Abstract:
River estuarine samples are a complex repository of terrestrial
and nearshore geophysical changes. Sediment core raised from the
Gad river estuary, situated in Maharashtra state of India, was
analyzed to understand their rock magnetic, particle size and
geochemical characteristics. These three parameters are closely
related to one another signifying detrital model of deposition,
devoid of soft diagenetic changes. The magnetic grain size and
particle grain size constraining the domain structure and physical
structure respectively, are both in consonance with chemical
elements that are amenable largely to physical comminution. The
integrated approach of this nature has been tried for the first time
on estuarine samples of India. Mineral magnetism utilises the
magnetic behaviour of a material to interpret environmental
processes acting upon it.
The study unraveled two-tier climate-induced modifications
to erosional activity, suggesting abrupt change at 20 cm depth in a
45 cm core. The magnetic characteristics are seen to be controlled
by the presence of ferrimagnetic minerals. Rock magnetic results
(clf, ARM, SIRM) and Curie temperature reveal that low-coercivity
magnetite (and/or titanomagnetite) is the dominant magnetic
mineral. clf is more at the bottom and less at the top. Magnetic
grain size is coarse at the top and fine at the bottom. Clay and silt
proportion is more at the top of the core and that of sand is more
at the bottom. Concentration of detrital geochemical elements like
Al, Ti and Cr are less at the top and more at the bottom of the core,
corroborated by chemical index of weathering and chemical index
of alteration. Thus, it is clear from clf that magnetic minerals are
more at the bottom and so are the detrital minerals like Al, Ti and
Cr, along with sand. This reveals energy conditions were vigorous
at the time of the deposition of these entities at the bottom, which
eased out at the top and are corroborated by the presence of clay
and silt, having correspondingly low Al, Ti and Cr with low clf.
Micromorphological features revealed abrasion pits, grooves
and bumped edges on sub-angular grains, which indicate moderate
transport distance under moderate to low energy conditions
revealing terrestrial origin of this material. Thus, similar studies
in other estuarine complexes can be used as a proxy to understand
monsoonal or environmental changes initiated by neotectonic
activity.
