Geology Reference
In-Depth Information
a renewed regression. If the rise is rapid, carbonate plat-
forms will drown, producing a drowning unconformity
surface (characterized by a rapid lithological change
from shallow-marine carbonates to deep shelf, slope
or basin deposits). Characteristic features of TST are
deepening-upward and increasingly open marine con-
ditions indicated by the composition of the biota, the
increase in bioclastic and oolitic carbonates as well as
the relatively small thickness of thin-bedded deposits.
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Highstand systems tract (HST):
Formed by regres-
sive deposits that accumulate when the sedimentation
rates exceed the rate of relative sea-level rise. The shal-
low shelf is widened, producing and shedding the great-
est quantity of carbonate sands having the best poten-
tial for reservoir rocks. Carbonate sand production at
shelf margins outpaces the relative sea-level rise or
standstill and the basinward migration of carbonates
result in aggrading prograding geometries. As accom-
modation space decreases, cycles become progressively
thinner. Characteristic features of HST are thick-bed-
ded or massive deposits, often light rock color, marine
biota, shoaling-upward patterns and a change from
open-marine to restricted conditions. Framework reefs
are generally regarded as characteristic features of HST
but may also form during the development of LST and
TST. Because TST or HST may be the result of a rela-
tive sea-level change or a change in sediment supply,
the relations of shoaling/deepening trends to sea-level
fluctuations must be critically evaluated.
very rapid flooding or a sudden increase in water depth.
A thin transgressive unit may be developed between
the top of the underlying parasequence and the upward-
shoaling part of the overlying sequence.
Parasequences are often organized into
para-
sequence sets
or bundles grouped into larger, third-or-
der depositional sequences. These sequences have a
well-defined internal structure in which subunits are
arranged into packages or parasequences with contrast-
ing stacking patterns. The packages of parasequence
sets are bounded by disconformities. Parasequence sets
display trends of shallowing, deepening or no change
in water depth. Lehrmann and Goldhammer (1999)
have argued for significant secular variations in the de-
velopment of parasequence and facies stacking patterns
during the Earth's history.
Parasequences correspond to successions developed
in response to periodic or episodic environmental
changes occurring in cycles of fourth-order (10
3
years)
to fifth-order (10
4
years). Many parasequences relate
in part to high frequency, Milankovitch-driven climate
changes and their associated sea-level fluctuations. This
is indicated by (1) estimates of cycle durations, (2) bun-
dling of carbonate cycles into sets of five, (3) spectral
analysis of time series constructed from cyclic strati-
graphic sections, and (4) evidence of high-frequency
sea-level drops in the platforms, generating disconfor-
mities, breccias, soils, or vadose fabrics on the tops of
parasequences (Koerschner and Read 1989; Gold-
hammer et al. 1990).
On the inner platform, parasequences consist of shal-
low lime sands or muds, capped by tidal flat facies,
and exposure surfaces. On the shelf edge and shoal com-
plexes of ramps, parasequences may be poorly cyclic
to non-cyclic and grainstone-dominated. In slightly
deeper water, parasequences show coarsening upward
from lime mud deposits and interbedded muds and
sands up into grainy caps.
Depositional sequences and systems tracts record
the interplay of accommodation (the space available
for potential sediment accumulation) and sediment sup-
ply. The separation of effects of accommodation, sedi-
ment supply and sea-level fluctuations needs diagnos-
tic facies criteria, which are more distinctive in silici-
clastic sediments (Posamentier et al. 1988; Posamentier
et al. 1992) than in carbonates, but can nevertheless be
effectively derived from outcrop, well data and micro-
facies.
Parasequences
Most sequences consist of small-scale, upward-shal-
lowing successions without significant breaks. These
parasequences
forming the smallest fundamental com-
posite building blocks of systems tracts have a thick-
ness of <1 to >10 m and can be delineated by the stratal
architecture of their component lithofacies, their bound-
ing surfaces or the contacts between each system.
Parasequences are bounded by minor marine flooding
surfaces, across which deeper water facies abruptly
overlie shallower water facies (Van Wagoner et al.
1988). In shallow-marine environments, parasequences
are characterized by an upper boundary that indicates
Sequence boundaries
Following the classical sequence stratigraphy ap-
proach, the most important criteria for recognizing se-
quence boundaries are incised valley and related uncon-
formities. These criteria are difficult to trace on car-
bonate platforms.
Sequence boundaries of platforms are often char-
acterized by hardgrounds and meteoric caps, in basins
by increasing terrigenous input, abundant allochtho-
nous sediment and coarse microfacies. Main sequence
boundaries are
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Emersion or subaerial surfaces. Sequence bound-
aries directly underlying deeper-water facies.
