Multi-proxy approach of the stratigraphy, geochemistry, and sedimentology of Eocene Margalla Hill Limestone: Case study from Muzaffarabad area, Sub-Himalayas, Pakistan

To investigate the sedimentological characteristics of the Margalla Hill Limestone (MHL), a multi-proxy approach including geochemical, mineralogical, and micropalaeontological investigations were carried out. Using detailed field observations and petrographic investigation of the studied section, five microfacies were dentified. Total organic carbon (TOC) values of the limestone are generally low (0.10-0.96%) and highly degraded, as observed from the low hydrogen index (i.e., 17-349 mg HC/g TOC). TOC is mainly Type-III and Type-IV organic matter that reflects a continental source or highly weathered marine organic matter (OM) with minor Type-II which corresponds to algal or bacterial OM. The values of T-max range from 419 to 512 degrees C for most of the samples, which lies within the mature stage. Major element abundances do not vary significantly in the studied unit and are mainly linked to variations in clay contents. Trace elements (V, Cr, Mn, Zn, Ni, Cu, Th, Rb, U, and Zr) of the MHL depict a mainly detrital origin for shales, and overall decrease in trace element's concentration suggests a change from physical to chemical weathering. The rate of sedimentation as determined by the Th/U ratio indicates a regular to irregular low rate of sedimentation, from base to top of the studied section. The redox conditions, as reflected in the variation of Cr (V/Cr), using the V/Cr ratio with Ni + V indicate that the MHL was deposited under oxic-dysoxic conditions. Chemical Index of Alteration, Chemical Index of Weathering, Plagioclase Index of Alteration, and Weathering Index of Parker showed that the source region was subjected to intense chemical weathering likely under tropical palaeoclimatic conditions with abundant rainfall. The limestone consists mainly of calcite followed by minor quartz and dolomite. Clay mineralogy revealed that the clay fraction is dominated by kaolinite, smectite, mica, and chlorite. The microfossils of various genera of benthic foraminifera have been identified. Based on benthic foraminifera, an Early Eocene (SBZ-8; Middle Ilrdian 2) age has been assigned to the MHL.

Automated summary

The sedimentological characteristics of the Margalla Hill Limestone (MHL) were investigated using a multi-proxy approach, revealing low organic carbon content, mainly Type-III and Type-IV organic matter. The study suggests a low rate of sedimentation under intense chemical weathering likely influenced by tropical paleoclimatic conditions with abundant rainfall.