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A Siluran reef complex at Delphi, Indiana, consists of two subcircular reefs occupying an area of about 4 sq mi (10.6 sq km). The reef is more than 400 ft (62 m) thick, has a volume of about 0.15 cu mi (0.64 cu km), and effected as much as 75 ft (23 m) of compaction-induced drape in the overlying Middle Devonian strata.
Stratigraphically, the complex extends upward from Salamonie (Middle Silurian) into Salina rocks (Upper Silurian). Growth of the complex proceeded through alternating periods of lateral expansion and restriction as reflected in the cross-sectional geometry of at least one of the reefs. These growth characteristics, whether related to sea-level fluctuation and/or salinity changes or to other factors, probably reflect the conditions that led to cyclic deposition of carbonate and evaporite rocks in the Michigan basin during Middle to Late Silurian time.
Present dips along reef flanks locally exceed 35°, but structural and stratigraphic analyses suggest that original depositional slopes may have been more gentle, that reef tops were never appreciably more than 200 ft (60 m) above the seafloor (although reef thicknesses of several hundred feet were attained before erosion), and that the central parts of the main reef masses were occupied by relatively rigid and volumetrically little changing structural cores.
The southwestern reef of the complex contains several biozones, which were verified in part by systematically collected faunal data and by trend-surface analysis. Biozones include: two central areas of highest organic-framework buildup characterized by corals and stromatoporoids and flanking zones characterized separately by echinodermal and other debris, pentamerid brachiopods, gastropods, and fine debris and chert. The zones are partly repetitive, however, and are related to both reef coalescence and alternating environmental conditions. The zonal distribution is similar to that already proposed for the large Silurian reef at Monon, Indiana, and somewhat resembles that proposed for the reef at Thornton, Illinois. These similarities and the fact of zonation in itself help to support he conclusion that the often debated Silurian buildups in the Great Lakes area satisfy all but the most rigid definitions of ecologic (organic-framework) reefs.
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