Fig. 3

Lithology and major sedimentary structures in the middle part of the Member IV of the Xiamaling Formation (Jizhentun section). a Sparsely distributed carbonate concretions in green-yellow silty shale; b Densely packed carbonate concretions in green-yellow silty shale; c Concretions wrapped by green silty shale laminae (arrows); d A concretion with sharp contact with the surrounding and lateral shale laminations; e and f Two concretions with lateral shale laminae (dash lines and arrows), and the laminae thickness in concretions is expanded by an order of magnitude relative to the laminae in adjacent shales