). Sediments produced during this regression may have also eroded and been exported downslope.
During the late Miocene (11.2-5.3 Ma), the platform was again flooded and carbonate production resumed in this area (Fig. F15D). The eastern edge of the edifice was again occupied by a reef buildup. Sediments shed toward the west were likely swept away by the strong currents flowing parallel to the platform flanks. In the platform center, this sedimentary pulse began with a phase of reef construction (Subunit IC) followed by a shallowing-upward succession consisting of rhodalgal floatstone (Subunits IB and IA; Fig. F15D). The latter accumulated in water depths that did not exceed 100 m. The uppermost sediments of Subunit IA include intertidal (beachrock) cements indicating extremely reduced accommodation space near the end of this depositional episode. Sediments were predominantly exported toward the southeast. The geometry of the southeastern margin evolved from a distally steepened to a more homoclinal ramp. The platform was exposed again during the latest Miocene as shown by the occurrence of a karst surface recovered at Site 1196 and the development of a pedogenic profile at Site 1199.
The southern platform was drowned during the early Pliocene and thereafter was swept by bottom currents that contributed to the formation of a well-developed hardground surface with a laminated crust (Fig. F15E). The adjacent slope areas of Sites 1197 and 1198 were the locus of extensive hemipelagic drift deposition dominated by pelagic foraminifer remains, which filled the topographic lows adjacent to the platform leading to the modern bathymetrically rather uniform seafloor (Fig. F15E).Discussion
Lithostratigraphic and biostratigraphic data obtained from the Neogene carbonate platforms from 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.Northern Carbonate Platform (MP2)
The MP2 platform began to grow in the late early Miocene on low-angle, southward-dipping clinoforms. What is most striking in the evolution of this platform is the rapid onset of carbonate growth in a distal periplatform setting (Fig. F14C). The initiation of neritic sedimentation over midslope deposits suggests a major sea level fall at that time. The nearly flat surface at the base of the platform further shows that the onset of carbonate production was nearly simultaneous along the area seismically imaged near Site 1193. During the subsequent growth, the platform was predominantly aggradational producing a steep-sided margin (Fig. F14D). Carbonate production ceased when sea level dropped, exposing the platform at ~11-12 Ma. This sea level drop shifted neritic sediment production onto the former slope, producing a lowstand platform. The high-relief margin and sufficient water depth inhibited the establishment and growth of a "healthy" platform. Consequently, the subsequent sea level rise was able to outpace neritic sediment production and a hardground developed on this lowstand edifice. Why the MP3 carbonate production phase did not initiate on the MP2 still remains to be understood.Southern Carbonate Platform (MP2/MP3)
This compound carbonate edifice was initiated during the earliest Miocene and became inactive during the latest Miocene. It apparently nucleated in a topographic low, questioning the paradigm that carbonate-platform nucleation requires positive antecedent topography. During its growth, this platform developed an asymmetrical architecture with an escarpment-like margin on the northwestern side and impressive high-angle prograding clinoforms on the southeastern margin. Little sediment was transported off the escarpment to the northwestern slope despite important platform aggradation during the early and middle Miocene (Fig. F15A, F15B, F15C).Biological Factors
The steep-sided geometry of the MP2 and MP3 platforms originally suggested that they were constructed by tropical to subtropical faunal assemblages including corals. In contrast, cores retrieved during Leg 194 document a cool subtropical fauna assemblage consisting primarily of red algae, bryozoans, and large 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 mud which is typical for aragonite-dominated systems.Physical Energy
The carbonate platform architecture observed on the Marion Plateau can be best explained by the dominance of currents. Exposure to predominant wind direction results in different platform margin geometries (e.g., leeward and windward margins of the Great Bahama Bank). The Marion Plateau example demonstrates that, similarly, exposure to bottom currents also results in distinctive platform architecture. In addition, seafloor currents influence sedimentation on the periplatform apron as they inhibit sedimentation in the upcurrent position and form wide low-angle clinoforms in a downcurrent position.
Rapid growth of carbonate platforms has been documented in many ancient examples. The results of Leg 194 suggest that sea level in conjunction with current-dominated sedimentation is a possible process for this phenomenon. The fact that the cool subtropical faunal assemblage produces platform geometries that are reminiscent of tropical carbonates further indicates that physical parameters may be more important for the platform architecture than the biological ones.
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