Geology Reference
In-Depth Information
•
Transgressive surfaces: Surfaces directly underly-
ing deeper-water facies.
are reflected by statistically-based patterns described
in Sect. 14.4 as 'dynamic microfacies types'. This
method assists in definiting sequence tracts and of hid-
den sequence boundaries.
•
Maximum flooding surfaces: Characterizing the
relatively deepest depositional environment.
Van Wagoner et al. (1988) and Schlager (1998) dis-
tinguished three major types of sequence boundaries:
Type-1 sequence boundary forms when the relative
sea level falls below the shelf break of the preceding
sequence. Type-1 sequence boundaries are character-
ized by the emergence of the platform out beyond the
shelf edge. This type is more common on rimmed
shelves than ramps. This type is characterized by dis-
tinct signatures that are recorded by microfacies crite-
ria (e.g. exposure; paleokarst, Pl. 129; pedogenic over-
print on marine deposits; Pl. 128/1).
Type-2 sequence boundary forms when relative the
sea level falls to somewhere between the old shoreline
and the shelf break. Only the inner shelf becomes ex-
posed. Type-2 sequence boundaries may have erosional
disconformities updip, but the sea level does not sig-
nificantly expose the margin. The sequence boundary,
therefore, is conformable. This type, associated with
small sea-level falls, is typical of ramps.
Type-3 sequence boundary forms when the sea level
rises faster than platforms can aggrade and a transgres-
sive systems tract with a significant marine hiatus over-
lies the preceding highstand tract (Schlager 1998,
1999). Marine erosion is particularly intensive on the
top of drowned platforms (see Sect. 15.6.2).
Sequence-related diagenesis:
Diagenetic features
are key criteria to sequence stratigraphy (Moore 2001;
Booler and Tucker 2002) and the recognition of sub-
aerial exposure phases (Fouke et al. 1995). Significant
porosity may develop beneath sequence-bounding un-
conformities and parasequence disconformities on car-
bonate platforms (Read and Horbury 1993). Normal-
marine parasequences exhibit first-generation iso-
pachous cement in grainstones. The rarity of marine
cement may indicate rapid carbonate accumulation dur-
ing a relative sea-level rise (keep-up carbonate system;
Sarg 1988). Arid climate would prevent meteoric va-
dose/phreatic cement and produce grain-to-grain con-
tacts and common pressure solution. Late highstand
conditions in peritidal sequences are recorded by me-
teoric-vadose cement and crystal silt.
Recognizing sequence boundaries
Discontinuities and unconformities are key elements
in recognizing sequence boundaries. Small-scale and
short-lived discontinuities as seen in thin sections and
outcrops can reflect large-scale variations in sea level.
Excellent examples of the potential of microfacies for
differentiating various orders of sea-level changes have
been provided by Evarts et al. (1995) and Hillgaertner
(1998), see Sect. 5.2.5.
16.1.2.2 Microfacies Data Applied to Sequence
Stratigraphy
Emersion (exposure) and subaerial surfaces
point-
ing to sea-level fall
are characterized by marine car-
bonates capped or penetrated by sediments exhibiting
terrestrial facies features (Sect. 15.1.2). These features
are documented by:
• Surfaces with desiccation marks occurring in su-
pratidal and upper intertidal zones (Sect. 15.5.1.2).
• Pedogenic structures (Sect. 15.1.1) including paleo-
sols capping or penetrating marine carbonate strata;
roots and alveolar structures; caliche pisoids and cali-
che crusts, sometimes with tepee structures truncated
by erosion.
• Paleokarst recorded by microkarst structures, karstic
solution cavities and joints in the underlying marine
limestone; speleothems. Surfaces with a network of
cavities penetrating tens of cm into the underlying lime-
stones and filled with sediment of different texture:
• Terrestrial sediments (red beds, green shales, detri-
tal sands or conglomerates) overlying marine carbon-
Relative sea-level fluctuations will affect the compo-
sition and distribution of microfacies types both of plat-
form and ramp carbonates as well as on slopes and in
basins. Sea-level fluctuations causing shallowing-up-
ward cycles and parasequences are clearly documented
by microfacies data and biota, allowing changes in
depositional water depths and accommodation space
to be estimated. Microfacies data are of particular value
in evaluating sequence boundaries (discontinuity sur-
faces; Sect. 5.2.5; Fig. 16.4), changes in paleo-water
depths (compare Sects. 9.3.4, 12.3 and 14.2) and shal-
lowing/deepening trends of the sea level, reflected by
changes in hydrodynamic energy (Sect. 12.1.1). Re-
cent studies (e.g. Della Porta 2003) use microfacies as
an integrated part of the interpretation of depositional
models in terms of sequence stratigraphy.
Shifts in the relative importance of grains due to
environmental changes caused by sea-level fluctuations
