Abstract:

The carbon and oxygen isotopes of marine carbonate rocks exhibit the inherent capacity to faithfully record the primary isotopic composition of seawater, serving as invaluable proxies for paleoenvironmental reconstructions. Based on this, this study systematically investigates the carbon and oxygen isotopic profiles containing 86 carbonate rock specimens retrieved from the Lower Ordovician strata of well Gucheng 8, situated in Gucheng lower uplift of the Tarim Basin. The results show δ¹³C values ranging from -2.32‰ to +0.53‰ , with an average of -1.10‰ , delineating four discernible stages. Simultaneously, the δ¹⁸O values span -8.80‰ to -7.15‰ , averaging at -8.04‰ , revealing three distinct“positive ten-negative shift”cycles. The computed paleoenvironmental characteristics, based on carbon and oxygen isotopes, underscore a salinity index (Z) consistently exceeding 120 during the Early to Middle Ordovician epoch in the study area. Concurrently, water temperatures exhibit fluctuationsbetween 16.7 ℃ and 26.9 ℃ , indicating an overall warm to hot marine environment with normal salinity levels. Isotopic variations further unveil nuanced sea-level dynamics. During the Penglaiba formation, the ancient sea level underwent a gradual rise followed by a subdued fall. In the Yingshan formation, sea levels exhibited continuous fluctuations with a gradual, marginal decline. Conversely, the Yijianfang formation documents a rapid rise in the ancient sea level, maintaining a sustained high position. These fluctuations closely parallel the global sea-level trends. Comparative scrutiny of δ¹³C curves across multiple Early to Middle Ordovician profiles (drilling wells) within the basin underscores analogous characteristics and variations. This concordance with global δ¹³C curves signifies that sedimentary patterns in the Tarim basin during the Early to Middle Ordovician were primarily dictated by sea-level changes, with limited influence by the peripheral orogeny and other tectonic activities.

Key words: Gucheng area, Lower and Middle Ordovician, carbon and oxygen isotopes, paleoenvironment