Geology Reference
In-Depth Information
Fig. 21.6
Subtidal (
a
through
e
) and intertidal microbialites: (
a
)
Shallow subtidal dome-shaped thrombolite structures with dark
microbial clots and dolomite-fi eld fenestral voids; Middle Cambrian
member 1 of the Mila Formation, east central Alborz Mountains in
northern Iran. (
b
) A branching columnar stromatolite from the
Silurian deposits of the southern Tabas Basin, east central Iran.
(
c
) Photomicrograph under normal light of a part of pisoidal form of
stromatolite (
oncoid
) formed by calcifi ed tubular cyanobacteria
(
Girvanella
); Negli Creek Member, Mississippian Kinkaid
Formation, east of Princeton, Kentucky. (
d
) Thin section photo-
graph of a part of an oncoid in a peloidal bioclast oncoid packstone.
The oncoid appears to have been formed by non-calcifi ed cyanobac-
teria; Negli Creek Member, Mississippian Kinkaid Formation,
southern Illinois. (
e
) A compound subtidal columnar stromatolite
complex from the Middle Cambrian member 2 of the Mila
Formation in central Alborz Mountains, northern Iran.
(
f
) Photomicrograph of fl at- to wavy-laminated stromatolite with
cement-fi lled planar birdseyes (see Sect.
21.5.2
) and molds of fi la-
mentous cyanobacteria (
arrows
); Middle Triassic middle member
of the Elika Formation in the Alborz Mountains, northern Iran
layers of sand- to coarse silt-sized and mud-size
sediments showing fl aser to wavy and lenticular bed-
ding. This millimeter- to centimeter-scale interlayering,
referred to as rhythmites (Reineck and Singh
1980
)
and tidal bedding or heterolithic stratifi cation (Demicco
and Hardie
1994
) form due to declining tidal current
energy and the resulting change in sand to mud ratio in
a landward direction, and represent deposition in the
