سازوکارهای دولومیتی شدن سکوهای کربناته ائوسن زاگرس (مثالی از سازند شهبازان، تاقدیس امیران، جنوب لرستان)

نویسندگان

1 دانشیار گروه زمین‌شناسی، دانشکده علوم، دانشگاه لرستان، خرم‌آباد، ایران

2 دانشجوی دکترا رسوب‌شناسی و سنگ‌شناسی رسوبی، دانشکده علوم، دانشگاه بوعلی‌سینا، همدان، ایران و کارشناس آزمایشگاه مرکزی دانشگاه لرستان، خرم‌آباد، ایران

چکیده

سازند شهبازان یکی از واحدهای کربناته ائوسن منطقه لرستان است که در این پژوهش مدل­های دولومیتی آن به روش­های سنگ­نگاری و زمین­شیمی (EDS) بررسی شده است. برش مورد مطالعه به ضخامت 71 متر در 65 کیلومتری جنوب باختری خرم­آباد واقع است و شامل سنگ­های کربناته، سنگ­های کربناته دولومیتی و دولومیت می­باشد. دولومیت­های سازند شهبازان بر اساس بافت به دولومیکرایت، دولومیکرواسپارایت، دولواسپارایت و پرکننده شکستگی­ها تقسیم می­شوند که به استثنای مورد اول، سایر دولومیت­ها ثانویه می­باشند. دولومیت­های نوع اول بیانگر تشکیل در محیط پهنه جزرومدی می­باشند. این دولومیت­ها در بیشتر موارد در انتهای توالی­های به سمت بالا کم عمق شونده دیده می­شوند که نشان دهنده افت سطح آب دریا هستند. دولومیت­های ثانویه در یک محیط دیاژنزی دفنی کم عمق تا متوسط در اثر تراوش شورابه­های کف حوضه­ای تبخیر شده به داخل سکوی کربناته سازند شهبازان شکل گرفته­اند. مقادیر پایین آهن (Fe) و مقادیر بالای استرانسیوم (Sr) و سدیم (Na) در دولومیکرایت­ها و تمرکز بالای Fe و منگنز (Mn) در دولومیـکرواسپارایت­ها و دولواسپارایت­ها به همراه شواهدی مانند لامینه­های جلبکی، اینتراکلست، تخلخل فنسترال و نبود کانی­های تبخیری بیانگر تشکیل این دولومیت­ها از مدل جزرومدی، تراوش و سپس دفن کم عمق تا متوسط می­باشد. مقادیر پایین Sr و تمرکز بالای Fe و Mn در دولومیت­های درشت بلور می­تواند بیانگر افزایش اندازه بلورهای دولومیت و تبلور دوباره­ی آن­ها در طی تدفین باشد. نبود دولومیت­های زین­اسبی احتمالاً بیانگر دخالت نداشتن محلول­ها و سیالات گرمابی در طی فرآیند دولومیتی شدن کربنات­های سازند شهبازان می­باشد.

کلیدواژه‌ها


عنوان مقاله [English]

Dolomitization mechanisms of Eocene Zagros carbonate platforms (an example from Shahbazan Formation, Amiran anticline, south of Lorestan)

نویسندگان [English]

  • A. Jamshidi 1
  • M. Sedaghatnia 2
1 Assoc. Prof., Dept., of Geology, Faculty of Science, Lorestan University, Khorramabad, Iran
2 Ph. D., student sedimentology and sedimentary rocks, Faculty of Science, Bu Ali Sina University, Hamedan, Iran, Expert of the Central Laboratory of Lorestan University, Khorramabad, Iran
چکیده [English]

Shahbazan Formation is one of the Eocene carbonate units of Lorestan region. In present study, dolomitization models this formation were investigated by petrographic and geochemical (EDS) methods. The section under study is 71 m in the thickness and located at 65 km southwest of Khorramabad including carbonate and dolomitic carbonate rocks, and dolomite. The dolomites based on the texture are categorized into dolomicrite, dolomicrosparite, dolosparite, and filling of fractures, which except dolomicrite, the others are secondary. Dolomites of first type represent the formation in the tidal zone environment. In most cases, these dolomites are seen at the end of upwardly shallowing sequences, which indicate a drop in sea level. The secondary dolomites were formed in a shallow to medium burial digenesis environment as a result of seepage of evaporated basin floor sediments into the carbonate platform of the Shahbazan Formation. Low amounts of Fe and high amounts of Sr and Na in dolomicrites, high concentration of Fe and Mn in dolomicrosparites and doloasparites along with evidences such as algal laminae, intraclasts, fenestral porosity and absence of evaporate minerals indicate the formation of this dolomite from the model of tidal, seepage and the following shallow to medium burial. Low values of Sr and high concentration of Fe and Mn in coarse crystal dolomites can indicate the increase in the size of dolomite crystals and their recrystallization during burial. The absence of saddle dolomites probably indicates the non-interference of hydrothermal solutions and fluids during the process of dolomitization of Shahbazan Formation carbonates.

کلیدواژه‌ها [English]

  • Diagenesis
  • Carbonate platforms
  • Dolomitization
  • Shahbazan Formation
  • Lorestan
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