Basic characteristics of tempestite sediments of Middle Sha 3 Formation in Gunan Subsag of Bohai Bay Basin and their geological significance

Abstract：This paper has proposed for the first time that tempestites were developed in the Middle Sha 3 Member of Shahejie Formation in Gunan Subsag. Based on core observation and analyses, the authors found two places of tempestites which were developed respectively in sediments of shallow lake and those of delta front. Some sedimentary structures were developed in this zone which were co-existent to serve as facies marks： Surface structures include gutter cast, truncated structure, surface of scour due to storms, whereas bedding structures include massive bedding, hummocky cross stratification, swale cross stratification, wavy bedding and wave-ripple bedding; in addition, life-escaping structures and contemporaneous deformation were also developed. The ideal vertical sequence of the tempestites in this zone is a Bouma-like sequence, and consists from bottom to top of five segments, namely SA, graded bedding; SB, parallel bedding; SC, storm wave oscillating effect; SD, wavy bedding; SE, mudstone segment. The discovery of tempestites is helpful to defining the shore-shallow lacustrine boundary, adds some new sedimentary facies types to this area, provides the new basis for lithostratigraphic division, and also supplies new space to the prospecting for ideal oil and gas reservoirs and source rocks.