2.1 INTRODUCTION
Before proceeding to the analysis of sedimentary rocks, their petrography, transportation, and deposition, it is appropriate to analyze the genesis of sediment particles. A
sedimentary rock is the product of provenance and process. This chapter is concerned
primarily with the provenance of sediment; that is to say the preexisting rocks from
which it forms and the effect of weathering on sediment composition. First, however, it
is apposite to consider the place of sediment particles within the context of what may
be called, for want of a better term, "the earth machine." Consider a geologist sitting in
a shady Saharan wadi idly hammering a piece of Continental Mesozoic "Nubian" sandstone. A piece of that sandstone falls from the cliff to the wadi floor. Within a second,
a sample of Mesozoic Nubian sandstone has suddenly become part of a Recent alluvial
gravel. The pebble deserves closer inspection. This sandstone clast is composed of a
multitude of quartz particles. The precise provenance of these cannot be proved, but
locally this formation overlies Upper Paleozoic sandstones and infills ancient wadis cut
within them. The Mesozoic sand grains are clearly, in part, derived from the Upper Paleozoic sandstones. Similarly it can be shown that the Upper Paleozoic sediment has
been through several previous cycles of erosion and deposition. The sand grains first
formed from the weathering of Pre-Cambrian granite.
This simple review introduces the concept of the sedimentary cycle and shows how
sediments are frequently polycyclic in their history. Now move from the specific event
to the other extreme. The concept of plate tectonics is reviewed in Chapter 10. At this
point, however, it is necessary to briefly introduce it, to better place sand grains in their
broader perspective. The basic thesis of plate tectonics is that the earth's crust is formed
continuously along linear zones of sea floor spreading. These occupy the midoceanic
rift valleys, areas of extensive seismic and volcanic activity. New crust, formed by vulcanicity, moves laterally away from the zone of sea floor spreading, forming a rigid lithospheric plate. Simultaneously, at the distal edge of the plate, there is a linear zone of
subduction marked by earthquakes and island arcs. Here old crust is drawn down and
digested in the mantle. Sediment forms initially, therefore, from volcanic rocks on the
midocean ridges. It may then be recycled several times, as seen in the Sahara, but inexorably individual particles are gradually drawn to the edge of the plate to descend and
be destroyed in the mantle. The history of a sediment particle may be likened to a ball
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Before proceeding to the analysis of sedimentary rocks, their petrography, transportation, and deposition, it is appropriate to analyze the genesis of sediment particles. A
sedimentary rock is the product of provenance and process. This chapter is concerned
primarily with the provenance of sediment; that is to say the preexisting rocks from
which it forms and the effect of weathering on sediment composition. First, however, it
is apposite to consider the place of sediment particles within the context of what may
be called, for want of a better term, "the earth machine." Consider a geologist sitting in
a shady Saharan wadi idly hammering a piece of Continental Mesozoic "Nubian" sandstone. A piece of that sandstone falls from the cliff to the wadi floor. Within a second,
a sample of Mesozoic Nubian sandstone has suddenly become part of a Recent alluvial
gravel. The pebble deserves closer inspection. This sandstone clast is composed of a
multitude of quartz particles. The precise provenance of these cannot be proved, but
locally this formation overlies Upper Paleozoic sandstones and infills ancient wadis cut
within them. The Mesozoic sand grains are clearly, in part, derived from the Upper Paleozoic sandstones. Similarly it can be shown that the Upper Paleozoic sediment has
been through several previous cycles of erosion and deposition. The sand grains first
formed from the weathering of Pre-Cambrian granite.
This simple review introduces the concept of the sedimentary cycle and shows how
sediments are frequently polycyclic in their history. Now move from the specific event
to the other extreme. The concept of plate tectonics is reviewed in Chapter 10. At this
point, however, it is necessary to briefly introduce it, to better place sand grains in their
broader perspective. The basic thesis of plate tectonics is that the earth's crust is formed
continuously along linear zones of sea floor spreading. These occupy the midoceanic
rift valleys, areas of extensive seismic and volcanic activity. New crust, formed by vulcanicity, moves laterally away from the zone of sea floor spreading, forming a rigid lithospheric plate. Simultaneously, at the distal edge of the plate, there is a linear zone of
subduction marked by earthquakes and island arcs. Here old crust is drawn down and
digested in the mantle. Sediment forms initially, therefore, from volcanic rocks on the
midocean ridges. It may then be recycled several times, as seen in the Sahara, but inexorably individual particles are gradually drawn to the edge of the plate to descend and
be destroyed in the mantle. The history of a sediment particle may be likened to a ball
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