Geology Reference
In-Depth Information
Fig. 21.10
Tepee structures and calcite pseudomorphs after
lenticular gypsum: (
a
) Tepee structure in thin-bedded and lam-
inated rocks of the tidal fl at facies; Middle Triassic member 2,
Elika Formation, eastern Alborz Mountains, northern Iran
(scale is 14 cm long). (
b
) A tepee structure (center of the pho-
tograph) in a tidal fl at succession; lower part of the Mississippian
lower St. Louis Limestone at Bussen Quarry, St. Louis County,
Missouri. The height of the teepee is about 50 cm. (
c
)
Photograph of a thin section from an arid upper intertidal
planar stromatolite showing a small tepee structure; Cave Hill
Member, Mississippian Kinkaid Formation, Buncombe Quarry,
southern Illinois. Note that all the light crystals are calcite
pseudomorphs after lenticular gypsum as seen in (
d
). Note also
the disruption of the laminae as a result of desiccation and
gypsum formation. (
d
) Photomicrograph of a part of sample
(
c
) under normal light showing calcite pseudomorphs after lenticu-
lar gypsum, microbial laminae and microcrystalline dolomite.
Circular objects in the dark microbial laminae are calcispheres
believed to be green algal sporangium. (
e
) Photomicrograph
under normal light of a microbial lamina containing calcite
pseudomorphs after gypsum covered by a sediment-rich lam-
ina composed of intraclastic dolomudstone; Cave Hill Member,
Mississippian Kinkaid Formation, Kinkaid Creek section,
southern Illinois
stromatolities that typically display fenestral fabric,
root casts, desiccation cracks and storm-generated
deposits (e.g. Shinn
1986
). A well preserved humid
tidal fl at and the associated coastal marsh facies has
been described by Mitchell (
1985
) from the Middle
Ordovician St. Paul Group in central Appalachians
(see Sect.
21.7.3
). Deposition of calcium carbonate
and/or gypsum in tidal fl at environments leads to
the formation of early diagenetic microcrystalline
dolomite by either direct precipitation or replace-
ment of the previously deposited calcium carbonate
(e.g. Hardie
1987
, Z. Lasemi et al.
1989
) .
