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《Acta Sedimentologica Sinica》 1984-02
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LOWER TERTIARY SANDBODY TYPES AND DEPOSITIONAL MODEL OF DONGPU FAULTED SALT LAKE BASIN

Zheng junmao ,Wang Defa SunYongchuan ( Wuhan College of Geology )  
Dongpu Basin is a faulted basin formed by the disintegration of North China platform due to the intense block movement in Late Mesozoic. Continental sandstone and mudstone interbedded with halite and gypsum rock, 7000m thick, were deposited in Early Tertiary. The centre of the salt lake is in the northern part of the basin, where sandbodies are well developed.According to the sandbody geometry, sedimentary sequence, grain size, rocks and minerals, as well as electric loggin g, four main types of sandbodies can be distinguished: ( 1 ) The small subaqueous alluvial fans. They are fan-shaped in plane and covers a maximum area of only tens of squre kilometers. The sedimentary sequence mainly consists of graded conglomerate and pebble sandstone which rapidly changes to wavy-evenly laminated siltstone. Petrologically, they belong to the low-maturity lithic sandstone with mixed texture. According to the analysis of grain size distribution, the initial dynamics resulting in small subaqueous alluvial fan comes from the intermittent flood. ( 2 )The subaqueous alluvial fans group-sand flats, the largest sandbodies in the basin, are composed of several coalescing fans.As a whole, these sandbodies are apron-shaped and occupy an area of hundreds of squre kilometers. They may be subdivided into proximal, median and distal parts,with the median fan sand flat dominating.The sequence of sand flat consists of(from bottom to top)graded and mas- sive sandstone, fine sandstone with parallel bedding,siltstone with wavy cross bedding, and evenly laminated muddy siltstone.The parallel bedding sandstone, formed by sheetflood under upper flow regime, is the main part in the squence. The types of sandstone are lithic sandstone and feldspathic lithic sandstone with mixed texture. The characteristie of grain size distribution shows that the sand flat has the nature of flood turbidity. (3) Density flow channels. They are striped and branched in plane. The sequence is composed of graded, cross and parallel bedding sandstones with sole mark dish structure and flame structure. Petrologically, they belong to lithic quar-tzose sandstone, with homogeneous texture. The grain size distribution clearly indicates the density flow deposition. ( 4 ) Lake shelf shoal. They are sheet-shaped in plane. The sequence consists of massive-wavy bedding sandstone and wavy - slight wavy bedding siltstone. The type of sandstone is the higher maturity lithic quartzose sandstone resulting from the wave reworking. The distribution of the sandbodies depends on the basin type, geologic history and material source. In Early Tertiary Dongpu Basin was a narrow rift-valley basincontrolled by block movement. At Sha-Ⅳ-Sha-Ⅲ-stage,only three types of sandbody developed as follows: (i) The small subaqueous alluvial fans were located both on east and west sides, perpendicular to the basin axis; (ii)the fans group was at the northern end,and(iii)the density flow channel at the southern end, both of them parallel to the basin axis. These deposits are related to the boundary fault activity and the development of valley respectively. At sha-Ⅱ stage, the lake shelf shoal sandbodies were formed in the central uplift zone and on its slope, because the valleys were fully filled and the central zone was uplifted. In the meanwhile,subaqueous alluvial fan successively developed on east and west sides of the basin, for the boundary faults coutinued to activate. To sum up ,sandbodies of Dongpu Basin have the following characteristics: ( 1 ) Theyare derived from nearby material sources, ( ii ) they are chiefly formed by flood turbidity current,and ( iii ) the subaqueous alluvial fans are dominant in the types of sandbody, and develop successively. Based on the basic conditions for the formation and distribution of sandbodies, the depositional model for the narrow faulted salt lake basin has been given. At present, oil and gas resources have been found in every type of sandbodies, but the sand flat is the richest in accumulation.The density flow channel sandbodies in Sha-Ⅳ member sho
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