Environmental Engineering Reference
In-Depth Information
packages diagnostic of those environments and reflect the grade or severity of
metamorphism. Grade is, in part, a trade-off between temperature and pressure. Principal
metamorphic facies and environments are shown in Figure 12.8 (see also Colour Plate 4
between pp. 272 and 273).
Metamorphic zones cover large tracts of continental crust through their formative
association with tectonic belts. The cumulative effects of six to eight global orogenic
episodes have created complex metamorphic belts around the cores of contemporary and
older orogens. Together 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, pp. 247-8).
THE ROCK CYCLE (3) SEDIMENTARY 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 (
autochthonous
) 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 proceed through the rock
cycle. Most sediments, however, eventually reach the oceans as land-sourced,
terrigenous sediment
. All are then recycled by deformation, magmatization and
metamorphism at plate
