Identification and Description of Parasequences in Upper May Member,Central Ordos Basin

Key words: 
• Central Ordos Basin /
• Upper May Member /
• parasequence /
• characteristies

Abstract: Based on various information,total 8 allocyclic parasequences are identified in Upper May Member,Central Ordos Basin and most of them are in good coincidence with the Gamma Ray logging curves. In this paper, each parasequence is described in detail. The upper parts of these meter-scale parasequences are mostly deposited in dolomite flat enriched with gypsum,which abound in various evidences of durable exposure. Controlled by sea-level fluctuation,δ¹³C values in each parasequence ascend and follow by reduction. As shown in parasequence No. 6,δ¹³C value varies upward from -3.O‰ to -0.7‰,and finally reaches to -1.7‰. But because each parasequence is located at the different part of the third-order sequence,the averages of δ¹³C values may change. These changes in higher-order sequence show a similar rule as that in parasequences. Different levels of sea-level fluctuation shaped sequences of different levels or even parasequences,thus indicated the similar rules in sequences and parasequences. On the basis of numerous test data, the content of gypsum and salt precipitated in different period of the formation of parasequences seem to change regularly, leading to the porosity and content of remnant gypsum steadily increase. While content of material which can not be dissolved in acid gradually descend,as it was mainly constituted by mud and organic matters. High porosity zone mostly distributes at the top or upper part of exposed parasequences.