Discovery of gas-discharged pits (gas pit) in the plains of the Yellow River delta

A kind of special sedimentary structures are developed in the overwater pla ins of the Yellow River delta. They look like funnels: round or nearly roun d, concave of pit-like, with a diameter ranging from several centimeters to 20 or 30 cm, and depth from several millimeters to over 20 cm. There is a vertical pipe (called gas discharging conduit) in the center, with a diamet er-of several millimeters to 1 cm, depth of several centimeters to more tha n 10 cm. There may be a lip-like relief (called "lip-like relief") on the p eriphery or some parts of the periphery, with ring structures showing horiz ontal stratification on the inner margin. Plant debris carbonized, plant fr agments or dark-colored minerals may sometimes be found in the center of th e pits, Such structures are usually developed in silt (with minor clay lami nations), often underlain by one or more thin layers of mud matter. Our stu dies find that they are genetically related with gas discharging of organic matter during biological degradation. Therefore, they are called "gas-disc harged pits".