Geology Reference
In-Depth Information
a setting predominantly influenced by marine energies
and elevated salinities. In some modern settings, however,
tidal influences can extend well into freshwater
settings. Low salinities can greatly influence biological
activity and patterns of sedimentation.
In hypertidal systems, tidal energies can propagate
for considerable distances into embayments and up
into fluvial settings. Many tidal bores consist mostly of
freshwater that is being pushed back upstream during
the initial flood tide. The high-energy conditions
related to bore passage can result in significant amounts
of erosion as well as inland transport and deposition of
suspended sediment. The notion of the potential scale
and importance of freshwater tidal systems can be
understood by ongoing research on modern analogs in
the lower reaches of the Amazon River system (Archer
2004, 2005
). The titanic outflow from the Amazon
mouth results in a freshwater cap that extends for
approximately 200 km out onto the Atlantic Ocean
continental shelf. No saline-water intrusion has ever
been documented in Amazon River waters and saline-
freshwater mixing occurs at considerable distance
from the coastline. On the upstream end, tidal influ-
ences can be measured more than 1,000 km inland from
the coast. The combined result is a 1,200-km long and
300-km wide, fresh-water tidal system (Archer
2005
).
Obviously, the Amazon is a mega-end-member depo-
sitional system and has the highest freshwater flux in
the world. For the Pennsylvanian basins discussed
herein, however, it may be a useful modern analog
both in terms of scale and in terms of the potential for
the lateral, inland extent of a vast freshwater tidal zone
(Archer and Greb
1995
).
At several localities in the EIB and WIB, tidal
rhythmites encase in situ lycopod trunks. These trees
rooted in the upper part of the peat and represent
non-peat accumulating forested wetlands that suc-
ceeded the underlying peat-forming wetlands, which
formed the coal as the water table rose. Lycopods
were apparently restricted to freshwater settings
(Habib and Groth
1967
; Phillips and DiMichele
1992
). That these lycopods were encased in rhyth-
mites indicates either the transgression of a hetero-
lithic tidal flat with brackish to marine water that
killed the trees, or if the trees remained living for
some time during initial burial, freshwater tidal con-
ditions. Based upon geochemical, petrographic, and
sedimentologic evidence, Kvale and Mastarlerz
(
1998
) determined that rhythmites above some EIB
coals were formed within freshwater settings.
16.3.4 Influences on Late
Paleozoic Tidal Ranges
During the Pennsylvanian Period the earth had a num-
ber of unique features that could have influenced tidal
modulation in the EIB and WIB. The basins under
discussion were close to the paleoequator and would
have had tropical climates (Heckel
1986
). Widespread
coals, especially in the upper Middle Pennsylvanian
(Desmoinesian), indicate the periodic establishment
of vast tropical rainforests within, and sometimes
extending between the EIB and WIB (e.g. Greb et al.
2003
). The tremendous size of the landmass of Pangea
may have resulted in the common occurrence of mega
paleorivers (Potter
1978
; Archer and Greb
1995
). The
relatively flat topography of the interior Pennsylvanian
basins meant that large areas could have been influ-
enced by water-level changes, both from the seaward
and the landward end. Also, a global paleo-ocean
spanned most of the planet during the Paleozoic Era
and unusual tidal resonances could have occurred
(Archer
1996b
).
16.3.5 Late Paleozoic Glaciations
Another factor that could have exerted major controls
on rhythmite deposition and preservation was glacio-
eustacy. Vast areas of Gondwanaland were affected
by continental glaciation and deglaciation, which was
manifested by high-frequency and high-magnitude
glacio-eustatic cycles (Heckel
1986, 1994
). Successive
alternation of Pennsylvanian facies have long been
interpreted as resulting from glacial-eustacy (e.g.
,
Wanless and Weller
1932
). Perhaps the resulting
oscillations in global sealevels created a potential for
strong, basinal resonances during specific periods of
time. Extensive paleovalleys were incised during
periods of sea level lowstand, and inferred glaciation.
During transgression, these would have been back-
filled and converted to estuaries. Funnel-shaped estu-
aries within low-relief basins with strong, basinal
resonances would favor tidal conditions and local
hypertidal systems, resulting in a variety of tidal
