Geology Reference
In-Depth Information
reported from Michigan (Wilkinson et al. 1980). Salt-
water and freshwater ooids exhibit differences in the
morphology, microfabrics and mineralogy of the corti-
cal layers, and in size ranges. The same is true for oolitic
carbonate particles formed in situ in hypersaline algal
mats (Friedman et al. 1973). These differences offer
the possibility of tracing the development of ancient
freshwater- and saltwater lakes (Sect. 4.2.5).
Lacustrine oncoids
have been described by various
names (algal nodules, water biscuits, lake balls, lacus-
trine pisoliths) for many years (Pia 1926, 1933, Schäfer
and Stapf 1978, Jones and Wilkinson 1978; Pl. 12/5).
The origin of these grains is related to the activity of
algae and microbes (Sect. 4.2.4.1). Cyanobacteria are
known or assumed to be essential in the formation of
these oncoids. Modern oncoids are not restricted to
freshwater environments but also occur in brackish and
hypersaline waters. Today, oncoids are rare in tidal and
shallow subtidal marine environments. Recent marine
oncoids are non-lithified, as opposed to modern hard
freshwater oncoids that are closely similar to Paleo-
zoic and early Mesozoic marine oncoids. This may be
related to the replacement of porostromate oncoids by
spongiostromate oncoids in marine settings during the
Jurassic.
lites of Shark Bay in western Australia. Spectacular
meter-sized stromatolites consisting of hydromagnes-
ite were described from the highly alkaline Salda Golu
Lake in Southern Turkey (Braithwaite and Zedef 1996).
The composite stromatolites originate from microbi-
ally induced precipitation of hydromagnesite, which is
possible because of the dissolution of magnesium and
calcium during the passage of water through ultrama-
fic rocks.
2.4.1.8 Fluvial Carbonates
Carbonate deposits originate in fluvial waters in vari-
ous settings including small creeks, large rivers or wa-
terfalls where tufa is formed. Stromatolites, oncoids and
ooids in present-day creeks and streams are usually
formed by the complex activity of algae, mosses and
microbes (Nickel 1985; Galli and Sarti 1989). Fluvial
oncoids and ooids differ in morphological and compo-
sitional criteria as well as in transport patterns (Ver-
recchia et al. 1997) from marine grains and are, there-
fore, valuable environmental indicators.
Basics: Nonmarine carbonates
Esteban, M., Klappa, C.F. (1983): Subaerial exposure envi-
ronment. - In: Scholle, P.A., Bebout, D.G., Moore, C.H.
(ed.): Carbonate depositional environments. - Mem. Am.
Ass. Petrol. Geol.,
33
, 1-54
Etheridge, F.G., Flores, R.M. (eds., 1981): Recent and an-
cient nonmarine depositional environments: models for
exploration. - Soc. Econ. Paleont. Min. Spec. Publ.,
31
,
349 pp.
James, N.P., Choquette, P.W. (1990): Limestone - The mete-
oric diagenetic environment. - In: McIlreath, I.A., Mor-
row, D.W. (eds.): Diagenesis. - Geoscience Canada, Re-
print Series,
4
, 35-73
Pia, J. (1926): Pflanzen als Gesteinsbildner. - 355 pp., Berlin
(Borntraeger)
Pia, J. (1933): Die rezenten Kalksteine. - Mineralogisch-
Petrographische Mitteilungen, Neue Folge, Ergänzungs-
band, 420 pp., Leipzig
Further reading:
K028
Stromatolites
occur both in freshwater and salt lakes.
The stromatolites are usually domes, passively paving
the relief of the substrate and reacting to environmen-
tal controls like currents. Stromatolites of less saline
lakes are composed of calcium carbonate whose pre-
cipitation is induced by algal/bacterial activity. Trap-
ping of clastic particles, widely known from modern
stromatolites, appears to be only of secondary impor-
tance. Stromatolites of saline lakes are well-known from
hypersaline lakes in Utah, Nevada and California as
well as from the San Salvador Island, Bahamas (Mann
and Nelson 1989). The stromatolites have complicated
microstructures consisting of micrite, pellets, filament
molds and casts as well as sparry calcite.
Lacustrine stromatolites contribute to the growth of
'algal mounds'
as demonstrated by the 'reefs' described
from the Great Salt Lake in Utah, which exhibit a non-
skeletal framework of algally induced aragonite pre-
cipitates, internal sediments and carbonate cements, and
cover about 100 square kilometers of the nearshore ar-
eas. The mounds extend from the shoreline down to a
depth of 4 m; they are surrounded by rippled ooid sand.
Coccoid bluegreen algae on the present-day living sur-
face are different from the filamentous algae recogniz-
able in the indurated stromatolites. The change in the
algal flora may be due to an increase in salinity. A similar
pattern is known from the famous intertidal stromato-
Pedogenic carbonates, caliche, paleosols
Esteban, M.C. (1975): Vadose pisolites and caliche. - Amer.
Ass. Petrol. Geol. Bull.,
60
, 1048-1057
Klappa, C.F. (1983): A process-response model for the for-
mation of pedogenic calcretes. - In: Wilson, R.C.C. (ed.):
Residual deposits. - Geol. Soc. London Spec. Publ.,
11
, 211-
220
Reeves, C.C. (1976): Caliche. Origin, classification, morphol-
ogy and uses. - 233 pp., Lubbock, Texas (Estacado Books)
Reinhardt, J., Sigleo, W.R. (1988): Paleosols and weathering
through geologic time: principles and application. - Geol.
Soc. America Spec. Paper,
216
, 181 pp.
Retallak, G.J. (2001): Soils of the past. Second edition. -
416 pp., Oxford (Blackwell)
