Geology Reference
In-Depth Information
Fig. 1.13
Stratigraphic chart for the Indiana portion of the Illinois
Basin showing stratigraphic intervals where good tidal rhythmite
records have been identified by the author. The
solid grey line
marks the boundary below which tidal rhythmites seem to be con-
trolled primarily by the synodic monthly cycle and above which
the tidal rhythmites appear to reflect the tropical monthly cycle
S
2
, O
1
, K
1
, and N
2
constituents using the dynamic model
allows much more specific comparisons to be made to
real-world analogues (in this case Booby Island tides)
than would otherwise be possible. In fact, utilizing this
approach within the Illinois Basin one can interpret the
dominance of diurnal (O
1
and K
1
) tides versus semidiurnal
(M
2
and S
2
) tides for various tidal rhythmite packages
that span the Mississippian-Pennsylvanian systems
(Fig.
1.13
). As Fig.
1.13
shows, tidal rhythmites older
than the upper Morrowan Blue Creek Coal appear to
have been deposited within synodically driven systems
dominated by M
2
and S
2
. Younger tidal rhythmites
appear to have been deposited within tropically driven
systems. This change from synodically driven to tropically
driven tidal systems may reflect the closure of the
Iapetus Ocean during the early Pennsylvanian and a
