Page 118 - Carbonate Facies in Geologic History

Illinois and Indiana Silurian Mounds and Reefs 105

ing Devonian time but sufficient remnants exist to assure that by Middle Silurian
time practically the whole continent was inundated by shallow seas producing
marine carbonate (Fig. IV -5). Patches of mounds and ecologic reefs of algae,
corals, and stromatoporoids are seen generally to rim the great central craton and
to lap over the eastern side of it in a wide belt mostly now exposed in Indiana,
Michigan, and Illinois. In addition, three cratonic basins are bordered by Silurian
reefy carbonate buildups: the Michigan basin, the Illinois (or Bainbridge basin),
and the West Texas-New Mexico (Tobosa) basin.
The Cordilleran geosyncline also contains a shelf margin of Silurian age in
central Nevada and in the northern Canadian Rockies. In none of these areas has
a barrier reef rim of any sort been demonstrated but only a relatively abrupt facies
change from fossiliferous carbonate into deeper water, graptolite-bearing, dark or
reddish argillaceous sediment. The Nevada slope facies has been well described
(Winterer and Murphy, 1960). Clear evidence of turbidites and slump deposits
exists here indicating steep slopes, but the shelf margin is so thoroughly dolomi-
tized that individual organic buildups are not observable and the real character of
the shelf margin is unknown. In most places a regional belt of limestone borders
the dolomitized craton but faunas are the same in this belt as within the interior.
Little is actually known about major Silurian shelf margins. Detailed descrip-
tion of Silurian buildups, therefore, comes almost solely from shelf areas, where
several different kinds of carbonate masses have been exhaustively studied for at
least thirty years, first biologically and more recently petrographically. Extensive
dolomitization, so characteristic of the North American Silurian, has hampered
the latter effort.

Illinois and Indiana Silurian Mounds and Reefs
The stratigraphic section in the northern middle western states, consists of about
100-200 m of marl, dolomite, and limestone. This covers an extensive shelf
between the Michigan and Illinois basins west of the Cincinnati-Findlay arch.
Only the northern edge of the shelf is exposed; the larger part lies in the subsur-
face of Illinois and Iowa. Fig. IV-6 shows the location of about 160 known carbon-
ate buildups in Niagaran (Wenlockian and Ludlovian) strata on this shelf and
around the Michigan basin. Shaver (1974) points out that many additional
buildups remain undiscovered.
Lowenstam (1950, 1957) divided the shelf into areas of pure carbonate, low
terrigenous, and high terrigenous sediments. These belts represent progressive
changes in deposition along a shallow shelf sloping into the Illinois basin. The
clastic free belt contains some evidence of tidal flat and restricted marine condi-
tions but normal marine waters covered much of the area. Water depth is proba-
bly indicated by average height of the buildups above sea floor which is only a few
meters in the northern outcrops and as much as 100 m in the subsurface of
Illinois. The buildups occur in irregular clusters in the non clastic and low clastic
belts, but no linear trends are obvious, except where they surround the Michigan
basin.