Formation phases of carbonate cements and sedimentary environments in lower Jurassic sandstones of the Lenghu V tectonic belt, North Qaidam Basin, China

Based on petrography and geochemistry, formation phases of carbonate cements and their sedimentary–diagenetic environments in Jurassic clastic rocks from the Lenghu V tectonic belt are determined. The results demonstrate that studied successions are mainly dark gray or black mudstone with interbedded thin gray siltstone and a few greywacke layers. Sandstones are lithic and feldspathic lithic greywacke with high kaolinite content. Calcite and ferrocalcite are the main carbonate cements. The carbon isotope values(δ13C) of the carbonate cements range from − 15.6 to 9.2‰ (average − 3.2‰) with a bimodal distribution. One peak is 4‰, which includes samples from the upper Xiaomeigou Formation (Fm) and the other one is − 12‰, which mainly represents the lower section. The oxygen isotope values(δ18O) have a wide range from − 18.5 to − 8.3‰ (average − 13.31‰). Combined with microscopic observations, two phases of carbonate cements are distinguished according to their isotopic characteristics. The earlier carbonate cements are mainly calcite and ferrocalcite, and they were precipitated in eodiagenesis stage. The carbon source of calcite was inorganic, and ferrocalcite was mixed with carbon from methanogenesis. This mixture led to a positive bias in the δ13C curve, which artificially inflated the paleosalinity. The later carbonate cements, formed in mesodiagenesis stage, were ferrocalcite. Decarboxylation of organic matters led to lower δ13C values. The major and trace elements data show that Sr, Ba, Fe and Mn have weak correlation with Al, Ti. So, they are effective to interpret the sedimentary environment. The Sr/Ba ratio < 1 indicates fresh water environment, Sr/Ca, Sr/Cu and Fe/Mn ratios suggest humid climate, and V/(V + Ni) ratio shows suboxic–anoxic environment. The carbon and oxygen isotopes, together with elemental geochemistry, show that the climate during the Early Jurassic was warm and wet and the studied successions were deposited in fresh water. The variations in elemental ratio curves indicate that climate became wetter and warmer later in the Early Jurassic.