Geology Reference
In-Depth Information
Fig. 21.5
Subtidal lagoon facies: (
a
-
b
) Heavily bioturbated
lime mudstone subtidal facies in the Lower Triassic lower mem-
ber of the Elika Formation of the Alborz Mountains, northern
Iran. (
a
) Photomicrograph of a vertically oriented thin section
showing abundant horizontal burrows in a lime mudstone (
b
)
Upper surface of a heavily bioturbated subtidal lime mudstone
bed (
coin
diameter is 2.5 cm). (
c
) Photomicrograph of a subtidal
lagoon facies composed of peloids, intraclasts and restricted
fauna (mainly dentritinid forams). Note a miliolid foram in the
upper right
; Lower Miocene Asmari Formation, Zagros
Mountains, southwest Iran. (
d
) Thin section photograph of a
peloidal bioclast oncoid packstone. Note the nearly concentric
oncoids (pisoidal form of microbialites) in the
upper right
. Note
also the geopetally fi lled gastropod shell mold in the
lower right
of the photograph; Negli Creek Member of the Mississippian
Kinkaid Formation, southern Illinois
intertidal sediments include tepee structures
(Fig.
21.10a-c
) and gypsum crystals or their pseudo-
morphs (Figs.
21.9d
and
21.10d, e
).
Tepees as defi ned by Adams and Frenzel (
1950
) are
structures having an inverted V-shaped profi le similar
to the American Indians tents. However, this form of
tepee is only occasionally present and it normally
appears as irregular and low ridges (Pratt
2002
) . Tepee
structures (Fig.
21.10a-c
) are common to peritidal
deposits and form as a result of desiccation, cementation
and crystal growth, thermal expansion, and contraction
of partially lithifi ed sediment in arid tidal fl at or
high energy shallow subtidal sediments (Kendall and
Warren
1987
) . They refl ect the polygonal antiform
ridges arched along polygonal cracks in a cemented
surface crust as seen in plan view (Demicco and
Hardie
1994
). In high energy subtidal settings, cemen-
tation of carbonate grainstone layers can lead to expan-
sion and development of centimeter-scale to giant
polygonal cracks that may be folded or thrusted at the
margins forming tepee structures (e.g. Kendall and
Warren
1987
; Lokier and Steuber
2008
) . Tepees
may also form as a result of brecciation of lithifi ed
sediment, regardless of their depositional setting,
by syndepositional fault movement and subsequent
cementation (Pratt
2002
) .
The lower intertidal sediment in high energy tide-
dominated coasts, commonly consists of planar lami-
nated and/or cross-bedded bioclast/peloid intraclast
grainstone facies that grade landward to heterolithic
