5.3 PRIMARY INORGANIC SEDIMENTARY STRUCTURES
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5.3.3.1 Massive Bedding
An apparent absence of any form of sedimentary structure is found in various types of
sedimentation unit. It is due to a variety of causes. First, a bed may be massive due to
diagenesis. This is particularly characteristic of certain limestones and dolomites that
have been extensively recrystallized. Secondly, primary sedimentary structures may be
completely destroyed in a bed by intensive organic burrowing.
Genuine depositional massive bedding is often seen in fine-grained, low-energy environment deposits, such as some claystones, marls, chalks, and calcilutites. Reef rock
(biolithite) also commonly lacks bedding. In sandstones massive bedding is rare. It is
most frequently seen in very well-sorted sands, where sedimentary structures cannot be
delineated by textural variations. It has been demonstrated, however, that some sands
which appear structureless to the naked eye are in fact bedded or cross-bedded when
X-rayed (Hamblin, 1962; Lewis and McConchie, 1994).
Genuine structureless sand beds may be restricted to the deposits of mud flows, grain
flows, and the lower (A unit) part of turbidites, though these may be size graded.
5.3.3.2 Flat-Bedding
One of the simplest intrabed structures is flat- or horizontal bedding. This, as its name
implies, is bedding that parallels the major bedding surface. It is generally deposited horizontally. Flat-bedding grades, however, via subhorizontal bedding, into cross-bedding.
The critical angles of dip that separate these categories are undefined. Flat-bedding occurs in diverse sedimentary environments ranging from fluvial channels to beaches and
delta fronts. It occurs in sand-grade sediment, both terrigenous and carbonate.
Flat-bedding is attributed to sedimentation from a planar bed form. This occurs
under shooting flow or a transitional flow regime with a Froude number of approximately 1. Sand deposited under these conditions is arranged with the long axes of the
grains parallel to the flow direction. Moderately well-indurated sandstones easily split
along flat-bedding surfaces to reveal a preferred lineation or graining of the exposed
layer (Fig. 5.10). This feature is termed parting lineation, or primary current lineation
(Allen, 1964). This sedimentary structure provides a paleocurrent indicator, indicating
the sense, but not the direction of current flow (Fig. 5.11). It is important to remember
that, like many of the bed sole markings previously described, parting lineation will not
be seen in friable unconsolidated sands, nor in low-grade metamorphic sediments.
5.3.3.3 Graded Bedding
A graded bed is one in which there is a vertical change in grain size. Normal grading is
marked by an upward decrease in grain size (Fig. 5.12). Reverse grading is where the
bed coarsens upward. There are various other types (Fig. 5.13). Graded bedding is produced as a sediment settles out of suspension, normally during the waning phase of
a turbidity flow (see Section 4.2.2). Though the lower part of a graded bed is normally
massive, the upper part may exhibit the Bouma sequence of sedimentary structures (see
Fig. 4.17, and Bouma, 1962).
The term "graded bed" is normally applied to beds measurable in centimeters or
143
5.3.3.1 Massive Bedding
An apparent absence of any form of sedimentary structure is found in various types of
sedimentation unit. It is due to a variety of causes. First, a bed may be massive due to
diagenesis. This is particularly characteristic of certain limestones and dolomites that
have been extensively recrystallized. Secondly, primary sedimentary structures may be
completely destroyed in a bed by intensive organic burrowing.
Genuine depositional massive bedding is often seen in fine-grained, low-energy environment deposits, such as some claystones, marls, chalks, and calcilutites. Reef rock
(biolithite) also commonly lacks bedding. In sandstones massive bedding is rare. It is
most frequently seen in very well-sorted sands, where sedimentary structures cannot be
delineated by textural variations. It has been demonstrated, however, that some sands
which appear structureless to the naked eye are in fact bedded or cross-bedded when
X-rayed (Hamblin, 1962; Lewis and McConchie, 1994).
Genuine structureless sand beds may be restricted to the deposits of mud flows, grain
flows, and the lower (A unit) part of turbidites, though these may be size graded.
5.3.3.2 Flat-Bedding
One of the simplest intrabed structures is flat- or horizontal bedding. This, as its name
implies, is bedding that parallels the major bedding surface. It is generally deposited horizontally. Flat-bedding grades, however, via subhorizontal bedding, into cross-bedding.
The critical angles of dip that separate these categories are undefined. Flat-bedding occurs in diverse sedimentary environments ranging from fluvial channels to beaches and
delta fronts. It occurs in sand-grade sediment, both terrigenous and carbonate.
Flat-bedding is attributed to sedimentation from a planar bed form. This occurs
under shooting flow or a transitional flow regime with a Froude number of approximately 1. Sand deposited under these conditions is arranged with the long axes of the
grains parallel to the flow direction. Moderately well-indurated sandstones easily split
along flat-bedding surfaces to reveal a preferred lineation or graining of the exposed
layer (Fig. 5.10). This feature is termed parting lineation, or primary current lineation
(Allen, 1964). This sedimentary structure provides a paleocurrent indicator, indicating
the sense, but not the direction of current flow (Fig. 5.11). It is important to remember
that, like many of the bed sole markings previously described, parting lineation will not
be seen in friable unconsolidated sands, nor in low-grade metamorphic sediments.
5.3.3.3 Graded Bedding
A graded bed is one in which there is a vertical change in grain size. Normal grading is
marked by an upward decrease in grain size (Fig. 5.12). Reverse grading is where the
bed coarsens upward. There are various other types (Fig. 5.13). Graded bedding is produced as a sediment settles out of suspension, normally during the waning phase of
a turbidity flow (see Section 4.2.2). Though the lower part of a graded bed is normally
massive, the upper part may exhibit the Bouma sequence of sedimentary structures (see
Fig. 4.17, and Bouma, 1962).
The term "graded bed" is normally applied to beds measurable in centimeters or