Geochemistry of Neoproterozoic limestones of the Shahabad Formation, Bhima Basin, Karnataka, Southern India

Major, trace and rare earth element (REE) geochemistry of carbonate rocks of the Neoproterozoic Shahabad Formation, Southern India were studied in order to investigate the depositional environment and source for the REEs. The PAAS (Post Archaean Australian Shale) normalized REE + Y pattern of Shahabad limestones have consistent seawater-like pattern i.e., i) LREE depletion (average (Nd/Yb)SN = 0.64 ± 0.08), ii) negative Ce anomaly, iii) positive Gd anomaly (average GdSN/Gd* = 1.05 ± 0.16), iv) superchondritic Y/Ho ratio (average Y/Ho = 38.13 ± 21.35). The depletion of LREE and enrichment of HREE are clearly indicated by the (La/Yb)SN, (Dy/Yb)SN and (Nd/Yb)SN ratios, which suggest the retention of seawater characteristics in these limestones. The negative Ce anomaly reflects the incorporation of REE directly from seawater or from the pore water under oxic condition, and also reveals the mixing of two-component systems with terrigenous clay (detrital) in the marine sediments. The terrigenous input in these limestones is confirmed by positive correlation of ΣREE with Al2O3, negative correlation of ΣREE with CaO and differences in Y/Ho ratios. V, Cr, and Sc, are positively correlated with Ti, and strong positive correlation of ΣREE with Fe2O3, Ni, Cr, Sc, and Y also indicate the presence of terrigenous materials in the Shahabad limestones

Significado diagenético de las composiciones isotópicas de carbono, oxígeno y estroncio en la Formación Mural (Aptiense-Albiense, zona Cerro Pimas, norte de Sonora, México)

A carbon, oxygen and strontium isotopic study on the predominantly carbonate succession of the shallow marine Mural Formation in the Cerro Pimas locality of northern Sonora (Mexico) was undertaken to understand isotopic variations in seawater during Aptian- Albian time. Petrographic study has been carried out on these limestones to support the isotopic study. The limestones show low content of TOC, both negative and positive δ13C values (-4.1 to +2.2‰ VPDB) and d18O values varying from -13.4 to -8.9‰ VPDB. The carbon isotope composition, δ13C vs δ18O plot and Mn/Sr ratio suggest that the δ13C measured values are primary in nature and remain unaltered during diagenesis. The carbon isotope curve shows a positive δ13C excursion followed by a negative event in the lower part of the Los Coyotes Member (Lower Albian) of the Mural Formation and confirm the global nature of the episode OAE 1b in the Cerro Pimas section. The 87Sr/86Sr ratios for limestones of the Mural Formation (0.707221 to 0.707340) are similar to the ratios for Late Aptian and Early Albian seawater (0.70726 to 0.70740). In addition, 87Sr/86Sr age of the Cerro La Ceja member (113.4 Ma; Late Aptian age) is comparable to that of published zircon radiometric age. The numerical age of Los Coyotes (112.0 Ma and 111.1 Ma) and Cerro La Espina (110.9 Ma and 110.7 Ma) members indicate also Early Albian age. The whole-rock Sr isotope ages in this study are consistent with the previously published radiometric and biostratigraphic ages.En este trabajo se presenta un estudio isotópico de carbón, oxígeno y estroncio de la sucesión predominantemente carbonatada de la Formación Mural que aflora en la localidad de Cerro Pimas, norte de Sonora (México) con el fin de entender las variaciones isotópicas del agua de mar dentro de la cual se depositó esta unidad durante el Aptiense-Albiense. Los datos isotópicos están apoyados por un estudio petrográfico de las rocas carbonatadas. Las calizas tienen un contenido bajo de carbón orgánico total, valores negativos y positivos de δ13C (-4.1 a +2.2‰ VPDB) y valores de d18O entre -13.4 y -8.9 ‰. La composición de los isótopos de carbón, la relación δ13C vs δ18O y la proporción Mn/Sr sugieren que los valores medidos de δ13C son de naturaleza primaria y no fueron alterados durante la diagénesis. La curva de isótopos de carbón presenta una excursión positiva de δ13C seguida por un evento negativo en la parte inferior del Miembro Los Coyotes (Albiense inferior) de la Formación Mural y confirma la presencia del episodio global OAE 1b en la sección estudiada de Cerro Pimas. La relación 87Sr/86Sr de las calizas de la Formación Mural (0.707221 a 0.707340) es similar a la relación isotópica del agua de mar del Aptiense tardío y Albiense temprano (0.70726 a 0.70740). Además, una edad 87Sr/86Sr de 113.4 Ma (Aptiense tardío) que hemos obtenido para el Miembro Cerro La Ceja es similar a una edad radiométrica U-Pb en circones previamente publicada. Del mismo modo, hemos obtenido edades 87Sr/86Sr en roca total para los Miembros Los Coyotes (112.0 Ma and 111.1 Ma) y Cerro La Espina (110.9 Ma and 110.7 Ma) que confirman una edad Albiense temprano que es consistente con edades radiométricas y bioestratigráficas previamente reportadas

Petrography and stable isotopic variations in Dalmiapuram Formation of Cauvery Basin, South India: implication on OAE1d

Petrography, carbon and oxygen isotopic study was carried out to interpret isotopic variations on the predominant carbonate sequence of the Dalmiapuram Formation of the Cauvery Basin, South India. The common petrographic types identified in the Dalmiapuram Formation range from wackestone to boundstone. The gray shale and limestone members show large variations in d<sup>13</sup>C and d<sup>18</sup>O values (Gray shale member: +1.44 to +2.40 ‰ VPDB, -3.05 to -5.92 ‰ VPDB, respectively; Limestone member: -6.07 to +2.93 ‰ VPDB; -7.08 to -0.39 ‰ VPDB; respectively). In the present study, the carbon and oxygen values are not correlated, which supports the fact that these limestones retain their primary isotopic signatures. In carbon isotope curve, one negative shift is identified in the gray shale member and a positive isotopic excursion is detected in the coral algal limestone (CAL). The observed positive isotopic excursion in the lower part of the CAL correlates with OAE1d and suggests the global nature of the late Albian OAE1d in the Cauvery Basin

Petrography and Stable Isotope Geochemistry of the Cretaceous El Abra Limestones (Actopan), Mexico: Implication on Diagenesis

Petrography and stable isotopes (carbon and oxygen) geochemistry of limestones from the El Abra Formation, Actopan, were studied to identify their digenetic environments. The major petrographic types identified are mudstone, wackestone, grainstone, and boundstone. Most of the studied samples show positive d13C values, except two samples (2 and 28), which are slightly negative values (-0.27‰ and -0.02‰). The organic remains identified in foraminiferal wackestone type can be responsible for the negative d13C values. The d18O values range from -12.41‰ to -4.02‰ and indicate meteoric diagenesis

An insight on the speciation and genetical imprint of bicarbonate ion in the groundwater along K/T boundary, South India

Groundwater stored in sand, clay, and underground rocks that act as huge aquifers of water have been vastly explored by human civilization for drinking and irrigation purposes. This underlying sand, clay, and rocks greatly influence the chemistry of groundwater by fluctuation in various concentrations of ions. The geochemistry of Ariyalur region was studied by taking 142 samples in both Pre monsoon (PRM) and Post monsoon (POM) seasons. Analysis for crucial cations and anions like K+, Na+, F-, Cl-, HCO3-, Mg2+, Ca2+, SO42-, PO43-, NO3-, and H4SiO4and the physico-chemical variables like pH, total dissolved solid (TDS), and electrical conductivity (EC) were carried out to understand the suitability of groundwater. From all these information, Piper-plot has been used to analyze the water type in both the seasons. The study revealed that in both the seasons, HCO3-covers the major factors followed by H2CO3which are responsible for the change of chemistry of groundwater and proved its importance in the study area. Hence to determine the relationship of HCO3-with other ions, various charts were used. To obtain the origin of this HCO3-and reactions related to it, correlation of different species like HCO3, CaHCO3, MgHCO3, NaHCO3, CaCO3, MgCO3, NaCO3, and H2CO3were analyzed along with major physico-chemical variables like pH and logpCO2. The factor analysis for these species revealed the processes and reactions taking place in the study area. The contour diagrams for all the species in both the season has revealed the origin of HCO3-and dissolution by comparing lithology and geology of the study area

Geochemistry of Sandstones from the Upper Cretaceous Sillakkudi Formation, Cauvery Basin, Southern India: Implication for Provenance

Major, trace and rare earth elements (REE) composition of sandstones from the Upper Cretaceous Sillakkudi Formation, Ariyalur Group, Cauvery Basin were studied to decipher their weathering and provenance history. Texturally, these sandstones are immature, poorly sorted and grain supported. Abundance of feldspars especially, plagioclase indicates rapid deposition of sediments from a nearby source rocks. Using the geochemical classification diagram the Sillakkudi sandstones are classified as fe-sand, quartz arenite, litharenite, sub-litharenite, sub-arkose, arkose, and wacke types,which is also supported by the petrography study. The transition trace elements like Co, Ni, and V are higher in theSillakkudi sandstones than upper continental crust (UCC) values. However, the Sillakkudi sandstones are lower in Cr (mean ~21) content than average UCC value (~ 35). The poor correlation between Cr and Ni (r = 0.08, number of samples n = 20) imply that these sandstones were derived from felsic source rocks. Similarly, the Eu/Eu* (0.35-1.73), La/Sc (1.93-9.36), Th/Sc (0.41-6.57), Th/Co (0.14-5.01), Th/Cr (0.23-2.94), and Cr/Th (0.34-4.28) ratios support a felsic source for the Sillakkudi sandstones.The significant enrichment of Zr, Hf, and Th in fe-sand, sub-arkose and litharenite could be related to the presence of heavy minerals, especially zircon. However, the zircon geochemistry did not affectthe REE distribution and its patterns in the Sillakkudi sandstones. The Chondrite normalized REE patterns of Sillakkudi sandstones are characterized by relatively flat HREE (Gd/YbCN = ~ 0.73-2.41; subscript CN refers to chondrite normalized value), enriched LREE (La/SmCN = ~ 3.39-5.82) and negative Eu anomaly (mean value Eu/Eu* = 0.80). The Gd/YbCN ratios (~0.73-2.50) are less than 2.5, which suggest that these Sillakkudi sandstones were derived from the less HREE depleted source rocks. The comparison of REE patterns and its Eu anomalies to the source rocks reveals that the Sillakkudi sandstones received a major contribution of sediments from Dharwar craton