Geology Reference
In-Depth Information
Plate 14 Pisoids and Vadoids
Pisoids are carbonate or non-carbonate grains structurally resembling ooids but differing in origin, environment,
internal structure and often a larger size (several millimeters to centimeters).
Discrimination between pisoids, oncoids and marine ooids is essential for recognizing non-marine or mar-
ginal-marine intervals, breaks in marine sedimentation (see Pl. 126) and the explanation of cyclic sequences
(-> 1). Vadoids originate in a vadose freshwater (ranging in salinity from freshwater streams to hypersaline
sabkhas) and in vadose-marine environments (e.g. supratidal areas, -> 4) . The grains are formed in various
climate zones, in pedogene settings (caliche pisoids, -> 6; Pl. 128), karst regions and caves (spelean pisoids,
'cave pearls', -> 8), and comprise fluvial pisoids as well as marine-vadose pisoids (-> 1; see Pl. 126).
Many carbonate vadoid grains are formed by chemical (e.g. cave pearls in speleothems), but also by micro-
bially induced precipitation (e.g. caliche pisoids in humid subtropical and tropical areas). Phosphatic, ferrugin-
eous, or siliceous pisoids are the result of the replacement of primary carbonate pisoids or concretional in-situ
formations (-> 7).
1
Vadose-marine pisoids
('walnutoids', Esteban and Pray 1983), formed by accretionary growth on a shelf crest within
shoaling upwards sequences in a protected hypersaline environment. The grains are linked by outer laminae and meniscus
cement. Successive laminae differ in thinner darker micritic and clearer, microspar lamellae. Nuclei are peloids or frag-
ments of preexisting pisoids. White areas represent neomorphic carbonate. Compare Pl. 126/1.
Note that the existence of pisoids, inverse graded bedding, polygonal fitting as well as radial-fibrous fans and mammillary
crusts need not necessarily indicate subaerial exposure. These pisolitic deposits are part of the shelf interior facies separat-
ing evaporitic dolomites on the landward side (lagoonal setting) and fossiliferous marine grain-, pack-, and wackestones
on the seaward side (open shelf). Interpretations of the depositional environment ranges from caliche formation in conti-
nental or coastal-spray-zone settings (Dunham 1969; Scholle and Kinsman 1974), products of marine inorganic precipi-
tation in inter- and subtidal settings (Esteban and Pray 1983), and grains formed in marine seepage or groundwater springs
(Handford et al. 1984). The seep-spring hypothesis is currently favored. SMF 26. Late Permian (Carlsbad Group,
Guadalupian): Mouth of Walnut Canyon, back-reef facies of the Permian Reef Complex, New Mexico, U.S.A.
2
'Coniatoids'
. The term has been suggested for carbonate crusts made up of uniformly thick coatings of aragonitic lamel-
lae occurring in the supratidal zone of the Persian Gulf (Purser and Loreau 1973). Criteria: Very densely-spaced finely
crystalline and fibrous laminae, sometimes forming discordant crusts on marine substrates. Note the irregular, non-sphe-
roidal shape of the grains and the marine nuclei (aggregate grains). Supratidal environment. Early Carboniferous (Viséan):
Czatkovice, Cracow area, southern Poland. Transgressive grainstone filling the relief of an underlying caliche horizon.
3
Broken and reworked pisoids
indicating erosion and redeposition of vadoids in high energy environments. SMF 26. Early
Cretaceous: Apennines, Italy,
4
Vadose pisoids.
Note the meniscus cement (arrow) between the pisoids. Late Permian (Zechstein): Subsurface, northern
Germany.
5
Coniatoids.
As compared with marine ooids, these grains differ in their large non-skeletal nuclei (-> 2). Early Carbonif-
erous (Viséan): Czatkovice, Cracow area, Poland.
6
Caliche pisoids
formed on top of lacustrine stromatolites. Note the significant upward growth. These pisoids were formed
within soils where vadose and shallow phreatic groundwaters became saturated with respect to calcium carbonate. Early
Permian (Rotliegendes): Oberweiler Tiefenbach, Rheinland-Pfalz, Germany.
7
Siliceous pisoids.
Bauxitic paleokarst surface. Note the shrinkage pores around the pisoids (arrows) and the morphologi-
cal similarities with other vadoids (e.g. -> 1). Cretaceous: Subsurface Ras al Khaimah, United Arabian Emirates.
8
'Cave pearl'
(cave pisolite, spaeloid) of a modern speleothem. These non-attached calcite cave deposits originate by
successive calcite precipitation from vadose-meteoric waters, forming strictly concentric lamellae around a nucleus;
these vadoids are characterized by radial crystal structures.
Note the slight deviations from spheroidal shape due to space competition with other grains. These grains grow in cup-
shaped depressions of caves. The grains are rotated by falling water drops (Homann 1969). Micritic pisoids with an
irregular internal lamination have been interpreted as 'subterranean oncoids' because of calcite precipitation related to the
metabolism of bacteria living within biofilms (Gradzinski 2001). Subrecent: Styria, Austria.
