Page 123 - Carbonate Facies in Geologic History

110 The Advent of Framebuilders in the Middle Paleozoic

wind
Height of buildup above sea floof ~
varies from 3m in lower t.v. I
Crinoid debris
to "O-70m in higher levels

Crinoid debris in lime Stromatactoid s formed by
mud with Syringopora bryozoan induced cavities by
slump or solution
Fig. IV -9. Idealized fully developed Middle Silurian ecologic reef common in the low clastics
belt of Lowenstam (1950). Boundstone of stromatoporoids developed on top of an accumula-
tion of crinoid-bryozoan debris in a micrite matrix. Mound facies follow pattern outlined in
Figs. XII-5 and VI-25: 1. Basal bioclastic pile of initial accumulation; 2. Micrite batTIest one
core-mud mound buildup below wave base; 3. Crestal boundstone, framebuilt organic reef;
4. Flank beds of crinoids derived from reef top dwellers, not framebuilders; 5. and 6. Talus
and organic veneer not recognized; 7. Capping grainstone of crinoidal sand

dwelling) forms. Lowenstam noted increasing abundance of stromatoporoids but that the
corals Favosites, Haiysites, Heliolites, and Syringopora form merely low bosses. Camerate
crinoids are abundant and brachiopods such as Chonchidium, rhynchonellids, atrypids, and
spirifers occur. Seven diverse trilobite genera are present and also many mollusks.
In the frame building (rough-water stage of Lowenstam, 1950, stage 4 of Textoris and
Carozzi, 1964) the buildup reached above wave base and the typical reef fauna flourished.
More and larger tabulate corals occurred with the stromatoporoids. New corals added
include Arachnophyllum, Thecia, Aiveolites, Fletcheria, and Coenites. Sponges disappeared.
Trilobites remained varied and are represented by eleven genera, although the convex
smooth-shelled Bumastus is by far the most abundant. Brachiopods are represented also by
many genera, including the heavy-shelled Trimerellacea and pentamerids. Conchidium oc-
curred on outer edges of buildups. The echinoderms were also important: blastoids and
cystoids (especially Caryocrinites) and dominant camerate crinoids occurred. Reef grazers or
carnivores are represented by 22 nautiloid genera! The lower reef stages representing quieter
water have orthocones but coiled and breviconic forms characterize the later stage of the
buildups, especially in the shelf ward, low clastic belt. Gastropods occur in varied shapes from
flat to turreted. The large probable dasycladacean Ischadites occurs also in this crestal stage.
The final and complete development of a typical Silurian shelf reef (stages 5 and 6 of
Textoris and Carozzi, 1964) was reached when the upward organic growth reached into wave
base and stopped. Lateral accretion of extensive flank beds forms a platform and to a limited
extent, outward core growth over the flank beds occurs on the windward side. Faunal
differentiation also marks the windward side and occasionally a capping bioclastic-oolitic
deposit developed. The flank beds, which are mainly crinoidal, may constitute much more
than half of the bulk of the buildup showing that bioclastic production by niche dwellers was
prolific on the reef top and lasted a long time. The very abundance of organic production at
this stage argues for a water depth measurable in meters across the top of the platform.
Fig. IV -9 is a summary diagram of growth history.
Study of the Thornton quarry south of Chicago by Ingels (1963), resulted in reconstruc-
tion of the fmal stage of growth of the reef crest and its relation to flank development on
windward and leeward sides. Exposures of this buildup permit only a limited view of the core
facies which is but a minor part of the rock volume, most of it being steeply dipping flank