The petrography and geochemistry of clastic rocks from the Upper Cretaceous Terani Formation of the Cauvery Basin, Southern India

Abstract

The petrography and geochemistry of clastic rocks from the Upper Cretaceous Terani Formation of the Cauvery Basin were studied to decipher their intensity of weathering, provenance, and tectonic history. Texturally, the Terani sandstones are moderately sorted with sub-angular and sub-rounded grains, indicating short transport and low maturity. The average Quartz–Feldspar–Rock fragment (Q–F–R) ratio of the Terani sandstone is Q89–F3–R8. Geochemically, the Terani clastic rocks are classified as sublitharenites, Fe-sand, shale, and Fe-shale types. The chemical index of alteration (CIA), plagioclase index of alteration (PIA), and chemical index of weathering (CIW) suggested moderate to high intensity of weathering in the source area. The enrichment of rare earth element (REE) contents in the Terani clastic rocks relative to UCC (Upper Continental Crust) indicates a higher concentration of heavy minerals. Likewise, the average values of Eu/Eu* (0.16), La/Sc (2.94), La/Co (2.15), Th/Sc (1.08), Th/Co (0.79), Th/Cr (0.12), and Cr/Th (8.39) revealed that the Terani clastic rocks were derived from a combination of felsic and intermediate source rocks. The chondrite normalized REE patterns of clastic rocks are characterized by a relatively flat HREE (Gdcn/Ybcn = 1.71), enriched LREE (Lacn/Smcn = 4.15), and negative Eu anomaly (Eu/Eu* = 0.16), which suggest the contribution of sediments with less HREE depleted source rocks from the Archaean group. A comparison of the REE pattern and Eu anomalies from this study with potential source rocks infers that the Terani Formation received a major contribution of sediments from the Dharwar Craton.