Mehrdad Sardar Abadi

Lower Carboniferous ramp sedimentation of the Central Alborz Basin, northern Iran: integrated sedimentological and rock-magnetic studies

Abstract The Lower Carboniferous Mobarak Formation in the Alborz Basin (northern Iran) was deposited along the northeastern margin of Gondwana in a carbonate ramp setting. This paper focuses on the Tournaisian stratigraphic interval of this formation that crops out at the Jaban section in the southwestern Central Alborz Basin. The following facies associations, representing different ramp palaeoenvironments, have been identified: (1) mudstone–wackestone outer-ramp facies; (2) crinoidal to skeletal grainstone–packstone mid-ramp facies; (3) peloidal to crinoidal grainstone–packstone inner-ramp facies; and (4) coastal facies, which include a variety of microbial laminated to oncoidal grainstones and mudstones with evaporitic pseudomorphs. This ramp profile was affected by frequent storms that were responsible for the formation of several skeletal to non-skeletal shoals in the distal mid-ramp to the most proximal inner-ramp areas. The development of the skeletal to non-skeletal shoals along the sea side of the ramp formed a semi-enclosed lagoon sensitive to the influence of both high tides and storm surges. The magnetic susceptibility (χin) of all the samples was measured and compared with that of the facies from which the sample was taken. There is a clear link between χin and the facies; the average χin values were higher for the distal facies than for the proximal facies. The χin profile of this Lower Carboniferous carbonate sequence reflects stratigraphic variations in response to relative changes in sea level and the input of detrital materials. In the context of the sequence stratigraphic framework, the average χin values for lowstand and transgressive systems tract deposits are higher than for the highstand systems tract deposits. The clear link between χin and facies indicates at least a partly preserved primary χin signal related to the detrital inputs. However, to obtain a better understanding of the nature and origin of the minerals carrying the χin, we performed hysteresis measurements on selected samples. It appears that the χin signal is mainly carried by low-coercivity ferromagnetic minerals such as magnetite, with a mixture of relatively coarse grains (detrital fraction) and ultra-fine grains (probably formed during diagenesis).

Foraminiferal proliferations in the Alborz Basin (northern Iran): global responses to Early Carboniferous glaciations

The Tournaisian interval of the Mobarak Formation in the Alborz Basin (Iran) preserves a specific bed with Earlandiidae and three foraminiferal zones that are restricted to specific intervals within the late Tournaisian and correlate with northern Eurasian biostratigraphic units. The bed with Earlandiidae dates to the early Tournaisian and corresponds with the lower Tournaisian and lower part of the upper Tournaisian of the Russian Stratigraphic Scale. The Granuliferella latispiralis–Latiendothyranopsis zone dates back to the earliest Ivorian (MFZ4?–MFZ5) and correlates with the G. latispiralis and Spinoendothyra costifera zones of the Urals. The Eotextularia diversa zone is of earliest late Ivorian age (MFZ6) and corresponds to the lower part of the E. diversa zone of the Russian Stratigraphic Scale. The Endospiroplectammina venusta–Eoparastaffella ex gr. rotunda zone is of latest Ivorian (MFZ7–MFZ8) age and correlates with the upper part of the E. diversa zone and the E. rotunda zone of the Russian Stratigraphic Scale. The entire early Tournaisian (Hastarian) portion is devoid of recognisable foraminiferal material, which is likely linked to a faunal shift of subtropical and temperate taxa to tropical latitudes in response to the glaciations at the Devonian–Carboniferous boundary. The establishment of the G. latispiralis–Latiendothyranopsis zone coincides with the first mondial Tournaisian foraminiferal radiation. The second and third episodes of foraminiferal diversification (E. diversa and E. venusta–Eoparastaffella ex gr. rotunda) are congruent with major foraminiferal shifts from Tethyan realms to higher latitudes in response to thermal periods. The occurrence of specific foraminiferal taxa in Alborz is strongly linked to transgressions and migrations of North Palaeotethyan biotic elements. The described Tournaisian cyclic patterns in the Alborz Basin share significant similarities with those in the North American, western European and Siberian realms, indicating a link with large-scale palaeoclimatic patterns. This cyclic system correlates directly with the pacing of global eustatic sea-level fluctuations caused by climate oscillations and follows the fourth-order ocean-level fluctuations as described from other, independent proxies.

Diagenetic factors controlling reservoir quality in the Faraghan Formation (Lower Permian), Darang Field, Southern Iran

The role of diagenesis in affecting (reduction or enhancement) reservoir quality in the Faraghan Formation in Darang #1 well was investigated. The Faraghan Formation is mainly composed of sandstone facies along with minor mudstone and siltstone and grades upward into mixed carbonate –siliciclastic facies and then to Dalan Formation carbonates. Depositional environment of this formation comprises various sub-environments of a delta setting in the lower part of the formation which grades upward into a shallow marine clastic environment in the upper part. The processes enhancing reservoir quality include dissolution of carbonate grains and cements and alteration of feldspar grains. Burial history diagram shows that after deposition during Lower Permain, the formation underwent a rapid burial up to 1000 meters below sea level in Upper Permian. Then a slight uplift (about 100 meters) and gradual burial followed up to Mid-Jurassic. Afterwards the formation experienced a series of rapid and moderate uplift from Mid-Jurassic to Tertiary when the formation uplifted to around 2500 meters above sea level. The suitable conditions resulting in dissolution took place in shallow buried sandstones. These conditions occurred two times during burial history: at the early stages of eodiagenesis and during telodiagenesis. The major processes deteriorating reservoir quality include compaction and cementation. Major cement types include carbonate cements (dolomite, siderite), clay cements (kaolinite, sericite, chlorite), silica cement and pyrite cement. The most abundant cement is the carbonate cement especially dolomite. The dolomite cement occurs as intergranular and poikilotopic forms. The most frequent clay cement is sericite which dominates in sitstones and lithicarenites. Silica cement, where present, fills all the pore spaces. The least frequent cement type is pyrite which is found in two forms of poikilotopic and framboidal. Where it is found in the form of poikilotopic cement, it massively fills the pore spaces and deteriorates the porosity and permeability of the sandstones.