Geology Reference
In-Depth Information
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Further reading
: K104
cific interest in refining carbonate sequence stratigra-
phy and the discussion of time involved in stratigraphic
breaks. Small-scale and short-lived discontinuities of-
ten reflect large-scale variations of relative sea level.
Terminology
Various names are in use for defining sedimentary
breaks and associated substrate types (Box 5.4).
5.2.1 Classification of Discontinuities
Practical classifications of the common discontinuity
types occurring in carbonate sequences were proposed
by Clari et al. (1995) for analyzing shallow- and deep-
water carbonates and Hillgaertner (1998) for studying
carbonate platforms.
The latter classification differentiates four discon-
tinuity surfaces taking into account the importance of
allogenic forcing of environmental changes:
(1)
Exposure surfaces
resulting from subaerial ex-
posure, commonly indicated by an overprinting of sub-
tidal carbonates due to a general lowering of sea level.
(2)
Condensation surfaces
comprising all disconti-
nuities related to stratigraphic condensation in subtidal
environments, and predominantly dependent on con-
trols by relative sea-level changes and changes in ac-
commodation.
(3)
Erosion surfaces
including discontinuities that
show evidence of subaqueous erosion.
(4)
Change surfaces
describing all surfaces indi-
cating changes in facies and/or texture.
5.2 Discontinuity Surfaces: From
Microfacies to Sequence Stratigraphy
Sedimentation is inherently a discontinuous process.
A gradual change in environmental conditions may be
accompanied by a continuous reaction of the deposi-
tional system, but any abrupt change or the passing of
a threshold lead to a discontinuity in sedimentation.
Discontinuities are commonly marked by a surface in
stratigraphic sections and/or by distinct level of facies
changes.
All surfaces indicating a break in sedimentation and/
or non-deposition in submarine and subaerial environ-
ments are called
discontinuity surfaces
(Heim 1924,
1934). The use of this term is purely descriptive in be-
ing independent of the process causing the formation
of the surface and the duration of the break. Conse-
quently, all stratigraphic gaps are included in this broad
definition, ranging from millimeter- and centimeter-
scale breaks in sedimentation revealed by microfacies
analysis, to small breaks on the scale of limestone beds
commonly marked as bedding planes, to major strati-
graphic unconformities which may be accompanied by
prolonged subaerial exposure.
Recognizing discontinuities and unconformities
should be a major goal in microfacies studies. Uncon-
formities represent the key element in sequence stratig-
raphy. Because sequence boundaries are defined as 'ob-
servable discordances in a given stratigraphic section
that show
evidence of erosion or non-deposition
with
obvious stratal terminations...' (Mitchum in Vail et al.,
1977), microfacies criteria pointing to breaks in sedi-
mentation (e.g. discontinuity surfaces, condensed de-
posits, hardgrounds, karst features et al.) are of spe-
In most cases small-scale erosion or facies changes
are related to locally restricted depositional processes.
But allogenic forcing associated with changes in cli-
mate and/or relative sea level can trigger changes in
wave base and hydrodynamic energy producing ero-
sion surfaces or surfaces characterized by an abrupt
change from carbonate to siliciclast-dominated sedi-
mentation.
5.2.2 Major Criteria of Discontinuities
Common criteria used to characterize subaerial and/or
submarine discontinuities in carbonate rocks include
geometry, lateral extent, surface morphology, biologi-
cal activity, mineralization, facies contrast, early di-
agenetic contrast, and biostratigraphy (Hillgaertner
1998). The evaluation of these criteria requires the study
of the rocks below and above the discontinuity, a thor-
ough analysis of the discontinuity surface in geological
