The hydrological implications of the weathered basalt horizon at Biyar
el Ghussein, NE Jordan
Nizar Abu-Jaber
Department of Earth and Environmental Sciences
Yarmouk University
Irbid 21163, Jordan
abujaber@yu.edu.jo
And
Department of Marine, Earth and Atmospheric Sciences
North Carolina State University
Raleigh, NC 27695-8208
USA
Abstract
The
weathering of basalt in the Biyar el Ghussein area is herein investigated. This weathering phenomenon gives important
insight into the reasons why shallow groundwater is present in this hot, arid
region. Although the vast majority of
basalt in the Tulul al Ashaqif area is essentially unweathered or show minor
signs of mechanical weathering, the studied weathered horizon shows signs of
significant chemical weathering.
Moreover, it is crosscut by numerous calcite veins, which are syngenetic
with the weathering.
Mineralogical
analyses of the weathering products show the presence of nontronite (an
iron-rich smectite) and faujasite (a zeolite), in addition to parent minerals
such as anorthite. Reactions leading to
the formation of these minerals are consistent with the chemical changes seen
in the waters of the adjacent shallow wells.
The isotopic composition of the carbonate veins suggest that they were
formed under ambient temperatures with waters similar to those seen in the
wells. The carbon isotopic data suggest
that the source of carbon is volcanogenic CO2.
These
data suggest that the formation of the weathering zone is a result of a
coincidence of shallow groundwater interacting with the basalt and rising
volcanogenic carbon dioxide. The carbon
dioxide lowers the pH of the water such that it is more aggressive towards the
basalt. This, in turn, leads to the
formation of clays, which inhibit downward flow of the shallow water, and thus
leads to a positive feedback mechanism, continuing the evolution of the shallow
aquifer and underlying aquitard.
