Geology Reference
In-Depth Information
came important in the Mississippian, as exemplified
by the deep-water 'Waulsortian' mud mounds. Stroma-
tolites gave way to thrombolites during the Viséan.
Microbial contribution to Pennsylvanian and Early
Permian reefs is indicated by the common association
of micrite and carbonate cement crusts with encrusting
organisms regarded to be microbial (e.g.
Archaeolitho-
porella
; Pl. 42/1). Similar associations occur in Middle
and Late Permian shelf-margin reefs, e.g. the Capitan
reef of Texas (Pl. 145/1). Shallow-water and deeper-
water stromatolite reefs are widely distributed in the
cratonic Zechstein basins of central Europe and the
Kungurian basins of the Urals. Comparable to the abun-
dance of microbial carbonates subsequent to the Fras-
nian/Famennian extinction event, microbial carbonates
and small microbial reefs dominated after the Permian-
Triassic extinction event during the Early Triassic. Mi-
crobes contributed to the construction of Middle and
Late Triassic reef frameworks (Harris 1993; Brachert and
Dullo 1994; Pl. 28/1, Pl. 116/2); some Late Triassic
reefs were almost exclusively formed by stromatolites
and microbial crusts. The importance of photic and aph-
otic microbes as reefbuilders increased again during
the Late Jurassic, where low-energy microbe-dominated
reefs and platform carbonates were widespread (Keupp
and Arp 1990; Schmid 1996; Leinfelder et al. 2002),
and continued being constituents of shallow-water and
deeper-water environments during the Early Cretaceous
(Strasser 1988; Neuweiler 1995). In the Late Cretaceous
microbialites were gradually substituted by encrusting
red algae. Some microbial-dominated reef carbonates
are still known from the Tertiary.
cessfully used in recognizing depositional environments
of Precambrian and Phanerozoic limestones and dis-
criminating shallow- and deeper-water settings (Hoffman
1974, George 1999, Riding and Awramik 2000).
Fig. 9.6 illustrates generalized distribution patterns
of Mesozoic microbialites based on about thirty case
studies. Marine deep-water microbialites, formed on
slopes and deeper ramps at water depths of 100 to
150 m, become important in the Late Jurassic (e.g. Gygi
1992), but were already present in the Precambrian
and Devonian. Aphotic stromatolites are known from
Late Triassic and Early Jurassic basinal settings (Böhm
and Brachert 1993).
9.1.5.3 Economic Importance of Stromatolites
Microbes contribute significantly to the precipitation
of a wide variety of authigenic minerals, including ox-
ides, carbonates, phosphates, sulfides and silicates (see
overviews by Mendelsohn 1976; Ferris 2000). Micro-
bial metabolic activity can alter Eh and pH conditions,
influence oxidation and reduction processes, and trig-
ger mineral precipitation and fixation of dissolved metal
ions, thus contributing to the formation of economi-
cally important ore deposits (Dexter-Dyer et al. 1984;
Westbroek and DeJong 1983; Schidlowski 1985; Fan
et al. 1999).
Many ore deposits are closely associated with stro-
matolites, particularly with stromatolite reefs. Examples
are deposits related to the Precambrian Banded Iron
Formation in America (James 1992), stratiform copper
deposits in Africa, or some lead-zinc deposits of the
Mississippi Valley type in North America (see papers
included in Wolf 1976 and Parnell et al. 1990). Cyano-
bacteria and other bacteria are directly or indirectly
implicated in the genesis of iron and manganese de-
posits (Nealson 1983; Yin 1990).
9.1.5.2 Paleoenvironmental Significance of
Microbial Carbonates
Microbial carbonates, together with associated en-
crusting organisms, offer a high potential for inter-
preting ancient environments and their major controls
by nutrients, light, water energy and water depth.
Nutrification is influenced by relative sea-level changes,
which result in shifts from oligotrophic to mesotrophic
organisms, including microbes (Wood 1993), an in-
crease in bioerosion, and platform drowning (Hallock
and Schlager 1986; Hallock 1988; Hallock et al. 1988).
Ecologic changes indicated by differences in micro-
bial carbonates allow sequence stratigraphy frameworks
of carbonate platforms to be refined and sea-level curves
to be evaluated (Whalen et al. 2002).
Differences in growth forms, morphotypes, micro-
fabrics and the association of microbialites with fossils
and specific sedimentary structures have been very suc-
9.2 Biogenic Encrustations
Encrusting organisms play a key role in carbonate sedi-
mentation and the development of reefs.
Skeletonized and non-skeletonized encrusting organ-
isms grow on various hard substrates represented by
hardgrounds (see Sect. 5.2.4.1), cobbles/boulders and
skeletons of organisms.
This subchapter addresses biogenic crusts developed
on free exposed surfaces of benthic host organisms, e.g.
the upper surface of colonial fossils, or in hidden and
shadow places, e.g. the undersides of colonies, or within
