The analysis of biogenic components and foraminiferal assemblages using the above methodology and the sedimentary features observed in cores and thin sections allowed six microfacies to be defined. These were labeled A through F, and their interrelationships were established (Fig. F7). The microfacies are given with their percentage in Figure F4 and are shown next to lithostratigraphic columns (Figs. F8, F9). The microfacies distribution at both Sites 1196 and 1199 does not follow a random pattern. Indeed, most of the microfacies are restricted to well-defined cored interval of lithostratigraphic subunits with respect to upcore variations of fossils assemblages (Fig. F5) and microscopic biogenic components (Figs. F8, F9). Microfacies uniformity suggests the stability of well-defined depositional settings and/or the prevalence of biosedimentological and environmental factors. As a consequence, an interpretation in terms of depositional setting is proposed with each microfacies description and is reported alongside the lithostratigraphic units (Fig. F5). Then, CFA is used to establish microfacies interrelationships in order to understand the upcore vertical microfacies succession with respect to environmental changes.
This microfacies is present in Subunit IIA at both sites and in the upper part of Subunit ID or Subunit IC (Figs. F8, F9). This mud-poor packstone to floatstone is defined by porcellaneous foraminifers (1%–33%), small agglutinated foraminifers (1%–19%), small hyaline foraminifers (0%–13%), and bivalves (0%–9%). Red algae is sometimes the major component (0%–83%).
Microfacies A is especially characterized by large porcellaneous forms with Austrotrillina howchini (0%–18%), alveolinids (0%–5%) including F. bontangensis, and soritids (0%–23%) (Pl. P1, figs. 1, 2). These represent the coarse portion of sediment, together with scattered, unbroken gastropods and bivalves up to 1 cm in size, delicate branches of red algae, associated serpulid colonies, and the rare large hyaline foraminifers Operculina, Lepidocyclina, and Miogypsina. Plates of the green algae Halimeda are locally abundant (0%–38%), as are rounded endoclasts.
All these bioclasts are mixed in a moderately sorted silt to medium sand composed of numerous peletoids and micritized red algae, miliolids, echinoderms (6%–33%) (echinoid plates and spines and ophiurids ossicles), and mollusks. Subordinate small bioclasts are delicate-branched bryozoan colonies (0%–16%), small hyaline benthic foraminifers including rotaliids (0%–16%), epiplanktonic foraminifers (0%–14%) (orbulinids and globigeriniids), and geniculate red algae (0%–10%). Bioturbation occurs only within mud-rich packstones. Early marine cementation sometimes occurs around grains as thin isopachous rims of Mg calcite blades.
The predominant fine-grained sediment in Microfacies A may be related to continuous winnowing by currents. Micritization and associated peloids suggest a long-time presence of grains. The variable mud and endoclast contents and unequal preservation of larger bioclasts attest to high- to moderate-energy settings. The geniculate red algae, soritids, alveolinids, and suspected sea-grass roots suggest sea-grass meadow areas. Using Austrotrillina and Flosculinella, Chaproničre (1975) and Betzler and Chaproničre (1993) assigned a shallow and protected platform environment to similar middle Miocene facies from Australia. Moreover, Chaproničre (1975) proposed a water depth of <30 m for the foraminiferal assemblage. The facies and depositional settings were also found by Fournier et al. (2004) in an Oligocene–Miocene isolated platform in the Philippines. Microfacies A also shows similarities with the miliolid–small rotaliniid facies of modern restricted platform and lagoon settings defined by Hallock and Glenn (1986). However, the Microfacies A environment appears to be related to more open-ocean conditions, as numerous echinoderm fragments and epiplanktonic foraminifers are present. Thus, Microfacies A can be assigned to an inner but not restricted platform setting.
This microfacies is only represented by three thin sections sampled in Subunits IIA and at either the upper part of Subunit IC or the upper part of Subunit IA (Figs. F8, F9). This microfacies is defined by the occurrence of scattered coral fragments of solitary and colonial taxa up to 1 cm in size (5%–40%) and abundant red algae (17%–59%). The corresponding coral assemblages are described in Table T1. The rudite-sized fraction of Microfacies B consists of thick-branched red algae, rhodoliths, micritized Halimeda plates (1%–24%), mollusks (2%–7%), and sporadic large benthic foraminifers (7%–13%). The silt to medium sand-sized portion of wackestone-packstone fabric shows dominant subangular to well-rounded red algae fragments, mixed with miscellaneous small benthic foraminifers (0%–7%), epiplanktonic foraminifers (1%–4%), and echinoid debris (1%–6%). A broad stratification with thin discontinuous laminae of micrite is visible in Unit I samples.
Microfacies B likely represents a high- to moderate-energy environment within or near coral patch reefs, indicated by poorly sorted rudstone to wackestone textures and by the coral assemblages given in Table T1. These corals are second reef frame builders. In Subunits IIA and IC they are characteristic of low-energy and well-oxygenated environments. At the top of Subunit IA, they hold a high-energy significance.
This microfacies is represented in Subunit IIB at Site 1199 (Fig. F9) and in one single sample at the base of a coral boundstone within Subunit IC at Site 1196 (Fig. F8). This microfacies exhibits upcore textural variations from floatstone-grainstone to packstone with common stratified geopetal deposits. It is characterized by Miogypsina (1%–12%) and, to a lesser extent, by Lepidocyclina (1%–13%) (Pl. P1, figs. 3, 4). It comprises an unsorted bioclastic sand with fragmented subangular to rounded branching red algae (32%–71%), geniculate red algae (0%–3%), branching bryozoan colonies (1%–8%), echinoderms (5%–20%) including ophiurids ossicles, and a noticeable amount of benthic foraminifers as large as 2 mm (6%–30%). Two foraminifer assemblages can be distinguished: one is an association of robust hyaline forms (common Miogypsina, Lepidocyclina, and Amphistegina and sporadically subordinate rotaliids and nummulitids such as Cycloclypeus, Operculina, and Operculinella), and the other one contains porcellaneous forms (alveolinids including F. bontangensis, soritids, and miliolids). Halimeda plates and bivalves (ostreids and pectinids) occur sporadically. Crusts made of branching red algae, lamellar bryozoan colonies, serpulid colonies, and encrusting hyaline foraminifers sometimes develop on a single side of large hyaline benthic forms (Lepidocyclina). Other small benthic and epiplanktonic foraminifers are also present.
The microfacies represents a high-energy setting indicated by the fragmentation and roundness of the individual bioclasts. The presence of large robust well-preserved hyaline benthic foraminifers suggests a high-energy, shallow-water environment (Hallock and Glenn, 1986). Microfacies C also contains a foraminifer assemblage of Miogypsina, Lepidocyclina, and Cycloclypeus. A similar microfacies from northwest Australia, dated early to middle Miocene, was reported by Chaproničre (1975) to a shallow water depth of <50 m.
The common geniculate red algae and the porcellaneous foraminifer assemblage are similar to those of Microfacies A, which we assigned to sea-grass meadow influences. Sedimentation breaks, as exemplified by geopetal deposits and encrusted Lepidocyclina, and recurrent textural variations are indicative of periodic decreases of energy. These sedimentary features may be related to periodic storm action and/or debris flows. Martín and Braga (1993) and Martín et al. (1993) also report these two processes for a similar red algal facies of outer-platform settings, dated middle Miocene, from the Queensland and Marion plateaus. Finally, the lack of coral in Microfacies C suggests an outer-platform setting far away from reef influence.
The microfacies occurs in the upper parts of both Subunits IA and IIA at both Sites 1196 and 1199 and at the top of Subunit ID at Site 1196 (Figs. F8, F9). This unsorted floatstone to mud-poor packstone is defined by large, robust, and partly reworked hyaline benthic foraminifers including Amphistegina (1%–16%) and the nummulitids Cycloclypeus (0%–5%) and Operculinella (0%–2%) (Pl. P1, figs. 5, 6). Microfacies D is also characterized by red algae (20%–79%) and Halimeda plates (1%–31%). All these bioclasts compose a coarse amount of rhodoliths a few centimeters in size (except in Subunit IIA), encrusting hyaline foraminifers (0%–16%), and branching red algae. Solitary and colonial coral (0%–5%), mollusks (0%–7%), and well-preserved Lepidocyclina specimens 2 mm in size (0%–4%) occur sporadically. Sponge clionids mostly affect all these bioclasts, whereas rare bivalve borings are only observed in rhodoliths. The fine portion of the sediment contains echinoderms (1%–18%), subangular red algae fragments, and epiplanktonic foraminifers (0%–8%). Most of the bioclasts are fragmented, rounded, and slightly micritized. A broad stratification may occur with faint upward-fining layers a few centimeters thick. Each layer of wackestone-packstone fabric contains medium sand-sized particles followed by discontinuous wavy laminae of micrite <1 mm thick. Thin micritic-rimmed cement can develop on top of bioclasts and geopetal micritic infillings of pores. These cements are sometimes buried by another geopetal micritic deposit.
Microfacies D indicates a high- to moderate-energy environment with occasional coral reef input. The presence of epiplanktonic foraminifers as well as the rich association of large hyaline benthic foraminifers and rarity of porcellaneous foraminifers suggests an outer-platform setting. The presence of several generations of geopetal deposit separated by micritic rims suggests sedimentation breaks. These sedimentary features with faint upward-fining layers may be characteristic of slope deposition controlled by sporadic storms. Indeed, Martín et al. (1993) reports a storm-influenced outer-platform setting at depths ranging from 30 to 80 m in an almost identical rhodolithic facies, dated middle Miocene from the Marion Plateau (Leg 133, Site 816). Marshall et al. (1998) also report depths ranging from 40 to 120 m for a comparable facies from a modern Australian environment. In the depositional environment inferred from the analysis of Microfacies D, the high percentage of Halimeda plates suggests the existence of meadows. Davies and Marshall (1985) and Drew and Abel (1988) reported finding Halimeda meadows at depths ranging from 20 to 100 m in the Holocene to modern environments of the Great Barrier Reef Province.
This unsorted floatstone to mud-poor packstone is present in the lower part of Subunit IA and in either Subunit IB or Subunit ID (Figs. F8, F9). It is characterized by especially large (up to 5 mm) and flat common Lepidocyclina (0%–11%), rare Amphistegina (0%–2%), and varying percentages of Halimeda plates (1%–41%). Red algae (40%–82%) is a major component with rudite-sized rhodoliths and delicate branches. The sand-sized portion of the sediment mainly contains red algae and echinoid fragments (0%–10%). The other bioclasts are rare and sporadic.
Microfacies E exhibits the same fabric as microfacies D with prevalent red algae and Halimeda plates. The restricted large benthic foraminiferal assemblage, the lower amount of Amphistegina, and the absence, for the most part, of porcellaneous benthic foraminifers, encrusting hyaline foraminifers, and coral suggest an outer-platform setting in a deeper position with respect to Microfacies D. Such a deep setting may explain the occurrence of the especially large and flat, well-preserved specimens of Lepidocyclina. Indeed, the increasing size and degree of flatness of the benthic foraminifers can be related to light attenuation with increasing habitat depth (Hallock and Glenn, 1986). Nevertheless, the nature of substrate can also be a controlling factor (Hottinger, 1983). Finally, the presence of Halimeda plates with a similar percentage as Microfacies D and absent corals and porcellaneous foraminifers may reinforce the hypothesis of in situ Halimeda meadows in outer-platform settings.
This unsorted floatstone to mud-poor packstone is mainly present in Subunits ID, IB, and IA (Figs. F8, F9). It contains abundant rhodoliths and branching red algae fragments (66%–99%) and a rudite-sized fraction of echinoderm debris with mainly echinoid plates and spines (1%–28%). The other rare and sporadic bioclasts are coral, nummulitids, alveolinids, small benthic and epiplanktonic foraminifers, and Halimeda plates.
Microfacies F, as well as Microfacies D and E, may represent an outer-platform setting of high to moderate energy with respect to its fabric and red algae abundance. The paucity of small bioclasts, including benthic and planktonic foraminifers, is most likely due to winnowing. Low bioclast diversity and the absence of coral may indicate that the depositional setting is far away from reef influence.
Microfacies interrelationships are established according to significant biogenic components (variables) and environmental controlling factors using CFA. Results are illustrated in Figure F7.
The first three axes calculated by the CFA explain 50.06% of the total variance of the data structure. These axes are controlled by 15 significant variables. The eigenvalues, relative inertia percentages of the axes, and the contributions of the variables are shown in Table T2. Examination of the factorial planes of axes 1 and 2 and axes 1 and 3 show a mostly continuous and homogeneous distribution of the samples (thin sections) (Fig. F7). Such a distribution indicates close interrelationships between microfacies with respect to significant variables, as shown in Figure F4.
Factorial axis 1 shows the opposition between a group of significant variables (Austrotrillina, soritids, miliolids, small hyaline foraminifers, small agglutinated foraminifers, bivalves, and serpulids) and a single variable (red algae) (Fig. F7). Within Microfacies A (porcellaneous foraminifers), the group of significant variables characterizes the biogenic composition of Subunit IIA and, to a lesser extent, Subunit IIB and that near the top of Subunits IC and ID (Figs. F8, F9). This first group of variables is also represented, but to a lesser extent, in Microfacies C and some thin sections of Microfacies B and D. On the contrary, red algae appears as a significant component of rhodolithic Microfacies D, E, and F, with the latter containing the highest percentage.
Factorial axis 2 classifies microfacies with respect to the opposition between the variables of red algae and echinoderms and the variables of Operculinella and Cycloclypeus (nummulitids) and encrusting hyaline foraminifers (Fig. F7). The upcore vertical succession along axis 2 of Microfacies F (red algae and echinoderms), E (Halimeda and Lepidocyclina), and D (Halimeda, nummulitids, and Amphistegina) is also visible at both Sites 1196 and 1199 in Subunits IA, IB, and ID (Figs. F8, F9).
Factorial axis 3 classifies microfacies with respect to coral and the variable pair Miogypsina-Lepidocyclina, which allows the discrimination of Microfacies B and C, respectively. Microfacies C is mostly represented within Subunit IIB at Site 1199 (Fig. F9).
A bell-shaped distribution of samples tends to appear on the factorial planes of axes 1 and 2 and of axes 1 and 3 (Fig. F7). Such a distribution may represent a possible polynomial relationship between axes, called the "arch" effect or "Guttmann" effect, and it is classically considered to be the manifestation of environmental gradients along a factorial axis (Hennebert and Lees, 1991). The Guttmann effect is visualized along axis 1 by the succession of Microfacies A–F.
Based on the microfacies descriptions and environmental interpretations, the bell-shaped distribution may correspond first to a depth gradient along axis 1. Axis 2 appears to be linked to axis 1 and does not hold any environmental significance. Indeed, Microfacies A (porcellaneous foraminifers) of the inner-platform setting is opposed to the three rhodolithic Microfacies D, E, and F of the outer-platform setting along axis 1. Microfacies C most likely holds an intermediate depth significance as it contains porcellaneous foraminifers, as does Microfacies A, and thus may register inner-platform influences. A gradual transition between depositional settings of Microfacies A and C may exist as both microfacies occur at Site 1199 in lithostratigraphic Unit II and are interlayered. This is also the case for Microfacies D–F as they are interlayered in Subunits IA and IB.
Moreover, axis 1 may correspond to both an energy gradient and a winnowing gradient. Indeed, the prevalent granulometry increases along the arch of the factorial plane of axes 1 and 2. Thus, the granulometry consists of silt to fine sand in Microfacies A, medium to coarse sand in Microfacies C, and coarse sand with additional numerous rhodoliths a few centimeters in size in Microfacies D through F. On the contrary, the fine portion of sediment including mud tends to disappear along the arch. The gradual disappearance of small bioclasts, including small benthic and planktonic foraminifers, from microfacies D to F can be also related to an increasing winnowing process.
Finally, axis 3 may represent a coral influence gradient (Fig. F7). Indeed, coral occurs in Microfacies B and in some thin sections of Microfacies D, whereas it is absent in Microfacies C.
The CFA ordering of Microfacies A–F along the arch is also associated with a reduction in the diversity of biogenic components, as shown in Figure F4. This phenomenon can be related to gradients of winnowing, depth, and coral influence. Indeed, an upcore vertical succession from Microfacies F and E to Microfacies D occurs in Subunits IA and IB at both Sites 1196 and 1199. This is consistent with the occurrence of common coral pebbles in the five uppermost cores at both sites in Subunit IA, whereas coral is absent in Subunit IB (Fig. F5).
Thus, the CFA ordering of Microfacies A–F with respect to the environmental gradients is representative of the upcore vertical biosedimentary evolution at both Sites 1196 and 1199 (Fig. F5). As a consequence, the microfacies evolution will be examined along with the site lithostratigraphic units with respect to the CFA ordering.