The combination of the microfacies results and the lithologic synthesis with the available seismic and stratigraphic data set from the Shipboard Scientific Party (2002a, 2002b) allowed us to refine the depositional architecture of the SMP on a regional scale (>5 km). In addition, we will define the geometry of some sedimentary bodies on a smaller scale (a few hundred meters).
Based on seismic analysis, stratigraphy, and biosedimentology, the Shipboard Scientific Party (2002a) established the depositional architecture at the scale of the SMP (>5 km). They described an asymmetrical platform with a northwestern flat-topped, reef-rimmed margin and an evolving southeastern margin from a distally steepened to a more homoclinal ramp (Fig. F3). In this pattern, they related the asymmetrical platform architecture to the strong influence of currents from the north to the south. In addition, the Shipboard Scientific Party (2002a) inferred a water depth increase and a coral reef–development decrease toward the southwest. Thus, Sites 1196 and 1199 were drilled in the flat-topped inner part of the platform behind a possible reef rim. Such a depositional architecture with rimmed platform and ramps has already been described or inferred from seismic analysis and biosedimentological studies in other Cenozoic carbonate platforms from Australia and the Indian Pacific Ocean (e.g., Chapronière, 1975; Betzler, 1997; Marshall et al., 1998; Noad, 2001; Fournier et al., 2004). Nevertheless, our analysis of the SMP's coral assemblages cannot reinforce the hypothesis of a well-developed reef rim because no primary framebuilders were found at Sites 1196 and 1199, as shown in Table T1.
The Shipboard Scientific Party (2002a, 2002b) considered the SMP's lithostratigraphic succession to be mostly monoclinal from one site to the other. They also demonstrated that the corresponding subunits showed very similar sedimentary facies, fossil assemblages, and thicknesses (Figs. F3, F5).
Our statistical analysis of microfacies partly confirms the lithostratigraphic subdivisions. Indeed, the composition and evolution of microfacies are mostly identical in each of the subunits represented within both sites (Fig. F5). Nevertheless, some exceptions remain for Subunits IC, ID, and IIB. Indeed, the microfacies composition and the fossil assemblages of these subunits signal the influence of coral reefs and rather support the alternative logging stratigraphy (Figs. F5, F6). In the logging stratigraphy, the boundary between logging Units 3 and 2 represents a gradual sedimentary change from rhodolithic floatstones (Microfacies D, E, and F) to coral boundstones-floatstones (Microfacies B and A) within Hole 1199A (Fig. F5). The boundary between the logging Units 3 and 4 and the dashed boundary situated just upcore in logging Unit 3 (Fig. F6) represent a gradual sedimentary change from large hyaline benthic foraminiferal floatstones (Microfacies C), coral boundstones-rudstones (Microfacies B), and porcellaneous foraminiferal grainstones (Microfacies A). Further, Subunit IIB and the coral upper part of Subunit IIIA (Hole 1196A) are correlated to the coral lower part of Subunit IIA (Hole 1199A), with respect to the logging stratigraphic pattern (Fig. F5). Finally, the lower part of Subunit IIIA (Hole 1196A) is correlated to Subunit IIB (Hole 1199A) (Fig. F5). At both sites, a dolomitic sucrosic facies appears at ~410 mbsf (Fig. F5). This facies most likely corresponds to the top of the old dolomitic platform, mostly dated early Miocene in age by the Shipboard Scientific Party (2002a) (Fig. F3). Further, the studied 410-m-thick stratigraphic succession may be subdivided into two parts with respect to the composition of microfacies and sedimentary rocks. The lower part would include Unit II and Subunit IIIA, and the upper part would correspond to Unit I.
A seismic profile a few hundred meters long provides well-defined seismic reflections in the 200-m-thick uppermost part of the platform succession at Site 1196 (Fig. F10). The analysis of these reflections suggests the existence of lens-shaped sedimentary bodies as thick as 30 m and prograding clinoforms in lithostratigraphic Unit I (Fig. F10). Such a sedimentary architecture supports the Shipboard Scientific Party's (2002b) observation of lateral variations as much as 20 m thick in Subunits IC and ID from Holes 1196A to 1199A, which lie at a distance of 20 m. Indeed, the difference of the thickness between the two holes was considered significant, as it exceeds the depth errors due to coring (Shipboard Scientific Party, 2002b).
Several sedimentary features encountered at both Sites 1196 and 1199 are also consistent with the seismic architecture. Thus, some cross-bedded stratifications underlined by rhodolithic layers were observed in core sections from Unit I (Shipboard Scientific Party, 2002b). These stratifications may correspond to the clinoforms suspected in the seismic profile (Fig. F10). The occasional grading of sediments with recurrent textural variations, the sedimentation breaks as exemplified by several generations of geopetal deposits, and the winnowing of the fine sand portion of sediment found in the rhodolithic Microfacies D, E, and F of Unit I can be characteristic of debris flows in slope settings. These sedimentary features may characterize the progradation of the clinoforms that are a response to strong currents that are inferred from the SMP's asymmetrical architecture by the Shipboard Scientific Party (2002a). These sedimentary features may also be associated with reef talus deposition. Indeed, an upcore gradual change in sedimentation occurs from rhodolithic floatstones to coral rudstones-boundstones in Subunits ID and IC at both sites. The presence of coral boundstones in Subunit IC at both sites points to the settlement of coral reefs at the SMP's scale. The available seismic data provide poor information of the geometry of these coral reefs. Nevertheless, some reflections may be interpreted as 200-m-long and 20-m-thick lens-shaped bodies (Fig. F10).
Although the association of both coral and rhodolithic facies have been documented in Cenozoic and modern platforms from Australia (e.g., Betzler, 1997; Davies, McKenzie, Palmer-Julson, et al., 1991; Davies and Peerdeman, 1998; Marshall et al., 1998), the geometry of the sedimentary bodies is poorly illustrated at a small scale. On the contrary, the geometry of a set of clinoforms, possibly similar to those of the SMP, is well known in some rhodolithic distally steepened ramps, dated late Miocene, from the Mediterranean region. Thus, Pomar et al. (1996) and Pomar (2001) reported a set of 2-km-long and 40-m-thick clinoforms in a 20-km-long prograding rhodalgal ramp from the Balearic Islands (Spain). In addition, Saint Martin et al. (1997) characterized a vertical stacking set of clinoforms 1–4 km long and 10–30 m thick in a 10-km-long prograding rhodalgal ramp from the Maltese Islands. Within this ramp, the clinoforms exhibit scattered lens-shaped coral reefs at the toes of their topsets.
Finally, the definition of the seismic profile did not really allow us to discern the sedimentary architecture of lithostratigraphic Unit II (Fig. F10). Nevertheless, the large hyaline foraminiferal deposits (Microfacies C) of Subunit IIB (Site 1199) and Subunit IIIA (Site 1196) indicate the existence of slope deposition as do the three rhodolithic microfacies (D, E, and F) from Unit I. On the contrary, no sedimentary features characteristic of slope deposition were observed in the porcellaneous foraminiferal deposits (Microfacies A) of Subunit IIA. Chapronière (1975) reported similar porcellaneous foraminiferal deposits in subhorizontal beds, dated middle Miocene, from western Australia.
The reconstruction of depositional environments is necessary to better understand the biosedimentary and paleoenvironmental evolution of the SMP at Sites 1196 and 1199. Based on the microfacies analysis, the corresponding paleoenvironmental interpretations, and the stratigraphic data, we defined an upcore vertical succession of six depositional environments at both sites (Fig. F5). Further, we proposed two reconstructions for Unit II–Subunit IIIA and Unit I, respectively (Fig. F11). The reconstructions correspond to the two main lithostratigraphic subdivisions of the vertical succession at both sites. They represent a theoretical zonation of platform deposits on the scale of the sedimentary bodies (at most 1 km in length) with repartitioned microfacies and the corresponding depositional environments. These reconstructions take into account the seismic architecture, the biosedimentological data synthesized in the site lithostratigraphic columns (Fig. F5), and the results of the microfacies analysis. Thus, the repartitioned microfacies in each reconstruction is inferred from the microfacies' interrelationships with respect to the environmental gradients defined by the CFA (Fig. F7) and from their vertical succession and interlayering next to the site lithostratigraphic columns (Fig. F5). The close interrelationships of the microfacies and the gradual sedimentary evolution of the corresponding deposits at Sites 1196 and 1199 suggest a lateral coexistence of the superimposed depositional environments. Finally, one must note that these reconstructions illustrate only a restricted area of the inner part of the SMP, limited at its northeastern side by a possible reef rim (Shipboard Scientific Party, 2002a).
The proposed reconstruction illustrates a theoretical platform setting with the depositional environments of outer platform, coral reef, and inner platform (Fig. F11). The corresponding deposits and microfacies are superimposed and show gradual changes along the vertical succession of Subunits IIIA–IIB and IIA at Sites 1196 and 1199 (Fig. F5). Nevertheless, the geometry of the sedimentary deposits remains in question, as no well-defined seismic data are available. Consequently, our reconstruction also takes into account some other Cenozoic environmental models from Australia and the Indo-Pacific region, which exhibit comparable facies and biogenic components (Chapronière, 1975; Hallock and Glenn, 1986; Betzler and Chapronière, 1993; Fournier et al., 2004).
With respect to our microfacies results, the inner platform corresponds to a moderate- to high-energy environment under open-ocean influences. The prevalent porcellaneous foraminiferal fine sand (Microfacies A) deposits are only represented in Subunit IIA. Chapronière (1975) assigned a maximum water depth of 30 m to a comparable foraminiferal assemblage from Australia. The deposits are winnowed, probably from the action of bottom currents. Muddier fine sands are present and may be related to a sheltering effect of sea-grass meadows and scattered coral patch reefs (Microfacies B). The coral reef environment is also of high to moderate energy with respect to the coral assemblages. The deposits are a mixture of porcellaneous foraminiferal fine sands and coral–red algal coarse sands (Microfacies B and D) that are encountered in Subunits IIIA–IIB (Site 1196) and Subunit IIA (Site 1199). The outer-platform environment shows episodic variations of energy and sedimentary breaks, which we attributed to slope deposition and/or storm action. The prevalent deposits are unsorted Miogypsina-Lepidocyclina sands (Microfacies C) represented in Subunits IIB (Site 1199) and IIIA (Site 1196). We assigned a maximum water depth of 50–60 m to this large, hyaline foraminiferal assemblage, following the models of Chapronière (1975) and Hallock and Glenn (1986). Finally, these deposits register the influence of coral reef and inner-platform environments as they contain rare coral and a porcellaneous foraminiferal assemblage similar to those of Microfacies A.
Thus, we relate the existence of the inner-platform environment to the development of coral reefs as shown by others studies for similar depositional settings (e.g., Hallock and Glenn, 1986; Betzler and Chapronière, 1993; Robertson, 1998; Fournier et al., 2004). In these studies, the coral reefs form a reef rim and sometimes surround an inner-platform area (Robertson, 1998; Fournier et al., 2004). Although a reef rim is reported at the northwestern side of the SMP (Shipboard Scientific Party, 2002a), the extension and the nature of the coral reef environment cannot be more precisely defined at Sites 1196 and 1199.
The proposed reconstruction illustrates a theoretical platform setting with the depositional environments of outer platform and coral reef (Fig. F11). The corresponding deposits and microfacies are represented in three subunits (IA, IB, and ID) and in single Subunit IC (Fig. F5). The reconstruction of the sedimentary architecture takes into account the available seismic data (Fig. F10). Thus, the deposits characterized by the three rhodolithic microfacies (D, E, and F) correspond to several sets of prograding clinoforms. The coral boundstones (Microfacies B) coincide partly with lens-shaped bodies we interpreted as possible coral reefs (Fig. F10). Nevertheless, the exact geometry of the sedimentary bodies still remains hypothetical and needs more detailed seismic data to be precisely established. Thus, our reconstruction also takes into account some other Cenozoic environmental models from the Mediterranean region, which show similar geometry, facies, and biogenic components (Pomar et al., 1996; Saint Martin et al., 1997; Robertson, 1998; Pomar, 2001).
With respect to our microfacies results, the outer platform consists of a high- to moderate-energy environment with common sedimentary breaks, which we attribute to slope deposition controlled by sporadic storms. The prevalent deposits are coarse sands with numerous rhodoliths as large as 10 cm (Microfacies F). These deposits sometimes include either additional derived reefal material with coral fragments and reworked Amphistegina-Nummulitids (Microfacies D) or additional large and flat Lepidocyclina (Microfacies E), possibly indicative of great depths (Fig. F11). In addition, high numbers of Halimeda plates are present within Microfacies D and E, possibly indicating the existence of Halimeda meadows, as is the case at depths ranging from 20 to 100 m in Holocene to modern environments from the Great Barrier Reef (Davies and Marshall, 1985; Drew and Abel, 1988). Martín et al. (1993) assigned a storm-influenced outer-platform setting at depths ranging from 30 to 80 m to such rhodolithic facies from the Northern Marion Plateau. The coral reef environment might be represented by the development of coral patch reefs in a moderate- to high-energy setting. The prevalent deposits are coral boundstones-rudstones (Microfacies B) and rhodolithic coarse sands (Microfacies D); both are encountered in Subunit IC. The other rare deposits are benthic foraminiferal sands (Microfacies A and C), which may be indicative of a sheltered reefal environment. The coral environment might represent a lateral extension of the reef rim of the northwestern side of the SMP.
Defining and ordering the six microfacies (A–F) with respect to their depositional signature from the inner platform, coral reef, and outer platform allowed the construction of a microfacies and environmental curve next to the lithostratigraphic columns of Sites 1196 and 1199 (Fig. F5). We recognized an almost identical succession of six depositional environments at both sites:
Thus, most of the changes of facies and environments clearly coincide with either the lithostratigraphic or logging stratigraphic subdivisions of the SMP proposed by the Shipboard Scientific Party (2002a, 2002b) (Fig. F5). The Shipboard Scientific Party (2002a, 2002b) described most of the retrieved boundaries as iron-stained micritic crusts a few centimeters thick. They interpreted the crusts as the result of hardground development following a possible exposure. Nevertheless, no unquestionable diagenetic features were found in thin sections to remove uncertainties about the existence of exposure (Shipboard Scientific Party, 2002b) Therefore, it has been stressed that additional diagenetic and isotopic studies will be necessary to refine the origin of the boundaries (Shipboard Scientific Party, 2002b).
The analysis of the facies and environmental evolution, with respect to the stratigraphic subdivisions at both sites, points to the existence of three main shallowing sequences that are intercalated with three abrupt deepening events (Fig. F5).
Sequence 1 corresponds to Subunits IIIA, IIB, and IIA and is represented by a gradual evolution of environments from (1) outer platform, (2) coral reef, and (3) inner platform. Using biostratigraphy from Boudagher-Fadel and Banner (1999), the upper part of the sequence (Subunit IIA) can be dated middle Miocene based on the foraminiferal assemblage of Austrotrillina howchini and F. bontangensis. On the contrary, the early middle Miocene age attributed to Subunits IIIA–IIB by the Shipboard Scientific Party (2002a, 2002b) cannot be determined with precision.
Sequence 2 corresponds to Subunits ID and IC, recording the succession from (4) an outer-platform environment to (5) a coral reef environment. The onset of this sequence is marked by an abrupt deepening with respect to the underlying inner-platform environment of Subunit IIA. The Shipboard Scientific Party (2002a, 2002b) reported iron-stained micritic crusts a few centimeters in thickness at the tops of both Subunits ID and IC. They attributed the boundary between Subunits ID and IC to the major late middle Miocene (N12–N14) sea level fall (Haq et al., 1988). This sea level fall was estimated in the Northern Marion Plateau at 86 m ± 30 mbsl (Shipboard Scientific Party, 2002a). However, we question the occurrence of such an event at this lithostratigraphic level. Indeed, our facies analysis instead showed a gradual biosedimentary evolution from Subunit ID to Subunit IC, which is more consistent with the alternative logging subdivisions of the Shipboard Scientific Party (2002b) (Figs. F5, F6). Moreover, the overlying Subunit IC contains unreworked larger benthic foraminifers Miogypsina spp. The last occurrence of this genus is positioned at the upper end of the planktonic Zone N12 or in the lower part of Zone N13 for the Indo-Pacific (Boudagher-Fadel and Banner, 1999; Boudagher-Fadel et al., 2000a). Thus, we propose instead to relate the surface between Subunits IC and IB to this second-order event. This surface would thus correspond to the Megasequence B/C seismic boundary, dated through seismic correlations to ~11 Ma by the Shipboard Scientific Party (2002a).
Sequence 3 occurs in Subunits IB and IA and is characterized by (6) an outer-platform environment. The onset of this sequence is marked by an abrupt deepening with respect to the underlying coral reef environment of Subunit IC. The lower part of the sequence is devoid of coral inputs, whereas its upper part shows the increasing influence of a coral reef environment. The Shipboard Scientific Party (2002a, 2002b) described at the top of Subunit IA an iron-stained irregular surface, overlain by laminated ferromanganese crusts and/or planktonic wackestone deposits, which they dated Pliocene in age. Further, they assumed that this surface coincides with the Megasequence C/D seismic boundary they dated to ~7.2 Ma and the drowning of the SMP. Thus, Sequence 3 occurred during the early late Miocene.
The correlation of the three shallowing sequences and the three deepening events to a global sea level curve remains problematic regarding potential high sea level variations during the middle and late Miocene (Haq et al., 1988). Sequences 1 and 2 of middle Miocene age record two shallowing phases that are intercalated with an abrupt deepening event. The sequences may correspond to two third-order cycles associated with the global middle Miocene sea level fall. The end of this event is represented at the SMP by an unconformity at the top of Sequence 2. Sequence 3, dated early late Miocene in age, represents a general shallowing consecutive to an abrupt deepening event with respect to the underlying coral reefs of Sequence 2. Thus, Sequence 3 may correlate to one of the third-order cycles following the major second-order sea level fall. Finally, the top of Sequence 3 was partly related to the Pliocene drowning of the Marion Plateau (Pigram et al., 1992; Pigram, 1993; Shipboard Scientific Party, 2002a).
In the adjacent Queensland Plateau, depositional sequences of carbonate platform were established and the sea level variations were estimated from the middle Miocene to the Pliocene (Davies, McKenzie, Palmer-Julson, et al., 1991; Brachert et al., 1993; Betzler et al., 1993, 1995; Betzler, 1997). Nevertheless, a precise correlation of the platform deposits from the Queensland and Marion plateaus remains difficult to establish with respect to the biostratigraphic resolution and core recovery. Only some global events can be recognized in both areas. Thus, two deepening events occurring in the early late Miocene and in the Pliocene are reported in the Queensland Plateau (Betzler et al., 1995; Betzler, 1997). They may correlate to the two deepening events present at the tops of Sequences 2 and 3 from the Southern Marion Plateau. Finally, the effect of the second-order sea level fall (late middle Miocene in age) was also detected in the Queensland Plateau, particularly from the basinward shift in onlap along seismic profiles (Davis, McKenzie, Palmer-Julson, et al., 1991).