Geology Reference
In-Depth Information
ogy (including mineralogy, bulk purity, fabric and tex-
ture), stratal permeability, bedding planes, fractures as
well as the existence of confined or unconfined aqui-
fers and structural conduits. Limestones are several
orders of magnitude more soluble than dolomites in
meteoric waters. Major extrinsic factors are climate
(rainfall and evaporation, temperature), base level (el-
evation and relief, sea level or local water bodies), veg-
etation and the duration of time involved in karst-
ification. Different climatic conditions produce differ-
ent karst types (James and Choquette 1988). Karst
formed in areas of warm temperatures and high rain-
fall is characterized by the development of soils and
terra rossa, abundant sinkholes, and dissolution-collapse
breccia. In semiarid and temperate climates both karst
and calcrete may occur together, depending on alter-
nating wet and dry conditions (Esteban and Klappa
1983).
The potential of microfacies studies for the recogni-
tion of ancient speleothems and karst and the signifi-
cance of paleokarst in basin analyses is discussed in
Sect. 15.2.
Microcodium , ichnofossils: crustacean and insect bur-
rows, bioturbation; terrestrial gastropods. (4) Stratifi-
cation - high- to low-angle beds, see Fig. 15.1. (5) As-
sociated facies - upward-shallowing succession, shelf/
reef - beach grainstone - eolian grainstone. Grains are
ooids or bioclasts, sometimes accumulations of particu-
lar foraminiferal tests (forming the 'miliolite' of the
Trucial Coast, a grainstone composed predominantly of
miliolid foraminifera).
Significance: The differentiation of wind-borne cal-
carenites from grainstones formed in marine environ-
ments is important because calcareous eolianites indi-
cate carbonate shorelines, sea-level fluctuations (ara-
gonite cements in eolianites indicate a later marine
transgression; White 1995, San Salvador Island, Baha-
mas), paleoclimatic conditions and paleowind direc-
tions.
2.4.1.5 Glacial Carbonates
The dissolution and recrystallization of glacially
transported carbonate debris may result in the forma-
tion of micritic and sparitic carbonates, leading to a
chemically distinctive carbonate matrix in meltout tills
or subglacial crusts within glacial-marine deposits
(Hallet 1976; Aharon 1988; Fairchild and Spiro 1990;
Fairchild et al. 1993), which are also recorded in Pre-
cambrian and Phanerozoic glacial deposits.
Enigmatic calcite pseudomorphs, known as glen-
donites and interpreted as pseudomorphs after ikaite, a
metastable hexahydrate of calcium carbonate, were re-
ported from some modern as well as ancient glacioma-
rine environments. Ikaite occurs in morgenstern-like
and stellate aggregates, several centimeters in size, and
indicates sub-zero temperatures at the seafloor. Glen-
donites are typically found in cold-water deposits dat-
ing from the Precambrian to the Pleistocene (Suess et
al. 1982; Shearman and Smith 1985; Kemper 1987).
2.4.1.4 Eolian Carbonates
Terminology: The term eolianite has been coined for
bioclastic wind-borne deposits that make up more than
90% of the Pleistocene deposits of Bermuda (Sayles
1931). The term is now used for eolian sands that have
been cemented by calcium carbonate in a subaerial en-
vironment.
The coalescence of dunes results in linear ridges with
large cross-bed sets which may stand a few tens of
meters above the source beach. Glacial, interglacial and
Holocene eolian carbonate dune deposits are known
from Bermuda, the Bahamas, northeast Yucatan, Cali-
fornia, the Trucial Coast, the Mediterranean (Mallor-
ca, Tunisia, Israel) and western Australia. Generally
modern eolianites are associated with coastal marine
deposits and pedogenic carbonates. Wind can transport
carbonate particles (ooids, microfossils) over wide dis-
tances (e.g. from Africa to the Caribbean).
Recognizing carbonate eolianites: Summaries of fea-
tures common among carbonate eolianites were given
by McKee and Ward (1983) and Abegg et al. (2001).
Important criteria are: (1) Grain texture - good overall
sorting, fine- to medium-grained, fenestrae, sorting dif-
ferences between laminae or beds. (2) Carbonate ce-
ments - vadose and phreatic Low-Mg calcite cements
(Pl. 2/2 and Pl. 31/5, 6, meniscus, pendant, needle-fi-
ber, blocky, drusy). (3) Biota - plant structures (rhizo-
concretions, calcified root systems of dune plants),
Non-marine carbonates formed in aquatic settings
comprise freshwater carbonates, lacustrine carbonates
and fluvial carbonates.
2.4.1.6 Travertine, Calcareous Tufa and
Calcareous Sinter
Origin: Travertine and tufa are freshwater carbonates
formed as a result of physical and/or biochemical CO 2
degassing around carbonate- and CO 2 -rich springs,
along streams and in pools, and often precipitated to-
gether with and on cyanobacteria, bacteria, algae (Pl.
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