Geoscience Reference
In-Depth Information
Metamorphic Environment
Figure 12.8
and their environments of
formation.
Source: Partly after Skinner and
Porter (1995)
Diagenesis
Low-grade Intermediate-grade
High grade
0
0
200
10
400
600
20
800
30
1000
E C L O G I T E
40
1200
0
100
200
300
400
500
600
700
800
Temperature (°C)
with ancient gneissose and schistose zones of the cratons,
they form up to 70 per cent of continental crust. By
substantially strengthening this crust, and in particular the
flysch,
mélange
and pelagic soft sediments of ocean basins
accreted via subduction, metamorphism has a major
impact on continental architecture. Its reorganization of
rock material extends and refines geological fractionation
and, in the process, creates distinctive suites of site-specific
geological resources used in the human domain (see box,
p. 279).
proceed through the rock cycle. Most sediments, however,
eventually reach the oceans as land-sourced,
terrigenous
sediment
. All are then recycled by deformation, magma-
tization and metamorphism at plate boundaries.
Sediments of pelagic (surface) and
benthic
(deep-water)
marine origin, formed by biogeochemical processes in the
oceans themselves, share a similar fate.
Denudation transforms rocks into disaggregated
minerals and lithic fragments or
detritus
, lacking cohesion,
and mineral solutions which are readily removed from
their source. At this stage they possess textural (particle
size, shape) and chemical properties diagnostic of their
parent rocks and, to some extent, the denudation process
involved. Three principal styles of sediment are recog-
nized.
Clastic sediments
(from the Greek word for
'broken') are formed by particles broken off parent rocks
and initially reflect fragment size and shape. A distinction
is drawn between
clasts
, or fragments larger than sand
size, and finer grains (sand, silt and clay sizes) which
precipitated primarily from dissolved salts, silicates and
carbonates. They form
biogenic sediments
when their
origins are organic, as do clastic shell and bone beds and
carboniferous (peat and coal) sediments (
Plate 12.6
).
Figure 12.10
shows the relation between sourcing process,
product and eventual sedimentary forms.
PROCESSES AND LANDSYSTEMS
Sediment sources
Sediments are the unconsolidated detrital and dissolved
remains of other rocks and organisms. Weathering and
erosion drive a
sediment cascade
from continental
denudation to eventual deposition in local (
autochtho-
nous
) or distant (
allochthonous
) sedimentary basins. The
geomorphic processes by which this occurs and the
transient sedimentary landforms they produce are covered
in later chapters. First, we need to establish the general
character of sediment bodies and their depositional
environments. Sediments retain some characteristics of
their source area and environment and acquire new ones
through in-transit refinement, moving under gravity or
transported by water, ice and wind. They become lithified
as sedimentary rocks, adding to continental crust when
retained as
terrestrial sediment
in continental basins, and
Sediment transport and deposition
Once in transit, the character of the original denudation
product is weakened and sediments begin to acquire the
signature of the transport environment. This occurs
