400
9 AUTOCHTHONOUS SEDIMENTS
of intraclasts. Intraclasts may range from sand size, via intraformational conglomerates
of penecontemporaneously cemented beach rock to, arguably, fore-reef slump blocks.
The most important of all grain types is skeletal detritus, individual grains of which
are termed biodasts (Plate 6C). As pointed out in the previous section, this is composed
of aragonite or calcite with varying amounts of trace elements. The actual crystal habit
of skeletal matter is varied too, ranging from the acicular aragonite crystals of lamellibranch shells to the single calcite crystals of echinoid plates. The size of skeletal particles is naturally very variable, ranging down from the largest shell to individual disaggregated microscopic crystals. Continued abrasion of skeletal debris by wave and
current action and by biological processes, such as boring, is responsible for the very
poor textural sorting characteristic of carbonate sediments.
Peloids are a third major grain type (Plate 6B). These were first defined as structureless cryptocrystalline carbonate grains of some 20-60/zm in diameter (McKee and Gutschick, 1969). Peloids form in many different ways. Pellets are peloids of fecal origin,
excreted by marine invertebrates. Pelletoids are peloids formed by the micritization of
skeletal grains through the action of endolithic algae. These colonize carbonate grain
surfaces, bore into them, and change their original fabric into structureless micrite (Taylor and Illing, 1969). Several other processes have been proposed for peloid formation (Maclntyre, 1985; Chafetz, 1986). The genesis of peloid formation is important because this grain type is sometimes a major constituent of limestone formations. Peloidal
deposits are especially characteristic of lagoons and other sheltered shallow inner-shelf
environments.
Lumps are another important carbonate grain type. These are botryoidal grains which
are composed of several peloids held together. They are sometimes termed "composite
grains" or grapestone. Grains such as these are probably formed by the reworking of
peloidal sediment that has already undergone some lithifaction. They are thus nascent
intraclasts (Illing, 1954).
Last of the grain types to consider are the coated grains. These are grains that show a
concentric or radial arrangement of crystals about a nucleus. The most common coated
grains are ooids. These are rounded grains of medium to fine grain size which generally
occur gregariously in sediments termed oolites, devoid of other grain types or matrix.
Ooids contain a nucleus. This is normally a quartz grain or a shell fragment. Modern
ooids are generally composed of concentric layers of tangentially arranged aragonite.
Modern oolites occur in high-energy environments, such as sand banks and tidal deltas.
Like their ancient analogs they are generally well sorted, matrix free, and cross-bedded.
These data all suggest that ooids form by the bonding of aragonite crystals around nuclei, such as quartz or skeletal grains, in a high-energy environment (Plate 6D). The
physicochemical processes that cause ooid formation are unclear, but it is noted that
they generally tend to form where cool dilute seawater mixes with warm concentrated
waters of lagoons and restricted shelves (e.g., the modern Bahamas platform). Recent
ooids are covered with a mucilaginous jacket of blue-green algae which serves as a site
for aragonite precipitation.
Pisoids are coated grains several millimeters in diameter (Fig. 9.2A). They form in
caverns (cave pearls). A rock composed of pisoids is termed a "pisolith." Vadose pisoliths form in caliche crusts beneath the weathered zones of soils ancient and modern
(Dunham, 1969).
9 AUTOCHTHONOUS SEDIMENTS
of intraclasts. Intraclasts may range from sand size, via intraformational conglomerates
of penecontemporaneously cemented beach rock to, arguably, fore-reef slump blocks.
The most important of all grain types is skeletal detritus, individual grains of which
are termed biodasts (Plate 6C). As pointed out in the previous section, this is composed
of aragonite or calcite with varying amounts of trace elements. The actual crystal habit
of skeletal matter is varied too, ranging from the acicular aragonite crystals of lamellibranch shells to the single calcite crystals of echinoid plates. The size of skeletal particles is naturally very variable, ranging down from the largest shell to individual disaggregated microscopic crystals. Continued abrasion of skeletal debris by wave and
current action and by biological processes, such as boring, is responsible for the very
poor textural sorting characteristic of carbonate sediments.
Peloids are a third major grain type (Plate 6B). These were first defined as structureless cryptocrystalline carbonate grains of some 20-60/zm in diameter (McKee and Gutschick, 1969). Peloids form in many different ways. Pellets are peloids of fecal origin,
excreted by marine invertebrates. Pelletoids are peloids formed by the micritization of
skeletal grains through the action of endolithic algae. These colonize carbonate grain
surfaces, bore into them, and change their original fabric into structureless micrite (Taylor and Illing, 1969). Several other processes have been proposed for peloid formation (Maclntyre, 1985; Chafetz, 1986). The genesis of peloid formation is important because this grain type is sometimes a major constituent of limestone formations. Peloidal
deposits are especially characteristic of lagoons and other sheltered shallow inner-shelf
environments.
Lumps are another important carbonate grain type. These are botryoidal grains which
are composed of several peloids held together. They are sometimes termed "composite
grains" or grapestone. Grains such as these are probably formed by the reworking of
peloidal sediment that has already undergone some lithifaction. They are thus nascent
intraclasts (Illing, 1954).
Last of the grain types to consider are the coated grains. These are grains that show a
concentric or radial arrangement of crystals about a nucleus. The most common coated
grains are ooids. These are rounded grains of medium to fine grain size which generally
occur gregariously in sediments termed oolites, devoid of other grain types or matrix.
Ooids contain a nucleus. This is normally a quartz grain or a shell fragment. Modern
ooids are generally composed of concentric layers of tangentially arranged aragonite.
Modern oolites occur in high-energy environments, such as sand banks and tidal deltas.
Like their ancient analogs they are generally well sorted, matrix free, and cross-bedded.
These data all suggest that ooids form by the bonding of aragonite crystals around nuclei, such as quartz or skeletal grains, in a high-energy environment (Plate 6D). The
physicochemical processes that cause ooid formation are unclear, but it is noted that
they generally tend to form where cool dilute seawater mixes with warm concentrated
waters of lagoons and restricted shelves (e.g., the modern Bahamas platform). Recent
ooids are covered with a mucilaginous jacket of blue-green algae which serves as a site
for aragonite precipitation.
Pisoids are coated grains several millimeters in diameter (Fig. 9.2A). They form in
caverns (cave pearls). A rock composed of pisoids is termed a "pisolith." Vadose pisoliths form in caliche crusts beneath the weathered zones of soils ancient and modern
(Dunham, 1969).