Annotati on of Outcrop Profiles
straightforward, and the researcher should be wary
of falling into the simplistic traps set in earlier fluvial
literature, for example, that lateral-accretion deposits indicate point bars in meandering channels. As
noted in Chap. 8, LA deposits occur in a wide range
of fluvial environments. Brierley and Hickin (1991)
carried out a detailed element analysis of a modern
gravel-sand river that showed braided, wandering,
and meandering reaches. The three fluvial channel
styles could not be distinguished from their deposits
on the basis of either lithofacies assemblages or architectural-element types. In Chap. 8, it is shown
that most element types occur in several different
fluvial settings. In order to distinguish them and to
arrive at reliable interpretations of channel style it is
necessary to take into account all available data on
element geometry and orientation. Detailed paleocurrent analysis is a very important part of this interpretive process.
Lithosomes of sixth-order and higher rank require classification and documentation using the
principles of lithostratigraphy or allostratigraphy
(Sect. 9.6). Increasingly, sequence concepts are now
being used to classify and describe such deposits
(Chap. 13). Large-scale channel belts and other discrete facies tracts may be mappable as individual
stratigraphic units and assigned member or formation rank. For example, DeCelles et al. ( 1991) defined
the surface enclosing individual fanglomerate bodies as sixth�order in rank, and designated the entire
assemblage of contemporaneous conglomerate
units as a formation�rank unit, enclosed by a seventh-order surface.
Deposits whose large-scale architecture is controlled by tectonic pulses or repeated base-level
changes may be suited to a stratigraphic approach
that employs allostratigraphic principles. As discussed in Chaps. 11 and 13, fluvial depositional systems tracts in the rock record are commonly
bounded by major erosion surfaces (of sixth-order
and higher rank), and may contain marine flooding
surfaces. These· surfaces are typically regional in extent, and indicate major allogenic controls on sedimentation. They provide the basis for subdividing
stratigraphic successions using regional contemporaneous or near-contemporaneous erosional or
flooding events. This is the formal basis for
allostratigraphy, as defined in recent stratigraphic
codes (see Miall 1990, Sect. 3.6; Sect. 9.6, this Vol.).
Both regional erosion surfaces and flooding surfaces
have been proposed as sequence boundaries by different authors (e.g., Galloway 1989a; Van Wagoner
et al. 1990), that is, as surfaces used to define the
95
beginning and end of sets of transgressive and retrogressive processes that are ·repeated in the rock
record. Regardless of the terminology used, the genetic concepts that underlie sequence-stratigraphic
principles are important aids for the regional
stratigrapher (Chap. 13). Employing such principles
can facilitate the understanding of regional stratigraphic processes more readily than the classic
methods of lithostratigraphy, which classify similar
facies together regardless of their age range1 and
separate dissimilar facies even where they are interbedded and clearly contemporaneous. Although basic geological mapping is more easily carried out
using lithostratigraphic methods, this older approach is much less useful as a basis for interpretation. Formal allostratigraphic terminology is now
being proposed for some such successions. For example, Plint (1990) subdivided a suite of shoreface
deposits (that included some fluvial units formed
during times of low base level) into a series of
allomembers defined by regional disconformities
and cycle boundaries. In another .study, referred to
at length in Chap. 13, Van Wagoner et al. (1990)
attempted to adapt existing lithostratigraphic terminology to a new sequence framework. A unit
originally called the Grassy Member was found to
contain a major regional disconformity as a result of
their sequence analysis, and was split into upper and
lower parts, each of which comprised part of two
separate stratigraphic sequences. The resutt is a
clumsy stratigraphic nomenclature that needs a
complete recasting in sequence terminology.
Research on nonmarine sequences is actively underway (Chap. 13), and a descriptive architectural
classification of sequence geometries and bounding
surfaces will likely be an eventual outcome. Terms
derived from the Exxon work on sequence stratigraphy, such as low-stand and high-stand systems tracts
(Posamentier and Vaill988; Posamentier et al.l988;
Van Wagoner et al. 1990), are interpretive in nature,
and only to be employed following thorough analysis, as discussed later. They cannot, in any case, be
applied without qualifications to rocks that are entirely nonmarine.
4.8 Annotation of Outcrop Profiles
A completed profile is analogous to a geological
map. It is basically a descriptive diagram of the exposed rocks. However, lil<.e a geological map, it may
contain a degree of interpretation. On geological
straightforward, and the researcher should be wary
of falling into the simplistic traps set in earlier fluvial
literature, for example, that lateral-accretion deposits indicate point bars in meandering channels. As
noted in Chap. 8, LA deposits occur in a wide range
of fluvial environments. Brierley and Hickin (1991)
carried out a detailed element analysis of a modern
gravel-sand river that showed braided, wandering,
and meandering reaches. The three fluvial channel
styles could not be distinguished from their deposits
on the basis of either lithofacies assemblages or architectural-element types. In Chap. 8, it is shown
that most element types occur in several different
fluvial settings. In order to distinguish them and to
arrive at reliable interpretations of channel style it is
necessary to take into account all available data on
element geometry and orientation. Detailed paleocurrent analysis is a very important part of this interpretive process.
Lithosomes of sixth-order and higher rank require classification and documentation using the
principles of lithostratigraphy or allostratigraphy
(Sect. 9.6). Increasingly, sequence concepts are now
being used to classify and describe such deposits
(Chap. 13). Large-scale channel belts and other discrete facies tracts may be mappable as individual
stratigraphic units and assigned member or formation rank. For example, DeCelles et al. ( 1991) defined
the surface enclosing individual fanglomerate bodies as sixth�order in rank, and designated the entire
assemblage of contemporaneous conglomerate
units as a formation�rank unit, enclosed by a seventh-order surface.
Deposits whose large-scale architecture is controlled by tectonic pulses or repeated base-level
changes may be suited to a stratigraphic approach
that employs allostratigraphic principles. As discussed in Chaps. 11 and 13, fluvial depositional systems tracts in the rock record are commonly
bounded by major erosion surfaces (of sixth-order
and higher rank), and may contain marine flooding
surfaces. These· surfaces are typically regional in extent, and indicate major allogenic controls on sedimentation. They provide the basis for subdividing
stratigraphic successions using regional contemporaneous or near-contemporaneous erosional or
flooding events. This is the formal basis for
allostratigraphy, as defined in recent stratigraphic
codes (see Miall 1990, Sect. 3.6; Sect. 9.6, this Vol.).
Both regional erosion surfaces and flooding surfaces
have been proposed as sequence boundaries by different authors (e.g., Galloway 1989a; Van Wagoner
et al. 1990), that is, as surfaces used to define the
95
beginning and end of sets of transgressive and retrogressive processes that are ·repeated in the rock
record. Regardless of the terminology used, the genetic concepts that underlie sequence-stratigraphic
principles are important aids for the regional
stratigrapher (Chap. 13). Employing such principles
can facilitate the understanding of regional stratigraphic processes more readily than the classic
methods of lithostratigraphy, which classify similar
facies together regardless of their age range1 and
separate dissimilar facies even where they are interbedded and clearly contemporaneous. Although basic geological mapping is more easily carried out
using lithostratigraphic methods, this older approach is much less useful as a basis for interpretation. Formal allostratigraphic terminology is now
being proposed for some such successions. For example, Plint (1990) subdivided a suite of shoreface
deposits (that included some fluvial units formed
during times of low base level) into a series of
allomembers defined by regional disconformities
and cycle boundaries. In another .study, referred to
at length in Chap. 13, Van Wagoner et al. (1990)
attempted to adapt existing lithostratigraphic terminology to a new sequence framework. A unit
originally called the Grassy Member was found to
contain a major regional disconformity as a result of
their sequence analysis, and was split into upper and
lower parts, each of which comprised part of two
separate stratigraphic sequences. The resutt is a
clumsy stratigraphic nomenclature that needs a
complete recasting in sequence terminology.
Research on nonmarine sequences is actively underway (Chap. 13), and a descriptive architectural
classification of sequence geometries and bounding
surfaces will likely be an eventual outcome. Terms
derived from the Exxon work on sequence stratigraphy, such as low-stand and high-stand systems tracts
(Posamentier and Vaill988; Posamentier et al.l988;
Van Wagoner et al. 1990), are interpretive in nature,
and only to be employed following thorough analysis, as discussed later. They cannot, in any case, be
applied without qualifications to rocks that are entirely nonmarine.
4.8 Annotation of Outcrop Profiles
A completed profile is analogous to a geological
map. It is basically a descriptive diagram of the exposed rocks. However, lil<.e a geological map, it may
contain a degree of interpretation. On geological