Paleoenvironment, Provenance and Chert Origin of the Lower Cretaceous Xiguayuan Formation (Luanping Basin, North China): Evidence From the Geochemistry of Lacustrine Sediments

Abstract

Industrial shale oil/gas flow have been discovered in the Luanping Basin, and the hydrocarbon potential of the Lower Cretaceous Xiguayuan Formation (Luanping Basin, North China) has attracted the attention of geologists in recent years. The paleoenvironmental conditions, provenance and chert origin of the Xiguayuan Formation were studied using the geochemistry of lacustrine sediments in the Caoying section. A pronounced positive correlation between the oxygen and carbon isotopes of autogenous carbonates indicates the Early Cretaceous Luanping paleolake was an alternating depositional environment of open and closed, with obvious evaporation. The V/(V + Ni) ratios, negative Ce and Eu anomalies suggest that the Xiguayuan Formation was deposited in anoxic conditions with fewer instances of euxinic conditions. Sr/Ba, Z and S values together demonstrate a brackish to saline waterbody under semi-arid and fluctuating semi-humid paleoclimatic conditions. Various indicators show that they are predominantly derived from felsic igneous rocks and minor probable basic source supply with minor sedimentary recycling. The cherts are developed in Unit 1 and Unit 2, compared with Unit 3, Unit 1 and Unit 2 show relatively open, less reductive, high salinity. A theoretical formation model of alternating rise and drop in lake levels in the basin was established to show syndepositional bedded chert-bearing sedimentary succession. The acid river dominate water and the evaporation in front fan delta environment maybe the main factor that lead silica precipitation.