Geology Reference
In-Depth Information
Laminae also have rare v-shaped polygonal cracks
and horizontal elongate fenestrae. Laminites are
interpreted to be evidence of a supratidal fl at-
type environment similar to the modern tidal fl ats
of Andros Island, Bahamas. Original studies by
Black (1933) on Andros Island, Bahamas, iden-
tifi ed nearshore surface mats of sticky, fi lamen-
tous blue-green algae that bind and trap sediment
washed in from the offshore subtidal carbonate
'factory', creating laminated sediments. Later
studies by Hardie & Ginsburg (1977) on Andros
Island found that thinly laminated sediments are
typically found in low-energy, fl at, supratidal
environments that are exposed subaerially 85%
of the time or more. Sediments associated with
these mats may contain fenestrae of various sub-
types, thin-bedded storm deposits, and well-
laminated muddy, organic-rich layers (microbial
mat) that alternate with layers of soft peloi-
dal sand (Hardie & Ginsburg, 1977; Demicco &
Hardie, 1994). By comparison, it is likely that
the laminated subfacies within Mendola Pass
cycles formed in a supratidal fl at-type environ-
ment via binding and trapping of transported sub-
tidal sediments onto fi lamentous microbial mats.
Additionally, certain examples (e.g. Mendola
Cycle 28 laminites) show evidence of a biological
origin, including centimetre-scale gravity-defying
stromatolite heads.
oscillations to 'touch down' directly onto the
subtidal environment and form diagenetic caps,
while slightly faster-subsidence at Mendola Pass
meant that the succession there remained in sub-
merged or in peritidal settings. Slightly faster
subsidence at Mendola Pass during this interval
serves to explain the 0.26 m difference in aver-
age cycle thicknesses between the two localities
(average cycle thickness of the Latemar interval
is 0.52 m; average cycle thickness of the Mendola
interval is 0.73 m). A tectonic arrangement of this
type would fi t with the sinistral transpressive
model of Doglioni (1987, 1988), who recognized
compressive tectonic features of both Anisian and
Ladinian age near the Latemar Platform, which
may have limited subsidence at the Latemar as
compared with Mendola Pass (see Doglioni, 1988,
p. 294, Fig. 1). It should be noted that this phase of
compression was both pre- and post-dated by rift-
ing phases that generated differential thickness in
sedimentary cover in the Dolomites, resulting in
an overall thickening of the Triassic section by at
least three times from the west near Mendola Pass
to the east near Tre Cime di Lavaredo (Laubscher &
Bernoulli, 1978; Doglioni, 1987, 1988).
While subfacies within Latemar and Mendola
cycles are notably different, the cycles nonethe-
less share several common traits. One distinct
similarity is that the facies within both succes-
sions 'shallow-up', i.e. the cycles at both localities
record a progressive fi lling of accommodation
space prior to being abruptly fl ooded. Further,
the depositional fabrics preserved within the
cycles are indicative of deposition within 0-10 m
of water depth (Hardie & Ginsburg, 1977; Shinn,
1986). Deposition in the shallow-marine realm
made these localities sensitive to environmen-
tal shifts, most notably sea-level oscillations.
Finally, the Latemar and Mendola cycles are both
of similar vertical scale (approximately 1 m), and
are 'bundled' into thinning-upward packages of
predominantly fi ve cycles, indicating that the
same process driving the development of vertical
stacking patterns was recorded at both localities
(Fig. 5). These patterns are analysed in further
detail below.
Comparison of Latemar vs Mendola cycles
The difference in subfacies successions in the
Mendola and Latemar cycles has several possible
explanations. First, it may be that the larger
size of the Mendola Pass platform allowed for
the development of 'protected' tidal-fl at environ-
ments. Isolation from open marine conditions
would have allowed for the accumulation of
muddy cycles with fl at laminite caps, similar
to those observed in the modern tidal fl ats of
Andros Island, Bahamas (Hardie & Ginsburg,
1977). This would be in contrast to Latemar,
which appears to have been a smaller isolated plat-
form. By comparison, the interior of the Latemar
Platform was affected by current and wave energy,
winnowing out mud and transporting more sedi-
ment off-platform, thereby creating the grainier
facies successions preserved there.
It is also possible that differential subsidence
affected vertical accommodation space at each
location. A slightly lower rate of subsidence at
Latemar during the interval under investiga-
tion may have allowed high-frequency sea-level
STRATIGRAPHIC CORRELATION
OF THE MENDOLA AND LATEMAR
SECTIONS
The 'Mendel Dolomit' was originally defi ned
lithologically by Richtofen (1874), but since that
