ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

tend entre la transgression Ouljienne (Stade isotopique 5e)
et la transgression Mellahienne (Stade isotopique 1). Elle a
ete consideree comme une seule phase pluviale synchrone
la regression generale wiirmienne.
Actuellement, des nouvelles donnees sur une region du littoral
atlantique marocain, permettent de revoir et de proposer
une nouvelle reconstitution tout en analysant les significations
eustatiques et climatiques de ces donnees nouvelles.
Elles montrent qu'au sein du Wiirm (Soltanien) il y
a eu plusieurs contrastes climatiques.
HELEN K. WATSON
Factors influencing the distribution of gully erosion
in the Mfolozi catchment,
South Africa, land reform implications
Department of Geography, University of Durban-Westville, p/bag X
54001, Durban 4000, South Africa
The Mfolozi catchment covers 100/0 of the province of
KwaZulu Natal. Over 400/0 of it is already severely eroded.
Its mean annual sediment load is extremely high and deposition
on its flood plain during extreme events has caused
very serious financial losses. There is concern that the catchment's
soil erosion scenario will be exacerbated by landuse
changes incumbent on the post apartheid government's
land reform programme. As peasant farmers particularly
perceive gully erosion as a problem, this study sought
to identify those parts of the catchment that are unsuitable
for allocation to them because they are either already
severely gully eroded or are susceptible to this form of erosion.
Two principle data sources were available for use.
Firstly, an unpublished geomorphological map showing
the location of eleven length classes of gullies. And secondly,
maps and associated memoirs delimiting and detailing
the topographic, substrate and rainfall characteristics
of the catchment's landtypes. Data on the Veld Type, bioclimatic
and landuse characteristics of these landtypes was
extracted from various additional sources. The density of
each gully class within each of the possible 16 landtypes
within each of the catchment's 43 subcatchments was recorded.
Statistical analysis of the resultant gully and
landtype data sets focused on the significance of differences
between and groupings within components of each set,
and on functional relationships between the two sets.
This study identified four and three of the 16 landtypes represented
in the catchment as susceptible and very susceptible
to gully erosion, respectively. It rated a total of 25
subcatchments as unsuitable for allocation to peasant farmers.
Ten of which are already severely gully eroded while
the remainder contain substantial portions of susceptible
landtypes. The findings of this study also suggest that most
of the gullies on white owned land are of natural origin
pre-dating the presence of Early Iron Age communities. By
contrast most gullies present in African communal lands
are more recent features caused by overgrazing.
JOHN A. WEBB \ S. MARSHALLSEA2 & P.F. GREEN 2
The thermal history of the Laura Basin,
North Queensland, Australia, and implications
for denudation of the region
1 School of Earth Sciences, La Trobe University, Bundoora,
Victoria 3083, Australia
2 Geotrack International Pty Ltd, 37 Melville Rd, West Brunswick,
Victoria 3055, Australia
The Laura Basin in northern Queensland contains up to 1
km of Middle Jurassic (Bathonian) to Early Cretaceous
(Albian) fluviatile and shallow marine strata, with a thin
covering of Cainozoic terrestrial sediments. Basement
comprises Middle-Late Palaeozoic sedimentary rocks and
granites.
Vitrinite reflectance (Vr) values from samples throughout
the Mesozoic sequence range from 0.6-1.00/0Ro, and there
is no increase in Vr with depth. The data indicate that all
the Jurassic-Cretaceous strata have been heated to temperatures
in excess of 100°C; these temperatures are also indicated
by apatite fission track analysis (Afta) on Mesozoic
samples from one well in the centre of the basin.
The timing of this heating is constrained by Afta data from
the well samples, and also from samples of Palaeozoic basement
underlying Jurassic sediments along the southern
margin of the basin. These data show that the Mesozoic
strata reached temperatures in excess of 100°C prior to
cooling at around 90 Ma.
In order to reach the temperatures indicated by both the
Vr and Afta data, the Mesozoic sequence must have been
overlain by about 2.5 km of sediment, assuming the present
geothermal gradient of 40 °C/km. If the gradient was
60 °C/km, the maximum believed reasonable for passive
margins apparently unaffected by intrusions or volcanism,
about 1.5 km of sediment must have covered the area.
The cooling recorded by the Afta data could represent the
removal of this overlying sediment by denudation. Since
the denudation occurred at around 90 Ma (Late Cretaceous),
the sediment eroded must have been Late Cretaceous
in age. However, a substantial thickness of sediment
of this age is unlikely ever to have been present in the Laura
Basin. Firstly, no onshore basins in Australia contain significant
thicknesses of Late Cretaceous strata (Fielding
1996). It has been proposed that during this time Australia
lay on a geoidal high, so that compared to other continents
flooded by Late Cretaceous eustatic high sea levels, Australia
was topographically elevated (Gurnis, 1994). Secondly,
there is only a narrow time window of 10-15 Ma
for deposition and removal of the entire Late Cretaceous
sediment package in the Laura Basin.
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