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mounds). Slopes between 5° and 25° are typical for
transitional mixtures between muddy and granular fab-
rics.
Other factors influencing slope stability are microb-
ially triggered cementation and automicrite covers.
Experiments with sand seeded with bacteria and fungi
show that microbes influence the angles of avalanche
and angles of repose and increase stability (Meadows
et al. 1994). Bacteria bind particles together with ex-
tracellular polymeric material, while fungi bind par-
ticles by holding them together with a network of hy-
phal filaments.
Microbial automicrite and synsedimentary cemen-
tation are essential controls on the stability and preser-
vation of steep and high carbonate slopes together with
textural composition (Keim and Schlager 1999). In the
Triassic of the Dolomites (Fig. 15.28), automicrite
formed by a combination of precipitation in microbes,
microbially mediated and inorganic precipitation ex-
tend from the outer part of flat platforms down to over
200 m on 25°-35° dipping slopes (Figs. 15.27, 15.28).
The microfacies of these automicrites is characterized
by peloidal boundstones (Fig. 15.27B). The autochtho-
nous micritic carbonate lithified almost instantly, and
together with fibrous cement, intermittently stabilized
the platform slope. Downslope transport of fragmented
automicrite layers (Fig. 15.27D) forming slumps and
debris flows contributed to the deposition of cipit boul-
ders in distal toe-of-slope positions.
Fig. 15.28. Western Sella Platform, Passo Pordoi, Dolomites.
Bedding geometry at the platform margin. Horizontal topsets
(bioclastic pack-, grain- and rudstones) showing planar bed-
ding and bed thickness in the range of decimeters to one meter
pass into the steeply dipping slope by thickening and steep-
ening in a basinward direction. The slope consists of irregu-
larly distributed breccias and blocks, and grain- and pack-
stone beds stabilized by automicrite. Primary depositional
dip angles reach 35°. The topsets of the Triassic (Late Ladin-
ian to Early Carnian) slope of the Schlern Dolomite are over-
lain by the Carnian Raibl Formation, a siliciclastic unit rec-
ognizable over the Alpine Chains. The Norian Dolomia
Principale is a peritidal complex. See Fig. 15.27 for pictures
of the microfacies. After Keim and Schlager (1999).
Schlager 1987; Kenter 1990). Declivity and geometry
of submarine slopes is controlled by the composition
of slope sediments. Calcareous sediments build steeper
slopes than siliciclastic sediment.
Steep slopes may be subdivided into three parts:
(1) An upper part dominated by avalanching of
coarse gravelly sandy sediment, low in mud (lithoclastic
rudstone with abundant marine cement, e.g. radiaxial
cement), separated from the middle and lower slope
by a distinct break. (2) Middle slope characterized by
scouring and deposition of gravelly debris flow (poorly
sorted lithoclastic rounded grain- to rudstone, gravel
and sand mixture). (3) Lower slope with sedimenta-
tion of concave muddy deposits, muddy sands or mud-
free sands (composite beds with mudstone and wacke-
stones, and fine-grained lithoclastic sand.
The controls on the stabilization of slopes are a cru-
cial problem in facies interpretation, particularly of
steep platform slopes or the slopes of mud mounds,
which can dip down with angles of between 30 and 75
degrees (e.g. Middle Devonian mud mounds, Morocco;
Wendt 1993). An important control on the stability of
slope angles are sediment fabrics reflected by the tex-
ture, size and abundance of grains (Fig. 15.26). Grain-
supported carbonates, devoid of fines, pile up to over
25° (e.g. Triassic of the Dolomites). Mud-supported
fabrics build slopes up to 5° (e.g. Carboniferous algal
15.7.5 Tracing Platform-Basin Transitions
Using Grain Composition Logs
15.7.5.1 Concept and Methods
Shedding of fine-grained carbonate material from plat-
forms, platform margins and upper slopes to adjacent
periplatform environments, connected with the trans-
port of the grains by turbidity currents and debris flows
lead to the formation of calciturbidites intercalated in
basinal carbonates. Compositional grain analysis and
statistical evaluation of the compositional data provide
important information on the history of exposure and
flooding of platforms, the timing of cyclic sedimenta-
tion, and stratigraphic platform-to-basin correlations.
The interpretation of compositional logs requires
knowledge of the depositional environments of carbon-
ate grains and thorough numerical analyses of grain
composition, frequency and association. Platform-
slope-basin correlations by compositional logs should
be combined with other methods, e.g.
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