پراکندگی زمانی و مکانی نهشته‎‍های آذر- آواری سازند قم، با تاکید ویژه بر جنوب شرقی استان کرمان: کاربرد آ‌نها در چینه‎‍نگاری رخدادی (تفرا)

سازند قم (الیگوسن میوسن پیشین) از توالی ضخیمی از مارن‎‍های دریایی، سنگ‌های کربناته، ژیپس و سیلیسی آواری تشکیل شده است. وجود همراهان آذرین، بارزترین تغییر رخسارۀ سازند قم است. این مقاله، اولین مثال منتشرشده از کاربرد چینه‎‍نگاری رخدادی (تفرا) در سازند قم و همچنین نخستین مطالعۀ جامع بر پراکندگی زمانی و مکانی نهشته‎‍های آذر آواری سازند قم و شامل دو بخش است. در ابتدا دربارۀ نهشته‎‍های آذر آواری سازند قم در دو برش بزدان و کقارکی در جنوب شرقی استان کرمان بحث می‌شود. آهک‌های توفی و واحدهای توفی هر دو برش بزدان و کقارکی، سن روپلین پسین را دارند که بیانگر وقوع یک رویداد آتشفشانی در منطقۀ شمال غربی دریاچۀ جازموریان در آن زمان است. در بخش دوم، دربارۀ پراکندگی زمانی و مکانی نهشته‎‍های آذر آواری سازند قم در سرتاسر قلمرو این سازند بحث شده و پراکندگی مکانی آ‌نها بر یک نقشۀ جدید نمایش داده شده است. واحدهای آذر آواری موجود در سازند قم در مناطق مختلفی از قلمرو این سازند (از جازموریان تا ماکو) گسترش درخور ‌توجه و منظمی دارد. به‌طور کلی می‌توان گفت این واحدها عمدتاً در سه منطقۀ زیر گسترش دارند: 1) منطقۀ جازموریان جیرفت سیرجان (پیش‌کمان)؛ 2) جنوب غربی نوار نطنز کاشان قم ساوه (عمدتاً درون‌کمان) و 3) منطقۀ زنجان (جنوب شرقی، جنوب، جنوب غربی، غرب و شمال غربی زنجان). ضخامت و سن این واحدها در برش‌های مختلف متفاوت است.

Temporal and spatial distribution of the Qom Formation pyroclastic deposits with special emphasis on SE Kerman Province: Their application in (tephra) event stratigraphy

AbstractThis article is the first published example of the application of (tephra) event stratigraphy in the Qom Formation as well as the temporal and spatial distribution of the Qom Formation pyroclastic deposits. The presence of igneous (volcanic) associations is the most obvious facies change of the Qom Formation. The tuffaceous limestones and tuff units of both Bozdan and Kagharaki sections (in the southeast of Kerman province) are late Rupelian in age, indicating the occurrence of a volcanic event in NW Jazmourian at the same time. The pyroclastic deposits of the Qom Formation have a significant and regular extent in different areas of the formation realm (from Jazmourian to Mako). In general, it can be said that the pyroclastic units of the Qom Formation are mainly distributed in the following three areas: 1) Jazmourian–Jiroft–Sirjan (forearc); 2) Southwest of the Natanz–Kashan–Qom–Saveh belt (mainly in intraarc); and 3) Zanjan region (SE, S, SW, W, and NW of Zanjan). The thickness and age of these deposits vary in different sections.Keywords: Qom Formation, Pyroclastic, Event stratigraphy, Tuff, Oligo–Miocene.   IntroductionThe Qom Formation (Oligocene–Early Miocene) comprises thick successions of marine marls, limestones, gypsum and siliciclastics that were deposited on extensive mixed carbonate–siliciclastic ramps (Reuter et al. 2009), in the Sanandaj–Sirjan forearc basin, Urumieh–Dokhtar magmatic arc (Intraarc basin) and Central Iran backarc basin (Mohammadi et al. 2013, 2015, 2019, Mohammadi and Ameri 2015; Mohammadi 2022). The presence of igneous (volcanic) associations is the most obvious facies change of the Qom Formation. Carbonate deposition of the Qom Formation took place under conditions of periodic to almost continuous influx of pyroclastic material. The presence of pyroclastic material throughout some stratigraphic sections indicates that at the time of deposition an islandarc volcanic complex was active nearby (Okhravi and Amini 1988). This article is the first published example of the application of (tephra) event stratigraphy in the Qom Formation and also the first comprehensive study on the temporal and spatial distribution of the Qom Formation pyroclastic deposits. Although geological investigations on the Qom Formation started more than 160 years ago, surprisingly little information is available concerning the pyroclastic deposits of the Qom Formation. The aims of this study are therefore to: 1) investigate the relative age of the pyroclastic deposits of the Qom Formation in the southeastern Kerman province; 2) investigate the temporal and spatial distribution of the Qom Formation pyroclastic deposits; 3) present a map showing the distribution of the pyroclastic deposits of the Qom Formation; and 4) use the pyroclastic units as a key unit in event stratigraphy. Material MethodsThis article consists of two parts. First, the Qom Formation pyroclastic deposits in two stratigraphic sections of Bozdan and Kagharaki (in the southeast of Kerman province) are discussed. Second, the temporal and spatial distribution of the Qom Formation pyroclastic deposits is discussed.Bozdan and Kagharaki (in the southeast of Kerman province) were studied bed by bed. Thin sections were prepared from lithified hard samples, while unconsolidated marly samples were washed, sieved, and screened for foraminifera contents. A total of 105 and 46 samples were collected from the Bozdan and Kagharaki section, respectively. All samples were studied in detail and particular attention has been paid to foraminifera. The age of the study sections as well as the age of the tuff/tuffaceous units was determined based on the vertical distribution of foraminifera, Qom Formation biozonation by Mohammadi (2022) and also the stratigraphic position. To accomplish the second part of the article, almost all related articles are reviewed and analyzed.   Discussion of Results Conclusions The Bozdan section is located 35 km northeast of south Roudbar City. Its lower contact with Eocene volcanic rocks is well exposed. Outcrops of the Qom Formation in the Bozdan section, 440 m thick, consist mainly of medium to thickbedded and massive limestones, reefal limestones, marls, tuffaceous limestones, and tuffs. A tuff unit of90 m thickness is intercalated in the lower part and separates the lower (30 m) and upper carbonate unit ( 320 m).The Kagharaki section is located in the southeast of Jiroft City. In the Kagharaki section, the tuff and tuffaceous layers are present merely in the lower 56m which consist mainly of medium to thickbedded and massive limestones, reefal limestones,tuffaceous limestones and tuffs.The presence of Nummulites in the tuffaceous limestones of the Kagharaki and Bozadan sections, as well as in the tuff unit of the Bozadan section is indicative of contemporaneous volcanic activity in the late Rupelian.The spatial distribution of the Qom Formation pyroclastic deposits is shown on a new map.In general, it can be said that the pyroclastic units of the Qom Formation are mainly distributed in the following three areas: 1) Jazmourian–Jiroft–Sirjan (forearc); 2) Southwest of the Natanz–Kashan–Qom–Saveh belt (mainly in intraarc); and 3) Zanjan region (SE, S, SW, W, and NW of Zanjan).In general, the pyroclastic units of the Qom Formation are absent in the following areas: 1) 1) in the north and northwest of Abadeh; 2) in the areas where the evaporite deposits of the Qom Formation are present; 3) Hamadan–Sanandaj–South Bijar area; and 4) southeast, south and southwest of Urumieh Lake.