Page 122 - Carbonate Facies in Geologic History

Illinois and Indiana Silurian Mounds and Reefs 109

This type of buildup forms the base of stromatoporoid-tabulate ecologic reefs, but may
occur alone. The Georgetown reef, cut by the Wabash River, was extensively cored and a
three dimensional picture can be ascertained (Fig. IV -8, Georgetown mound).
Some lime mud mounds may be as much as one mile in diameter and are simple, low,
conical forms consisting of pure limestone or dolomite, surrounded by vuggy, argillaceous,
somewhat cherty, dolomite-wackestone with less than 10% bioclasts. The mound rock is
massive wackestone-packstone with a bioclastic content varying from 5 to 50%.
A diverse fauna contains ostracods, trilobites, brachiopods, corals, stromatoporoids,
cephalopods, gastropods, bivalves, and sponges. This is a very similar biota to that of the
normal, bedded Silurian limestone; no vertical or lateral differentiation of fauna exists in
these mounds. The dominant particle types are crinoids and cryptostome bryozoans includ-
ing many fenestrates and small stick-like forms. Corals are mostly small solitary rugose and
colonial tabulates. Stromatoporoids are small, cabbage-head types and not framebuilders.
Algae are rare-only a few Solenopora. Stromatactoid structures occur throughout. Petro-
graphic study indicates that these sparry calcite-filled cavities are caused by collapse and
separation of mud away from bryozoan fronds and as well are fillings of cavities formed by
protective umbrellas of bryozoans. They are not considered dissolved organic frame builders
as interpreted by Lowenstam, Lees, Lecompte, Bathurst, and other researchers (see Chap-
ter V).
3. Crinoidal-tabulate coral mud mounds in Iowa (Gower Formation, Philcox 1970,
Fig. XII-4)
These are essentially masses of lime mud, replete with crinoids and scattered colonies of
Favosites, Halysites, and rarer plate-like stromatoporoids. The accumulations are from 5 to
30 m high. They built up close to wave base where extensive flank beds developed in a
marginal (windward side) zone.
Early in the mound history, crinoids may have induced mud accumulations which were
differentiated on the windward, north, and west facing sides, but these gradually fused to-
gether leeward and upward into a main mound. On the steeper, windward sides of the
mound, a blanket of tabulate corals developed. At a late stage, the amalgamated mass
developed into a platform near wave base. Then colonies of organisms developed as wedge-
shaped biostromes on the windward side ofthe platform. These consist of algal stromatolites,
beds of rhynchonelled brachiopods and thickets of the interwoven dendroid coral, Amplexus,
with scattered colonial tabulates. This organic veneer grades down-slope into a laminated
dense dolomite, a quiet-water lithofacies which is also developed in the lee of the mound
complex.

4. Full development of Silurian ecologic reefs
The development into wave-resistant framework-built bodies is described thoroughly by
Lowenstam (1950, 1957) for buildups in the low clastic belt of Illinois and Indiana. His studies
included the subsurface as well as outcrops. Later petrographic study of outcropping reefs in
this belt was conducted by Textoris and Carozzi (1964) and a detailed investigation, with
some imagination, was made by Ingels on the Thornton reef (1963). Despite the different
approaches, and the cumbersome separation of paleontology from petrography, a coherent
picture emerges. These buildups tended to form farther down the paleoslope and in water of
greater depth than mounds previously described. Some of them grew to the appreciable
height of more than 30-70 m above the sea floor.
Several growth stages are recognizable. The buildups began with crinoidal wackestone
accumulations below wave base, much like that of the Gower Formation of Iowa. Faunally,
this corresponds to Lowenstam's pioneer population of Syringopora, Favosites, dendroid
tabulates, and few stromatoporoids. Much sedimentary infill of argillaceous lime mud occurs
and also networks of stromatactoid structure. Crinoids consist mainly of Pisocrinus. Several
genera of ornate trilobites and four genera of sponges are recognized.
Stromatactoid mudstone-wackestone and calcisilt replete with bryozoans, forms the next
important stage and is identical to the total mound development seen at Georgetown and
most of that at Wabash and Mattock (stages 2 and 3 of Textoris and Carozzi, 1964). A
gradual increase occurs in the varied reef fauna which culminates in the true framework of
wave-resistant organisms at the top and includes an impressive list of reef-associated (niche-