Page 114 - Carbonate Platforms Facies, Sequences, and Evolution

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again.
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upon
Catalan
the
emergence
Basin
LST
episode
Benifallet
after
apparently
the
lagoonal
La
TECTONIC CONTROL
F.
unit.
TIME- several million years
SYSTEMS TRACTS
suffered
for the development of the La Riba
GLOBAL EUSTASY CONTROL
Riba
a
TIME- several/many million years
And
and regional subsidence, as through global eustasy.
Superimposed on these third-order (106 yr) relative
changes of sea-level, which give rise to depositional
Unit,
results in the formation of the sequence boundary. A
then
denudation and gradual loss of relief, and deposition,
evaporites (if arid), of a lowstand systems tract. With
systems tract as the magnitude of regional subsidence
fifth order (105 and 104 yr) sea-level changes, perhaps
Calvet, M.E.
both space and time into alluvial plains and lakes, and
sinks, and siliciclastics are deposited in fans, passing in
declines and relative sea-level begins to fall. The latter
clastics give way to carbonates as relative sea-level rises
may develop as the lowstand systems tract is established
major paleokarst, evaporite horizon or fluvial sandstone
through regional subsidence, establishing a transgressive
give rise to parasequences and parasequence sets. In the
carbonate aggradation and progradation of the highstand
resulting from the effects of Milankovitch rhythms, which
of systems tracts will be produced through tectonic rifting
Fig. 25. Simplistic sketch suggesting that a similar sequence
sequences there may well be shorter time-scale, fourth and
tectonic model, rifting leads to the creation of sources and
systems tract. Stratal onlap of the TST facies is followed by
short-lived
the
mud mounds
with
The
flats
units
these
tary,
1000yr-
Hardie,
or
in
sea-level
available,
is
the
subsequence)
rise,
sand
rapid sea-level
carbonate
with
that
greatly reduced,
Tucker and J.M. Henton
probably structural,
glacioeustatic
regional subsidence,
the
when
plate-tectonic
rises,
long periods of time.
and
shoals
their
ramps
to be admitted, but to
and
rate
boundaries.
Muschelkalk
to the
is
but one
of the
induced
prograde
ence following a phase of rifting.
terrigenous
adherents
They
tectonic
consist
SHALLOWING -UPWARD
much
or
1986 and Tucker & Wright,
of
the depositional sequences themselves.
Lower
(see
clastic
eustatic
FOUR TH-FIFTH-ORDER
lower
and
represent
aggrade.
influx
CYCLES
mechanisms
reviews
the La Riba Reef Unit. The lower and upper l;lound­
east/southeast may ac­
changes 1 ) than that of the regional thermal subsid­
of global extent. Both rises and falls can be expected
reflooding of the basin. The effect of some barrier,
type cyclicities in sedimentary strata requires a se­
shorter time-scale sea-level changes responsible for
count for the rapid sea-level changes before and after
demonstrate Milankovitch­
aries of the La Riba Reef Unit are parasequence (or
much
1990). An or­
Atlantic). An increase in plate tectonic activity and
bonate cycles is a contentious issue, with sedimen­
On a much larger scale than western Europe, the
( <1 em
and new ocean formation in the Jurassic (Tethys and
paleokarstification, and this was followed by rapid
The origin of small-scale shallowing-upward car­
the main phase of continental rifting and separation
was
eustatic effects from relative sea-level rises due to
important feature of
Triassic was also a time of crustal extension before
in Triassic times. It is a problem to distinguish these
Upper
metre-scale,
changes would lead to eustatic changes in sea-level
seafloor spreading rates causing ocean basin volume
sea-level
Muschelkalk were formed during the transgressive
and highstand phases of two major periods of relative
by a stillstand or slow fall, during which time tidal
at a time of relative tectonic
The
shallowing-upward cycles (see Figs 5 & 13). These
quiescence. As discussed earlier, many stratigraphic
parasequences are apparently the result of relatively
quence with a sufficiently large number of cycles so
to initiate the cycle, followed
exposure horizons at the tops of cycles may represent
all
of
bital forcing mechanism is gaining popularity, it has