Document Type

Honors Project

Comments

Special thanks to Connie Soja, Ray Rogers, and Minjin Chuluun.

Abstract

Mongolia lies at the center of a very complex accretionary terrane and suffers from a lack of research due to its remoteness and limited access. Understanding the many individual terranes in the area is important in piecing together Mongolia’s geologic history. This study aims to contribute new data from a southern Mongolia Upper Ordovician sequence for the purpose of describing depositional conditions during this time period. These data can be combined with other studies to improve understanding of the tectonic processes that resulted in the amalgamation and formation of central Asia.

Magnetic susceptibility and stable carbon isotopes were studied in a sequence of late Ordovician carbonate and clastic strata from the Shine Jinst Area of southern Mongolia where the Daravgai and Gashuunovoo formations are exposed. The rocks at this locality are typical of carbonate shelf deposits and primarily exhibit interbedded carbonate and shale. Six sections capped by faults were measured in this structurally complex area.

Magnetic susceptibility is used as a proxy for sea level and tectonic effects that drive terrestrial weathering and detrital input to the marine system. Preliminary hyteresis measurements identify the primary iron-carrying mineral as hematite. The basal section of the Daravgai Fm. exhibits anomalously high values that average 4.01x10-8 m3/kg. The two overlying sections exhibit values averaging 1.29x10-8 m3/kg and include no major excursions. Missing section in the Daravgai Fm. make climate and sea level interpretations problematic. Meter-scale magnetic susceptibility oscillations were identified in the Gashuunovoo Formation that likely represent transgressive/regressive cycles, which are reflected lithologically as siliciclastic pulses. Significant positive excursions were identified which alternate with low value, relatively stable periods averaging 2.08x10-8 m3/kg. These data can be used in future studies for lateral correlation of other sections near Shine Jinst.

Stable carbon isotope ratios are used to interpret climate trends and attempt regional and global correlation of the section. The Daravgai Fm. exhibits anomalously low carbon isotope values in the basal section averaging -2.79 ‰ followed by relatively stable values averaging 1.61 ‰ in the overlying two sections. The lower part of the Gashuunovoo Fm. exhibits an increasing trend and a major positive excursion averaging 2.32 ‰. This excursion likely corresponds to one of the five major excursions identified worldwide in the late Ordovician, and is tentatively correlated with a lower Ashgill excursion. Carbon isotopic data for the upper sections of the Gashuunovoo Fm. would enhance the dataset and strengthen the possibilities for correlation of the sequence.

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