Geology Reference
In-Depth Information
bonates and to recognize 'warm-mode' and 'cold-mode'
paleolatitudes are improving rapidly.
Bioerosion.
The biological destruction of carbonate
substrates by borers, raspers and crushers is currently
being intensively studied. Important objectives of these
studies, which are also significant in microfacies analy-
sis, are the application of boring morphology and bor-
ing patterns for paleobathymetric reconstructions and
the estimation of the quantitative contribution of bio-
eroders for the marine carbonate budget (Sect. 9.3).
Carbonate grains
. Increasingly new more informa-
tion is available concerning the biological and non-bio-
logical controls on the origin of carbonate grains, the
site of deposition of carbonate grains, and on the dis-
tribution and dominance of grain types during the Phan-
erozoic. Characteristic grain type associations and the
specific composition of grain types (e.g. ooids, oncoids,
pisoids) provide hints on paleolatitudes and environ-
mental controls for both marine and non-marine car-
bonate deposition (Sect. 4.2; Sect. 12.2).
Microbial carbonates
: Carbonate deposits produced
or localized by microbial communities are known from
marine, marginal-marine, freshwater and terrestrial
environments. The role of bacteria and cyanobacteria
Plate 1 A First Glance at a Thin Section
The plate exhibits a thin section of a Mesozoic limestone from the Austrian Alps. Although the first appearance
may be puzzling, a thorough investigation of the microfacies criteria reveals an amazing history for this rock.
1
The thin section shows fossils and dark-colored areas. The fossils correspond to organisms once living in a reef and
contributing to the formation of a reef limestone. The dark-colored areas represent various types of fine-grained sediment.
The reef limestone contains calcareous sponges, serpulid worms (SW), tube-like microproblematica (
Microtubus
, M),
and bivalves. Sponges and encrusting
Microtubus
and serpulids formed an organic framework. The sediment within the
framework (S1) consists of tiny, densely packed microcrystalline particles (peloids) and some ostracods and foraminifera.
Both elements can only be seen under greater magnification. The sponges are represented by non-chambered inozoans
(IS), chambered sphinctozoans (SS) and chaetetid sponges (CS).
Microtubus
occurs encrusted on and within sponges, and
is concentrated near the periphery of the framework, but the outermost part is a microbial crust (MC) formed by bacteri-
ally induced carbonate precipitation. The surface of the microbial crust outlines a distinct relief. The eye-catching large
bivalve shell (BI) was heavily attacked by boring organisms including boring bivalves (BB) and microborers drilling tiny
holes (MB). The bivalves are infilled with dark calcareous sediment (S2) different in composition from the sediment
within the organic reef framework. The conspicuous circular section might represent a pelagic fossil (H,
Heterastridium
).
The reddish fine-grained sediment (S3) yields densely packed skeletal grains including sponge spicules and pelagic shell
debris as well as some radiolarians and pelagic ostracods.
This sediment and the reef limestones are covered by a thin Fe/Mn crust (arrows) forming small microbial
microstromatolites. The overlying sediment S4 contains (white) skeletal elements of crinoids, mostly parts of arms asso-
ciated with pelagic shell debris. Radiolarians, small ammonites and pelagic algal cysts are too small to be visible on the
plate.
Interpretation:
Microtubus
indicates a Late Triassic (Norian to Rhaetian) age for the reef limestone. The composition of
the reef-building assemblage characterizes reefs formed in protected low-energy settings. Environmental change is indi-
cated by the peripheral microbial crust covering the reef fabric. The reef limestone and the microbial crust were transected
by a fissure that remained open for some time as indicated by the calcite tapestry. Later on the fissure was infilled by the
open-marine pelagic sediment S3. The Fe/Mn crust covering both the surface of the microbial crust and the fissure fill
marks a discontinuity surface corresponding to a hardground and indicating a gap in the sedimentary record. The microfacies
of sediment S4 reflects the renewed onset of pelagic sedimentation. The sediments S3 and S4 are Liassic in age. The time
hidden in the pelagic sedimentary record, however, can not be determined from the thin-section data.
Solution took place within the reef limestone as shown by solution voids (V) within the originally aragonitic sponges.
In contrast primary calcitic fossils, e.g.
Microtubus
, are well preserved. These voids and the cement type of the calcite
tapestry within the fissure point to freshwater diagenesis, an interpretation that also is supported by stable isotope data.
The small-scale microfacies data of the thin section reflect the large-scale developments in sedimentation at the Triassic-
Jurassic boundary in the Northern Calcareous Alps: Platform and reef development - platform destruction related to
subaerial exposure and drowning - transition to open-marine pelagic sedimentation.
The sample comes from the Tropfbruch quarry in Adnet near Salzburg, Austria, where Rhaetian reef limestones (Fig.
8.2) are overlain by Early Jurassic carbonates. See also the polished palte of this outcrop on page XXIII.
The story behind this thin section is rather difficult, but the following chapters will answer to all of your questions
and give the information needed.
