Geology Reference
In-Depth Information
21.7.5.2 Middle Elika Member
The middle member (up to 700 m thick) consists of
very thin- to thick-bedded dolomite and dolomitic
limestone and overlies the lower unit with a distinct
interregional unconformity. In the type locality
(Fig.
21.25
), it is 200 m thick and consists almost
exclusively of peritidal facies (Fig.
21.31
) including:
(1) laminated fenestral dolomudstone with desiccation
cracks, tepee structures and calcite pseudomorphs after
gypsum/anhydrite (supratidal facies) (Figs.
21.10a
and
21.18c
); (2) wavy to fl at-laminated stromatolite with
desiccation cracks (upper intertidal facies) (Fig.
21.6f
).
In some localities of the central Alborz Mountains,
domal stromatolite/thrombolite bioherms related to
upper subtidal/intertidal depositional settings have
been recognized near the base of the member; (3) lami-
nated peloid bioclast packstone to mudstone/
dolomudstone with desiccation cracks that may display
heterolithic stratifi cation (lower intertidal facies); (4)
fenestral bioclast peloid/intraclast grainstone (beach
ridge facies); (5) erosive-based and laterally discon-
tinuous layers of bioclast/peloid intraclast grainstone
that grade upward to mudstone with tabular bedding
and herringbone cross-bedding (intertidal channel
facies) (Fig.
21.16b, c
); and (6) gray peloid bioclast
mudstone to packstone (lagoonal facies).
Tidal fl at deposits constitute the bulk of the middle
Elika and occur in the transgressive and highstand sys-
tems tract (Fig.
21.31
). They are characterized by their
light tan to cream color, thin to very thin bedding and
common presence of micritic dolomite, desiccation
cracks, tepee structures, laminations, birdseyes, gyp-
sum/anhydrite casts/molds and collapse breccias.
Short-term seaward progradation resulted in numerous
shallowing-upward high frequency cycles that are
superimposed on long-term third-order cycles
(Fig.
21.31
). In sharp contrast to lower Elika, the
Middle Triassic middle Elika was deposited primarily
under a fair weather condition.
upper parts and include (1) heavily bioturbated gray
bioclastic peloid lime mudstone-packstone with a
restricted fauna (subtidal lagoon) (Figs.
21.3d
, and
21.5a, b
) that may be overlain by (2) an iron oxide-
stained, 20-70 cm thick, bioclast (mainly small gas-
tropods) grainstone (Fig.
21.13b
) that may contain
fenestral fabric and exposure features (beach ridge)
and/or (3) very thin- to medium-bedded, laminated
tan to light brown lime mudstone/dolomudstone with
desiccation cracks (intertidal) or graded grainstone to
lime mudstone containing bioclast/peloid and intrac-
last grains (intertidal channel). These facies are inter-
layered with numerous erosive-based, commonly
graded intraclastic storm facies of various thicknesses
(Fig.
21.4b-d
) recording deposition under a storm
dominated platform. Peritidal facies are arranged
into 4th to 5th-order shallowing-upward cycles
and comprise the main part of the highstand systems
tract exhibiting progradational stacking pattern
(Fig.
21.29
).
In the northern Tabas failed rift basin (Fig.
21.25
,
locality 4), the lower Elika equivalent Sorkh Shale
Formation consists almost entirely of tidal fl at facies
(Fig.
21.30
) intercalated with numerous storm beds
and comprise the transgressive and highstand systems
tracts of a depositional sequence (Ghomashi
2008
,
Y. Lasemi et al.
2008
) . Southward, the carbonate-rich
peritidal deposits grade into mixed-carbonate and
quartz sandstone containing bimodal, spherical, and
well-rounded sand grains with polished (frosted) surface,
tabular bedding with internal laminations, herringbone
cross-laminations/cross bedding, fl aser and lenticular
bedding. In the Eslamabad section (locality 5 in
Fig.
21.25
), sandstone beds are overlain and underlain,
with sharp contacts, by shallowing-upward carbonate
tidal fl at cycles forming pure siliciclastic-pure carbon-
ate double cycles (Fig.
21.19d
). Sedimentary struc-
tures, super mature rounding and vertical association
with carbonate tidal fl at facies suggest deposition in
tidal fl at setting. The frosting and bimodal size distri-
butions of the quartz grains suggest proximity to a des-
ert environment (e.g. Klein
1977
). The sand grains are
interpreted as coming from aeolian dune sands which
were transported over the carbonate tidal fl at (at the
time of emergence) and were subsequently reworked
in the intertidal environment during the next platform
fl ooding (Y. Lasemi et al.
2008
) .
21.7.6 Tidalites of the Upper Miocene
Asmari Formation
The Upper Oligocene to Lower Miocene Asmari
Formation (James and Wynd
1965
) was deposited in
the elongate Persian Gulf-Mesopotamian foreland
