Geology Reference
In-Depth Information
tance of tens of kilometers or more may point to major
oceanographic controls (Di Stefano et al. 2002).
Surface morphology:
Flat, sharply-cut surfaces in
shallow-marine environments are commonly caused by
high-energy abrasion of lithified sediment. Irregular
wavy and crinkled surfaces may indicate minor ero-
sion or the former existence of microbial mats. Irregu-
lar surfaces also can be caused by differential compac-
tion of strata differing in composition and texture.
Rough surfaces can originate from burrowing and bio-
erosion. Stylolitization superimposed on preexisting
discontinuity surfaces may lead to jagged surfaces.
Biological activity:
The distribution of burrowing,
boring and encrusting organisms is crucial for under-
standing changes in sedimentation rates and resulting
substrate types. The intensity of bioturbation and the
types of trace fossils below and above the discontinu-
ity surface assist greatly in differentiating pre-omis-
sion, omission, and post-omission phases (Bromley
1975). Lithification of marine discontinuity surfaces
is indicated by boring organisms that cut sharply
through fabrics and different microfacies types of un-
derlying rocks. Encrusting microfossils attached to
hardgrounds indicate retardation or breaks in sediment
input.
Mineralization:
Discontinuity surfaces showing
in
situ
crusts of iron and manganese oxides and phosphates
as well as authigenic glauconite (Pl. 22/2) indicate
breaks in sedimentation, commonly in deep subtidal
and bathyal environments. In contrast, aluminium-iron
oxide crusts are found to be the result of paleosol for-
mation (terra rossa) and alteration during subsequent
marine flooding (Wright 1994). Penetrative staining of
strata underlying discontinuity surfaces by iron oxides
can indicate oxidation due to pedogenesis and karstifi-
cation.
Facies contrast:
A sharp change in microfacies
across a surface must not necessarily represent an im-
portant break in sedimentation. However, a drastic en-
vironmental change can be inferred from a superposi-
tion contradicting Walther's law (Clari et al. 1995), e.g.
the superposition of limestones exhibiting microfacies
associations typical of platform and reef carbonates by
limestones showing evidence of pelagic sedimentation.
Changes in grain sizes (reflecting changes in water en-
ergy) and in the lithologic composition (e.g. siliciclas-
tics versus carbonates) may provide other clues to the
discontinuity character of surfaces.
Diagenetic contrasts:
Diagenetic evidence of dis-
continuities is given by sharp differences in the diage-
netic style at the discontinuity, e.g. vadose diagenesis
or early diagenetic changes due to meteoric waters in
the underlying and contrasting marine phreatic diagen-
esis in the overlying strata. Cement stratigraphy, dif-
ferent compaction features in the overlying and under-
lying strata as well as stable isotopes and trace ele-
ments offer other clues to the existence of exposure
surfaces or breaks in the diagenetic style. Isotope and
trace element data assist greatly in recognizing sub-
marine lithification of hardgrounds, identifying uncon-
formities and the genetically interpretating subaerial
and submarine discontinuities (Marshall and Ashton
1980; Videtich and Matthews 1980).
Biostratigraphy:
Generally biostratigraphic and
chronostratigraphic data are the only means for esti-
mating the time involved in discontinuities. In basinal
sections with rather monotonous sedimentation pat-
terns, fossils can even be only means of identifying
stratigraphical discontinuities. The main limitation,
however, is time resolution, which is commonly too
low for the majority of discontinuity surfaces occur-
ring in carbonate sequences. Most discontinuities in
shallow-marine platform carbonates are below bio-
stratigraphic resolution. Better results have been ob-
tained in the context of studying deep-marine carbon-
ates.
5.2.3 Microfacies Criteria and Significance
of Exposure Surfaces
Detecting exposure surfaces in limestones by means
of microfacies includes the following criteria:
• Conspicuous differences in facies types reflected
by differences in rock color, lithologic composition,
texture and fossil content below and above the expo-
sure surface,
• Meteoric-vadose diagenesis at bounding surfaces,
characterized by vadose cements in the underlying
rocks and contrasting marine phreatic cements in the
overlying rocks. Vadose zones extend downwards in
the strata for a few tens of centimeters only. The sur-
face can be accentuated by stylolitization. Stable iso-
tope trends across exposure surfaces exhibit signifi-
cant shifts (Sect. 13.2.4),
• Dedolomitized horizons: The alteration of dolomite
to calcite takes place in several diagenetic settings (see
Sect. 7.8.3) but is a common feature of near-surface
vadose zones (Kenny 1992). Open dolomolds may in-
dicate subaerial exposure and dissolution by karst wa-
ters (Braun and Friedman 1970),
• Microkarst caused by dissolution of CO
2
-enriched
meteoric waters is indicated by sharp-cut and intensely
stained surfaces displaying a microrelief with millime-
ter- to centimeter-sized cavities (Sect. 15.2.1). The ab-
