Geology Reference
In-Depth Information
mounds. It is common in protected environments and
cryptic habitats.
•
Coverstone
refers to a texture characterized by in-
situ
sedentary tabular or lamellar reef organisms cov-
ering, protecting, and stabilizing broken debris (Pl. 78/
4). This type was included by Embry and Klovan in the
bindstone
category together with reef carbonates char-
acterized by encrusting organisms that bind debris to-
gether, forming a secondary reef framework.
For the needs of a field geologist, Cuffey's (1985)
expansion of the differences in the relationship of reef
organisms and texture may appear to be too rigorous.
The terms, however, have the advantage of reminding
us of the complexity of reef-building, which could not
be described by the use of more widely defined terms
such as 'boundstone' or 'framestone'. Cuffey's terms,
therefore, will be especially valuable for studies deal-
ing with the biological controls on reef building.
ern reefs are gorgonian fans, alcyonarian whips, or the
bladelike coenostea of hydrocorallines. Sea grass is also
involved in baffling and trapping processes by its blades
or roots.
However, geologists may be baffled by bafflers. An-
cient organisms believed to have acted as bafflers com-
prise sponges, bryozoans, bivalves and algae as well
as some corals (Fagerstrom 1987). It is virtually im-
possible to demonstrate that the original organisms
baffled sediment; therefore, baffling is inferred from
functional morphology (erect and dominant upward
growth; flexible, semirigid or rigid skeletons) and close
intercolonial and intracolonial spacing. But both crite-
ria may be misleading (Flügel and Krainer 1992). If
organisms are attributed to the baffler guild, additional
criteria should be considered (e.g. composition of the
fine-grained sediment between and around the 'poten-
tial bafflers', see Pl. 41/1; role of extrinsic factors).
Do rock names assist in reliable environmental in-
terpretations ?
The question can be discussed by using
the term bafflestone, which implies the process of baf-
fling. '
Baffling
' refers to current reduction and gravity-
induced dropping of sediment grains to the substrate.
In reef limestones baffling organisms are rarely found
in their growth position. Examples of bafflers in mod-
Controls on growth forms used in limestone classifi-
cations:
Growth forms of organisms used to define tex-
tures of reef carbonate types are commonly thought to
be related to water energy. But what controls the or-
ganic growth types used in textural carbonate classifi-
cations? Common reef fabrics may be placed into four
groups, characterized by the morphology of the domi-
Plate 42 Classification of Autochthonous Carbonates: Framestones
Framestones (a subdivision of boundstones) are defined by the occurrence of sessile benthic fossils that are
densely spaced and preserved in life position. The morphology and distribution of the fossils should fit into an
imaginary three-dimensional organic framework. Organisms contributing to the formation of framestones have
changed during the Phanerozoic and include corals (Fig. 8.2), coralline sponges (this example), stromatopor-
oids, rudist bivalves, and calcareous red algae.
Framestone.
The thin section of a reef limestone, cut obliquely to the growth direction of most of the sessile organisms,
exhibits fossils, cement-filled interspaces and areas with fossils and micrite. Most of the fossils are coralline sponges, except
for a few mollusk shells (MS) and crinoid columnals (C). Sponges include various 'sclerosponges' (S), inozoid sponges (IS)
characterized by the absence of segmentation, as well as segmented sphinctozoid sponges (SS). The growth forms of these
sponges correspond somewhat to densely spaced bundles of cylinders united by organic encrustations. Encrusters are
sclerosponges,
Archaeolithoporella
(A) and
Tubiphytes
(T) - two microfossils of unknown systematic position (see Pl. 98/8).
Isolated sponges as well as the concentrations of fossils were initially bound by microbial crusts and marine carbonate
cement. The originally aragonitic cements and the biogenic encrustations formed a rigid, highly porous framework structure
(accentuated by white spot lines). Large voids between the organic framework were partly occluded by marine-phreatic,
fibrous isopachous cements (IC) and botryoidal cements (BC) as well as sparry calcite mosaics (SCM). The remaining
intergranular voids were occluded by large calcite crystals formed in a meteoric-vadose zone during exposure of the reef. The
composition and diagenesis of these reef rocks are similar to Late Permian reefs in Texas and New Mexico (cf. Pl. 145), both
of which are very important hydrocarbon reservoirs.
The name framestone is substantiated by the existence of an organic framework made by in-situ fossils and forming a rigid
structure that was strengthened by coeval marine cementation. A massive growth form, as indicated in the original definition
of the name, is not compulsory, because small densely growing sessile organisms can build frameworks as well.
This sample comes from one of the very last reefs formed prior to the huge Permian-Triassic mass extinction. SMF 7-
F
RAMESTONE
. Uppermost Permian (Changsingian): Jiantinba, western Hubei, South China. Crossed nicols.
Picture courtesy of Fan Jiasong (Beijing)
