Page 77 - Carbonate Sedimentology and Sequence Stratigraphy

68 WOLFGANG SCHLAGER

➤ supratidal - above normal high tide, flooded only by MEGABRECCIAS
storms;
➤ terrestrial - beyond the reach of the sea. Breccia beds are common on the slopes and in the basins
Transitions between these environments are gradual. The adjacent to the T and M factories, i.e. in facies belts 1, 3 and
supratidal/terrestrial boundary in particular has been 4. In the field, these breccias often are very conspicuous
blurred in inhabited coastal lowlands as humans have set- and rival reefs as eye catchers. The beds are usually much
tled in the supratidal marshlands and turned them into ter- thicker than the background sediment and may contain out-
restrial environments. For carbonate sedimentologists, how- size clasts, exceeding 10 m in diameter or, in elongate slabs,
ever, the distinction between supratidal and terrestrial is of 100 m in length. The term “megabreccia” has been widely
prime importance. Carbonate depositional systems build used for these beds (see Wrigth and Burchette, 1996, p.365,
into the supratidal zone by themselves, without any out- and Spence and Tucker, 1997 for overviews). There is gen-
side forcing. A change to terrestrial conditions, on the other eral agreement that the material was transported by variable
hand, requires external forcing in the form of a relative sea- combinations of sliding, slumping, debris flows and turbid-
level fall. Theoretically, it should also be possible to trans- ity currents, with prominent roles of sliding and slumping
form a supratidal flat into a terrestrial environment by long- at the beginning and the end of the event, debris flows gov-
distance progradation of the shoreline without relative fall erning the main phase of transport, and turbidity currents
of sea level. However, I am not aware of any hard data developing late and emplacing graded beds on top of the
demonstrating this effect and its significance in the geologic coarse breccia beds and downstream of them.
record. Sedimentary structures and postulated transport mecha-
The formation of soil is a diagnostic feature of the terres- nisms are not significantly different from those of coarse sili-
trial environment. It requires alteration of rock or marine ciclastic deposits in tectonically active settings. The peculiar
sediment by a complex array of abiotic and biotic processes. twist with carbonate megabreccias is that they frequently oc-
These alterations characteristically take on the order of 10 3 cur in tectonically quiet settings and lack evidence of deep-
4
to 10 years to produce geologically observable results (e.g. cutting, tectonically driven submarine erosion. Yet the size
Birkeland, 1999, p. 215). of the clasts borders on that of tectonic klippen. I think that
Soils develop a variety of characteristic textures as well two peculiarities of the T and M factories largely explain
as chemical signals (e.g. Figs 4.14, 4.15, 4.16 ; overviews in the megabreccias. First, both factories tend to build steep
Esteban and Klappa, 1983; Immenhauser et al., 2000). slopes. Second, lithification does not require long time and
Carbon isotopes of carbonate cement are useful chemical deep burial; rather, lithification often takes place at the sea
fingerprints of soil. The rationale is as follows. In natural floor or under shallow burial, controlled as it were by the
substances, carbon consists of a mixture of two stable iso- content of metastable carbonate minerals, pore-fluid com-
topes, 12 Cand 13 C. Organisms take up proportionally less position and microbial activity. This variability of diagenetic
13 C. Soils obtain most of their carbon from organic processes pathways frequently produces alternations of hard and soft
and are, therefore, depleted in 13 C. Cement precipitating intervals in a wide range of spatial scales. In this set-up,
from water that percolated through soils inherits this finger- small masses starting to move on the upper, steeper slopes
13
print and shows a low δ C. (Fig. 4.14; Allan and Matthews, may destabilize larger masses on the lower, flatter slopes;
1977) cushions of fluidized sediment may facilitate gravity trans-
Cements related to exposure may contain fluid inclusions port on the rise and basin floor with only few degrees of dip
of very low salinity, again indicating fresh-water lenses or (see Hine et al. ,1992, for a Pleistocene example and p. 119f
percolation of rainwater through the rocks (e.g. Goldstein et for sequence-stratigraphic consequences).
al., 1990; Fouke et al., 1995; Immenhauser et al., 1999).
Besides soils, the morphology of the exposure surface ENVIRONMENTAL MESSAGES FROM ORGANISMS
may provide evidence of exposure. In all but the most
Fossil organisms are an important source of environmen-
arid climates, carbonates develop a dissolution morphology,
tal information and the literature on this subject is extensive.
known as “karst”, with sinkholes, caves and irregular corro-
Most of it is beyond the scope of this book with its focus on
sion surfaces (chapter 3). However, similar morphologies
develop on patchily lithified sea floors swept by currents. physical sedimentology and large-scale anatomy of carbon-
Finally, biota indicating fresh-water or dry conditions is ate rocks. However, even with this bias organisms are a very
an excellent indicator of terrestrial exposure. The seeds of important source of information.
Characean algae are particularly useful because they are In examining distribution patterns of organisms in Pha-
easily preserved, have characteristic shapes and do not re- nerozoic carbonate rocks we must remember that what we
quire detailed taxonomic identification because the entire see is the result of two controls: changes of environmental
group is restricted to brackish or fresh-water conditions. conditions, and changes imposed by organic evolution. As
Microcodium is a very characteristic microfeature in Creta- sedimentologists we strive to subtract purely evolutionary
ceous and Cenozoic soils, probably formed by intracellular effects and isolate the environmental information in order to
formulate models that are as widely applicable as possible.
calcification of root cells (Kozir, 2004).
However, environmental change is an important control on