فهرست مطالب مصطفی قماشی

The Ravar terrane, is situated at the south of the Tabas block. At the Ravar series Mesozoic strata, especially the Jurassic carbonate- evaporite deposits known as Pectinid limestone, Magu gypsum and Bido red evaporates are the most important parts of this deposits and have a variety of facies, distribution, and suitable exposure. Upper Jurassic Sequence stratigraphy of the mentioned deposits were messed up and even their position on the geological maps are not coherently elaborated. So investigation on the sedimentary environment and tectonic controller factors will be a great help in understanding the sequence stratigraphy position of these deposits. Thickness of mostly shale, interbedded Pectinid limestone, gypsiferous marl, red gypsum to white and finally red evaporates sequences were measured 470, 725, 50, more than 1450 (possibly contains Hojedk and Baghamshah Formations), and 1300 m, respectively. Eleven indicator facies were identified in the studied deposits which ended in tidal flat, lagoon, shoal and open marine belts. These facies illustrate environmental characteristics of carbonate-evaporite sequences. The facies changes, display the shallow carbonate homocline ramp setting.

After studying satellite images and field surveys in the Ravar region of Kerman, 5 sections of Lakarkuh-1 and 2, Horjond, Khorand and East of Bahabad were selected as the most suitable geological sections. Total of 68 thin sections from four sections of LakarKuh-1 and 2, Horjand and Khorand were examined by petrography. Of course, it is worth mentioning that no samples were taken from the Bahabad section and these were examined only to compare the thickness and lithological facies and especially evaporite deposits.

After extensive field surveys, five geological sections with the most complete outcrops were finally selected. The reason for the decrease in the thickness of evaporative deposits from 1450 m in the Khordand, 1300 m in the Bahabad, 725 m in the Horjond, 450 m in Lakarkuh-1 and 50 m in LakarKuh-2 can probably be due to the local changes in sea water levels and availability of more evaporative conditions. In order to identify and interpret gypsum and anhydrite facies and related fabrics, several sections have been selected in the Lakarkuh and Horjond areas. First, after introducing field photos, some of the important identified fabrics were examined. The microfacies investigation of 4 sections has led to the identification of 11 microfacies with the acronyms A, B, C and D, which belong to the 4 facies belts of intertidal, lagoon, shoal and open sea environments. Finally, according to the identified facies, a sedimentary environment model was presented.

The unusual and high thicknesses of gypsum in the Ravar region often have a tectonic origin and are mainly related to the formation of disharmonic folds. Evidence such as single nodules, enterolytic, and fenestral textures in evaporites show that these factories formed simultaneously with deposition or in the early stages of diagenesis. Three facies’ belts including intertidal zone facies (A1, A2 and A3 facies), high tidal zone facies (A4 and A5), and lagoon (B1) have been identified. The sedimentary model presented for the studied deposits in the shallow parts of a carbonate-evaporative system, which was probably a hemoclinical ramp. Examination of evaporitic facies in Lakarkuh and Horjond regions indicates that these facies are often deposited in sabkha and intertidal and lagoon environments.

The Oxfordian – Kimmeridgian rocks of the Ravar series and the Bido Formation with the age of Upper Jurassic, are distributed in the south of Tabas Block. The most important diagenetic processes in the studied thin sections are namely hematitization, micritization, dolomitization, cementation, evaporite nodules and compaction. Interpretation of paragenetic sequences of studied samples show the effect of different processes in the marine, meteoric and burial diagenetic environments. Major and minor elements and comparison with previous studies, indicate that aragonite was an original carbonate mineralogy for the Upper Jurassic carbonate rocks in the Ravar area. Low Sr / Mn and Sr/ Ca versus Mn show higher alteration in the studied samples which mainly show an open diagenetic system with high water- rock interaction.

After field observations, 78 samples of carbonate rocks were selected from four geological sections of north of Lakar- Kuh -1, 2, Horjond, Baghamshah, and Khorand. After microscopic observations, 21 carbonate samples from studied intervals were selected for elemental investigations in the geochemistry lab of the Shahid Beheshti University. In order to determine original carbonate mineralogy, these data were compared with Mozduran Formation carbonates, Gordon warm water tropical limestones of Tasmania, the Ilam, Fahliyan Formations and Qaleh- Dokhtar carbonates with original aragonite mineralogy. Low Sr/Mn ratio show high water – rock interaction.

Activity of microscopic organisms in these environments is associated with micritization in the lagoon environments. Bladed cement forms in the marine environments and equant, blocky and drusy calcite cements are formed in meteoric and mainly in burial environments. With increasing depth of burial, chemical compaction has formed dissolution seams and stylolite. Hemititization and dolomitization processes are formed in the late diagenetic stages.
Geochemistry of major elements (Na, Sr, and Mn) such as high Sr/Na ratio with the average of > 2 show aragonite was an original carbonate mineralogy. High Mn values suggest the effect of meteoric diagenesis on the studied samples. Higher Sr values in some of the samples may indicate original aragonite mineralogy and low impact of diagenetic processes. The high Sr/Ca and Mn variations in the studied carbonates represent the effect of meteoric diagenesis in closed and open diagenetic systems.

Sr/Mn versus Mn variations in samples fall within and very close to aragonite field of the Mozduran Formation due to higher impact of meteorite diagenesis. This probably shows the effect of meteoric diagenesis with higher water - rock interaction (W/R) in an open diagenetic system. Relatively low Sr/Mn values show an open diagenetic system for studied carbonate samples. The Sr/Ca ratio in the studied carbonates show a relatively open diagenetic system with high water- rock interaction.

The Ravar zone, a wedge-shaped terrane, situated at the southern Tabas block, is confined between two main boundary faults, the Nayband fault in the east and the Kuhbanan fault in the west.  The Mesozoic strata, especially the Jurassic layers, have a variety of facies, distribution, and exposure. Structural restoration and stratigraphical correlation between the Jurassic layers on the basis of the field and lab studies in the Ravar zone, brought new result.  The stratigraphy, structure, and magmatism of the middle Jurassic to late Cretaceous successions are similar to the deposits which have been evolved in the continental rift zone is being transited to a passive margin.  The sedimentary successions are classified into two groups; the clastic deposits of the Hojedk and Baghamshah Formations, and carbonate-evaporate of the Pectinid layers, and alkali basalts of the Ravar Series which are considered as the syn-rift series. The second group composed of lower Cretaceous red marl and evaporate as well as upper Cretaceous marine limestone well-thought-out as the post-rift series. These two series are separated by pronounce angular unconformity and basaltic volcanic rocks in the Ravar zone. Deepening towards the west, indicates that the main ocean, in the Cretaceous time, must be at the west side, somewhere at the west of the Tabas block. The Nain and Dehshir-Baft ophiolites could be the best candidates for the remnants of this ocean.

The Sorkh Shale Formation (Lower Triassic) crops out in parts of Tabas Block. This formation is composed of limestone, dolomite, sandstone, shale and gypsum. Its lower contact with the Permian rocks and its upper contact with the Shotori Formation is marked by distinct erosional unconformity. To determine the depositional environments of the Sorkh Shale Formation, stratigraphic sections of Godar-e-Sorkh (southeast of Tabas), Godar-e-Gachal (southwest of Tabas) and Islam-Abad (southeast of Zarand) have been studied. Field and laboratory studies have shown that the Sorkh Shale Formatin consists of carbonate (lagoonal to tidal flat environments), silisiclastic (lagoonal, tidal flat and coastal plain environments) and evaporite (playa environment) facies. Both carbonate and siliciclastic storm deposites (tempestite) are also recognized. The Sorkh Shale Formation have been formed in a homoclinal clastic-carbonate ramp similar to the peresent day Persian Gulf. The Sorkh Shale succession in the Tabas Block indicates a depositional sequence (3rd_order cycle) bounded by type 1 unconformities. Northward increase in carbonate content, northward paleocurrent directions and higher thickness of the Lower Triassic rocks indicate that the Sorkh Shale Formation was deposited in a failed rift along the Kalmard/Kuhbanan and Nayband Faults.