Page 367 - Carbonate Facies in Geologic History

354 Summary

oxide, normally deposited in most marine sediment is commonly slowly reduced
during the decay of organic matter entrapped in the sediment. In areas of slow
sedimentation bacterial decay on the sea floor can, before burial, oxidize all
organic matter whose absence within the sediment after burial prevents the ulti-
mate reduction of the ferric iron. Pink encrinites, such as the Moccasin (Ordovi-
cian) of the Appalachians, Niagaran of the Michigan basin, Early Cretaceous of
the Oman geosyncline, and the Hierlatzkalk (Lias) of the Austroalpine Alps are
all basinal sediments, mostly in geosynclines. Nonferruginous sediments are natu-
rally a light color in an oxidizing environment.
The geosynclinal rock types outlined above are normally cherty, the chert
being in beds and nodules crudely parallel to the bedding. The silica is derived at
early diagenetic stages from solution of the opaline silica of microplankton and
spicules entombed in the sediment. It precipitates in replacement and void fillings,
first as crystobalite which later alters to chalcedony. Petrographic study often
shows much mutual replacements of silica and calcite, inferring multiple stages of
varying pH conditions. The replacement and cavity filling begins very early in
diagenesis and continues over a long time-period.
Sedimentary structures are simple, the strata away from the slopes are thin-
bedded and commonly laminate; bioturbation is rare or absent. In places thick
beds of breccia occur, channelling down into the even, planar beds and shovelling
them up, crumpling and contorting them. Syngenetic conglomerates and breccias
formed by seafloor solution and concretion growth are unique. Unlike terrige-
nous clastic sediments, even the thin lime mudstones show some early consolida-
tion. Flame structures, load casts, and convolute bedding are rare to absent.
Flaser (ball and flow) structure and evidence ofresedimentation is known in slope
areas. Sedimentary structures have been described in detail by Schlager and
Schlager (1973).
The biota of geosynclinal strata is chiefly pelagic and identical with that
described below under starved basin limestones.

Belt 1 B. Cratonic Basin Carbonates (Starved and Mostly Euxinic) (Fondothem)

These sediments are formed in centers of deep intracratonic and marginal cra-
tonic (miogeosynclinal) basins, areas removed from coastlines or from the in-
fluence of carbonate-producing shelf areas. The deposition is dependent on the
amount of influx of fine argillaceous and siliceous material and the rain of decay-
ing plankton, for the water is too deep and dark for benthonic production of
carbonate. Contributions from the land are slight in a typical carbonate-produc-
ing shelf or basin and come mainly from wind-blown material; consequently a
starved and somewhat deep basin commonly results.
Sediment is formed generally in euxinic conditions developed below oxygena-
tion level and below wave base. The water depth is at least 30 m and generally
several 100 m. The bottom water flowing off of the surrounding shelves may
become hypersaline and dense, preventing easy turnover. This situation causes
oxygen deficiency which, coupled with the constant rain of decaying plankton,