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5 SEDIMENTARY STRUCTURES
roughly oriented parallel with the paleocurrent. In ideal circumstances it has been possible to find the tool which cut these markings at their downcurrent end. Tools that have
been found include pebbles (especially mud pellet stripped up from the bottom), wood
and plant fragments, shells, astonished ammonites, and fish vertebrae (e.g., Dzulinksi
and Slaczka, 1959).
Flutes, grooves, and tool marks are three of the commonest sole markings found as
interbed sedimentary structures. All are erosional and all are best seen in, but not exclusive to, turbidite facies. A variety of other sole markings have been described and
picturesquely named. Reviews of these can be found in Potter and Pettijohn (1977),
Dzulinski and Sanders (1962), and Dzulinski and Walton (1965). It was pointed out in
Section 4.2.2 that several detailed studies of turbidite formations show a correlation between current direction and sedimentary structures. Proximal turbidites tend to occur
within channels. Moving downcurrent channels give way to tabular basin floor turbidites
under which the sequence of erosional bottom structures grades from scour-and-fill to
flute marks to groove marks and finally tool marks at their distal end. These changes
are related to the downcurrent decrease in flow velocity of any individual turbidite.
5.3.3 Syndepositional (Intrabed) Structures
Syndepositional structures are those actually formed during sedimentation. They are
therefore, essentially constructional structures that are present within sedimentary
beds. At this point it is necessary to define and discuss just what is meant by a bed or
bedding. Bedding, stratification, or layering is probably the most fundamental and diagnostic feature of sedimentary rocks. Layering is not exclusive to sediments. It occurs
in lavas, plutonic, and metamorphic rocks. Conversely, bedding is sometimes absent in
thick diamictites, reefs, and some very well-sorted sand formations. Nevertheless, some
kind of parallelism is present in most sediments. Bedding is due to vertical differences
in lithology, grain size, or, more rarely, grain shape, packing, or orientation. Though bedding is so obvious to see it is hard to define what is meant by the terms bed and bedding
and few geologists have analyzed this fundamental property (Payne, 1942; McKee and
Weir, 1953; Campbell, 1967).
One of the most useful approaches to this problem is the concept of the sedimentation
unit. This was defined by Otto (1935) as "that thickness of sediment which appears to
have been deposited under essentially constant physical conditions." Examples of sedimentation units are a single cross-bedded stratum, a varve, or a mud flow diamictite. A
useful rule of thumb definition is that beds are distinguished from one another by lithological changes. Shale beds thus typically occur as thick uninterrupted sequences. Sandstones and carbonates, though they may occur in thick sections, are generally divisible
into beds by shale laminae. Here are two more arbitrary but useful definitions:
1. Bedding is layering within beds on a scale of about 1 or 2 cm.
2. Lamination is layering within beds on a scale of 1 or 2 mm.
Using these dogmatic definitions, the synsedimentary intrabed structures are of five
categories: massive, flat-bedded, cross-bedded, laminated, and cross-laminated. The
morphology and origin of these are now described.
5 SEDIMENTARY STRUCTURES
roughly oriented parallel with the paleocurrent. In ideal circumstances it has been possible to find the tool which cut these markings at their downcurrent end. Tools that have
been found include pebbles (especially mud pellet stripped up from the bottom), wood
and plant fragments, shells, astonished ammonites, and fish vertebrae (e.g., Dzulinksi
and Slaczka, 1959).
Flutes, grooves, and tool marks are three of the commonest sole markings found as
interbed sedimentary structures. All are erosional and all are best seen in, but not exclusive to, turbidite facies. A variety of other sole markings have been described and
picturesquely named. Reviews of these can be found in Potter and Pettijohn (1977),
Dzulinski and Sanders (1962), and Dzulinski and Walton (1965). It was pointed out in
Section 4.2.2 that several detailed studies of turbidite formations show a correlation between current direction and sedimentary structures. Proximal turbidites tend to occur
within channels. Moving downcurrent channels give way to tabular basin floor turbidites
under which the sequence of erosional bottom structures grades from scour-and-fill to
flute marks to groove marks and finally tool marks at their distal end. These changes
are related to the downcurrent decrease in flow velocity of any individual turbidite.
5.3.3 Syndepositional (Intrabed) Structures
Syndepositional structures are those actually formed during sedimentation. They are
therefore, essentially constructional structures that are present within sedimentary
beds. At this point it is necessary to define and discuss just what is meant by a bed or
bedding. Bedding, stratification, or layering is probably the most fundamental and diagnostic feature of sedimentary rocks. Layering is not exclusive to sediments. It occurs
in lavas, plutonic, and metamorphic rocks. Conversely, bedding is sometimes absent in
thick diamictites, reefs, and some very well-sorted sand formations. Nevertheless, some
kind of parallelism is present in most sediments. Bedding is due to vertical differences
in lithology, grain size, or, more rarely, grain shape, packing, or orientation. Though bedding is so obvious to see it is hard to define what is meant by the terms bed and bedding
and few geologists have analyzed this fundamental property (Payne, 1942; McKee and
Weir, 1953; Campbell, 1967).
One of the most useful approaches to this problem is the concept of the sedimentation
unit. This was defined by Otto (1935) as "that thickness of sediment which appears to
have been deposited under essentially constant physical conditions." Examples of sedimentation units are a single cross-bedded stratum, a varve, or a mud flow diamictite. A
useful rule of thumb definition is that beds are distinguished from one another by lithological changes. Shale beds thus typically occur as thick uninterrupted sequences. Sandstones and carbonates, though they may occur in thick sections, are generally divisible
into beds by shale laminae. Here are two more arbitrary but useful definitions:
1. Bedding is layering within beds on a scale of about 1 or 2 cm.
2. Lamination is layering within beds on a scale of 1 or 2 mm.
Using these dogmatic definitions, the synsedimentary intrabed structures are of five
categories: massive, flat-bedded, cross-bedded, laminated, and cross-laminated. The
morphology and origin of these are now described.