Geology Reference
In-Depth Information
Box 4.20.
Significance of pisoids and vadoids.
Paleoenvironmental proxies
Water energy levels
: Differences in shape and lamination of vadoids (e.g. cave pearls) allow the evaluation of con-
tinuous or discontinuous and high- and low-energy environments.
Transport processes
: The analysis of the association of vadose pisoids with other coated grains allows transport
processes within schizohaline peritidal zones to be recognized (Peryt and Piatkowski 1977).
Salinity
: Many pisoids found in peritidal carbonates were formed in waters of increased salinity. Other pisoids are
indicative of non-marine salinity, e.g. freshwater or brackish waters.
Sealevel fluctuations
Pisolitic limestone beds intercalated within marine sequences may indicate breaks in sedimentation caused by drops
in sea level, and associated with karstification phases. Pisoids are common constituents of the supratidal and inter-
tidal members of shallowing-upward cycles.
Depositional settings
Caliche pisoids, cave pisoids, lacustrine pisoids and meteoric-vadose and marine-vadose pisoids can be used to
recognize different subenvironments of terrestrial and marginal marine settings (Esteban and Pray 1983). Standard
microfacies 25 underlines the importance of pisolitic carbonate rocks.
Paleoclimate
Micro- and ultrastructure of cave pisoids as well as caliche pisoids reflect seasonal changes and longer-term climatic
changes.
Economic importance
Hydrocarbon reservoirs
: Pisoid limestones are important reservoir rocks, e.g. in the Permian of West Texas and in
the Mississippian of North Dakota (Gerhard 1985). The productive facies include pisolitic wackestones and grain- to
packstones. Porosity is enhanced by subaerial and vadose dissolution related to pisoid formation.
gest a completely different interpretation of the pisoids.
Pisoids were regarded as vadose concretions produced
by subaerial vadose exposure and diagenesis in a cali-
che soil. The caliche hypothesis gained wide accep-
tance, but started being discussed again. Texas pisoids
were compared with modern pisoids found in sabkha
environments of the Persian Gulf (Scholle and Kins-
man 1974). Assereto and Folk (1976) and Loucks and
Folk (1976) concluded that the pisoids were produced
by marginal marine supratidal environments. Intensive
research by Lloyd Pray and his group led to a complete
rejection of the caliche and the vadose origin hypoth-
eses (Esteban and Pray 1976, 1983). The authors inter-
preted the pisoid depositional setting as a shelf crest of
a carbonate marginal mound separating an evaporitic
lagoon from a more open-marine area. Pisoids and piso-
lites formed from the intensive precipitation from very
shallow hypersaline waters in a peritidal environment.
Thick pisolite development commonly occurred as parts
of shoaling upward sequences of inter-tepee depres-
sions.
As shown by the pisoids of the Permian Reef Com-
plex, differences in shape, nuclei, lamination and tex-
ture of pisoids reflect differences in the depositional
environment (Noe 1996). Mobile transported pisoids,
formed on shallow subtidal environments, are charac-
terized by spherical and ellipsoidal shapes, symmetri-
cal cortices, nuclei of bioclasts and broken pisoids, and
a normally graded texture. Pisoids formed in situ within
peritidal settings exhibit a polygonal fitted fabric, asym-
metrical cortices, downward bent laminae, sediment
inclusions in the peripheral laminae, and inverse grad-
ing. Similar criteria occur with pisoids formed in situ
in the vadose zone (e.g. caliche pisoids), but these grains
are distinguished by their association with meniscus
and dripstone calcite cements and mammillary cement
crusts.
Non-carbonate pisoids
Most modern pisoids consist of calcite or aragonite,
but ferruginous, phosphatic and siliceous pisoids (Pl.
126/3) are also known (Carozzi 1960; Jenkyns 1970;
Bhattacharyya and Kakimoto 1982; Burgess 1983;
Soudry and Southgate 1989; Abreu 1990). Non-carbon-
ate pisoids were formed in more than one mode (Sect.
4.2.5).
The source of the iron of ferruginous pisoids has
been related to volcanism, as well as to solution and
weathering processes. Pisoids occurring within laterite
and bauxite profiles are explained by dissolution and
reprecipitation controlled by climatic variation, chem-
istry of the groundwater in terms of its pH and Eh, and
differences in weathering of the parent rocks and in
transport processes (Pl. 126/4).
Significance of pisoids
Box 4.20 summarizes the significance of pisoids in
microfacies analysis.
