Page 100 - Sedimentology and Stratigraphy

6

Continents: Sources of Sediment

The ultimate source of the clastic and chemical deposits on land and in the oceans is the
continental realm, where weathering and erosion generate the sediment that is carried as
bedload, in suspension or as dissolved salts to environments of deposition. Thermal and
tectonic processes in the Earth’s mantle and crust generate regions of uplift and sub-
sidence, which respectively act as sources and sinks for sediment. Weathering and
erosion processes acting on bedrock exposed in uplifted regions are strongly controlled
by climate and topography. Rates of denudation and sediment flux into areas of sedi-
ment accumulation are therefore determined by a complex system that involves tectonic,
thermal and isostatic uplift, chemical and mechanical weathering processes, and erosion
by gravity, water, wind and ice. Climate, and climatic controls on vegetation, play a
critical role in this Earth System, which can be considered as a set of linked tectonic,
climatic and surface denudation processes. In this chapter some knowledge of plate
tectonics is assumed.

6.1 FROM SOURCE OF SEDIMENT ticulate matter by a variety of mechanisms. Eventually
TO FORMATION OF STRATA the sediment will be deposited by physical, chemical
and biogenic processes in a sedimentary environment
In the creation of sediments and sedimentary rocks the on land or in the sea. The final stage is the lithification
ultimate source of most sediment is bedrock exposed on (18.2) of the sediment to form sedimentary rocks,
the continents (Fig. 6.1). The starting point is the uplift which may then be exposed at the surface by tectonic
of pre-existing bedrock of igneous, metamorphic or sedi- processes. These processes are part of the sequence of
mentary origin. Once elevated this bedrock undergoes events referred to as the rock cycle.
weathering at the land surface to create clastic detritus In this chapter the first steps in the chain of events
and release ions into solution in surface and near- in Fig. 6.1 are discussed, starting with the uplift of
surface waters. Erosion follows, the process of removal continental crust, and then considering the pro-
of the weathered material from the bedrock surface, cesses of weathering and erosion, which result in
allowing the transport of material as dissolved or par- the denudation of the landscape. The interactions