6.2 SEDIMENTARY FACIES
187
able to most geologists (saving the geosemanticists). Logically, most reefs are a variety
of the carbonate shelf environment, but their abundance and geologic importance justifies giving them independent status. Submarine channel and fan environments result
from turbidity currents and associated processes. (see Section 4.2.2). Pelagic deposits are
those literally "of theopen sea." These are largely fine-grained chemical deposits, argillaceous, calcareous, or siliceous. Pelagic sediments were deposited away from terrestrial
influence, but this may have been due either to great water depth or distance from land.
The designation "pelagic" evades this dilemma.
The classification of depositional environments shown in Table 6.4 has certain limitations. Nevertheless it forms a useful framework for the discussion of the main sedimentary models that are described later in this chapter.
6.2 SEDIMENTARY FACIES
Having examined the concept of sedimentary environments and their classification, it is
now appropriate to discuss their ancient products. Geologists first began to study sedimentary rocks in some detail in the early part of the penultimate century. In 1815, William Smith published his "Geological Map of England and Wales, with Part of Scotland." The principles on which this map was based were that strata occurred in sequences
of diverse rock types, that these strata had a lateral continuity which could be mapped,
and that they were characterized by different assemblages of fossils. As the discipline
of stratigraphic paleontology progressed, it became apparent that some fossils appeared
to be restricted to certain geological time spans, while others were long ranging but appeared to occur in certain rock types. The implications of these facts were realized by
Prevost in 1838. He proposed the name "formation" for lithostratigraphic units. By
using biostratigraphy he demonstrated that different "formations" were formed in the
same geological "epoch," and that similar formations could occur in different epochs.
Almost simultaneously, Gressly (1838), working in the Alps, reached similar conclusions. He coined the name "facies" for units of rock which were characterized by similar lithological and paleontological criteria. His original definition (as translated by Teichert, 1958b) reads:
To begin with, two principal facts characterize the sum total of the modifications which I call facies or aspects of a stratigraphic unit: one is that a certain lithological aspect of a stratigraphic
unit is linked everywhere with the same paleontological assemblage; the other is that from such
an assemblage fossil genera and species common in other facies are invariably excluded.
Gressly took the word "facies" from the writings of Steno (1667) and, though he was
not aware of the distinction of zonal and facies fossils, he formulated a number of laws
that govern the vertical and lateral transitions of facies. By the mid-nineteenth century
it was established that bodies of rock could be defined and mapped by a distinctive combination of lithological and paleontological criteria. These rock units were called "formations" or "facies" by different geologists. Over the years, however, these two terms
assumed different meanings. "Formation" became ever more rigorously defined, while
"facies" became used in ever broader meanings.
187
able to most geologists (saving the geosemanticists). Logically, most reefs are a variety
of the carbonate shelf environment, but their abundance and geologic importance justifies giving them independent status. Submarine channel and fan environments result
from turbidity currents and associated processes. (see Section 4.2.2). Pelagic deposits are
those literally "of theopen sea." These are largely fine-grained chemical deposits, argillaceous, calcareous, or siliceous. Pelagic sediments were deposited away from terrestrial
influence, but this may have been due either to great water depth or distance from land.
The designation "pelagic" evades this dilemma.
The classification of depositional environments shown in Table 6.4 has certain limitations. Nevertheless it forms a useful framework for the discussion of the main sedimentary models that are described later in this chapter.
6.2 SEDIMENTARY FACIES
Having examined the concept of sedimentary environments and their classification, it is
now appropriate to discuss their ancient products. Geologists first began to study sedimentary rocks in some detail in the early part of the penultimate century. In 1815, William Smith published his "Geological Map of England and Wales, with Part of Scotland." The principles on which this map was based were that strata occurred in sequences
of diverse rock types, that these strata had a lateral continuity which could be mapped,
and that they were characterized by different assemblages of fossils. As the discipline
of stratigraphic paleontology progressed, it became apparent that some fossils appeared
to be restricted to certain geological time spans, while others were long ranging but appeared to occur in certain rock types. The implications of these facts were realized by
Prevost in 1838. He proposed the name "formation" for lithostratigraphic units. By
using biostratigraphy he demonstrated that different "formations" were formed in the
same geological "epoch," and that similar formations could occur in different epochs.
Almost simultaneously, Gressly (1838), working in the Alps, reached similar conclusions. He coined the name "facies" for units of rock which were characterized by similar lithological and paleontological criteria. His original definition (as translated by Teichert, 1958b) reads:
To begin with, two principal facts characterize the sum total of the modifications which I call facies or aspects of a stratigraphic unit: one is that a certain lithological aspect of a stratigraphic
unit is linked everywhere with the same paleontological assemblage; the other is that from such
an assemblage fossil genera and species common in other facies are invariably excluded.
Gressly took the word "facies" from the writings of Steno (1667) and, though he was
not aware of the distinction of zonal and facies fossils, he formulated a number of laws
that govern the vertical and lateral transitions of facies. By the mid-nineteenth century
it was established that bodies of rock could be defined and mapped by a distinctive combination of lithological and paleontological criteria. These rock units were called "formations" or "facies" by different geologists. Over the years, however, these two terms
assumed different meanings. "Formation" became ever more rigorously defined, while
"facies" became used in ever broader meanings.