沉积间断与地层完整性估测

HIATUS AND STRATIGRAPHIC COMPLETENESS ESTIMATION

沉积间断的广泛存在，给年代地层对比、生物演化序列的研究，灾害性或幕式事件沉积层的分析造成了不利的影响，降低了研究结果的精度。根据沉积间断的形成时期，可将其划分为同生期沉积间断和后生期沉积间断；根据沉积间断的分布范围，可将其划分为微型、小型、中型、大型和巨型５个级别。一般而言，微型和小型沉积间断大多形成于同生期，大型和巨型沉积间断则主要由后生期的全球性海平面升降、古地理和古气候变化以及全球构造活动所形成，而中型沉积间断（在沉积盆地范围内发育）既可在同生期也可在后生期形成。地层完整性是指剖面上所有沉积物代表的总时间与地层剖面代表的总时间之比。由于地层剖面中或多或少地发育有沉积间断，因此地层剖面的完整性是一个大于０小于１的小数。如何准确地测定地层剖面的完整性是目前仍未能解决的地质学难题，近１０余年来，国外有许多研究者潜心于这一领域的研究，从不同的角度提出了各自的测定方法，这些方法包括：长期与短期沉积速率比值法、剖面厚度与全球同期平均沉积厚度比值法、潜穴化石定性估测法以及古地磁法等。尽管这些方法各有其不足之处，但却为今后的研究打下了坚实的基础。

Extensive occurrence of hiatus in geological record imposes bad effects on the study of chronostratigraphic correlation,organic evolution,catastrophic or episodic depositional layers,and on analysis precise.Hiatus could be classified into syn depositional and post depositional ones according to their formation stages,and into micro,small,median,large and giant ones according to their scales.The micro and small hiatus generally formed in syngenetic stages;the large and giant ones resulted from global eustatic movements,paleogeographic and paleoclimatic changes,tectonic activities in epigenetic stage;the median ones formed in both the syngenetic and epigenetic stages.The completeness of a stratigraphic section is defined as the fraction of time intervals of some specified length(t) that left a record.The value of the completeness of any section is less than one but greater than zero.How to determine the completeness of a section is still a unsolved problem.In recent ten years or more,some research workers proposed a number of approaches such as the ratio between long term and short term sedimentation rates,the ratio of thicknesses between a section and globle average synchronous deposits,tired trace fossiles and paleomagnetic data methods.These methods have laid down a firm foundation for future work.