Geology Reference
In-Depth Information
4.3.1 Intentions and Methods
Fig. 4.30), Powers (1953), and Folk (1955) are the most
widely used comparison scales. Descriptive terms for
roundness classes used by most authors and by the Shell
Standard Legend (1995) are very angular, angular, sub-
angular, subrounded, rounded and well rounded. The
sphericity of grains is described as very elongated, elon-
gated, slightly elongated, slightly spherical, spherical,
very spherical (Shell Standard Legend 1995). The chart
enhances rapid measurement and closely spaced dis-
crimination of grain characteristics (examples in Pl. 26/
3, Pl. 27/2, Pl. 33/1, 2, Pl. 114/1, 3, Pl. 115/2).
Another useful roundness scale, particularly for cal-
careous shell material, was provided by Pilkey (1967;
Fig. 4.31), who studied sands from the southeastern U.S.
Atlantic coast. The author concluded that the round-
ness of shell material larger than 2 mm obviously re-
flects wave energy. High-energy beaches produce strik-
ingly well rounded skeletal grains, whereas much of
the coarse shell material in low-energy coastal settings
Rounding of carbonate grains is common in grainstones
as well as in fine to coarse breccias and conglomerates
(Sect. 5.3.3). In grainstones, only fossils and extraclasts
are good abrasion indicators. Ooids and peloids are
round to begin with, and intraclasts are rounded almost
immediately. Rounding is the result of many complex
factors. The roundness of fossils and bioclasts depends
on the inherent roundness, the roundability of the skel-
etal grains (related to the microstructure and architec-
ture of shells), grain sizes (coarser particles round faster
than finer ones), the vigor of surf action, and the length
of time an area is exposed to surf action. Bioclasts that
can be used in morphometric studies of carbonate grains
include foraminifera, calcareous algae, mollusk shells
and echinoderms.
The roundness is a function of grain composition,
size, type of transport process, and transport distance.
Hard, resistant grains, such as quartz, are rounded less
readily during transport than less durable grains such
as carbonate grains and limestone pebbles, which be-
come well rounded by fluvial transport over distances
of just a few kilometers in contrast to quartz pebbles.
Downstream roundness increases more rapidly for lime-
stones than for other lithologies (Mills 1979). Experi-
mental abrasion of limestone pebbles indicate no sig-
nificant difference with regard to the effect of the abra-
sion of fine- or coarse-grained limestones during cal-
culated transport over distances between 6 and 142 km
(Kuenen 1956). Less durable micritic limestones tend
to reach their ultimate ellipsoidal shapes in the final
stage of transport.
Methods:
Many parameters have been proposed to
describe the shape and roundness of rock particles (Re-
views: Barrett 1980; Diepenbroek et al. 1992; Hofmann
1994), but many of them are based on three-dimen-
sional measurements and can not be applied directly to
two-dimensional thin-section data. Most shape studies
of modern sediments concern quartz grains or sedimen-
tary pebbles. Because large grains and pebbles are com-
monly more easily rounded than small grains, many
models developed for pebble- to cobble-sized grains
must be considered with caution in the context of mi-
crofacies analyses of carbonate grains. However, a stan-
dardized characterization of the morphometric data of
carbonate grains is recommended.
Grain roundness in thin sections is commonly esti-
mated by reference to a two-dimensional visual com-
parison scales or charts. These consist of sets of grain
images of known roundness and are recommended for
rapid estimates of rounding and sphericity. The charts
of Krumbein (1941) and Krumbein and Sloss (1955;
Fig. 4.30.
Comparison chart for estimating roundness and
sphericity of sand-sized grains
(adapted from Krumbein and
Sloss 1963). Roundness is roughly shown as an increase (from
left to right) in the shape from angular to round (angular,
subangular, subrounded, rounded, well rounded). Sphericity
is the degree to which the shape of a sedimentary particle
approaches that of a sphere. Sphericity values correspond to
the ratio of the nominal diameter of the grain to the dianmeter
of a circumscribing sphere (generally the longest diameter of
the grain). A perfect sphere has a sphericity of 1.0. Round-
ness values are derived from the Wadell coefficient, com-
puted as the ratio of the average radius of the curvature of the
particle image to the radius of the maximum inscribed circle.
A perfectly rounded particle (an ooid) has a roundness value
of 1.0. Less rounded grains have values less than 1.0. The
chart is used in the description of transported bioclasts and
clasts in limestones. When a match or near match is found,
both the roundness and the sphericity of the particle have
been determined. Measurements are made directly from thin
sections or from photographs. It is helpful to examine at least
100 carbonate grains per sample and to take a critical look at
the type of grains. The letters accompanying the images fa-
cilitate the rapid assignment to a specific roundness/spheric-
ity class. See Pl. 114/3 and Pl. 115/2 for examples.
