The Clay-Mineralogical Record of the Last 10 Million Years off Northeastern Australia

Abstract

Clay mineral assemblages for the last 10 m.y. are described for Site 823, at 16°S in the Queensland Trough, to the northeast of Australia. Largely unaffected by diagenetic influences, these mostly express the evolution of northeastern Australian continental environments during the late Neogene: (1) beginning during the late Miocene at about 7.0 Ma is an increase of illite derived from rocky substrates at the expense of smectite from deeply weathered soils; this increase was the result of increasing aridity in the Australian interior and globally cooler temperatures, associated with increases in Antarctic glaciation; (2) concomitant and further increases of kaolinite fluxes to the Queensland Trough during the late Miocene-early Pliocene largely reflect an increase in rainfall in northeastern Australia; (3) increases in both soil-and rock-derived minerals probably intensified as a result of late Neogene uplift of the eastern highlands; (4) clay-mineral associations during the Pliocene and Pleistocene display minor variations only and probably resulted in part from differential settling and sea-level changes; (5) similar trends of clay-mineral variations occur at both ODP Site 823 and DSDP Site 588 (Lord Howe Rise). Less abundant kaolinite relative to illite at Site 588 nevertheless suggests a southward decrease of continental humidity and/or of the eastern highlands uplift; (6) influences of global climate and oceanic and atmospheric circulations on clay-mineral associations dominated during the late Miocene and were progressively replaced by influences of more regional environmental variations during the Pliocene and especially the Pleistocene.

Regional trends in clay mineral fluxes to the Queensland margin and ties to middle Miocene global cooling

“…Two other regional middle to late Miocene clay mineral records located at intermediate waters depth (~1500 m water depth) have been investigated for clay mineralogy: Site 588 (Lord Howe Rise, Stein and Robert, 1985) and Site 823 (Townsville Trough, Chamley et al, 1993). These two records only overlap with our own record over a 5 my interval between 12 and 7 Ma, and there is a 2 my gap in the Marion Plateau record.…”

“…The goal of the present work is therefore not to compare these records in detail, but to contrast our finding with the conclusions of these previous studies. We note that the most prominent change in clay minerals identified at Sites 588 and 823 is an increase in the K/S ratio (Chamley et al, 1993). This change takes place around 7.1 Ma at the deeper locations (Fig.…”

“…However, the middle Miocene cooling that occurred around 14 Ma (McGowran and Li, 1998) induced increased aridity over the Australian continent (Stein and Robert, 1985;Martin, 1998), retraction of vegetation to the coastal areas (Truswell, 1993;Kershaw et al, 1994), and a recession of continental weathering intensity in Queensland (Li and Vasconcelos, 2002). Provided that the continental proxies are reliable, the parallel between kaolinite increase on the margin and increased aridity and cooling in Queensland is at odds with a bclassicalQ interpretation of kaolinite as indicator of more tropical climate (Chamley, 1989;Bolle et al, 2000;Adatte et al, 2002; (e) Kaolinite to smectite ratio for Site 588 (Stein and Robert, 1985) and Site 823 (Chamley et al, 1993) versus time. The dark line is an interpolation between data from the two sites.…”

“…8f). Based solely on clay mineral proxies, Chamley et al (1993) interpreted the increase of kaolinite at Sites 588 Fig. 11.…”

“…To investigate temporal trends in mineralogy and compare our data set with other records, we calculated three indices commonly used in clay mineral studies (Robert and Chamley, 1987;Chamley et al, 1993;Adatte et al, 2002, Fig. 7, Table 2b, see Section 2): the calcite to detritus (C/D) ratios, the kaolinite to smectites (K/S) ratios and the kaolinite to micas + (K/ C) ratios.…”