Geochemistry and depositional environment of the Mesoproterozoic Xiamaling shales, northern North China

Abstract

The Mesoproterozoic Xiamaling shales from Xiahuayuan area, southeastern Xuanlong sag, Northern China, were examined the elemental analysis by applying the inductively coupled plasma-mass spectrometry (ICP–MS). The Xiamaling shales have good to excellent oil hydrocarbon generation potential with type II kerogenas demonstrated by high content of alginate macerals, TOC, HI, S1 + S2 and extractable organic matter (EOM). The Xiamaling shales were deposited under a marine environment according to boron (B) (56–111 ppm) and other trace elements concentration, which is consistent with the geological settings. These shales can be classified into three types: (1) Type I is a coastal facies, which is influenced by fresh riverine water and has low B content of 56–61 ppm; (2) Type II is a marginal marine with B content of 73–79 ppm; and (3) type III has high B content of 111 ppm, typical of a brackish, and open marine environment.The Xiamaling shales have high B (>50 ppm), low Na, Ca, Ce, Mn, and rare earth elements (REEs). According to the elemental composition and ratios, the Xiamaling sediments were derived from the felsic source, which has suffered from moderate to strong chemical weathering based on the CIA, ICV, and PIA values. The Xiamaling marine shales were mostly deposited under dysoxic-anoxic conditions. The paleoclimate during the deposition of the Xiamaling shales was warm and humid based on the cross plots of Sr/Ca vs Mn/Ca values and Sr/Cu vs Rb/Sr ratios. Their total REE (ΣREE) contents increase from coastal towards the open marine zone. The Xiamaling shales display a negative Ce anomaly, representative of marine shale, and show a negative Eu anomaly, which is representative of lacustrine oil shale and the Post-Archean Australian Shale (PAAS) and is different from typical marine shale of younger age, indicating the influx of terrestrial sediment.