S. Ramasamy

Geochemistry of Sandstones from the Upper Miocene Kudankulam Formation, Southern India: Implications for Provenance, Weathering, and Tectonic Setting

ABSTRACT Petrographic, major, trace, and rare earth element compositions of sandstones from the upper Miocene Kudankulam Formation, Southern India, have been investigated to determine their provenance, tectonic setting, and weathering conditions. All sandstone samples are highly enriched in quartz (Q) but poor in feldspar (F) and lithic fragments (L). The major-element concentrations of these sandstones reveal the relative homogeneity of their source. Geochemically, the Kudankulam sandstones are classified as arkose, subarkose, litharenite, and sublitharenite. The CIA values (chemical index of alteration; mean value 44.5) for these sandstones and the A-CN-K diagram suggest their low-weathering nature. Similarly, their Fe2O3* + MgO (mean 2.7), Al2O3/SiO2 ( 0.09), K2O/Na2O ( 2.2) ratios and TiO2 contents ( 0.3) are consistent with a passive-margin setting. The Eu/Eu* ( 0.5), (La/Lu)cn ( 21), La/Sc ( 5.9), Th/Sc ( 1.9), La/Co ( 5.7), Th/Co ( 1.8), and Cr/Th ( 5.3) ratios support a felsic source for these sandstones. Chondrite-normalized REE patterns with LREE enrichment, flat HREE, and negative Eu anomaly also are attributed to felsic source-rock characteristics for Kudankulam sandstones. Total REE concentrations of these sandstones reflect the variations in their grain-size fractions. The source rocks are probably identified to be Proterozoic gneisses, charnockites, and granites of the Kerala Khondalite Belt, which must have been exposed at least since the late Miocene. Finally, the unusual Ni enrichment in the Kudankulam sandstones, unaccompanied by a similar enrichment in Cr, Co, and V, may be related to either the presence of pyrite in the sandstones or, more likely, the fractionation of garnet from the source rocks during transportation.

Geochemistry of Upper Miocene Kudankulam Limestones, Southern India

Concentrations of major, trace, and rare-earth elements (REE) were measured in shallow marine limestone samples of the upper Miocene Kudankulam Formation, southern India, in order to investigate the geochemical variations among various litho-units. The CaCO3 content is higher in algal limestone (AL; 92 ± 1, n = 3) and clastic limestone (CL; 90 ± 2, n = 3) than sandy shell limestone (SSL; 81 ± 1, n = 3). All trace elements exhibit lower concentrations than post-Archean Australian Shale (PAAS) values, except one SSL sample. Large variations in ΣREE content are observed among CL, SSL, and AL (~14-142, ~68-124, and ~38-98, respectively). Almost all limestone samples analyzed from the Kudankulam Formation show a small negative cerium anomaly (Ce/Ce* ~0.8-0.9), except one AL sample, which lacks this cerium anomaly (Ce/Ce* ~1.04). Variations in Ce anomalies and ΣREE contents in Kudankulam limestone samples are mainly controlled by the amount of terrigenous sediments and diagenetic behavior. Shale-normalized REE patterns and (La/Yb)s, La/Sc, La/Th, and Th/Sc ratios suggest that the detrital sediments present in the limestones were probably derived from felsic source rocks. The observed low content of U (0.9 ± 0.5, n = 9) and U/Th (0.2 ± 0.1, n = 9) ratio in these limestones are probably related to an oxygen-rich environment.

Carbon and Oxygen Isotopic Signatures in Albian-Danian Limestones of Cauvery Basin, Southeastern India

Abstract The Albian-Danian limestones of Cauvery Basin show a wide range of d13C and d18O values (–13.2 to +1.1% and –9.0 to –2.5%, respectively). The cement samples show negative carbon and oxygen isotope values (–18.9 to –3.9% and –9.0 to –4.3%, respectively). The petrographic study reveals the presence of algae, molluscs, bryozoans, foraminifers and ostracods as major framework constituents. The limestones have microspar and equant sparry calcite cements. The pore spaces and vugs are filled with sparry calcite cement. The bivariate plot of d13C and d18O suggests that most of the samples fall in the freshwater limestone and meteoric field, while few samples fall in the marine limestone and soil calcite fields. The presence of sparry calcite cement, together with negative carbon and oxygen isotope values, indicates that these limestones have undergone meteoric diagenesis.

Carbon, oxygen and strontium isotopic signatures in Maastrichtian-Danian limestones of the Cauvery Basin, South India

A petrographic, carbon, oxygen and strontium study of the carbonate succession of the shallow marine Kallankurichchi and Niniyur formations of the Cauvery Basin, Tamil Nadu, India was conducted to understand the isotopic variations in seawater during Maastrichtian-Danian. The limestones from both the Kallankurichchi and Niniyur formations show large variations in Mn and Sr concentrations and high Mn/Sr ratios indicate alterations of primary isotopic signatures during shallow burial diagenesis. The limestones of both the Kallankurichchi and Niniyur formations show negative δ13C (−4.73 to −0.49‰ VPDB; −5.63 to −1.87‰ VPDB; respectively) and −18O values (−8.89 to −3.66‰ VPDB; −8.56 to −5.41‰ VPDB; respectively). The carbon and oxygen isotope composition, δ13C vs. δ18O plot and Mn/Sr ratio suggest that the measured δ13C and δ18O values have been significantly altered during diagenesis. The limestones from both the Kallankurichchi and Niniyur formations show large variations in 87Sr/86Sr values (0.709310 to 0.711962; 0.708280 to 0.708398, respectively) which are higher than 87Sr/86Sr ratios of the contemporary Lower Maastrichtian (87Sr/86Sr: 0.707760) and Danian (0.707819 to 0.707833) seawaters. The elevated 87Sr/86Sr ratios in the limestones of the Kallankurichchi Formation suggest that these limestones were significantly modified by pore fluids during meteoric diagenesis. The observed large fluctuations in 87Sr/86Sr ratios in the Niniyur Formation resulted from variations in riverine input. One sample from the Niniyur Formation exhibits an unaltered 87Sr/86Sr ratio (0.707828) which is interpreted to indicate an age of 65.02 Ma.

Facies, biostratigraphy, diagenesis, and depositional environments of Lower Cretaceous strata, Sierra San José section, Sonora (Mexico)

We used petrofacies analysis, carbon, oxygen and strontium isotope data to interpret the isotopic variations in the carbonate rocks of the Mural Formation of Sonora (Sierra San Jose section), Mexico. The petrographic study reveals a range of lithofacies from wackestone to packstone. The analyzed limestones show significant negative δ18O values (-18.6 to -10.9 VPDB) and δ13C values ranging from negative to positive (-2.6 to +2.5‰ VPDB). The absence of correlation between δ13C and δ18O values suggests a primary marine origin for the δ13C values of limestones from the Sierra San Jose section. The limestones have large variations in 87Sr/86Sr values (0.707479 to 0.708790). Higher 87Sr/86Sr ratios in various levels of the studied section suggest that most of the sediments were derived from the Proterozoic basement of the Caborca block during Early Cretaceous time. A decrease in 87Sr/86Sr ratios at certain levels indicates an influx of lesser amounts of radiogenic Sr that could have been caused by contribution of sediments from the Triassic and Jurassic volcanic rocks.

Provenance and Tectonic Setting of the Proterozoic Clastic Rocks of the Kerur Formation, Badami Group, Mohare Area, Karnataka, India

The clastic rocks of the Kerur Formation Group were analyzed for major, trace and rare earth elements to infer the tectonic settings and provenance signatures. The lithology of the Kerur Formation includes basal arenite (BA), lower conglomerate (LC), quartz arenite (QA) and upper conglomerate (UC). The CIA and PIA values and A–CN–K diagram indicate a moderate to intense chemical weathering for the clastic rocks of the Kerur Formation. The bivariate plot and multidimensional diagram based on major-trace elements suggesting the clastic rocks were derived mainly from the passive margin setting. A silicic source for clastic rocks of the Kerur Formation is indicated by higher ratios of SiO2/Al2O3, ΣLREE/ΣHREE, elemental ratios, bivariate and ternary plots. REE patterns and Eu anomalies support that TTG (Tonalite-Trondhjemite gneiss), granodiorite, granitic rocks of the Dharwar Craton and the basement rock (schist) of the Kaladgi-Badami Basin could be the source rocks for the Kerur Formation. REE modeling suggests that the average composition of the Kerur Formation represents a mixture of sediments derived from a provenance consisting of 40% TTG, 30% granite, 20% granodiorite and 10% schist.

Paleo-Redox Conditions of the Albian-Danian Carbonate Rocks of the Cauvery Basin, South India: Implications for Chemostratigraphy

Abstract To understand the paleo-redox conditions that occurred during the Albian-Danian, we have analyzed major and trace elements from limestone samples collected from three sections of the Cauvery Basin. In the Vadugarpettai quarry section, coral algal limestone (CAL) and bedded limestone show small variations in CaO content (54.4–55.5%; 51.3–54.1%, respectively) than marl and gray shale (GS) (31.9–49.7%; 26.9–40.3%, respectively). In the Vellipirangiyam quarry section, oyster Gryphaea limestone (OGL) shows larger variations in CaO content (42.5–53.9%) than Inoceramus limestone and fossiliferous limestone (52.00–52.50%; 52.70–52.80, respectively). The concentrations of immobile trace elements (Zr and Th) varied significantly between different litho-units of the sedimentary rocks of Albian-Danian age. In addition, Al2O3 shows a positive relationship with immobile trace elements such as Zr and Th, suggesting a terrigenous origin for these elements. Enrichment of U and V contents is higher in the GS and particular intervals of CAL and marl bedded limestone (MBL) of the Vadugarpettai quarry section, lower and upper parts of the OGL of the Vellipirangiyam quarry section, and lower and middle parts of the limestone sequence of the Periakurichchi quarry section. The GS and lower part of MBL (Vadugarpettai quarry section); lower and upper parts of the OGL (Vellipirangiyam quarry section); and lower, middle, and upper parts of the Periakurichchi quarry section also show enrichment of Mo. Furthermore, high values of U/Th were noticed in the CAL and MBL of the Vadugarpettai quarry section, OGL of the Vellipirangiyam quarry section and certain intervals of the Periakurichchi quarry section. Such variations in U, V, and Mo contents and the U/Th ratio in the studied sections suggest that the Cauvery Basin experienced oxic to suboxic-anoxic conditions during Albian-Danian.

Provenance of the Gondwana sediments, Palar Basin, Southern India

Palar Basin is considered as one of the pericratonic rift basins of East Coast of India. An integrated petrography, clay mineralogy, and heavy mineral studies of sandstone and shale from the Palar Basin South India, Tamil Nadu, have been carried out to decipher their provenance, palaeoclimate, and tectonic setting. The mineralogical make-up of the clastic rocks is largely composed of quartz followed by feldspar, rock fragments with minor amounts of chlorite, glauconite, zircon, garnet, and opaque. Palar sandstones are coarse to medium-grained and mineralogically immature (poorly sorted and few are moderately sorted). The detrital quartz grains consist of well rounded and rounded quartz, monocrystalline quartz with uniform or straight extinction, and polycrystalline grains with five crystals of straight to slightly curved intercrystalline boundaries. Few of the monocrystalline quartz contain inclusions of sillimanite, rutile, and zircon needles and low content of plagioclase and k-feldspar. These petrographic analyses of the sandstone suggest derivation from metamorphic, igneous, and reworked sediments of different grades which infer that the recycling and the clasts were subjected to both long and short transport distances. The recycled orogen provenance suggests that the sandstone has been derived predominantly from low-lying granite and gneissic sources. The framework petrography exhibits (a) higher percentage of quartz, (b) predominance of monocrystalline grains, (c) feldspar affinity, and (d) paucity of rock fragments, and low F/R ratio suggests that the sediments deposited were in a passive continental margin tectonic setting. The presence of euhedral, angular, and subrounded grains of zircon, rutile with reddish and dark boundaries, the high and moderate ZTR Index (zircon-tourmaline-rutile) and the ratio of RuZi (rutile/zircon) and GZi (garnet/zircon) low values reveal that the Palar Basin sediments were sourced from the combination of metamorphic and igneous (granite and gneisses) rock types. The presence of high amount of rock derived clay (illite, chlorite) is predominant than soil derived (kaolinite, smectite) clay minerals. The high content of illite indicates the source of the clay minerals from pre-existing rocks of granite and gneisses, subjected to physical weathering over chemical weathering in a temperate climate (hot/humid) with moderate hydrolysis.