When the horizontal level cannot fall down the slope-break, an imperfect horizontal fluctuating cycle is formed, a system of lake expand and lake shrink (dimidiate tracts).

• 当水平面未下降到坡折带以下时，构成一个不完整的水平面升降旋回，可形成包括湖扩展、湖萎缩2个体系域的层序（二分体系域）。