یافته‌های نوین زمین‌شناسی کاربردی

یافته‌های نوین زمین‌شناسی کاربردی

سنگ­ نگاری و مدل دولومیتی شدن سازند شهبازان در تاقدیس خرم ­آباد، شمال لرستان

نویسندگان
1 دانش‌آموخته دکترا، گروه زمین‌شناسی، دانشکده علوم، دانشگاه لرستان، خرم‌آباد، ایران
2 دانشیار گروه زمین‌شناسی، دانشکده علوم، دانشگاه لرستان، خرم‌آباد، ایران
چکیده
در این پژوهش، روند دولومیتی شدن سازند شهبازان (ائوسن میانی - پسین) با استفاده از مطالعات سنگ­نگاری و زمین­شیمیایی مورد بررسی قرار گرفت. ناحیه مورد مطالعه در شمال لرستان و در 25 کیلومتری شمال شهرستان خرم­آباد واقع شده است. در این برش، سازند شهبازان با سنگ­شناسی غالب سنگ­آهک و دولومیت دارای ضخامت 85 متر می­باشد که مرز پایینی آن به صورت هم­شیب بر روی سازند آواری کشکان و مرز بالایی آن توسط کربنات­های سازند آسماری به صورت ناپیوستگی پیوسته­نما پوشیده شده است. بر پایه مطالعات سنگ­نگاری بر روی 80 مقطع نازک رسوبی و مطالعات ژئوشیمیایی بر روی 26 نمونه به روش (EDS) دو گروه از دولومیـت­ها شناسایی گردید. دولومیت­های اولیه (دلومیکرایت­ها­) با بافت موزائیکی و اغلب بی­شکل در اندازه­های کمتر از 10 میکرون و دولومیت­های ثانویه (دلومیکرواسپارایت­ها و دلواسپارایت­ها) با بافت نیمه شکل­دار تا شکل­دار در اندازه­های بزرگ­تر از 10 میکرون و حتی بزرگ­تر از 250 میکرون مشاهده شدند. در دولومیت­های مورد مطالعه یک روند افزایشی از عناصرFe و Mn و یک روند کاهشی از عنصر Sr از دلومیکرایت­ها به سمت دلومیکرواسپارایت­ها و دلواسپارایت­ها مشاهده می­شود. وجود برخی شواهد مانند تخلخل چشم پرنده­ای، لامینه­های جلبکی و اینتراکلست­ها و نبود کانی­های تبخیری در نمونه­های مورد مطالعه نشان داد، دولومیت­های اولیه در یک مدل جزرومدی تشکیل شده­اند و سپس بر اثر تراوش شورابه­های کف حوضه­ای در پلت­فرم کربناته سازند شهبازان دولومیت­های ثانویه در یک مدل دیاژنزی دفنی کم عمق تا متوسط گسترش یافته­اند. تمرکز نسبتا بالای Na در دولومیت­های مورد مطالعه (1/0تا 4/1 درصد وزنی) بیانگر تاثیر شورابه­های کف حوضه­ای بر روی آن­ها می­باشد. نسبت Ca/Mg در تمام دولومیت­های مورد مطالعه، بیشتر از حد استاندارد (6/1) بوده و دلالت بر غیراستوکیومتری بودن آن­ها دارد. این موضوع را می­توان با توجه به سن سازند شهبازان در این بخش از حوضه­ی رسوبی لرستان توجیه نمود.
کلیدواژه‌ها

عنوان مقاله English

Petrography and dolomitization models of the Shahbazan Formation in Khorram Abad anticline, north Lorestan

نویسندگان English

N. Maleki Sadeghi 1
R. Zarei sahamieh 2
1 Ph. D. (graduated), Dept., of Geology, Faculty of Science, Lorestan University, Khorramabad, Iran
2 Assoc. Prof., Dept., of Geology, Faculty of Science, Lorestan University, Khorramabad, Iran
چکیده English

In this research, the dolomitization process of Shahbazan Formation (Middle-Late Eocene) was investigated using petrographic and geochemical studies. The studied area is located in the north of Lorestan and 25 km north of Khorram Abad city. In this section, the Shahbazan Formation with limestone and dolomite lithology is 85 meters thicknesses, its lower border is on the slope of the Kashkan clastic formation and its upper part is discontinuously formed by the carbonates of the Asmari Formation was covered continuously. Based on petrographic studies on 80 sedimentary thin sections and geochemical studies on 26 samples by EDS method, two groups of dolomites were identified. Primary dolomites (dolomicrites) with a mosaic and often amorphous texture in sizes less than 10 microns and secondary dolomites (delomicrosparites and dolosparites) with a semi-shaped to shaped texture They were observed in sizes larger than 10 microns and even larger than 250 microns.In the studied dolomites, an increasing trend of Fe and Mn elements and a decreasing trend of Sr element can be observed from dolomicrites towards dolomicrosparites and doloasparites. The existence of some evidences such as bird's eye porosity, algal laminae and intraclasts and the absence of evaporite minerals in the studied samples showed that, the primary dolomites were formed in a tidal model, and then the secondary dolomites were expanded in a shallow to medium burial diagenesis model due to seepage of the basin floor in the carbonate platform of the Shahbazan Formation. The relatively high concentration of Na in the studied dolomites (0.1 to 1.4% by weight) indicates the influence of basin floor sediments on them. The ratio of Ca/Mg in all the studied dolomites is higher than the standard limit (1.6) and indicates that they are non-stoichiometric. This issue can be justified considering the age of Shahbazan Formation in this part of Lorestan sedimentary basin.

کلیدواژه‌ها English

Petrography
Dolomite
Geochemistry
Shahbazan Formation
Khorramabad
Lorestan
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  • تاریخ دریافت 08 مرداد 1403
  • تاریخ بازنگری 14 مهر 1403
  • تاریخ پذیرش 21 آبان 1403