Page 44 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 2: PRINCIPLES OF CARBONATE PRODUCTION 35

that partly explain this discrepancy and Fig. 2.31 shows an determining the proportion of mud (i.e. silt and clay-size
outcrop example. Height and steepness of carbonate sea material), sand, gravel and the presence of organic frame-
cliffs depend on the internal anatomy of the rock. Homo- work; this step, in combination with the depositional struc-
geneous hard limestones or dolomites are apt to form near- tures, is the key to the hydrodynamic environment of depo-
vertical cliffs, often undercut by an intertidal notch carved sition. (2) Determination of the kinds of grains in the coarse
out by bio-erosion. Homogeneous but soft rocks cannot sup- fractions, using hand lens or microscope. In siliciclastics,
port a steep cliff and for inhomogenous rocks with alterna- grain kind is the clue to the source area of the material. In
tions of hard and soft layers, the shore profile depends on carbonates, it offers information on the depositional envi-
the orientation and the angle of the stratigraphic dip. Steep ronment, particularly water chemistry and ecology.
cliffs can be cut into inhomogeneously bedded formations Formal classification systems honor both aspects – grain
only if the dip is shoreward. Seaward dipping packages size, and grain kind of the coarse fraction. Figs. 2.32, 2.33,
tend to develop a dipslope, possibly with a slight undercut 2.34 summarize the most common classification system by
immediately above the water line. Dunham (1962) with modification by Embry and Klovan
The overview in Fig. 2.30 shows that vertical cliffs are (1971). The system requires identification of fine matrix (silt,
most likely to be found on poorly stratified, hard limestones, clay), particles (sand and coarser), pore space (open or filled
such as reefs and mounds. Unfortunately, such rocks are by clear cement), and organic framework. The principles for
also most likely to conceal vertical stratigraphic boundaries defining the classes are, in descending order
because they abound with steep depositional boundaries ➤ Presence and kind of hard organic framework.
and a cliff is easily masked when marine material is plas- ➤ Presence of particles coarser than sand.
tered on it during the next transgression. ➤ Proportion of particles vs. fine matrix and, where both
Despite these problems we should intensify the search for are abundant, the distinction between grains floating in
fossil sea cliffs in the carbonate record. They are reliable in- the matrix (matrix support) and grains resting upon one
dications for lowstands of the sea, providing much stronger another (grain support).
evidence for sea-level movements than the alternation of Several features in the Dunham classification of Fig. 2.32
transgressive and highstand systems tracts as will be shown merit discussion. The distinction between matrix support
in chapters 6 and 7. and grain support cannot be reliably answered by examin-
ing 2D sections of the rock. However, fairly good guesses
CARBONATE ROCKS – THEIR DESCRIPTION AND are possible. The eye can be trained to recognize grain-
CLASSIFICATION supported fabrics by examining grainstone sections. Dun-
ham (1962) provides a series of examples illustrating the
The carbonate factories produce sediment in the size range of textures resulting from irregular grain shapes.
range of clay to boulder, similar to siliciclastics, plus sed- A second point concerns wackestones. They consist of
iments that are hard upon formation because of organic fine matrix, presumably originally mud, with significant ad-
framebuilding or cementation within the depositional envi- mixtures of sand or coarser material. This may be a pri-
ronment. These “framestones” or “bindstones” are a spe- mary depositional fabric, for instance a deposit from a flow
cialty of carbonate rocks. that had reached the limit of its transport capacity and was
Description of rock texture generally proceeds in two forced to deposit many grain sizes simultaneously. How-
steps: (1) A crude characterization in terms of grain size by ever, wackestones can just as easily be the result of burrow-

Fig. 2.32.— Summary of Dunham’s (1962) textural classification of limestones consisting mixtures of sand and mud. After Flügel
(2004), modified.