در مطالعۀ حاضر، توالیهای ریزشونده بهسمت بالا در سازند کربناته- تبخیری کنگان، در بخش مرکزی خلیج فارس بررسی شدند. این رسوبات روند کلی توالی تمپستایت را نشان میدهند. مقایسۀ این توالیها با نهشتههای حاصل از سایر سازوکارهای موجود برای تشکیل رسوبات دارای دانهبندی تدریجی نشان میدهد این رسوبات حاصل وقوع طوفان در زمان رسوبگذاری سازند کنگان در منطقۀ مرکزی خلیج فارس هستند. بررسیها نشان میدهند این توالیها در شیب کم محیط رمپ کربناته تشکیل شدهاند و ساختهای ریزشی در آنها مشاهده نمیشوند؛ جورشدگی خوبی در بخشهای میانی این نهشتهها مشاهده میشود. مقیاس این نهشتهها در حد چند سانتیمتر تا چند ده سانتیمتر است و رسوبات آواری در آنها مشاهده نمیشوند. تکرار درخور توجه این توالیها به همراه دلایل یادشده در بخشهای میانی تا بالایی سازند کنگان، سازوکار طوفانی را برای تشکیل آنها تأیید میکند. بررسی این نهشتهها به شناخت شرایط آبوهوایی ابتدای تریاس در حوضۀ خلیج فارس کمک بسیاری میکند.
عنوان مقاله [English]
Analysis of carbonate tempestite deposits of the Kangan Formation in the central Persian Gulf
This study focuses on fining-upward sequences of carbonate-evaporite series of the Kangan Formation in the central Persian Gulf area. The studied sequences shows a typical succession of tempestite deposits. Comparison of these units with sedimentary deposits of other graded bedding forming mechanisms, showed that these strata have been formed during the occurrence of storms at the time of Kangan deposition in this region. Formation of these units in shallow depths of gently sloped ramp setting, well sorting especially at the middle of the unit, absence of slumping related structures in these sediments, centimeter to decimeter scale, lack of terrigenous sediments and their considerable repetition in middle to upper parts of the Kangan Formation confirms the role of storms in their formation during lower to middle Triassic in the Persian Gulf Basin. Study of these strata could be a great help for understanding the climate change at the Triassic time in Persian Gulf Basin.
Event stratigraphy have been considered in many studies and many models have been proposed for their sediments deposition. The Bouma sequence is the most well-known model for these deposits which is the result of turbidity currents. There are not many studies about such deposits in carbonate formations. A total of four models have been proposed for deposition of fining-upward sequences in carbonates. They include 1) turbidity currents by rivers which could not be used for carbonates as they are not affected by runoffs. 2) Tempestites in carbonate formations. The term introduced by Aigner in 1982 and now has a wide application in carbonate storm deposits. 3) Slumping and their resulted local fining-upward sequences. 4) Internalites which are the result of wave breakdown in mid to outer ramp settings. The former have been introduced recently but their use in interpretation of fining-upward sequences is increasing in recent years. With respect to each model mechanism, the effective factors and so the depositional situations could be reconstructed. Tempestites have been frequently formed in the Triassic due to the warm and arid climate. The Persian Gulf basin was in 10 degree south in that time. So, the Kangan Formation was selected for more considerations to find possible tempestite deposits. This study focuses on the event deposits in this formation using cores and thin sections’ data. With respect to the reservoir quality of this formation in our country, determination of its sedimentary environment and processes are so important.
Material & Methods
Macroscopic studies was carried out on 540 m cores of the Kangan Formation in three wells. A total of four plugs prepared from each meter of core. Thin sections were prepared from trims of plugs. One third of each thin section stained with Alizarin Red-S to distinguish calcite from dolomite (Dickson 1965). Facies named according to Dunham classification (Dunham 1962).
Discussion of Results & Conclusions
Fenestral mudstone, stromatolite boundstone and anhydrite have been deposited in peritidal environment. These deposits have unique structures such as bird’s eye or anhydrite nodules. Massive to layered anhydrites shows deposition in warm and dry climate sabkha environment. Mudstones associated with anhydrites have no allochems. The facies have been dolomitized in most cases. Dolomites are fine and xenotopic. Stromatolite boundstone exhibit fine lamination and fenestral voids. Thrombolite has been formed in the base of the formation after the Permian–Triassic mass extinction.
The lagoon environment has low energy and limited water circulation. Low diversity of fauna, lack of exposure related structures, frequency of micrite and peloids denote to lagoon environment. Both small and large allochems are visible. Bioturbation and micritization are obvious.
Ooid and peloid grainstones build the main part of the shoal depositional setting. These facies have been deposited in high energy environment which is obvious from sand size ooids, lack of lime mud (micrite) and well sorting. Cross-bedding could be seen on core samples. Intraclast/bioclast grainstone is related to sea-ward shoal sub-environment.
Gradual change in facies and lack of a complete build-up reveal that these facies have been deposited in a ramp setting. There are also no slumping related structures.
Core and thin sections’ studies show that the studied sequences begin with an erosional surface. Erosional related evidence including rip-up clasts and flame structures are evident. The basal facies show clear fining-upward sequence. Toward the top, cross-laminations are visible which have been developed within the well sorted grainstones. There are no silt or fine sand size particles in this sequences and so the parallel lamination never developed at the topmost intervals. The uppermost facies are consist of massive micrites with anhydrite cement or nodules.
Tempestite deposits are developed in shallow to deep carbonate and siliciclastic environments. They are thicker with larger size particles in shallow settings. They are also more mature with basal erosional surface in such environments. The fining-upward sequence and erosional surface show that the environmental energy decreases after a sudden increase. Presence of anhydrite shows that the environment was arid and warm during deposition of these units.
Results of this study show that there have been many storms in present Persian Gulf location at Early and Middle Triassic time. The resulted sequence starts with an erosional surface and continues with parallel lamination in high current regime. The uppermost units could not be distinguished and have a massive structure due to high micrite content. The main difference of these sequences with turbidites lies in their mechanism. Their high frequency and shallow environment, lack of slumping related structures and terrigenous sediments as well as low clay content and high frequency of intraclasts and pleoids demonstrate a storm mechanism for their deposition. The result is tempestite deposits in the Kangan Formation.