Page 104 - Carbonate Facies in Geologic History

Clastic Content in Carbonates 91

the wide distribution of various chert types and recognition of the depositional
facies in relationship to chronostratigraphy were never developed and study of
chert residues along with that of heavy minerals in terrigenous sediments declined
somewhat in interest, although both techniques are still useful in subdividing
thick carbonate strata.
Two types of detrital siliceous grains occur extensively though in trace
amounts in major carbonate strata. Zones of rounded, frosted, rather large,
quartz grains are known from many sections and probably represent coastal dune
and beach sand reworked in the shallow marine environment. Such zones are
known widely in the Lower Ordovician platform composed of shallow shelf and
intertidal deposits surrounding the Canadian shield of North America. Some of
these zones of quartz grains occur now in areas hundreds of km from the original
sandy coastlines of the great carbonate producing sea which lay across the shield.
The southern shelf of the modern Persian Gulf has up to 10% of such grains
distributed from large sand dunes into the sea along the coast south of the Qatar
Peninsula (Shinn, in Purser, 1973). Quartz sand is so resistant that it may be
widely distributed in the carbonate environment without appreciable abrasion or
solution.
The other siliceous detrital grains common in carbonates are silt-size angular
quartz and feldspars which are also widely distributed, probably windblown.
Again, the Persian Gulf Holocene sedimentary regime serves as a model (Kukal
and Saadallah, in Purser, 1973). Vast dust storms in the Gulf are capable of
blowing silt and clay size material completely across the sea (a distance of
400 km). Probably a large part of the fine material of the axial part of the Persian
Gulf east of the Qatar Peninsula (both carbonate and terrigenous) is wind-blown.
In the geologic record such silts may occur as concentrates in intertidal beds,
which often also contain rounded, frosted quartz sand grains and traces of detrital
thorium. Such beds cause pronounced high gamma readings. Wind distribution
of radioactive detrital material may be indicated in the cyclic Late Devonian
Duperow strata of the Williston basin of North Dakota, which contain argilla-
ceous silty dolomite beds in sabkha evaporite strata (Chapter X). These beds con-
tain radioactive silt grains whose distribution is indicated on Fig. 111-4. Strati-
graphic evidence indicates a sandy source area to the south and the intensity of
the gamma ray deflections increases in this direction.
The same strong gamma ray deflections or "kicks" are common in thick, and
otherwise pure, carbonates in Ordovician, Silurian, and Mississippian strata of
the Williston basin (Porter and Fuller, 1959; Krumbein and Sloss, 1963, p.382).
Similar thin zones of high gamma ray response are known in Siluro-Devonian
strata in the West Texas basin. In all these examples such markers are widely
traceable across the basins, and the ubiquitous, silty, terrigenous material is prob-
ably windblown.
Beds of altered volcanic ash (bentonites or metabentonites) may be important
units in carbonate strata, particularly useful as key beds ("time markers") in
correlation. These have been widely traced in the Middle Ordovician of the
Appalachians (Dott and Batten, 1971, Fig. 10-18). Kay (1953) used these benton-
ites to effect correlations between the eastern black shale and western limestone
facies of the Trenton group in New York state. Volcanic activity in the mobile