An 11 million-year-long record of astronomically forced fluvial-alluvial deposition and paleoclimate change in the Early Cretaceous Songliao synrift basin, China
Abstract In the Cretaceous Songliao Basin, northeastern China, boreholes have recovered detailed geologic data from sub-basins that developed during the early rifting of the basin. Among these, the Lower Cretaceous Shahezi Formation (K1s) in the Songshen 4 (SS4) Well is represented by an 836-m-thick succession of fluvial-alluvial deposits, with coals distributed throughout the upper part of the formation. Gamma-ray (GR) logging data are quantitatively related to grain size-defined litho-facies that recur throughout the succession, with high GR values in mudstones and low GR values in sandstones. Cyclostratigraphic analysis of the GR log reveals depositional cycling at frequencies that are consistent with those of the Earth's astronomical parameters. The results also indicate that occurrence of coal beds occur every ~100 kyr, when development of vegetation at the depositional site was promoted by humid climate. We propose a model of astronomically forced hydroclimate change to explain the cycling of depositional environments between fluvial and alluvial conditions. The model describes a principally fluvial environment that episodically experienced, at one extreme, advancing alluvial conditions involving a drop in base level, lake recession and increased aridity (conglomerates and coarse sands), and at the other extreme, an advancing lakeshore with a rise in base level and increased humidity (fine sands, silts, muds and coal). A floating astronomical time scale was established by calibrating long orbital eccentricity (405-kyr) cycles interpreted along the GR series. The results indicate that the duration of the K1s is 11.14 Myr (27.5405-kyr long orbital eccentricity cycles), corresponding to an average sedimentation rate of 6.55 cm/kyr. The recognition of astronomical forcing in this early Cretaceous fluvial-alluvial-dominated depositional setting offers a new framework for interpreting the sedimentary cyclicity and dynamics of early rift basins and for developing time scales.