露头剖面层序中的海平面变化研究

SEA-LEVEL CHANGE IN SPECIAL REFERENCE TO OUTCROP SEQUENCES

在以往的沉积学研究中，常常用露头创面中的相对水深变化来讨论海平面的变化规律。本文通过对塔里木盆地北部寒武纪反映相对海平面变化的上超点变化曲线与肖尔布拉究地区寒武系露头剖面中相对水深变化曲线的对比研究发现，上超点变化曲线与相对水深变化曲线具有明显的差异。前者在寒武纪呈持续上升趋势，而后者除早期有一快速上升外，总体呈持续下降趋势，这说明相对水深变化虽与海平面变化存在着本质的联系，但单凭露头剖面中相对水深变化的研究是难以对海平面变化作出正确估价的。为了能从露头剖面的地层层序中来了解海平面的变化规律，本文作者应用改进后的Fischer图解和数学方法对肖尔布拉克寒武系剖面进行了海平面变化的重建．结果发现，它们与上超点变化曲线的变化趋势完全一致。这充分说明了海平面变化对沉积层序的控制作用，同时也告诉我们，不能简单地利用露头剖面中的相对水深变化来研究地质历史时期的海平面变化规律。只有在排除了不同沉积背景上基底沉降速率和沉积物供给速率对沉积层序的控制效应后，才能从露头剖面的地层层序中获取正确的海平面变化信息。

Sea-levcl change in thc geological history is one of the most important aspects involved in sedimentology and paleooceanography. and also the theoretical basls and key subject of sequence stratigraphy. Sea level changc during the Cambrian in northern Tarim Basin is discussed in this paper as well. Evidence from seismologic stratigraphy and sedimentology of outcrop sections indi-cates that the curvc showing thc relative change of coastal onlap indicates a steady rise and that of the relative change of water depth is gradually decreased. They are quite different. In order to un-derstand the rule of sea-levcl change from the outcrop section, thc tendency of sea-level change has been reconstructed with thc mathematical method and the developed Fischer Plot, and the results show that the rcconstructed tendency of sea-level change is the same as that of the change of coastal onlap. reflecting the relative change of sea level, which provides evidence suggesting that the sea-level change has an important control over depositional sequences, but it is difficult to eval-uate the sea-level change only on the relative change of water depth from the outcrop section. It is only after ellmlnating the effects of hasernent subsidence and sediment productivity that the correct information on sca-level change can be obtained from the outcrop section-