Abstract
Authigenic grain-coating chlorite is widely distributed in the clastic rocks of many sedimentary basins around the world. These iron minerals were mainly derived from flocculent precipitates formed when rivers flow into the ocean, especially in deltaic environments with high hydrodynamic conditions. At the same time, sandstone sequences with grain-coating chlorites also tend to have relatively high glauconite and pyrite content. EPMA composition analysis shows that glauconites with “high Al and low Fe” content indicate slightly to semi-saline marine environments with weak alkaline and weakly reducing conditions. By analyzing the chlorite-containing sandstone bodies of the southern Sichuan Xujiahe Formation, this study found that chlorite was mainly distributed in sedimentary microfacies, including underwater distributary channels, distributary channels, shallow lake sandstone dams, and mouth bars. Chlorite had a tendency to form in the upper parts of sandstone bodies with signs of increased base level, representing the influence of marine (lacustrine) transgression. This is believed to be influenced by megamonsoons in the Middle and Upper Yangtze Region during the Late Triassic Epoch. During periods of abundant precipitation, river discharges increased and more Fe particulates flowed into the ocean (lake). In the meantime, increases or decreases in lake level were only affected by precipitation for short periods of time. The sedimentary environment shifted from weakly oxidizing to weak alkaline, weakly reducing conditions as sea level increased, and Fe-rich minerals as authigenic chlorite and glauconite began to form and deposit.
Highlights
Grain-coating chlorite, known as pore-lining chlorite, is widely distributed in many sedimentary basins around the world
The Yanchang Formation in the Ordos Basin was deposited in a continental sedimentary environment, the Upper Jurassic of the Songliao Basin is from a marine sedimentary environment, and the Xujiahe Formation in the Sichuan Basin is in a hybrid marine–continental sedimentary environment
Authigenic chlorite in Xujiahe Formation sandstones has the following characteristics: (1) Because chlorite had already developed between grain contacts, it can be concluded authigenic chlorite formed on grain surfaces before mechanical compaction; (2) authigenic chlorite growths were found on the surface of quartz overgrowths and silica cement which filled in intergranular pores, indicating that authigenic chlorite may have formed after silica cementation; (3) chlorite was not found in intergranular dissolved pores or mouldpores, while chlorite has been found on the surface of feldspar grains with dissolved pores, suggesting authigenic chlorite formed before dissolution
Summary
Grain-coating chlorite, known as pore-lining chlorite, is widely distributed in many sedimentary basins around the world. The Yanchang Formation in the Ordos Basin was deposited in a continental sedimentary environment, the Upper Jurassic of the Songliao Basin is from a marine sedimentary environment, and the Xujiahe Formation in the Sichuan Basin is in a hybrid marine–continental sedimentary environment All of these sandstones were deposited in high salinity environments. Grain-coating chlorite formation was thought to be mainly controlled by sedimentary facies and formed in high hydrodynamic conditions and weak alkaline environments with abundant Fe and Al sources (Huang et al 2004). The study examines the characteristics of paragenetic minerals (glauconite, pyrite) and the vertical distribution patterns of authigenic chlorite in various sandstone bodies to study the formation environment, genetic mechanism, and other factors which affected the distribution of authigenic grain-coating chlorite
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