Page 368 - Carbonate Facies in Geologic History

Belt 2. Shelf Facies (Deep Undathem) 355

increases the tendency toward stagnant, reducing environments. Few burrowers
can exist here and fine laminated sediment results.
a) Prevailing rock types: Dark thin beds of limestone with dark shale or silt
and some thin-bedded anhydrite. In some evaporite basins thick halite may con-
stitute a later fill-in deposit.
b) Colors: Dark brown or black (up to several percent bituminous organic
matter). In some basins reddish colors prevail (see preceding discussion under
leptogeosynclinal facies).
c) Grain types and depositional texture: Lime mudstones and calcisiltites, mi-
cropeloids and microbioclasts. Crinoidal accumulations are also present.
d) Bedding and sedimentary structures: Very even planar mm lamination, rip-
ple cross-lamination, small scale rhythmic bedding consisting of even beds of
limestone intercalated with thin shales.
e) Terrigenous clastics: Somewhat admixed with carbonates and also in-
terbedded in thin layers; quartz silt and shale. This material is windblown as well
as water-carried. Chert is very common, probably derived during early diagenesis
from opaline organisms and later from solution and replacement of quartz silt by
carbonate.
f) Biota: Exclusively nektonic-pelagic fauna preserved in local abundance on
the bedding planes. Mass mortality of pelagic organisms is believed responsible
for these accumulations. Megafauna includes graptolites, planktonic bivalves,
ammonites, and sponge spicules. Microfauna is thoroughly admixed with fine
sediment. It includes calcareous calpionellids, tintinnids and calcispheres, and
siliceous radiolarians and diatoms.

Belt 2. Shelf Facies (Deep Undathem)

The water is tens of meters or even a hundred meters deep, generally oxygenated
and of normal marine salinity, with good current circulation. The depth is suffi-
cient to be below normal wave base but intermittent storms affect bottom sedi-
ments. Such shelves are generally wide and sedimentation is quite uniform. This is
the typical realm of deeper neritic sedimentation and may consist of carbonate or
shale. Whereas the stratigraphic record is replete with interpreted examples of
widespread neritic shelf deposits, there is no modern model for this type of sedi-
mentation. Interpretations are based mainly on the rock record. Further, this
facies belt is very similar to Belt 7 which represents an open circulation shelf
existing "inside" a shelf margin barrier. Distinction between Belts 2 and 7 is
important but at present far from clear.
a) Prevailing rock types: Very fossiliferous limestone interbedded with marl.
Well segregated beds.
b) Colors: Gray, green, red, and brown due to variable oxidizing and reducing
conditions.
c) Grain types and depositional texture: Bioclastic and whole fossil wacke-
stone. Occasional beds of winnowed bioclastic grainstone and coquina. Much
pelleting of micrite matrix. Some calcisiltite.
d) Bedding and sedimentary structures: Sediment thoroughly burrowed, beds
homogenized. Thin to medium, wavy to nodular beds. Ball and flow structures