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In-Depth Information
Fig. 15.23. Platform-derived block within slope sediments.
This block, exposed in the road cut at the Pordoi Pass, is a
spectacular example of the deposition of lithified platform-
margin material on carbonate slopes and in basinal sediments
of the Triassic in the Dolomites. The gravity-displaced block
is embedded within beds of the S. Cassiano Formation that
consists of volcaniclastic sediments and limestone turbidites.
Rotated geopetal fabrics within the boulders argue against it
being an 'in situ' patch reef. The block is a boundstone with
biogenic crusts, growth cavities and encrusting biota. The
rounded block has a maximum height of 3 meters. Rounding
of the block and of its constituent boulders is partly due to
organic envelopes developed during resting phases in block
transport. The platforms acting as a source of the allochtho-
nous carbonates prograded some kilometers over the S.
Cassiano Formation. Late Triassic (Carnian).
Plate 137 Microfacies of BaseofSlope Carbonates: Triassic of the Dolomites (Italy)
Excellent examples of ancient carbonate slopes are exposed in the Triassic of the Dolomites, where carbonate
platforms prograded by the redeposition of shallow-water material on the slopes. Microfacies analysis reflects
the source and depositional pattern of the allochthonous sediment.This plate shows a thin section (with enlarged
details of it) of a single 6 cm thick limestone bed. Common criteria of carbonate slope and base-of-slope
deposition and the distinct changes in platform-derived gravity flow sediment and pelagic sediment. Allochtho-
nous sedimentation is represented by the turbiditic influx of shallow-water derived grains formed on a platform
and resedimented in deep water (units A, C and D2). The pelagic background sedimentation and distal turbidites
are recorded in unit B as well as in unit D1. Note the sharp boundaries between the major units.
Middle Triassic (Buchenstein Formation, Ladinian): Seceda platform, Western Dolomites, Italy (-> 1-6).
The basinal Buchenstein (Livinallongo) Formation consists of flat, medium- to thin-bedded marls and lime-
stones that intergrade upslope with allochthonous limestones consisting of material shed from the margins of a
platform. The carbonates of the Buchenstein Formation exhibit several microfacies types, including radiolarian
packstones and wackestones, fine-laminated dark mudstones, and calciturbidites with shallow-marine and slope-
derived lithoclasts and fossils. The varying composition of the calciturbidites reflects platform progradation and
high-frequency eustatic oscillations (Maurer et al. 2003).
1
Vertical sequence consisting of resedimented shallow-marine carbonate grains and pelagic deep-marine sediment.
Unit A1: The basal lithobioclastic grainstone consists of lithoclasts, skeletal grains and peloids. Larger lithoclasts are
peloid packstones (PP) as well as bindstones (B). Both microfacies are common in Middle Triassic platform interior and
reef mound settings. Bioclasts are dasyclad green algae (D), foraminifera, ostracods, and rare echinoderm debris, 'Tubi-
phytes' (T) and small coated debris. Note fining-upward pattern. The unit corresponds to part of a proximal calciturbidite.
Unit A2: The upper part of unit A differs from the lower part by the admixture of pelagic biota ('filaments', e.g. thin
pelagic bivalve shells; see Pl. 113/2) and echinoderm fragments. Unit B: Radiolaria packstone. The abundant spumellarians
occur with some broken filaments, a few monaxon sponge spicules (SS) and rare ostracods (O). Darker patches are caused
by burrowing. Unit C: Lithobioclastic packstone. In comparison to unit A, lithoclasts (L) are smaller and better sorted.
Bioclasts are represented by some mollusk shells (S) and rare echinoderms. Larger grains are strongly micritized bind-
stone clasts. Unit D1: This unit is characterized by a chaotic accumulation of densely packed pelagic bivalve shells
(coquina floatstone). Platform clasts, including thick-shelled bivalves (B), bindstone lithoclasts and coated grains, are
subordinate. Unit D2: Alternation of platform-derived material and pelagic grains. Platform-derived grains are bindstone
lithoclasts and thick-shelled bivalves (B). Parallel alignment of filaments and sponge spicules point to the existence of
weak bottom currents. A part of the variations in turbidite composition is related to hydrodynamic sorting.
2
Detail of part A1 (not shown in -> 1). Lithobioclastic grainstone. Grains are fragments of the enigmatic encrusting fossil
'Tubiphytes' (T; see Sect. 10.2.6), pelmicrite clasts and small echinoderms (top left corner).
3
Detail of part A2 (not shown in -> 1). Characteristic sections of 'Tubiphytes' , variously sized peloids, filaments (black
arrow) and a few radiolarians (white arrow).
4
Detail of part B . Radiolaria packstone. Densely packed, variously sized spumellarian radiolaria, rare ostracods (O) and
monaxon sponge spicules (SS).
5
Detail of the upper part of unit C and the lower part of unit D1 . Micritic platform clasts overlain by pelagic filaments.
6
Detail of the upper part of unit D1 and the lower part of unit D2 . The irregular orientation pattern of the filaments
indicates burrowing (BU).
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