Chalheghareh section
At section Chalheghareh (Fig.
9
), deposits of the Qom Fm.
cover reddish continental conglomerates and sandstones
with intercalated paleosoils of the Lower Red Fm. The
contact is not exposed.
The marine succession of the Qom Fm. starts with
conglomerates, cross-bedded, coarse-grained sandstones
with small nummulites, rhodolites (Lithothamnion), coral-
linacean debris and fragmented echinoids intercalating
with parallel and cross-bedded corallinacean (Lithotham-
nion) packstones, rudstones and grainstones with abundant
rotaliid and nummulitid foraminifers, as well as pecti-
nid bivalves (Pecten tietzei, Amussiopecten labadyei,
Aequipecten submalvinae) (1–28). Grainstones of bed 3
include a reddish colored zone, and bed 6 is a stromato-
lithic limestone with wavy mm- to cm-scale lamination. In
bed 12 shells of large oysters are accumulated. Upsection
(22–28) the coarse siliciclastic content decreases and in-
terbeds
of
marl
and
argillaceous
limestone
with
Fig. 10
Thin section microphotographs illustrating the depth zona-
tion of biotic assemblages from Oligo-/Miocene ramps in the
Esfahan-Sirjan and Qom basins. a Gastropod (potamidids) coquina
with a high amount of detritic quartz, lithic grains and coral (Porites)
clasts indicating a nearshore environment in the vicinity of mangroves
(Abadeh 38). b Miliolid-grainstone with Archaias and Borelis. The
dominance of porcellaneous foraminifers points to a shallow,
restricted inner ramp environment with seagrass vegetation (Qom
15). c Foraminiferal-corallinacean rudstone with Sporolithon, Borelis,
heterosteginid and rotaliid foraminifers from a shallow inner ramp
setting (Abadeh 61). d Corallinacean-bryozoan floatstone of a
moderately deep environment on the middle ramp. Bryozoans are
represented by erect delicate branching, and unilaminar encrusting
growth forms (Qom 18). e Lepidocyclina-bryozoan floatstone. The
large size and flat discoidal shape of Lepidocyclina tests is typical for
a deep low light environment on the outer ramp. Associated with the
lepidocyclinids are erect delicate branching bryozoans (Abadeh 53).
f
Bryozoan-packstone composed of erect delicate branching, erect
bilaminar, and erect foliaceous colonies from the outer ramp. This
facies represents the deepest environment, where erect bryozoans
have the highest abundance and diversity, and where light dependent
biota is completely missing (Qom 28)
Int J Earth Sci (Geol Rundsch)
123
lepidocyclinids occur. A series of marls with intercalated
limestones follows (29–66). The latter are concentrated in
the middle segment of this unit (30–48) and comprise
Lepidocyclina (eulepidinids) floatstones as well as oyster
coquinas (Hyotissa hyotis) that are accompanied by erect
branching and nodular growing bryozoans (36, 38). Bry-
ozoans, mainly of the erect rigid fenestrate growth type,
become more frequent in the upper part of the unit. In
particular, bed 58 is formed by a bryozoan floatstone that is
characterized by a diverse fauna with large fragments of
erect growth forms including fenestrate, foliaceous and
delicate branching, as well as flexible growth types.
Associated planktic foraminifers belong to Globigerinoides
primodius, Gs. immaturus, Gs. quadrilobatus, Globigerina
praebulloides, Gg. cf. ottnangiensis, Globigerinella obesa,
Cassigerinella boudecensis, Tenuitella clemenciae, Tenui-
tellinata angustiumbilicata. Operculina complanata and
lepidocyclinids [E. dilatata, E. cf. formosoides, L. (N.)
morgani-type, E. undulosa?, L. cf. elephantina?] are the
most abundant larger foraminifers, while Heterostegina
(Vlkerkina) assilinoides and Amphistegina are frequent.
Additionally, miogypsinids (Miogypsinoides formosensis,
M. bantamensis) occur above bed 55. Sponge spicules and
pectinids are also present. In beds 29–49 the pectinid fauna
comprises Pecten tietzei, Amussiopecten labadyei, A. sub-
pleuronectes and Costellamussiopecten suzannae, whereas
the pectinid fauna in beds 59–63 becomes restricted to
P. tietzei and A. subpleuronectes. A marly floatstone with
isolated, in situ, dome-shaped corals (Porites, Goniopora,
bed 65) and rhodolites (Spongites, Lithoporella, Litho-
thamnion, Sporolithon), as well as a bed of wackestone
(66) terminate this unit.
Upsection, rippled oolithic and peloidal, bioclastic
(echinoids, mollusks, corallinaceans) grainstones (67–72)
follow, occasionally containing monospecific potamidid
gastropod faunas. These beds are followed by thick marly
layers grading into siltstones (73–85) and mollusk-domi-
nated coqunias (86, 87). Intercalated grainstones are often
cross-bedded and associated with mud-pebbles. The ben-
thic microfauna of the marls consists of ammonias and
pararotalias, and rare sponge spicules (78, 84). Plankton is
represented by Globigerina praebulloides, Gg. cf. cipero-
ensis, Gg. officinalis, Gg. ouachitaensis, Globoturbo-
rotalita woodi, Globigerinoides primordius, Cassigerinella
boudecensis,
and
Tenuitella
munda.
Argillaceous
limestones (88–92) containing bivalves (e.g., Costell-
amussiopecten pasini) and echinoids follow above. Here,
the benthic foraminiferal fauna is more diversified, yielding
Cibicidoides, Hanzawaia, Eponides, Nonion, and Textu-
laria. In bed 92 thin gypsum crusts are intercalated. The
sedimentary succession continues with green marls (93–94)
containing agglutinating benthic foraminifers (Jadammina,
Ammotium,
Ammobaculites)
as
well
as
planktic
foraminifers (Globigerina officinalis?, Cassigerinella chi-
polensis,
C.
boudecensis,
Tenuitella
gemma),
and
ostracods. Thin gypsum interbeds occur at the top of the
marls (94), grading into a thick evaporite unit (95). Its base
consists of thin (\5 mm) gypsum layers with oscillation
ripples that alternate with cross-bedded, fine-grained, red-
dish siliciclastics. Upsection the siliciclastic intercalations
become thinner and finally disappear. A single bed contains
large gypsum clasts (Ø 40 cm). The evaporites are covered
by green marls (96–97), which are devoid of benthos while
plankton is present.
The uppermost unit of section Chalheghareh is formed
by
fossiliferous
limestones
(grainstone,
packstone,
rudstone; 98–106) composed of peloids, mollusks, echi-
noderms, corallinaceans and foraminifers (Operculina,
Austrotrillina, few soritids, miogypsinids). They further
contain rhodolites, gastropods and infaunal bivalves. Some
beds are characterized by mass occurrences of the irregular
echinoid Parascutella (104, 105) and the teredinid bivalve
Kuphus melitensis in life position (Fig.
5
c). Moreover,
Kuphus coquinas occur (103, 105), which can be traced for
hundreds of meters (in another section located about 8 km
in WNW direction similar Kuphus layers appear in the
same stratigraphic position). The top surface of bed 105 has
a reddish color and the top of the limestone succession
shows karst fissures infilled with violet siltstone (Fig.
5
f). It
is assumed that it represents the boundary to the above
following Upper Red Fm.
Sedimentary evolution of the Qom formation
Depositional environments in the Esfahan-Sirjan fore-arc
basin
Abadeh section
Section Abadeh includes three depositional sequences
(Fig.
4
). Terrestrial sediments of the Lower Red Fm. are
deposited during the lowstand of the first sequence.
Flooding of the platform is indicated by nummulitic sili-
ciclastics deposited in a nearshore environment. With
increasing water depth, the coarse siliciclastic content
decreases and pure siliciclastics become substituted by
argillaceous limestones and marls, for which offshore
conditions are indicated by plankton assemblages (1–5).
A deeper offshore environment with reduced light levels is
also suggested by Schuster and Wielandt (
1999
) for the
associated solitary coral assemblage.
A subsequent sea-level drop is indicated by an alterna-
tion of marls with thin interbeds of sandstones (6–15) that
may represent a lowstand fan terminating the first
Int J Earth Sci (Geol Rundsch)
123