Geology Reference
In-Depth Information
Pisoids
formed under low-energy conditions exhibit
irregular shapes and discontinuous lamination. High-
energy pisoids are characterized by regular spherical
shapes and concentric laminations (Sect. 4.2.6).
Aggregate grains
are usually formed in low-energy
to moderate-energy environments with fluctuating wa-
ter circulation (Sect. 4.2.7).
Intraclasts
originate both in low-energy and high-
energy environments (Sect. 4.2.8.1). Imbrication pat-
terns point to controls by weak currents (Sect. 5.1.2).
Flat-pebble conglomerate patterns indicate medium to
high-energy currents.
Morphometric criteria of grains
Changes in
grain roundness and sphericity
(Sect.
4.3.2) in association with changes in
grain size and sort-
ing
may be used in recognizing different hydrodynamic
levels (Fig. 4.32, Fig. 8.6). Of particular interest are
rounded fragments of fossils that were not originally
round (Fig. 4.31) and angular or rounded resediments
(intraclasts and extraclasts). Note that the roundness of
skeletal grains depends on many factors, including the
architecture and microstructure of the shells, grain size,
water energy and the length of transport.
Grain size
Covariation of grain size and sorting (Sect. 6.1.1.2),
grain size parameter diagrams and the maximum grain
diameter (Sect. 6.1.2.2) give some clues to depositional
energy levels if the limitations discussed in Sect.
12.1.1.2 are considered.
Fig. 12.2.
Controls on growth forms and growth patterns of
colonial reef builders
by water energy and sedimentation
rates. The relationship between the external shape and the
external growth banding geometry of the colonies can be used
to infer water roughness and the relative rates of sedimenta-
tion. Enveloping and non-enveloping growth forms result
from the position of the living surface during growth. These
positions are expressed in corals by successive growth bands,
in stromatoporoids by latilaminae (Sect. 10.2.3.1). Envelop-
ing forms indicate increasing stability to water roughness by
a decreasing height to width ratio. Progressive burial in sedi-
ment increases stability related to ragged margins. The latter
is a measure of the capacity of the organism to keep pace
with sedimentation. Ragged margins point to high-energy
conditions, smooth margins to low-energy conditions. After
James and Bourque (1992).
Fabrics
Biofabrics and grain orientation
are valuable indi-
cators of water energy levels (Sect. 5.1.2) and bottom
currents. High-energy conditions are reflected by im-
brication (Pl. 18/1)), cross-bedding (Pl. 18/3) and
stacked shells. Low-energy and moderate energy con-
ditions are indicated by parallel oriented thin platy skel-
etal grains (Pl. 18/5). Orientation patterns of intraclasts
and flat-pebble conglomerates can be used in recog-
nizing flow-current directions.
Some
depositional breccia
are indicators of high-
energy conditions, e.g. peritidal breccia and forereef
breccia (Sect. 5.3.3.3).
slightly agitated, moderately agitated and strongly agi-
tated conditions (Sect. 12.1.1.2).
Water Energy Indices
Textural criteria (size, sorting and roundness of
grains, matrix types) and compositional data (fossils,
carbonate and non-carbonate admixtures) as well as
sedimentary structures allow inferred water energy to
be recognized by five
energy indices
that describe car-
bonates deposited under quiet, intermittently agitated,
Growth forms of sessile organisms
The external geometry and the growth patterns of
reef-building organisms have received considerable
attention as proxies for prevailing hydrodynamic con-
ditions. Growth forms of corals and stromatoporoids
