Source of organic matter and depositional environments of the middle Paleocene Lakhra coals, Sindh Province, Pakistan.

The middle Paleocene sub-bituminous coals and lignites in the Lakhra coalfield, Sindh Province, Pakistan were investigated to clarify source of organic matter and depositional environments in the Indus Delta. Here we present multiproxy approach such as C/N and C/S ratios, n-alkane indicators such as average chain length (ACL), carbon preference index (CPI), and aquatic index (Paq), as well as pristane/phytane (Pr/Ph) ratio, oleanane/hopane (Ole/Hop) ratio, and Ts/(Ts + Tm) ratio to evaluate the peat accumulation conditions at the Lakhra coalfield. The TOC, TN and TS contents range from 8.15 to 55.1%, 0.08 to 0.93% and 2.14 to 11.4%, respectively. The C/N ratios (51.2-96.5) indicate a dominant control of organic matter by terrestrial higher plants to the peat bog. The values of C32 hopanes 22S/(22S + 22R) and C29 sterane 20S/(20S + 20R) are 0.14-0.36 and 0.26-0.29, respectively, showing that the maturity level is immature. The depositional environment was evaluated to be lagoonal brackish water pond or marsh where depositional condition was anoxic to dysoxic based on the datasets of high sulfur contents (2.1-11.4%), low to high C/S ratios (0.7-26) and low Pr/Ph ratios (0.14-1.00) without UCM. The Lakhra coals and lignites are characterized by high C/N ratios (51-97), high oleanane/C30-hopanes ratios (<1.30), medium to high CPI ( ～2.0-9.8), medium ACL (27.6-28.7) and low to medium Paq values (0.13-0.34). According to these results, main origin of the coals was probably angiosperm reed/grass. The coals are also minorly accompanied by aquatic/ submerged plants and specific mosses without Sphagnum. The Lakhra coals are not abundant in regular steranes and characterized by high contents of diasteranes probably due to fine clay rich depositional environment.