Geology Reference
In-Depth Information
Fig. 14.8.
Devonian coral-stromatoporoid limestone
. A comparison of Devonian reef and platform carbonates from various
parts of the world with regard to the biotic composition and growth types of sessile organisms, biofabric, sedimentary texture
and microfacies displays astonishing similarities. These coincidences can be used to designate rough basic facies types
(Hladil 1986; Machel and Hunter 1994), which are successful tools in recognizing facies zones in outcrop and subsurface
samples. The thin section shows a reef limestone with a widely distributed association consisting of well-preserved tabulate
corals (
Alveolites
- A), rugose corals (
Tyrganolites
- T) and stromatoporoids (
Stachyodes
- S,
Clathrocoilona
- C,
Amphipora
- AM). Larger growth voids are infilled with tiny micritic clasts, peloids and a few microfossils forming a grainstone-
packstone matrix. Remaining void space is occluded by blocky calcite. The criteria of the sample correspond to those of the
facies zone IVf of the Machel-Hunter Model (see Fig. 14.9). Both assignments point to a forereef environment situated
adjacent to the active zone of reef growth. A ramp model would place the sample in a mid-ramp position (Fig. 14.11). The
SMF classification would attribute the sample to the SMF Type 7. This assignment indicates 'organic reef, platform margin',
but allows no furthergoing differentiation. Late Devonian (Late Frasnian): Moravian Karst, Czech Republic. After Hladil
(1986).
Practical advice:
If you want to extract maximum information from this and similar limestones you should consider the
following criteria:
Fossils:
How are they preserved? Try to differentiate fossils, if possible on a generic or even better, a species level! Determine
or estimate the taxonomic diversity! Estimate the frequency of the fossils and their contribution to rock-building! Which
fossils are abundant, which are rare? Are sessile organisms in life position, are they overturned, transported or how are they
oriented? Are the fossils bored or encrusted? Are there recurrent encrustation patterns? Determine growth forms of the
sessile organisms and their guild membership (constructors, bafflers, binders, encrusters, dwellers)! Which organisms are
commonly intergrown?
Matrix:
Describe the matrix with regard to the texture (mud-, wacke-, pack- or grainstone), size, sorting, grading and roundness
of matrix grains and type and amount of micrite!
Porosity and cements:
Porosity type (interskeletal, intraskeletal, shelter pores); cement types and fabric (use polarized light
and cathodoluminescence!)
