2
1 INTRODUCTION
Curiously enough, modern sedimentology was not born from the union of petrography and stratigraphy. It seems to have evolved from a union between structural geology and oceanography. This strange evolution deserves an explanation. Structural geologists have always searched for criteria for distinguishing whether strata in areas of
tectonism were overturned or in normal sequence. This is essential if regional mapping
is to delineate recumbent folds and nappes. Many sedimentary structures are ideal
for this purpose, particularly desiccation cracks, ripples, and graded bedding. This approach reached its apotheosis in Shrock's volume Sequence in Layered Rocks, written
in 1948. On a broader scale, structural geologists were concerned with the vast prisms
of sediments that occur in what were then called geosynclinal furrows. A valid stratigraphy is a prerequisite for a valid structural analysis. Thus it is interesting to see that
it was not a stratigrapher, but Sir Edward Bailey, doyen of structural geologists, who
wrote the paper "New Light on Sedimentation and Tectonics" in 1930. This seminal paper defined the fundamental distinction between the sedimentary textures and structures of shelves and those of deep basins. This paper also contained the germ of the turbidity current hypothesis.
The concept of the turbidity flow rejuvenated the study of sediments in the 1950s and
early 1960s. While petrographers counted zircon grains and stratigraphers collected
more fossils, it was the structural geologists who asked "How are thick sequences of
flysch facies deposited in geosynclines?" It was modern oceanography that provided the
turbidity current as a possible mechanism (see Section 4.2.2). It is true to say that this
concept rejuvenated the study of sedimentary rocks, although in their enthusiasm geologists identified turbidites in every kind of facies, from the Viking sandbars of Canada
to the alluvial Nubian sandstones of the Sahara.
Another stimulus to sedimentology came from the oil industry. The search for stratigraphically trapped oil led to a boom in the study of modern sediments. One of the first
fruits of this approach was the American Petroleum Institute's "Project 51," a multidisciplinary study of the modern sediments of the northwest Gulf of Mexico (Shepard
et al., 1960). This was followed by many other studies of modern sediments by oil companies, universities, and oceanographic institutes. At last, hard data became available
so that ancient sedimentary rocks could be interpreted by comparison with their modern analogs. The concept of the sedimentary model was born as it became apparent that
there are, and always have been, a finite number of sedimentary environments that deposit characteristic sedimentary facies (see Section 6.3.1). By the end of the 1960s sedimentology was firmly established as a discrete discipline of the earth sciences, Through
the 1960s the main focus of research was directed toward an understanding of sedimentary processes. By studying the bedforms and depositional structures of recent sediments, either in laboratory flumes or in the wild, it became possible to interpret accurately the environment of ancient sedimentary rocks (Laporte, 1979; Selley, 1970, 1996;
Reading, 1978,1996). Through the 1970s and 1980s sedimentological research expanded
in both microscopic and macroscopic directions. Today a distinction is often made
between macrosedimentology and microsedimentology. Macrosedimentology ranges
from the study of sedimentary facies down to sedimentary structures. Microsedimentology covers the study of sedimentary rocks on a microscopic scale, what was often
termed petrography. The improved imaging of sediments by scanning electron microscopy and cathodoluminescence brought about greater understanding of the physical
1 INTRODUCTION
Curiously enough, modern sedimentology was not born from the union of petrography and stratigraphy. It seems to have evolved from a union between structural geology and oceanography. This strange evolution deserves an explanation. Structural geologists have always searched for criteria for distinguishing whether strata in areas of
tectonism were overturned or in normal sequence. This is essential if regional mapping
is to delineate recumbent folds and nappes. Many sedimentary structures are ideal
for this purpose, particularly desiccation cracks, ripples, and graded bedding. This approach reached its apotheosis in Shrock's volume Sequence in Layered Rocks, written
in 1948. On a broader scale, structural geologists were concerned with the vast prisms
of sediments that occur in what were then called geosynclinal furrows. A valid stratigraphy is a prerequisite for a valid structural analysis. Thus it is interesting to see that
it was not a stratigrapher, but Sir Edward Bailey, doyen of structural geologists, who
wrote the paper "New Light on Sedimentation and Tectonics" in 1930. This seminal paper defined the fundamental distinction between the sedimentary textures and structures of shelves and those of deep basins. This paper also contained the germ of the turbidity current hypothesis.
The concept of the turbidity flow rejuvenated the study of sediments in the 1950s and
early 1960s. While petrographers counted zircon grains and stratigraphers collected
more fossils, it was the structural geologists who asked "How are thick sequences of
flysch facies deposited in geosynclines?" It was modern oceanography that provided the
turbidity current as a possible mechanism (see Section 4.2.2). It is true to say that this
concept rejuvenated the study of sedimentary rocks, although in their enthusiasm geologists identified turbidites in every kind of facies, from the Viking sandbars of Canada
to the alluvial Nubian sandstones of the Sahara.
Another stimulus to sedimentology came from the oil industry. The search for stratigraphically trapped oil led to a boom in the study of modern sediments. One of the first
fruits of this approach was the American Petroleum Institute's "Project 51," a multidisciplinary study of the modern sediments of the northwest Gulf of Mexico (Shepard
et al., 1960). This was followed by many other studies of modern sediments by oil companies, universities, and oceanographic institutes. At last, hard data became available
so that ancient sedimentary rocks could be interpreted by comparison with their modern analogs. The concept of the sedimentary model was born as it became apparent that
there are, and always have been, a finite number of sedimentary environments that deposit characteristic sedimentary facies (see Section 6.3.1). By the end of the 1960s sedimentology was firmly established as a discrete discipline of the earth sciences, Through
the 1960s the main focus of research was directed toward an understanding of sedimentary processes. By studying the bedforms and depositional structures of recent sediments, either in laboratory flumes or in the wild, it became possible to interpret accurately the environment of ancient sedimentary rocks (Laporte, 1979; Selley, 1970, 1996;
Reading, 1978,1996). Through the 1970s and 1980s sedimentological research expanded
in both microscopic and macroscopic directions. Today a distinction is often made
between macrosedimentology and microsedimentology. Macrosedimentology ranges
from the study of sedimentary facies down to sedimentary structures. Microsedimentology covers the study of sedimentary rocks on a microscopic scale, what was often
termed petrography. The improved imaging of sediments by scanning electron microscopy and cathodoluminescence brought about greater understanding of the physical