Geology Reference
In-Depth Information
or mineral grains. Nuclei formed by skeletal grains are
rare, but may occur in peritidal settings. Pisoids may
have more than one nucleus.
Box 4.18.
Environments of pisoid formation.
Non-marine
• Pedogenic horizons (Burgess 1983; Chafetz and But-
ler 1980; Calvet and Julia 1983; caliche and soil
pisoids see Sect. 2.4.1.1)
• Speleothems (Kirchmayer 1964; Donahue 1969;
Jones and MacDonald 1989; Tietz 1988; cave pearls
see Sect. 2.4.1.3)
• Cold- and hot-water springs and pools (travertine
pisoids formed by inorganic precipitation and bacte-
rially triggered carbonate deposition (Chavetz and
Meredith 1983; Folk and Chavetz 1983)
• Spring-fed pools (Schreiber 1981)
• Freshwater and salt lakes (Jones and Wilkinson 1978;
Handford et al. 1984; lacustrine pisoids see Sect.
2.4.1.7)
• Fluvial environments (Braithwaite 1979; Jones and
Wilkinson 1978; see 2.4.1.8)
Transitional marine and shallow-marine
•
Cortex:
Many pisoids exhibit a distinct concentric
structure consisting of very densely spaced dark and
light laminae (Pl. 14/7, Pl. 126/1). The light laminae
consist of microspar. Dark laminae are interpreted as
microbially induced (Esteban and Pray 1983, Jones and
MacDonald 1989). The number of laminae is variable
and ranges from distinctly more than in ooids to only
few. Caliche pisoids often have few laminae, freshwa-
ter and vadose pisoids many laminae (Esteban 1976),
but many exceptions exist.
Texture:
Inverse grading is a common feature of au-
tochthonous pisoids formed in situ. Common associa-
tions of pisoids are fenestral carbonates, tepees, diage-
netic grainstones and vadose mammillary crusts, char-
acterized by bent laminae at the bottom surface of
pisoids.
Beach zones
•
Coastal sabkhas and playa lakes (Jones and Renault
1994)
•
Supra- and intertidal zones (Purser and Loreau 1973,
see 2.4.2.2)
Diagenesis:
Modern pisoids formed in hypersaline
settings (e.g. Persian Gulf) consist of aragonite and/or
High-Mg calcite. A brick-like microfabric of micro-
sparitic laminae seen in many ancient pisoids is caused
by meteoric-phreatic inversion of originally aragonitic
laminae into granular calcitic laminae without a dis-
tinct dissolution phase (Assereto and Folk 1976). Ex-
periments and studies of ancient peritidal carbonates
suggest that intense fenestral alteration of original mi-
critic sediments in the peritidal environment can lead
to in situ formation of pisoids and diagenetic grain-
stones (Mazzullo and Birdwell 1989).
•
Subtidal shelf crest
Several settings of pisoid formation are common in
the geological record:
Cave pisoids
or 'cave pearls' are grains formed by
chemical precipitation on a preexisting nucleus in a pool
of water within a cave. They originate in high-energy
splash pools and low-energy rimstone pools. The
pisoids from splash pools are commonly spherical to
subspherical (Pl. 14/8), exhibit a polished surface and
typically have an exotic nucleus surrounded by dis-
tinct laminae exhibiting a radial microfabric, and yield-
ing an extinction cross under crossed nicols. Pisoids
from rimstone pools have an irregular shape with a
rough surface, rarely have a nucleus, and commonly
exhibit indistinct and irregular porous laminae. The
crystal fabric of pisoids formed in speleothems is, at
least partly, controlled by organic compounds and bac-
terial contribution (Jones and Renault 1994).
Modern pisoids: Environments and controls
Recent pisoids form under a wide range of environ-
ments including non-marine, transitional marine and
shallow-marine settings (Box 4.18).
Pisoids originate from
1
Chemical precipitation from carbonate-saturated
agitated water (e.g. cave pearls of speleothems; or
thermal pisoids, e.g. Mammoth Hot Springs, Yellow-
stone). The concentric laminae originating from
chemical precipitation are abraded by contacts with
other grains.
Vadose pisoids
may originate in hypersaline transi-
tional environments or in meteoric environments. Char-
acteristics of these grains are in situ growth, e.g. indi-
vidual grains that coalesce and build composite grains
(Pl. 14/1), and asymmetrical growth extending upwards
(Pl. 14/6). Repeated desiccation of vadose pisoids can
lead to fracture and grain breakage; reworked pisoid
fragments are abundant in supra- and intertidal depos-
2
Chemical and biochemical precipitation in low-en-
ergy environments (e.g. vadose pisoids, fluvial
pisoids, pisoids formed in periodically desiccated and
sometimes hypersaline tidal zones, and
3
concretionary growth in arid and semiarid pedogenic
layers (caliche pisoids).
