Page 14 - Sedimentology and Stratigraphy

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Introduction: Sedimentology

and Stratigraphy

Sedimentology is the study of the processes of formation, transport and deposition of
material that accumulates as sediment in continental and marine environments and
eventually forms sedimentary rocks. Stratigraphy is the study of rocks to determine the
order and timing of events in Earth history: it provides the time frame that allows us to
interpret sedimentary rocks in terms of dynamic evolving environments. The strati-
graphic record of sedimentary rocks is the fundamental database for understanding
the evolution of life, plate tectonics through time and global climate change.

1.1 SEDIMENTARY PROCESSES tion in water also contributes to sediments in some
situations.
The concept of interpreting rocks in terms of modern Formation of a body of sediment involves either the
processes dates back to the 18th and 19th centuries transport of particles to the site of deposition by grav-
(‘the present is the key to the past’). ‘Sedimentology’ ity, water, air, ice or mass flows or the chemical or
has existed as a distinct branch of the geological biological growth of the material in place. Accumula-
sciences for only a few decades. It developed as the tion of sediments in place is largely influenced by the
observational elements of physical stratigraphy chemistry, temperature and biological character of the
became more quantitative and the layers of strata setting. The processes of transport and deposition can
were considered in terms of the physical, chemical be determined by looking at individual layers of sedi-
and biological processes that formed them. ment. The size, shape and distribution of particles all
The nature of sedimentary material is very varied in provide clues to the way in which the material was
origin, size, shape and composition. Particles such as carried and deposited. Sedimentary structures such as
grains and pebbles may be derived from the erosion of ripples can be seen in sedimentary rocks and can be
older rocks or directly ejected from volcanoes. Organ- compared to ripples forming today, either in natural
isms form a very important source of material, ranging environments or in a laboratory tank.
from microbial filaments encrusted with calcium car- Assuming that the laws that govern physical and
bonate to whole or broken shells, coral reefs, bones and chemical processes have not changed through time,
plant debris. Direct precipitation of minerals from solu- detailed measurements of sedimentary rocks can be