Sedimentological and geochemical characteristics of the late middle Eocene dolostone succession, Kachchh, western India

Abstract

Dolostones form by many processes, and their origins are enigmatic. The present study focuses on the origin of the newly recorded 16.5‐m‐thick late middle Eocene dolostone succession of the Kachchh Basin, western India. To achieve this target, a comprehensive field, petrographic, scanning electron microscopy and geochemical studies have been carried out. This dolostone succession shows a sharp contact with the underlying middle Eocene Fulra Limestone Formation. The petrographic study shows a dominance of unimodal, planar‐euhedral idiotopic mosaics of dolomite rhombs along with replacement texture. The X‐ray diffraction pattern confirms it as non‐stoichiometric type dolomite (average molar% MgCO3 = 47.19). SEM‐EDS study deciphers perfectly developed zoned planar‐e dolomite rhombs (40–100 μm) with minor variation in the concentrations of elemental Mg (47.67% to 43.33%) and Ca (52.33% to 56.67%). Among the major oxides, CaO and MgO are the dominant components, indicating that the major mineral phase is dolomite. The positive correlation of CaO with SiO2, Al2O3, and Fe2O3 and a negative correlation of MgO and ∑REE with them suggest that the CaO was mainly contributed from the rivers, while the MgO was contributed from the lagoonal brine/seawater. The low concentration of Sr (144–234 ppm) in the studied dolostone is compatible with the marginal marine mixing origin. The ∑REE content in the dolostones is comparatively lower (18.22 ppm) than the average value of typical marine carbonates (28 ppm), thus suggesting meteoric mixing conditions. The superchondritic Y/Ho values (35.21–100) and Post‐Archaean Australian Shale‐normalized REY patterns of the dolostone facies exhibit typical shallow (bathymetry <50 m) seawater‐like patterns. A strong positive correlation between ΣREE and Y, and low Er/Nd ratios (~0.1) in the dolostone facies indicate incorporation of minor terrigenous influx during dolomitization in a meteoric‐marine mixing condition. The authigenic uranium (0.4–1.7) and slightly negative Ce anomaly suggest that the dolostone succession was precipitated in oxic to dysoxic conditions. The prevalence of planar (euhedral and subhedral) dolomite texture indicates that the dolomitization occurred in a shallow marine environment under low salinity and temperature conditions. Thus, the absence of evaporitic association and physical compaction or deep burial features and the presence of large channelized vugs, mouldic pores along with partially replaced shallow marine fossils, anhydrite cement, and marginal‐marine values of Sr, Na, Ba, Mn, and Fe support the meteoric‐marine mixing model of dolomitization under shallow marine condition of this dolostone succession.

Role of cementation and compaction in controlling the reservoir quality of the Middle to Late Jurassic Sandstones, Jara dome, Kachchh Basin, western India

“…Dolomitization takes place normally due to increase in Mg‐concentration, high temperature, evaporative condition, high CO 2 pressure, high Mg/Ca ratio, SiO 2 , and organic acid effects (Tucker & Wright, 1990). The occasional development of dolomite cement attribute the local incursion of surface water rich in Mg‐ions while mixing with circulating basinal pore water (Morse & Wright, 1990; Singh, Srivastava, & Kanhaiya, 2019).…”

Microfacies and diagenetic overprints in the limestones of Middle Jurassic Fort Member (Jaisalmer Formation), Western Rajasthan, India: Implications for the depositional environment, cyclicity, and reservoir quality

“…Dolomitization took place during or after the introduction of late, post‐compaction cement, and subsequently the replacement process must be of burial diagenetic origin, though not necessarily at great depth and resulted in volume reduction and porosity enhancement (McIlreath & Morrow, 1990; Singh, Srivastava, & Kanhaiya, 2019). Since the dolomite was formed during shallow burial diagenesis within a few metres to tens of metres from the sediment water interface.…”