5.3 PRIMARY INORGANIC SEDIMENTARY STRUCTURES
155
bipolar. Sometimes symmetrical ripples contain cross-laminae that dip only in a shoreward direction. These are termed wave-formed current ripples (Fig. 5.26).
Asymmetric ripples, by contrast to symmetric ones, show a clearly differentiated lowangle stoss side and steep-angle lee side. Internally they are cross-laminated, with the
cross-laminae concordant with the lee face. Asymmetric ripples are produced by unidirectional traction currents as, for example, in a river channel. It may be hard to interpret the origin of some individual ripples. Wave and current action can alternately
modify bed forms during a tidal cycle or a fluvial flood phase. Normally it is wave action that molds a previously formed asymmetric current ripple (Allen, 1979).
Both asymmetric and symmetric ripples can occur with isolated lenses of mudstone.
This is termed tlaser bedding (Reineck and Wunderlich, 1968; Terwindt and Breusers,
1972). With gradually increasing sand content, flaser bedding can grade into beds composed entirely of cross-laminated sand in which ripple profiles are absent, though they
are sometimes preserved on the top of the bed. Various terms have been proposed for
these sedimentary structures, including cross-lamination, climbing ripples, and ripple
drift bedding. Jopling and Walker (1968) have defined a spectrum of ripple types, that is
related to the ratio of suspended to traction load material which is deposited (Fig. 5.26).
Normal traction currents deposit sand on the lee side of the ripple only. With increasing suspended load, sedimentation also occurs on the stoss side. This generates a series
of ripple profiles whose crests migrate obliquely upward downcurrent. With excessive
suspended load, sinusoidal ripple lamination develops from the vertical accretion of
symmetric ripple profiles. Jopling and Walker point out that these symmetric ripples
that deposit continuous laminae of sediment are distinct from the isolated symmetric
ripples formed by wave oscillation.
Particular attention has been paid to trying to differentiate cross-lamination of nonmarine and marine origins (e.g., Flemming and Bartholoma, 1995). It has been suggested
that draping clay laminae on ripple foresets indicate subtidal deposition; the Hjulstrom
effect (see Section 4.1) permits the preservation of the draping laminae formed from
clay that settles out at slack water (Visser, 1980). Clay drapes have, however, also been
observed on modern intertidal flats (Fenies et al., 1999) and in interdune sabkhas (Glennie, 1970, 1987).
Having described ripple morphology in cross-section, now consider them in plan.
Ripples seen in modern sediments or exposed on ancient bedding surfaces show a diversity of shapes. Certain dominant types tend to occur and these have been named
(Fig. 5.27). Simplest of all are the straight-crested ripples; these include ripples with both
symmetric and asymmetric profiles. Straight-crested or rectilinear ripples can be traced
laterally for many times further than their wavelength. They are oriented perpendicular to the direction of wave or current movement that generates them. Sinuous ripples
show continuous but slightly undulating crest lines.
The second main group of ripples, as seen in plan, are those whose crest lengths
are generally shorter than their wavelength. These are exclusively asymmetric current
ripples. Two important varieties can be recognized. Lunate ripples have an arcuate crest,
which is convex upcurrent. Linguoid ripples have an arcuate crest, which is convex downcurrent. In plan view, successive linguoid or lunate ripples may be arranged en echelon,
out of phase with one another, or in phase, if the crests all lie on the same flow axis. In
the same way that trough cross-bedding originates in the migrating hollows of complex
155
bipolar. Sometimes symmetrical ripples contain cross-laminae that dip only in a shoreward direction. These are termed wave-formed current ripples (Fig. 5.26).
Asymmetric ripples, by contrast to symmetric ones, show a clearly differentiated lowangle stoss side and steep-angle lee side. Internally they are cross-laminated, with the
cross-laminae concordant with the lee face. Asymmetric ripples are produced by unidirectional traction currents as, for example, in a river channel. It may be hard to interpret the origin of some individual ripples. Wave and current action can alternately
modify bed forms during a tidal cycle or a fluvial flood phase. Normally it is wave action that molds a previously formed asymmetric current ripple (Allen, 1979).
Both asymmetric and symmetric ripples can occur with isolated lenses of mudstone.
This is termed tlaser bedding (Reineck and Wunderlich, 1968; Terwindt and Breusers,
1972). With gradually increasing sand content, flaser bedding can grade into beds composed entirely of cross-laminated sand in which ripple profiles are absent, though they
are sometimes preserved on the top of the bed. Various terms have been proposed for
these sedimentary structures, including cross-lamination, climbing ripples, and ripple
drift bedding. Jopling and Walker (1968) have defined a spectrum of ripple types, that is
related to the ratio of suspended to traction load material which is deposited (Fig. 5.26).
Normal traction currents deposit sand on the lee side of the ripple only. With increasing suspended load, sedimentation also occurs on the stoss side. This generates a series
of ripple profiles whose crests migrate obliquely upward downcurrent. With excessive
suspended load, sinusoidal ripple lamination develops from the vertical accretion of
symmetric ripple profiles. Jopling and Walker point out that these symmetric ripples
that deposit continuous laminae of sediment are distinct from the isolated symmetric
ripples formed by wave oscillation.
Particular attention has been paid to trying to differentiate cross-lamination of nonmarine and marine origins (e.g., Flemming and Bartholoma, 1995). It has been suggested
that draping clay laminae on ripple foresets indicate subtidal deposition; the Hjulstrom
effect (see Section 4.1) permits the preservation of the draping laminae formed from
clay that settles out at slack water (Visser, 1980). Clay drapes have, however, also been
observed on modern intertidal flats (Fenies et al., 1999) and in interdune sabkhas (Glennie, 1970, 1987).
Having described ripple morphology in cross-section, now consider them in plan.
Ripples seen in modern sediments or exposed on ancient bedding surfaces show a diversity of shapes. Certain dominant types tend to occur and these have been named
(Fig. 5.27). Simplest of all are the straight-crested ripples; these include ripples with both
symmetric and asymmetric profiles. Straight-crested or rectilinear ripples can be traced
laterally for many times further than their wavelength. They are oriented perpendicular to the direction of wave or current movement that generates them. Sinuous ripples
show continuous but slightly undulating crest lines.
The second main group of ripples, as seen in plan, are those whose crest lengths
are generally shorter than their wavelength. These are exclusively asymmetric current
ripples. Two important varieties can be recognized. Lunate ripples have an arcuate crest,
which is convex upcurrent. Linguoid ripples have an arcuate crest, which is convex downcurrent. In plan view, successive linguoid or lunate ripples may be arranged en echelon,
out of phase with one another, or in phase, if the crests all lie on the same flow axis. In
the same way that trough cross-bedding originates in the migrating hollows of complex