Deposits of the Qom Formation (Oligocene-Miocene) in Rameh section are located in northeastern Garmsar and contain dominantly 420 m Limestone, pebble-rich to sandy Limestone and marl Limestone. This formation is unconformably overlaid and underlaid with siliciclastic deposits of Upper Red Formation and Lower Red Formation. Field observations, along with laboratory investigations, have resulted in identifying tidal flat, lagoon, shoal and open marine environments in the studied formation. Open marine facies association consists of bioclast mudstone, bioclast wackestone, bioclast packstone and bioclast roadstone ; shoal facies association consists of ooid grainstone, bioclast grainstone and coral boundstone ; lagoon facies association is composed of dolomitic mudstone, intraclast bioclast wackestone and bioclast packstone ; and tidal flat facies association is sandy dolomudstone and stromatolite boundstone. The qom formation rocks in Rameh section are deposited in a rimmed shelf carbonate ramp. This formation undergoes various diagenetic processes including dissolution, porosity, cementation, micritization, compaction and dolomitization.
The Qom Formation is composed of a vast outcrop in Central Iran and has attracted a lot of attention due to its high hydrocarbon reservoir. The facies diversity of this formation has resulted in noncompliance between its stratigraphic units in different areas. This study is primarily concerned with analyzing depositional facies, depositional environment and digenetic processes dominating the Qom Formation in Rameh section in northeastern Garmsar. Field and microscopic observations led to identification of facies, depositional environment and diagenetic processes of the Qom Formation in the studied section.
Geographical Setting of Rameh SectionAbdolabad-Rameh region is 214,000 square acre and located in northeastern Garmsar (Semnan province), 60 kilometers of Abdolabad village. It is formed by geographical coordinates of 52˚42'20"E and 35˚21'12"W. It is surrounded by Kabir River in the north, Kavir-e-Bozorg in the south, Sorkhe in the east and Ivanaki in the west (
To identify the facies characteristics and the conditions dominating the depositional environment of the formation, the geological maps of Semnan province (at 1:250,000 scale) [
The Qom Formation has 420 m thickness and is located in Rameh section. It is consists mainly of Limestone, sandy Limestone, pebble-rich Limestone and Marl. This formation is unconformably overlaid and underlaid with Upper Red Formation and Lower Red formation. It hosts four Facies association is, namely A, B, C and D, as follow:
This facies association contains four facies as follow:
Facies A1: Bioclast lime mudstone: This facies contains less than 10% fossils scattered as echinoid, benthic foraminifera, bivalves and red algae. Dolomitization process is also noted here. This facies is reported to have weak-sorted allochems (
Facies A2: Sandy bioclast wackestone: This facies includes medium-to-thick bedded
limestones, containing 10% red algae, 10% bivalves and 20% bryozoan, brachiopod, and a small portion of quartz, which are scattered within a lime matrix (
Facies A3: Sandy bioclast packstone: This facies consists of skeletal fragments like 20% red algae, 10% brachiopod, 10% bryozoan, 10% tubularia and rotalia as well as 10% quartz (
Facies A4: Bioclast roadstone: This facies is mostly covered by 30% to 60% bioclast fragments. These fragments are weak-sorted and varied in types, including 40% red algae, 10% echinoid and a small amount of benthic, miliolid, strachod and gastropod foraminifera (
Interpretation:
Abundant number of allochems like brachiopod, echinoderm, bivalves, sponge, clacisphere and bryozoan can be found in this open marine Facies association. These organisms are sensitive to salinity, and can only survive in an open marine environment [
This facies association consists of four facies as follow:
Facies B1: Ooid grainstone: This facies is composed of 60% ooids. The skeletal allochems like miliolid and bivalves can be found here as well. Compound ooids also exist there, in which two or three ooids form the core of the larger ooids. The core of most ooids is composed of the bioclasts like brachiopod and echinoid. The pore space between allochems is filled by cement (
Facies B2: Bioclast grainstone: The proportion of bioclasts is more than 50% Skeletal allochems are composed of 50% bivalves, 20% milioids, 10% red algae, 7% gastropod, and less than 5% strachod, echinoid, peloid and green algae. The pore space between allochems is filled by cement (
Facies B3: Coral boundstone: This facies consists mostly of coral, which is located between dark, thin-to-medium bedded limestones. Other allochems include 30% crinoid, 25% brachiopod and small amount of bivalves, bryozoan and benthic foraminifera. Some of these allochems are greater than 2.5 cm in size. The matrix of this facies is covered by limemud, small amount of calcite cement and considerable amount of organic matter, making it dark (
Interpretation
Lack of a lime-mud matrix is the most salient characteristics of this facies. The presence of grainstones suggests a high-energy environment like shoal. The abundance of ooids and paucity of oncoids indicate deposition in shallow and high-energy environment like shoal [
in shoal environment and above the wave effect line [
This facies association is mainly composed of three facies, as follow:
Facies C1: Dolomitized mudstone
This massive-to-thick bedded facies is often made up of bivalve-rich limemud, and its matrix is dolomitized. These dolomitized mudstones contain intraclasts and bioclasts (
Facies C2: Bioclast intraclast wackestone
This facies contains 14% intraclast. Other allochems included in this facies are 10% bivalves together with scant amount of miliolid, benthic foraminifera and gastropod. The skeletal fragments do not have high density and floats in a matrix of limemud (
Facies C3: Bioclast packstone
This facies contains 20% miliolid, 10% red algae, 10% green algae, 10% peloid, 10%
gastropod, 10% bivalves and a small portion of intraclast in limemud (micritic) matrix (
Microfacies C3a: Bioclast packstone miliolid: This facies hosts 25% miliolid, 25% benthic foraminifera, 15% red algae, 10% bivalves and strochod (
Interpretation
The limemud matrix of the facies association C, which is neomorphized in some facies, is the sign of a low-energy environment. Distribution of benthic foraminifera in recent environments is governed by various factors like temperature, salinity, water turbulence, light penetration, sedimentation rate and water depth [
This facies association is composed of two facies as follow:
Facies D1: Sandy dolomudstone: This facies contains microcrystalline, equant and euhedral dolomites, with 20 to 50 micron in size. In some facies, dolomites have transparent margins while their central part is dim. Also, this facies contains 15% fine-to- medium-grained, poorly sorted angular quartz. It also has parallel lamination (
Facies D2: Stromatolite boundstone: They are the simplest forms of stromatolites, usually found in restricted tidal flat environments. They show desiccation polygons and have laminoid fenestrate or elongated cavities (
Interpretation
This facies association hosts abundant limemud and sand-sized clastic particles and silt. The deposition of micritic facies together with fine-grained skeletal and non-skeletal fragments has occurred in this facies association is, suggesting low-energy environments and calm waters. The presence of dolomicritic facies in this facies association points to its formation in a shallow environment dominated by evaporation conditions. The paucity of fossils in this facies association indicates lack of suitable environmental conditions and limited water circulation [
The analysis of depositional environment, as the best way in determining the conditions and quality of deposition during their formation, is used to examine factors affecting depositional environment. The results of petrographic and geochemical studies were employed to present the facies or depositional models to understand depositional environments easily [
Carbonate rocks of the Qom Formation in Rameh section have undergone various
diagenetic processes including dissolution, porosity, cementation, compaction, micriti- zation and dolomitization (
Intergranular and bird’s eye porosities are the important porosities of the Qom Formation (
Various types of cements are contained in the Qom Formation, as follow:
- Blocky cement: This cement has coarse grains and fills the intergranular pores and the cavities of the Qom Formation (
- Clear syntaxial cement: Clear syntaxial overgrowth cement commonly grows in optical continuity with crinoid grains (
- Drusy cement: This cement mostly fills the cavities resulting from dissolution of skeletal fragments (
In the Qom Formation, the micritization often influences bivalve shells and brachiopods (
Physical compaction is characterized by the fracture of bivalve shell and the subsequent interparticle porosity (
Dolomitic cement is one of the most important kind of cement in the Qom Formation.
Time | Eogenesis | Mesogenesis | Telogenesis | |||||
---|---|---|---|---|---|---|---|---|
Environment Processes | marine | meteoric | mixing | burial | uplift | |||
Micritization | ||||||||
Compaction | Physical | ***** | ***** | |||||
Cemical | ||||||||
Cementation | Equant mosaic | ***** | ||||||
Syntaxial rim | ***** | |||||||
Dissolution | ***** | |||||||
Dolomitization | Type I | |||||||
Type IV | ||||||||
Fracturing |
Dolomite crystals are dark, anhedral and compact. They lack fossils, yet have Bird’s eye porosity. In this type of dolomites, the early sedimentary textures are found in terms of green or blue algal coatings, which may result in Bird’s eye porosity (
Boring: Boring process usually affects bivalve shells in the Qom Formation (
The Qom Formation in Rameh section hosts 420 m thin-to-massive limestone, pebble-rich to sandy limestone and marl limestone. This formation is unconformably overlaid and underlaid with siliciclastic deposits of Upper Red Formation and Lower Red Formation. According to field observations and laboratory investigations, the Qom Formation in Rameh section contains 12 facies, occurring in tidal flat, lagoon, shoal and open marine environments in a rimmed shelf carbonate ramp. This formation undergoes various diagenetic processes including dissolution, porosity, cementation, micritization, compaction and dolomitization.
Sardarabadi, S., Jahani, D. and Ghadimvand, N.K. (2016) Facies, Depositional Environment and Diagenesis of the Qom Formation in Rameh Section (Northeastern Garmsar). Open Journal of Geology, 6, 1240-1256. http://dx.doi.org/10.4236/ojg.2016.610091