Abstract:

Slope break is a geomorphologic concept, which refers to a zone where there is an abrupt change in topographic slope.Slope break can develop not only in sedimentary basins,but also in denudation areas. It would be of stratigraphic sequences in marine sedimentary basins shows that the type I sequences in a basin with shelf slope break margin or growth fault (faulted slope break)margin,unconformity and incised valley can be formed above the slope break,while basin-floor fan,slopee fan and other aktian deposits,as well as lowstand wedges can be deposited below the slope break during the lowstand.This sedimentary system constitute a typical LST sediments.The concept of structural slope break has been advanced in recent years in the rift lacustrine basins. Structural slope break refers to the sedimentary slope break caused by the long activity of syndepositional structures. The structural slope break, on the one hand, has a strong impact on the accommodation and deposition in the basin, and on the other hand, controls the spatial distribution of the sedimentary facies.There are various kinds of faulted slope breaks,such as salient-gentle slope margin, gentle slope-subsag margin,salient-steep slope margin, steep slope-subsag margin, etc.And it has been further clarified that incised valleys, lowstand river-delta and lake-floor fans can be developed in the LST in the lake basin, which would constitute the potential reservoirs in deeper part in the basin.The distribution of these sediments have clearly been controlled by the structural slope breaks in the basin margin.The study of the Paleogene in Dongying sag shows that there are multistage slope breaks ,such as faulted, flexural, depositional and erosional slope breaks in the half graben-like rift lacustrine basin. The large syndepositional faults that act as the boundary of tectonic subunits, such as salient, slope (fault bench) and subsag, would often become faulted slope breaks.As a result,there will be various types of faulted slope breaks successively developed from the steep slope to gentle slope belts,including salient-steep slope, steep slope-subsag,salient-gentle slope, gentle slope-subsag margins.Nearby these slope breaks,some smaller syndepositional faults can become auxiliary faulted slope breaks and constitute various complex plane combination patterns. Above and below the faulted slope break on the edge of salient may develop erosional slope break. Flexural slope break are mainly associated with syndepositional anticline and nose,which are developed best on both limbs of central anticline-belt and on delta front.The most typical depositional slope break is that developed along the long-axis on Dongying delta front.Faulted slope break controls the development of lowstand fan and compound lithologic-structural traps, depositional slope break controls the distribution of lowland fan,highstand slumping turbidity fan and lithologic traps,while flexural and erosional slope breaks control not only the development of sandbody,but more apparently control the development of stratigraphic overlap, truncation wedge-out and stratigraphic traps.Finally, different kinds of slope breaks developed in the rift lake basin distribute extensively and appears multistage slope breaks.The slope break controls significantly the development of stratigraphic depositionnal and non-structural traps. Combination of oil and gas reservoirs usually occur near the slope break.A thorough study of the genetic type and distribution of slope breaks and their control on sedimentary facies and reservoirs in the maturely explored area as Dongying sag would certainly establish a vast domain in the study of rift basins in eastern China.

Key words: slope break, subtle trap, lowstand system tract, rift basin, Dongying sag, Tertiary