Geology Reference
In-Depth Information
21.7.3
Tidalites of the Middle Ordovician
St. Paul Group in Central Appalachians
The St. Paul Group (150-200 m thick) is a part of the
Cambro-Ordovician platform carbonate succession in
the central Appalachians, USA. In Maryland and
Pennsylvania, the St. Paul succession consists mainly
of lagoonal, tidal fl at and freshwater coastal marsh
facies (Mitchell 1985 ; Hardie 1986 ; Demicco and
Hardie 1994 ) including: (1) thin-bedded interlayered
LLH (Logan et al. 1964 ) stromatolite, unfossiliferous
mudstone containing molds of cyanobacteria and
intraclast packstone facies (coastal freshwater lake/
supratidal marsh); (2) laminite facies (0.2-1 m thick)
characterized by interlaminated planar- to wavy pelo-
idal mudstone and crinkly stromatolite boundtone
with birdseyes and desiccation cracks (supratidal
levee); (3) bioturbated mudstone-wackestone facies
(0.1-1.5 m thick) with low diversity fauna (mainly
ostracods) and no internal layering (intertidal pond);
(4) coarse peloidal sand- to pebble-sized micritic
intraclast grainstone-packstone up to 0.3 m thick
(meandering point bar deposit); (5) thick-bedded bio-
turbated lime mudstone facies with low to moderate
diversity fauna (restricted to semi-restricted inner
shelf lagoon to intertidal); (6) thick-bedded biotur-
bated wackestone/packstone and grainstones with
diverse fauna including tabulate corals and bryozoans
(open outer shelf lagoon).
In the St. Paul tidal deposits, bioturbated mudstone
capped by mud cracked laminite facies dominate the
lower part of tidal fl at facies succession and are pro-
gressively replaced upward by thin-bedded and lamin-
ite facies that become more abundant upward near the
boundary with the overlying coastal freshwater lake
facies. According to Mitchell ( 1985 ) , the closest mod-
ern analog of the St. Paul facies is the tidal fl at system
of the Bahamas including the Great Bahama Bank,
Andros Island tidal fl at and the inland freshwater
marsh described by (Shinn 1983a, 1986 ) . The absence
of reefs and ooid grainstone, dominance of bioturbated
peloidal mudstone, low diversity fauna, abundance of
stromatolites and the absence of any trace of evapo-
rites in the supratidal facies, all suggest the existence
of a low energy, rainy tidal fl at during the deposition of
the St. Paul Group in central Appalachians (Mitchell
1985 , Demicco and Hardie 1994).
Fig. 21.27 Map of Illinois and neighboring states showing an
outline of the Illinois Basin (From Buschbach and Kolata 1991 )
and the locations of sections used to prepare the composite sec-
tion shown in Fig. 21.28 . Numbered localities: ( 1 ) Kinkaid
Creek section, Jackson County, Illinois; ( 2 ) Buncombe quarry
section, Johnson County, Illinois
21.7.4 Mississippian Tidalites
in the Illinois Basin
The intracratonic Illinois Basin (Kolata and Nelson
1991 ) covers parts of the states of Illinois, Indiana,
Kentucky and Tennessee (Fig. 21.27 ) in the mid-
continent of the United States (Buschbach and Kolata
1991 ) . During the Mississippian (Early Carboniferous),
several hundred meters of shallow marine carbonates
were deposited on carbonate ramp platforms that
opened into the deep Ouachita Trough to the south.
Extensive peritidal facies have been described from
the Cave Hill Member of the Upper Mississippian
Kinkaid Formation (Y. Lasemi 1980 , Y. Lasemi and
Carozzi 1981 ). The Cave Hill Member (Swann 1963 )
is bounded by two limestone units namely the Negli
Search WWH ::




Custom Search