Geology Reference
In-Depth Information
Shallowing-upward theme of the cycles
indicating subaerial exposure during sea-level fall
means that Latemar deposition was not only able
to 'keep up' with subsidence, but also experienced
extended periods of time with the platform top in
peritidal and supratidal conditions; environments
not known for exceptionally fast accumulation
rates.
The Mendola and Latemar cycles are late Anisian-
early Ladinian-aged shallowing upward, metre-
scale peritidal platform carbonate cycles that
formed as a result of repetitive environmental
changes over time. The presence of the shallowing-
upward cycles allows for physical measure-
ment and statistical analysis of their stacking
trends (i.e. trends in the vertical arrangement of
cycle thicknesses). The cyclic successions have
stacking patterns that not only appear similar to
the eye (Fig. 8), but cross-correlate with a cor-
relation coeffi cient exceeding 95% (Fig. 7), sug-
gesting a shared external driver. Site-specifi c
controls (e.g. autogenic sedimentary processes, local
subsidence) exerted comparably less infl uence
on the development of the cyclic successions, but
could explain the differing average cycle thick-
nesses between the successions (the average cycle
thickness of the Latemar interval is 0.52 m; the
average cycle thickness of the Mendola interval
is 0.73 m).
The case for allocyclicity
The fact that correlated cycle stacking patterns
are recognized at separate locations that are
constrained by biostratigraphy is strong evi-
dence for the existence of a common allocyclic
mechanism driving the development of both suc-
cessions. The presence of allocycles at Latemar
and Mendola Pass sections requires that mecha-
nisms existed during the Mid-Triassic that acted
to generate both sea-level oscillations and resul-
tant carbonate depositional cycles at both locali-
ties. In the case of the Latemar controversy, it can
be established to a fi rst order that shallowing-
upward depositional cycles have been identifi ed
and cross-correlated to the Mendola Pass section,
implying that the driving mechanism is at the very
least regional in its stratigraphic infl uence.
Recording of cyclic processes
Both Mendola and Latemar successions formed in
environments conducive to recording rhythmic,
cyclic sea-level oscillations. The shallow-marine
realm (0-5 m water depth) is almost certainly the
depositional environment in which these depos-
its formed, where repeated fl ooding and exposure
of the sediment-water interface was recorded
within each depositional facies succession. This
is especially true at Mendola Pass where facies
within shallowing-upward cycles are consistent
with those found on modern carbonate tidal fl ats
(Hardie & Ginsburg, 1977). Strong and consistent
sedimentological evidence for early subaerial
exposure of Latemar cycle caps provides evidence
for a similar facies-water-depth relationship at
Latemar (Preto
et al
., 2004).
The signifi cant statistical correlation between
the Mendola and Latemar stacking patterns pro-
vides additional support for the same sea-level
oscillations affecting both depositional environ-
ments. This observation is important because
constantly deep-submerged environments, as
favoured by Blendinger (2004) for the Latemar
cycles, would 'miss beats', not have a non-random
bundling pattern, and not be readily correlatable
to a clearly peritidal counterpart at Mendola (Preto
et al
., 2005). The presence of Mendola cycle cap
laminites and Latemar cycle cap dolomite crusts
DISCUSSION
Comparative sedimentology
Comparison of Holocene and Pleistocene dated
allocycles to the more distant past record is use-
ful for several reasons. First, composite sea-level
oscillations regarded as drivers for shallow-
marine carbonate sedimentation in the Holocene
and Pleistocene are grouped into fi ve approx-
imately 20 kyr oscillations per 100 kyr oscilla-
tion (Broecker
et al
., 1968; Bloom
et al
., 1974),
a bundling pattern that conspicuously matches
the cycle stacking at Latemar and Mendola
Pass successions. The fact that sea-level oscil-
lations within the Milankovitch band form
dateable carbonate depositional cycles demands
consideration. Second, if the composite allocyclic
forcing mechanisms that formed depositional
cycles in the Mid-Late Triassic are still active
today, then those same mechanisms ought to be
recorded as depositional cycles today. Discovery
of modern or recent depositional cycles with mil-
lennial periodicities and bundling patterns sim-
ilar to that of the Latemar pattern would better
point the cyclostratigraphic community towards
