The mineralogy of the bulk sediment is almost identical at all sites. Major changes coincide with the boundaries of the three main lithologic units (Exon, Kennett, Malone, et al; 2001).
The lower siliciclastic unit (Unit III) extends from the late Maastrichtian (drilled at Site 1172) to the latest Eocene (at all sites). The bulk mineralogy (Tables T2, T3, T4, T5, T6; Figs. F2, F3, F4, F5) is largely dominated by quartz, in association with significant amounts of clay minerals, minor amounts of feldspars and pyroxenes, and sporadic traces of dolomite. These minerals are essentially terrigenous (Robert, in press). Trace to minor amounts of pyrite and gypsum are present in most intervals, in probable relationship with abundant organic matter (0.5–5 wt% total organic carbon) (Exon, Kennett, Malone, et al., 2001), dysoxic conditions, and/or further diagenesis. Similar mineralogical assemblages are commonly found in rift graben sediments like those of the Lower Cretaceous Otway Group of southeast Australia (Duddy, 1983; Little and Phillips, 1995). Significant abundances of biogenic calcite (and traces of aragonite at Site 1168) are locally present, especially in the upper middle and upper Eocene part of the sequences. Opal-CT is abundant at all sites with the exception of Site 1168 on the WTM. High contents of opal-CT coincide with sharp decreases of siliceous microfossils in smear slides (Exon, Kennett, Malone, et al., 2001) and cease abruptly downhole as quartz increases. Transitions to increased contents of opal-CT and quartz correlate with decreased porosity at all sites (Exon, Kennett, Malone, et al., 2001) and may result from silica diagenesis (Robert, in press).
The transitional unit (Unit II) is latest Eocene to earliest Oligocene in age and is characterized by the preservation of abundant diatoms followed later by occurrences of glauconitic siltstones and sands (Exon, Kennett, Malone, et al., 2001). Bulk mineral assemblages in Unit II (Table T2; Figs. F2, F3, F4, F5) are rather similar to those in the lower siliciclastic Unit III. However, biogenic carbonates start increasing in Unit II, with fluctuations in abundance. The increase in biogenic carbonates (mostly calcite) and decrease in terrigenous minerals continue in the lower part of the upper biogenic Unit I.
The upper biogenic unit (Unit I) extends from the earliest Oligocene to the Pleistocene (Exon, Kennett, Malone, et al., 2001). Bulk mineral assemblages are largely dominated by biogenic carbonates, which essentially consist of calcite with minor and sporadic amounts of aragonite at Sites 1168 and 1170 (Table T2; Figs. F2, F3, F4, F5). Opal-CT is also present intermittently, especially at Sites 1168 and 1170, where it occurs concomitantly with siliceous microfossils (Exon, Kennett, Malone, et al., 2001). Small quantities of clinoptilolite are recorded locally, especially at Site 1168. Trace to minor amounts of pyrite and gypsum are present in a few samples. The contents of quartz, clay minerals, and feldspars are highly variable and intermittently associated with traces of pyroxenes and dolomite. In fact, very low contents of these typically terrigenous minerals are recorded in most of the unit (Figs. F3, F4, F5). The highest terrigenous contents are recognized at the very base of Unit I in the early Oligocene, and terrigenous content decreases rapidly upward. The strong relative decrease in terrigenous minerals and concomitant increase in biogenic carbonates from the transitional Unit II to the Oligocene part of Unit I is probably related to regional subsidence in the Tasmanian area (Exon et al., in press). One exception is Site 1168 on the WTM, close to continental drainage basins and in a less biologically productive area, where terrigenous minerals remain abundant throughout the Oligocene and decrease in the early Miocene to a middle Miocene minimum (Fig. F2). Significant increases in quartz and clay minerals are visible in the late Pliocene and Pleistocene of the northern Sites 1168 (WTM) and 1172 (ETP), probably related to intensified eolian processes (Robert, in press).
The clay mineral assemblages are essentially terrigenous (Robert, in press) and show significant regional differences (Tables T7, T8, T9, T10, T11; Figs. F2, F3, F4, F5). However, major variations in the clay assemblages coincide at all sites and with major changes in regional tectonics and global climate.
The Maastrichtian to upper Paleocene sequence was drilled at Site 1172 (ETP) only, where largely predominant smectite is associated with very minor amounts of chlorite and illite (Fig. F5). The contents of chlorite and illite increase slightly in the late Paleocene, where it is associated with significant occurrences of kaolinite, but smectite remains dominant. However, at Site 1171 on the STR (Fig. F4), late Paleocene abundances of chlorite and illite are significantly higher (and smectite lower). The composite late Paleocene clay mineral assemblage coincides with a transition in the direction of tectonic movements (from northwest–southeast to north–south) and with the formation of basins in the STR area (Exon et al., in press). Chlorite, illite, and kaolinite decrease in the early Eocene, and smectite dominates largely in the middle Eocene of Site 1172 on the ETP, and Sites 1171 and 1170 on the STR.
Late middle and late Eocene-age sediments of southern Sites 1170 and 1171 (STR) are characterized by increased contents of illite, mostly as brief intervals of large dominance of the mineral (Figs. F3, F4). Significant amounts of glauconite in some intervals of Unit II could interfere with illite identification, as they both have relatively similar mineralogical structures and X-ray signals. However,
No such increases in illite are observed at northern Sites 1168 and 1172. Upper middle and upper Eocene sediments at Site 1168 on the WTM are characterized by intervals of abundant to dominant kaolinite (associated with smaller amounts of illite and trace amounts of random mixed-layered clays), separated by an interval of abundant smectite (Table T7; Fig. F2). At Site 1172 on the ETP, which was attached to the passive western margin of the Tasman Sea, only a slight increase in kaolinite is visible over this interval (Table T7; Fig. F5). The upper middle and upper Eocene interval was a time of intense activity along the Tasman Fracture Zone (Exon et al., in press).
The lower Oligocene to lower Miocene interval is characterized at all sites by the large dominance of smectite, associated with relatively small and variable amounts of chlorite, illite, and kaolinite. The abundance of kaolinite is higher at the northern sites, especially at Site 1168 on the WTM (Table T7; Figs. F2, F3, F4, F5). The Oligocene was a time of regional subsidence of the Tasmanian area (Exon, Kennett, Malone, et al., 2001; Exon et al., in press), favorable to increased formation and erosion of smectite (Robert, in press).
An early Miocene increase in kaolinite is visible at Site 1168 on the WTM, where it lasts until the middle Miocene (Fig. F2). There is no significant early Miocene variation in the clay assemblage at Site 1172 on the ETP (Fig. F5). At southern Sites 1170 and 1171 on the STR, the clay assemblages remain largely dominated by smectite, with minor and variable increases in chlorite and illite (Figs. F3, F4).
The middle Miocene is characterized by an increase in smectite at northern Site 1168 on the WTM (Fig. F2), whereas increased contents of illite followed by kaolinite occur at southern Sites 1170 and 1171 on the STR (Figs. F3, F4). At Site 1172 on the ETP, the middle Miocene marks the beginning of a slightly increasing trend in kaolinite (Fig. F5). These changes in the clay mineral assemblages of the Tasmanian area occurred as the East Antarctic Ice Sheet expanded, early in the middle Miocene (Kennett, 1977). The late Miocene is characterized by increased contents of chlorite, illite, and/or kaolinite that are more visible at northern Sites 1172 (ETP) and, especially, 1168 (WTM). These changes occurred as the West Antarctic Ice Sheet developed in the late Miocene (Kennett, 1977).
Beginning in the latest early Pliocene, increased contents of chlorite, illite, and/or kaolinite are visible at all sites (Table T7; Figs. F2, F3, F4, F5). Late Pliocene and Pleistocene clay mineral assemblages in the Tasmanian region are rather similar in nature and abundance at all sites. They are also similar to the clay assemblages recognized on Lord Howe Rise in the adjacent Tasman Sea (Robert et al., 1986; Stein and Robert, 1986). Homogenization of clay assemblages off Australia coincided with the development of aridity in southern Australia (Martin, 1991), probably as wind processes exerted more control on the distribution of clay particles (Robert, in press).