Page 124 - Carbonate Facies in Geologic History

Illinois and Indiana Silurian Mounds and Reefs 111

INTERREEF

\
\
I I
, I
,
,
.,,, \
~ \ \ '. '
." '
. ,
\
\
PREVAILING WAVE ,
AND ,
WINO OIRECTlON

---20---
Isobath
o 200 !.OOm in fathoms
"", -_ .. ' ..... _....1'

Fig. IV -10. Paleoecologic map of Thornton Silurian reef complex in South Chicago, Illinois,
at a level 100 m above its base. From Ingels (1963, Fig. 16). Map reconstructs the key biofacies
for the buildup as conceived to result from growth of ecologic reef into wave base. Illustration
with permission of author and American Association of Petroleum Geologists

beds (35 off the central core area). The asymmetry of the flat conical elliptical mass is clearly
0
shown, the flanking strata tailing off in the leeward direction.
Small satellite mounds are mapped in flank positions surrounding the main core which is
composite and grew outward and upward in both windward and leeward directions. Wave-
resistant frame-built core on the windward side of the reef is indicated by coral-stromatopo-
roid ridges around the center. The belt of such boundstone is four times as wide on the