Geoscience Reference
In-Depth Information
Hawes
N
Yo r k
20 km
Fig. 1.2
Composition
of suspended sediment
samples recovered from
the River Ouse (UK) and
four of its tributaries that
have different underlying
geologies. (Adapted from
Walling et al. 1999.)
Carboniferous
Permian & Triassic
Jurassic
1.2.1 Detrital minerals
the different tributaries have distinct mineralog-
ical and magnetic signatures that demonstrate
variations in the relative importance of differ-
ent rock units as sources for fluvial sediment
(Fig. 1.2). Local variations are attributed to
variations in the rates of erosion and sediment
supply from the different geological units
(Walling et al. 1999).
In reality, these detrital minerals rarely
undergo a simple source to sink transport route,
but instead are subject to numerous phases
of weathering, transport, deposition, storage,
lithification, reworking and redeposition. For
example, detrital sands in the Orinoco drainage-
basin of South America are derived from similar
bedrock material, but the nature of relief and
chemical weathering markedly alter the grain
composition (Johnsson et al. 1991). Material
derived from steep, orogenically-active terrains
undergoes limited chemical weathering, whereas
Detrital minerals, such as quartz and feldspar,
along with heavy minerals, form a primary
component of many terrestrial and marine
sediments. These minerals are initially released
by weathering processes and are progressively
eroded and transported into, and through, a
range of sedimentary environments. As a result,
initial mineralogical composition of the bedrock
often influences the relative abundance of the
individual minerals that are released. This con-
trol is clearly illustrated in studies of suspended
sediment compositions within river catchments
where individual tributaries are underlain by
bedrock of differing geological compositions. In
the River Ouse catchment (north-east England),
for example, individual tributaries drain areas
of differing geology (Carboniferous, Permian/
Triassic and Jurassic). Suspended sediments in
