Page 83 - Carbonate Sedimentology and Sequence Stratigraphy

74 WOLFGANG SCHLAGER

An observation-based model of shoaling autocycles in - mud banks, tidal passes, delta lobes etc. typically form
shoal-water carbonates was proposed by Ginsburg (1971) rhythmic patterns in space. Migration of these spatial pat-
and summarized by Hardie and Shinn (1986) (Fig. 5.2). The terns can also produce a rhythmic sediment record (e.g.
model is compatible with observations in the Holocene but Reading and Levell, 1996 , 15). Fig. 5.3 shows a siliciclas-
the Holocene record provides incomplete proof of the model tic example where shore-parallel migration of subtidal mud
because it covers only part of the postulated cycle. The most banks leads to stepwise accretion of the beach, separated by
critical parts, - the halting of progradation because of lack phases of erosion.
of supply and the subsequent turnaround into a transgres-
sive phase that floods the supratidal flats - have not been ob- ORBITAL RHYTHMS
served. Whether this can happen fast enough and on a suffi-
ciently large scale to match the observations in the geologic The Earth’s orbit around the Sun is perturbed by the mo-
record is open to debate. Holocene observations do show tions of the Moon and the other planets; these perturbations
that prograding carbonate tidal flats tend to develop high induce subtle changes in the solar radiation received by the
beach ridges on the seaward side. When this defense sys- Earth that modulate the climate. There are three important
orbital perturbations (Fig. 5.4):
tem is overwhelmed, large parts of the landward flats must
➤ Changes in eccentricity. The Earth’s orbit is an el-
be flooded almost simultaneously - one of postulates of the
lipse whose elongation changes with important cycles
Ginsburg model. The resulting autocycles oscillate between
around 100 and 400 ky and weaker modulations around
shallow marine and supratidal; based on extensive evidence
1.2 and 2 My.
from the Hoolocen, the system is assumed to not be able to
➤ Changes in tilt (or obliquity) of the Earth’s rotational
build into the terrestrial environment.
axis. At present, the axis of rotation is not perpendicular
◦
Besides this non-linear response of an entire system in the to the plane of orbit but tilted by 23 . Important cycles
time domain, we also find space rhythms within a system of tilt are 40 and 54 ky.
1 DROWNING (subsidence >> carbonate production)
Fig. 5.2.— Ginsburg’s autocyclic
shoreline transgresses
model of sedimentation on carbon-
ate platforms assumes steady sub-
minor reworking of
surface sediments sidence on a seaward dipping plat-
form and a depth-dependent carbon-
ate production; production is near-
2 PROGRADATION ( carbonate production > subsidence ) zero in supratidal flats (shown in
shoreline progrades black) and high in the shallow-marine
lagoon (dotted). Cycle starts with
max. depth 10 m
rapid transgression and formation of
lagoon carbonate factory and dispersal a lagoon; sediment production in the
system in full operation lagoon starts slowly (start-up phase
in Fig. 1.12); after several thousand
CONTINUOUS SUBSIDENCE intertidal subtidal grates shoreward, the inner part of
years sediment production in the la-
3
PROGRADATION STALLED ( carbonate production ≈ subsidence )
carbonate factory too small for
goon increases rapidly, sediment mi-
efficient operation
supratidal cap
the lagoon fills up to tidal flats and
TIME 4 DROWNING (subsidence >> carbonate production) up phase in Fig. 1.12).The progra-
these flats begin to prograde (catch-
A-B-C shallowing upward sequence
dation of tidal flats continually re-
duces carbonate production as pro-
ductive lagoon floor is replaced by
shoreline transgresses
non-productive tidal flats.
water to shallow for efficient operation of carbonate factory
Progra-
dation ceases when the lagoon has
become too small to support further
growth of the tidal flats. At this point,
the system will stagnate until subsi-
dence lowers the beach ridges on the
5 PROGRADATION ( carbonate production > subsidence ) seaward side of the flats and a new cy-
omission surface (subtidal cle starts with rapid transgression of
shoreline progrades directly on supratidal)
the supratidal flats. After Hardie and
Shinn (1986), modified.
stacking of A-B-C shallowing upward sequences