Geology Reference
In-Depth Information
Plate 41 Classification of Autochthonous Carbonates: Bafflestones and Bindstones
Many textures used in the classifications of limestones are strongly biologically controlled. The term
bound-
stone
introduced by Dunham (1962) designates 'carbonate rocks showing signs of being bound during deposi-
tion' (e.g. reef limestones or stromatolites). This rather comprehensive term was later split up into several new
terms, all of which demand a decision of the user about the potential control of sessile organisms on the forma-
tion of specific rock textures. Studying Devonian reef carbonates, Embry and Klovan (1971) proposed three
terms to replace the name boundstone (see Fig. 8.5). The terms are widely used by geologists and palecologists
interested in ancient reefs, despite considerable problems regarding the genetic meaning. The term
boundstone
should only be used if no decision on the mode of binding during deposition can be made.
Framestones
(see Pl. 41; Fig. 8.2) are characterized by massive in-situ fossils that constructed a rigid three-
dimensional framework during deposition. The in-situ fossils therefore form the supporting framework of the
rock, with matrix material occurring in interstices of the fossils.
Bindstones
(-> 2) contain in-situ, tabular or lamellar fossils that encrusted and bound sediment during depo-
sition. In bindstones the matrix, not the in-situ fossils, form the supporting framework of the rock.
Bafflestones
(-> 1) contain in-situ stalk-shaped fossils which trapped sediment during deposition by acting as
baffles (i.e. reducing the rate of flow of water, thus causing deposition). The ingredients for recognizing a baffle-
stone are the presence of a large number of in-situ stalk-shaped fossils, and a good imagination on the part of the
geologist.
Plates 41 and 42 exemplify the application of the three terms and demonstrate the advantage of using a few
more sophisticated reef rock terms.
1
Bafflestone
. The picture displays a vertical to slightly oblique section of a tabulate coral colony belonging to the group
Auloporida and the genus
Multithecopora
Sokolov. Species of this genus are characterized by colonies composed of
cylindrical very thick-walled branching corallites that are not in contact and arranged parallel to each other. The corallites
of the erect colonies of
Multithecopora syrinx
(Etheridge) are arranged subparallel to each other or grow in various
directions.
The corals occur in situ within mini-mounds found in a shelf sequence formed during a relatively sea-level highstand
(Flügel and Krainer 1992). Maximum height of the mounds is 30 cm, maximum width 90 cm. Except for the tabulate
corals, the mounds yield only a few foraminifera and ostracods.
The fine-grained sediment of the mound is represented by (a) homogeneous micrite (M) within and between the
auloporid colonies and in the center of the mounds, and (b) inhomogeneous calcareous siltite (S) between corallites and
colonies, reflecting the allochthonous background sedimentation, also indicated by siltstone beds overlying and underly-
ing the mounds. The micrite was formed prior to the siltite, which increases in frequency towards the mound margin. The
delicate upwards branching growth form of the coral implies that these organisms were able to filter out carbonate mud,
owing to baffling of gentle currents. A slight increase in water energy as indicated by changes in the orientation of corals
enabled the siltite sediment to be trapped between the corals leading to a more rapid accumulation of the mound sediment
as compared with the adjacent area.
The name bafflestone is justified because of (a) the occurrence of stalk-shaped fossils in life position (as shown by the
geopetals (GP), and (b) abundant fine-grained sediment baffled and trapped by the organisms. SMF 7-B
AFFLESTONE
.
Middle Carboniferous (Kasimovian): Carnic Alps, Italy.
2
Bindstone
. Vertical section of an irregularly laminated fenestral limestone. The limestone is part of a peritidal cyclic
sequence consisting of subtidal, intertidal (this example) and supratidal members. The texture is characterized by alternat-
ing peloidal layers (PL), thin clotted micrite layers (ML) and intercalated horizontally elongated, laminate fenestral pores (F).
The peloidal layers consist of fine and coarser micrite grains which are grain- or mud-supported. Mud-support occurs in
association with micrite layers. Smaller micrite grains are well-rounded, larger grains are subangular. Some larger par-
ticles are composite grains (CG) and may represent 'black pebbles' (see Sect. 4.2.8). Spar-filled interspaces between
smaller grains correspond to tiny tubes representing relics of microbial and algal filaments (white arrow). Many spar-
filled fenestrae exhibit flat bottoms and slightly digitated roofs. Isolated grains within the fenestrae (black arrows) indi-
cate a partial breakdown of the roofs.
The vertical intrusion of a large clast (C) as well as the irregular shape of some fenestrae (F) indicate the soft consis-
tence of the sediment. Despite the absence of in-situ lamellar fossils, this limestone is a bindstone because the grains were
bound together by microbial activity concentrated in the micritic layers. The binding effect was enhanced by synsedimen-
tary cementation in intergranular pores. SMF 21-B
INDSTONE
. Late Triassic (Pantokrator limestone, Norian): Didymi Moun-
tains, Peloponnesus, Central Greece.
