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its (Pl. 14/4). Features characteristic of vadose and
phreatic pisoids are a radial fibrous fabric, perched in-
clusions, grain-to-grain contacts, polygonal fit of grains
(Pl. 126/2), pisoids linked by cement crusts (Pl. 126/
1), inverse grading (Pl. 126/2), tepee structures, and
irregular non-spherical pisoid shapes.
cur? Pisoids have been interpreted variously as marine,
schizohaline, lacustrine, fluvial, spring-surface fed sur-
face pool, speleothemic, phreatic and vadose. Phreatic
and vadose as well as schizohaline and marine records
of pisoids are common from the Permian, Triassic and
Jurassic (Box 4.19).
Vadose pisoids have many characteristics in com-
mon with caliche pisoids. Joint features are, for ex-
ample, the downward elongation of laminae, and the
development of fibrous or blocky calcite cements be-
tween the grains. Both hypersaline and caliche pisoids
are associated with fenestral fabrics (commonly grain-
supported for vadose pisoids, mud-supported for cali-
che pisoids).
Features which can be used to differentiate vadose
pisoids are (1) the regional geological setting of pisoid
accumulation; (2) presence of fossils; (3) spar crystals
with a triangular cross section, indicating precipitation
from fresh water; and (4) fabrics which point to the
original mineralogy (e.g. brick-like textures or elon-
gated rays with square termination interpreted as ara-
gonite relicts formed in hypersaline settings (Assereto
and Folk 1976; Esteban 1976; Esteban and Pray 1983;
Chafetz and Butler 1980; Hay and Wiggins 1980).
Primary marine pisoids are known from Paleozoic
and a few Triassic and Jurassic occurrences. In con-
trast to the rather scarce record from the Phanerozoic,
marine pisoids appear widespread in Proterozoic car-
bonates, commonly associated with ooids, stromato-
lites and intraformational conglomerates (Swett and
Knoll 1989). This disparity is related to changes in the
degree of calcium carbonate supersaturation caused by
the Late Precambrian and Early Cambrian radiation of
calcified algae and animals.
The Permian Reef Complex case study: An instruc-
tive example of the varying interpretation of pisoid
limestones in the ancient record are the pisoids occur-
ring in bedded carbonates of the famous Permian Reef
Complex (Pl. 14/1, Pl. 34/6, Pl. 126/1). Early workers
considered the pisoids to have been formed by algae;
they compared the grains with modern oncoids and as-
sumed that the pisoids were formed and deposited in
lagoons as free subaquatic sedimentary particles. Dun-
ham (1969) and Thomas (1965) were the first to sug-
Caliche pisoids originate in situ at the surface or
within surface layers by evaporation. The sizes ranges
from about 2 mm to several centimeters. Many caliche
pisoids are characterized by weak or obscure lamina-
tion, accretion and centrifugal growth, laminae of vari-
able thickness, alternation of light and dark laminae,
composite nuclei (pisoid fragments or lithoclasts), or
absence of an observable nucleus, circumcracking struc-
tures, association with micritic crusts, a clotted peloi-
dal texture and a mud-supported fabric (Fig. 4.27; Pl.
128/6).
Box 4.19. Selected case studies of ancient pisoids. A com-
prehensive list of studies of modern and ancient pisoids
and pisolites can be found in Peryt (1983).
Pleistocene: Braithwaite 1983; Burgess 1983; James
1972.
Tertiary: Gradzinski and Radomsi 1976; Misik 1980;
Surdam and Stanley 1979; Swineford et al. 1958.
Cretaceous: Enos 1974; Tisljar 1983.
Jurassic: Melas and Friedman 1987; Goldberg and
Friedman 1974.
Triassic: Assereto and Folk 1976,1980; Bernoulli and
Wagner 1971; Ciarapicca and Passeri 1983; Dozet
1991; Henrich 1984; Wagner 1987
Permian: Pisoids in the Permian Reef Complex, Texas
- Dunham 1969; Esteban 1976; Esteban and Pray
1977, 1983; Loucks and Folk 1976; Kendall 1969;
Mazzullo and Birdwell 1989 (discussion: Wright
1990); Pray and Esteban 1977; Peryt 1981, 1983,
1984; Clark 1980; Folk and Siedlecka 1974.
Carboniferous: Gerhard 1985; Misik 1980; Wright
1981.
Devonian: Wardlaw and Reinson 1971.
Silurian: Kahle 1974; Frank et al. 1993.
Precambrian: Cao Ruiji and Xu Yaosong 1983;
Radwanski and Birkenmayer 1977; Simonson and
Jarvis 1993; Swett and Knoll 1989.
Fluvial pisoids and lacustrine pisoids typically show
variable irregular shapes and sizes, alternation of fine-
grained detritic laminae and laminae consisting of
coarse cement; association with laminated crusts, or-
ganic detritus and crystal sand.
Unusual settings in which pisoids are formed are
oilfield waters (Kemp 1959), landfills (Maliva et al.
2000), power stations (Knatz 1966), and old mines
(Hahne et al. 1968).
Ancient pisoids
The interpretation of ancient pisoid limestones
hinges on two questions: What processes caused pisoid
formation and in what environment did formation oc-
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