Seasonal to sub-seasonal palaeoenvironmental changes in Lake Sihetun (Lower Cretaceous Yixian Formation, NE China)

Sedimentary properties of the fossil-bearing deposits of Lake Sihetun (Yixian Formation, Lower Cretaceous) were investigated on a high-resolution, sub-millimetric scale. Data were obtained from three excavations and 50 thin-sections. Lake evolution is subdivided into four phases, of which Phases 2 and 3 provided suitable conditions for excellent fossil preservation. Six microfacies are recognized within these two phases: (1) allochthonous, siliciclastic laminae (26.1 μm thick on average), (2) chrysophycean cyst accumulations, (3) tuffaceous silt, (4) lacustrine chemical precipitates, (5) tuff and (6) normal-graded, sandy to silty siliciclastics. Phase 2 is characterized by Microfacies 1–5 and Phase 3 by Microfacies 6. Biofilms are common, and mass occurrences of framboids (pyrite pseudomorphs) are occurring in sediments of Phase 2. Varves can be verified for Microfacies 2 and 4 (179 μm thick). Thicknesses of Microfacies 2 highly fluctuate depending on the occurrence of seasonal heavier rains, which led to deposition of tuffaceous silt layers (Microfacies 3). Meromictic conditions dominated Phase 2, but recurrent mixing is demonstrated by short-lived colonization events by a benthic invertebrate fauna. The transition of Phase 2 to holomictic Phase 3 is marked by a dramatic increase in sediment yield associated with a change from dry to humid climates. Fossil preservation differs according to the diverse physical and chemical conditions during lake evolution. Phase 2 yields framboid replacement of organic tissue and excellent preservation of growth increments of clam-shrimp carapaces, which are poorly preserved in Phase 3. Conversely, though exhibiting a different mode of preservation, insect fossils are superbly preserved as dark stains sealed by silica coatings during Phase 3.