Geology Reference
In-Depth Information
4.2.6 Pisoids and Vadoids - Simply 'Larger
Ooids' or Carbonate Grains on their Own?
(e.g. cave pearls in high-energy splash pools, Pl. 14/8),
or pisoids originating in geyser waters). Low-energy
'quiet-water' pisoids show irregular layers at the pe-
riphery and strong deviations from the spherical shape
(e.g. pisoids formed in low-energy rimstone pools of
caves). The shape may be asymmetrical, and exhibit
downward or upward thickening (Pl. 14/6, Pl. 126/2)
as well as lateral elongations whose laminae vanish
within the surrounding sediment or connect single
pisoids, indicating in-situ growth. Broken and recoated
pisoids are common, indicating alternating periods of
fracturing, reworking and pisoid growth. A common
feature of vadoids is polygonal fitting resulting from
vadose compaction.
Terminology
Grains which are structurally similar to ooids, but
have a diameter larger than 2 mm, are often called
pisoids (from
pisum
, Latin for pea). Different names
have been used for these grains, e.g. 'diagenetic oo-
ids', caliche ooids (Fig. 4.27), cave pearls, vadose oo-
ids etc. The size limit of 2 mm is highly artificial, but
also considers, that most marine ooids do not grow to
large sizes, because large grains can not be held in sus-
pension long enough to be coated. Many pisoids are
several centimeters in size, but others are smaller than
the critical boundary allows.
Nucleus and cortex:
The nuclei are formed by frag-
ments of pisoids and cement crusts, lithoclasts, peloids,
The term
pisoid
, has different meanings, depending
on the genetic concepts inferred. Some authors use the
name in order to describe rounded carbonate grains built
by algae; these 'algal pisolites' however should be ad-
dressed as oncoids or rhodoids (Sect. 4.2.4). Other au-
thors favor the term pisoid, when they want to express
a non-marine origin for a coated carbonate or non-car-
bonate grain. It may be characterized by concentric lay-
ers made up of calcite or aragonite around a nucleus,
commonly a lithoclast or pisoid fragments. The term
vadoid
introduced by Peryt (1983) underlines the sub-
stantiated or inferred formation of many (but not all)
of these grains within a freshwater vadose or marine
vadose environment, as indicated by specific cement
types. Vadoid is a useful term if the vadose origin is
sufficiently substantiated. A specific variety of pisoids,
first recognized in back-reef strata of the Permian Reef
Complex in New Mexico and West Texas, has been
called
walnutoid
(Esteban and Pray 1983; Pl. 34/6, Pl.
126/1). The name is derived from the striking resem-
blance in size and shape to nuts of the Texas walnut
tree.
Comparison with other grains
Pisoids are generally larger than most marine oo-
ids, and characterized by dense or irregular laminations
around a non-skeletal nucleus. Oncoids differ in the
common presence of encrusting biota.
Fig. 4.27.
Caliche pisoids
.
A
- Holocene caliche with pisoids
(center) and micritic crusts. Note the indistinct lamination
around organically coated nuclei. The matrix contains abun-
dant terrigenous quartz grains. Fuerteventura, Canary Islands.
B
- Ancient caliche with finely laminated layers which have
been transformed into pisoids with indistinct boundaries. In
contrast to stromatolites formed by biogenic activity, the lami-
nar structures fill in the depressions of the microrelief. Such
fabrics, together with in situ brecciation, desiccation cracks
and the sparseness or absence of fossils, are good indicators
of subaerial exposure of marine sequences and periods of non-
deposition during which paleosols were formed in a semiarid
climate. Middle Triassic (Wetterstein Formation, Ladinian):
Reisskofel, Carinthia, Austria. Scale is 5 mm for A and B.
Criteria
Descriptive criteria of pisoids are the shape, nucleus,
cortex, and structure of the cortex laminae as well as
the size, sorting and texture of the grains.
Shape:
The shape of pisoids is environmentally con-
trolled: Pisoids formed in continuously agitated water
exhibit increasing roundness in the peripheral layers
