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Greenhouse climate during ordovician and miocene?
Part III - - Abstracts GREENHOUSE
CLIMATE
DURING
45
ORDOVICIAN
AND MIOCENE?
T. Schwarz, M. Wipki and K. G e r m a n n Technische Universitiit Berlin, Germany
Laterites are soils formed under conditions of intense chemical weathering. Their formation requires conditions (high precipitation, temperature, etc.) which are presently given in tropical areas of low latitudes. In the geological past, however, there have been periods of preferred laterite formation also in high latitudes which are attributed to a 'greenhouse climate'. Two examples of such high latitude laterites are presented here, one formed during Cambrian-Ordovician and one during Tertiary (Miocene) times. In northwest Sudan (Tawiga) a bauxitic laterite has been discovered which is overlain by upper Ordovician-Lower Silurian sandstones. The parent rock of this laterite consists of mafic members of an ophiolite complex of Late Precambrian age (570 Ma) which is exposed further north. Lateritization occurred in a nearshore environment on the northern edge of Gondwana, where marine transgressions reached far into the continent. One striking feature of the Tawiga laterite is its position near to the paleo-pole which was probably located near to Morocco during middle Ordovician. In upper Ordovician an extensive continental glaciation covered northern Africa with an ice sheet which probably reached also to northern Sudan.
GENESIS
OF 'LATERITIC'
Similar conditions occurred in the Tertiary of Central Germany (Vogelsberg) where a bauxitic laterite is covered by and intermixed with loess. Here, lateritization can be dated more precisely to mid-Miocene. The occurrence of laterites is restricted to a basaltic parent rock of mid-Miocene age. The paleogeographical position of the Vogelsberg laterite does not differ much from its present position which is situated north of Frankfurt. Both of the described laterites have in common a close temporal and spatial relation with glaciation, but in both cases conditions were favorable for the formation of appreciable amounts of bauxite minerals. The Vogelsberg laterite probably formed 'only' ten million years before continental ice sheets of the Pleistocene glaciation stood only some one hundred kilometers away. The Tawiga laterite formed on rocks which are 470 Ma old, while glaciation reached its maximum around 440 Ma. The Vogelsberg laterite yields gibbsite contents of up to 70%, while at Tawiga the boehmite was preferably formed with contents of up to 30%. Both show evidence of intensive chemical weathering.
DURICRUSTS
IN WESTERN
AUSTRALIA
R.P. Bourman* and A.J. C o n a c h e r t * Department of Geology, University of South Australia, Underdale, 5032, Australia t Department of Geography, University of Western Australia, Nedlands 6009, Australia
'Laterite' is generally regarded as iron- and aluminum-rich material, which formed as a result of intensive tropical weathering conditions over much of the Australian continent in the past, particularly in the Tertiary and Mesozoic, and is thus thought to be useful as a morphostratigraphic marker for correlation of land surfaces and as an indicator of former climates (Twidale, 1983). Whilst this may be true in a very general sense, there are many discrepancies. A major purpose of this investigation is to clarify these discrepancies. The paper provides preliminary results from an ongoing research program, which seeks to quantify the physical, chemical and mineralogical characteristics of iron- and aluminum-rich, indurated crusts (variously called ferruginous duricrust, lateritic duricrust, laterite or ferricrete), in different landscape positions and on
different parent materials in the southwest of Western Australia (Bourman, 1989, 1993a; Bourman et al., 1987). The hypothesis that the macro-, meso- and microcharacteristics of the duricrusts as well as their chemical and mineralogical compositions are indicators of their environments of formation, and thus represent 'tape recordings' of past landscapes, will be tested. Although the 'laterites' of Western Australia have been studied extensively (e.g. Playford, 1954; Mulcahy, 1960, 1973; Finkl and Churchward, 1973; Gilkes et al., 1973; Schmidt and Embleton, 1976; Davy, 1979; Butt, 1982; Mann and Oilier, 1985; Anand and Gilkes, 1987; Smith et al., 1987; Brimhall et al., 1988), there are still many unresolved problems as reviewed by Bourman (1993b). Questions considered by this paper will include:
CLIMATE
DURING
45
ORDOVICIAN
AND MIOCENE?
T. Schwarz, M. Wipki and K. G e r m a n n Technische Universitiit Berlin, Germany
Laterites are soils formed under conditions of intense chemical weathering. Their formation requires conditions (high precipitation, temperature, etc.) which are presently given in tropical areas of low latitudes. In the geological past, however, there have been periods of preferred laterite formation also in high latitudes which are attributed to a 'greenhouse climate'. Two examples of such high latitude laterites are presented here, one formed during Cambrian-Ordovician and one during Tertiary (Miocene) times. In northwest Sudan (Tawiga) a bauxitic laterite has been discovered which is overlain by upper Ordovician-Lower Silurian sandstones. The parent rock of this laterite consists of mafic members of an ophiolite complex of Late Precambrian age (570 Ma) which is exposed further north. Lateritization occurred in a nearshore environment on the northern edge of Gondwana, where marine transgressions reached far into the continent. One striking feature of the Tawiga laterite is its position near to the paleo-pole which was probably located near to Morocco during middle Ordovician. In upper Ordovician an extensive continental glaciation covered northern Africa with an ice sheet which probably reached also to northern Sudan.
GENESIS
OF 'LATERITIC'
Similar conditions occurred in the Tertiary of Central Germany (Vogelsberg) where a bauxitic laterite is covered by and intermixed with loess. Here, lateritization can be dated more precisely to mid-Miocene. The occurrence of laterites is restricted to a basaltic parent rock of mid-Miocene age. The paleogeographical position of the Vogelsberg laterite does not differ much from its present position which is situated north of Frankfurt. Both of the described laterites have in common a close temporal and spatial relation with glaciation, but in both cases conditions were favorable for the formation of appreciable amounts of bauxite minerals. The Vogelsberg laterite probably formed 'only' ten million years before continental ice sheets of the Pleistocene glaciation stood only some one hundred kilometers away. The Tawiga laterite formed on rocks which are 470 Ma old, while glaciation reached its maximum around 440 Ma. The Vogelsberg laterite yields gibbsite contents of up to 70%, while at Tawiga the boehmite was preferably formed with contents of up to 30%. Both show evidence of intensive chemical weathering.
DURICRUSTS
IN WESTERN
AUSTRALIA
R.P. Bourman* and A.J. C o n a c h e r t * Department of Geology, University of South Australia, Underdale, 5032, Australia t Department of Geography, University of Western Australia, Nedlands 6009, Australia
'Laterite' is generally regarded as iron- and aluminum-rich material, which formed as a result of intensive tropical weathering conditions over much of the Australian continent in the past, particularly in the Tertiary and Mesozoic, and is thus thought to be useful as a morphostratigraphic marker for correlation of land surfaces and as an indicator of former climates (Twidale, 1983). Whilst this may be true in a very general sense, there are many discrepancies. A major purpose of this investigation is to clarify these discrepancies. The paper provides preliminary results from an ongoing research program, which seeks to quantify the physical, chemical and mineralogical characteristics of iron- and aluminum-rich, indurated crusts (variously called ferruginous duricrust, lateritic duricrust, laterite or ferricrete), in different landscape positions and on
different parent materials in the southwest of Western Australia (Bourman, 1989, 1993a; Bourman et al., 1987). The hypothesis that the macro-, meso- and microcharacteristics of the duricrusts as well as their chemical and mineralogical compositions are indicators of their environments of formation, and thus represent 'tape recordings' of past landscapes, will be tested. Although the 'laterites' of Western Australia have been studied extensively (e.g. Playford, 1954; Mulcahy, 1960, 1973; Finkl and Churchward, 1973; Gilkes et al., 1973; Schmidt and Embleton, 1976; Davy, 1979; Butt, 1982; Mann and Oilier, 1985; Anand and Gilkes, 1987; Smith et al., 1987; Brimhall et al., 1988), there are still many unresolved problems as reviewed by Bourman (1993b). Questions considered by this paper will include: