Page 366 - Carbonate Facies in Geologic History

Belt 1 A. Turbidite and Leptogeosynclinal Deep Water Facies (Fondothem) 353

and reviewed below. These thin, calcareous and siliceous sediments, rich in ferric
iron, are considered to be products of basins removed from land and protected
from much terrigenous influx. The soluble iron supposedly derives from a few
rivers draining a tropical land. The hinterland contained extensive salt flats and
probably not much run off occurred. Pelagic sedimentation existed widely and at
varying depths. Several local Jurassic troughs within the Austroalpine nappes of
Austria have been carefully studied (the Unkenthal by Garrison, 1967, and Garri-
son and Fischer, 1969; the Berchtesgaden area by Jurgan, 1969; the Glasenbach
near Salzburg, Bernoulli and Jenkyns, 1970; and the Tauglboden by Schlager and
Schlager, 1973). Bernoulli and Jenkyns (1974) reviewed these sediments thor-
oughly.
The following rock types are known, mostly in starved geosynclinal troughs
and on the deeply submerged swells within them.

1. Radiolarites (112, Chapter IX): Siliceous, very fine-grained laminites with radiolaria
and other tiny siliceous organic particles. Very fine, paper-thin layers of siliceous shale are
interbedded with radiolarian layers.
2. Red biomicrite and red nodular limestone (117, ChapterIX): Beds of ferruginous
micrite with less than 5% argillaceous material, with pelagic microfauna; largely dissolved
former aragonitic bioclasts; some ferruginous manganese crusts. Peculiar conglomerate and
rubble beds are caused in part by in situ solution on the sea floor, of calcitic sediment
deposited at about the compensation depth for aragonite. Characteristically such sediment
occurred on swells (high areas) within the geosyncline and may be traced in some areas as
slumped and resedimented masses moved downslope to trough positions. A related microfa-
cies is ferruginous pisolite in a red biomicrite matrix.
3. Light-colored pelagic lime mudstones with thin interbedded allodapic units (114,
Chapter IX). Such strata are evenly and planar bedded. Much silt-size and finely ca1carenitic
material as well as micrite exists.
4. Dark basinal micrite and spiculitic limestone with some micropeloids (112, Chapter-
IX). This limestone is similar to the light-colored pelagic facies but contains higher amounts
of organic matter. It is commonly cherty and contains Chondrites traces (burrows). Probably
such beds represent the distal ends of turbidites.
5. Pelagic micropeloid beds and tiny coated particles with ammonoids, Radiolaria, and
calpionellids (J 13, Chapter IX). These beds may be allodapic and are distal turbidites.
6. Bioclastic grainstones and packstones composed of ammonites, small gastropods,
pelagic bivalves, and echinoderms. Most of such sediment must be a product of current
deposition, representing bars or megaripples formed at some depth on the sea floor. Two
special types are listed below, numbers 7 and 8.
7. "Filamentous" microcoquinoids of very thin shells of pelagic bivalves such as Halobia,
Daonella, Posidonia (1 15, Chapter IX).
8. Red to pink encrinites. This characteristic rock type represents accumulated echino-
dermal debris in a nonreducing environment. The significance of the red color of this and other
rock types in deep water environments is discussed below.
9. Spiculite: accumulation of very light, mostly hexactinellid sponge spicules of opaline
silica. Probably the spicules washed down into the basin centers from the slopes.
10. Sporadic debris flows of calcareous breccia, ranging from coarse-grained to micro-
breccia. Fragments were mostly consolidated before deposition and were derived from
nearby shelves and slopes.
11. Ferro-manganese crusts and nodules on limestone; these are products of very slow
sedimentation and concretionary growth in deep water. Manganiferous nodules are present
in the modern sea floors at great depth and are not uncommon in Austroalpine Tethyan
Mesozoic strata.
The red-purple color common in some of these above sediment types is an
indirect result of the slow sedimentation (Fischer in Mesolella et al., 1974). Ferric