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Jodry (1969) has pointed out that these lateral facies changes may not have
been strictly contemporaneous if the relationship has been obscured by dif-
ferential compaction. The reefs and associated carbonates have been exten-
sively dolomitized.
A well-documented Niagaran reef is the Thornton reef complex in Illinois
(Ingels, 1963) about 15 km south-west of the-southern tip of Lake Michigan,
which is exposed in a commercial dolomite quarry. It is roughly circular in
plan, with an area at the surface of about 5 km2; and it is about 175 m thick
at its maximum development. The foundation of the reef on its southern
flank is a bioclastic calcarenite, and Ingels suggested that coral colonies co-
alesced to form the initial reef growth. Subsequently, rising sea-level led to
upward and outward growth. The water depth was inferred to have been
about 60 m outside the reef, which implies subsidence of about 120 m relative
to sea level during the growth of the reef. The prevailing winds were west-
south-westerly (by present geography) and this led to ecological zoning with
biotopes and lithotopes trending south-east to north-east.
There are hundreds of subsurface reefs, but these cannot of course be
known in the same detail as those that are exposed at the surface, and quarried.
First oil production from a reef in Illinois came from the Marine Pool, and
this reef was studied by Lowenstam (1948, 1950). Drilling revealed it to
have a horseshoe shape, open to the north; and Lowenstam interpreted it to
be a reef with detrital bars caused by prevailing southerly winds. Only one
well penetrated the reef itself. Around the reef are terrigenous clastics of the
Mocassin Springs Formation. The clays in these contiguous sediments are
usually red, and Lowenstam suggested that they accumulated below the
euphotic ceiling. Near the reef, however, they are green: here they probably
accumulated above the euphotic ceiling. The Mocassin Springs sediments are
thicker to the north of the reef, suggesting redistribution by wave action to
the lee of the reef.
Perhaps the most interesting reef trend is that studied by Gill (1979) to
the east of Lake Michigan, in the north-west of the Michigan basin.
The Michigan basin is remarkable in that it is a relatively undeformed area
of Middle Silurian carbonates and evaporites, with many hundreds of reefs,
that occupies the general area of the Great Lakes of North America. In the
central part of the basin (Fig. 12-3) the environments of the carbonates formed
concentric rings, one of which consisted of almost continuous reefs - like a
barrier reef. Within this ring was another of patch reefs, which was to develop
into an area of pinnacle reefs in which petroleum would accumulate. In the
central area was deeper water in which carbonates, mainly, accumulated.
The basin was subsiding during the Middle Silurian, the central part more
rapidly than the margins, with the result that the height of the pinnacle reefs
tends to increase towards the centre of the basin (Fig. 12-4). Despite the sub-
sidence, the ring of continuous reefs appears to have isolated the interior of
the basin from the sea because the period of reef growth was followed by the