DEBRIS FLOW SEDIMENTS OF SANDY CLASTIC ON THE GENTLE SLOPE AREA OF PRODELTA IN OLIGOCENE, DONGYING BASIN
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摘要: 东营盆地渐新世沙河街组第三段中部前三角洲暗色泥岩中,有许多长条状透镜状砂质碎屑岩体。它们的岩性以具有大量内碎屑为显著特征;并且,在泥质薄层和泥质撕裂块内,具有流纹构造;在韵律组成上,表现出上部为刚性筏、下部为层流段沉积的基本的二分性结构。本文认为这些是泥石流(碎屑流)沉积物,与浊流沉积物具有不同的特点。文内还将湖盆中内碎屑泥石流细分为砂质泥石流、粉砂质泥石流、碳酸盐质泥石流和泥质泥石流,并按组构和岩性讨论了沉积物的韵律分段及其成因意义。
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关键词:
- 混石流(碎屑流)沉积 /
- 湖相沉积 /
- 内碎屑刚性筏 /
- 韵律分段 /
- 东营盆地
Abstract: The 3rd Member of Shahejie Formation Oligocene is the sediments during the deep depressing period of Dongying Basin, Shandong Province. At that time, the basin centre was a west-dip gentle slope of prodelta where a dark mudstone with maximum thickness up to 600 meters was deposited. A lot of elongate and/or lenticular clastic rock bodies have been discoveried in this set of dark mudstone. The studies based on drilling data indicate that the lithological characters of the clastic rock body are as following:( 1 ) It predominantly is sandstone with intraclastic texture and mud-block silt-block rudstone and mud-block grain rock with intraclastic texture as well as dolosiltite, and it is common that angular and stripped argillaceous rent block mixed into these rocks.( 2 ) When the mud is more contentrative(forming argillaceous rent block,especially thin interbeds), the rhyotaxitic structure in the mud is common.( 3 ) The rhythm of the rock consists of the rigid raft in the upper part and the laminar flow zone in the lower part. All the characters indicate that sediments are transported and deposited by cohesive plastic(Bingham substance) flow(namely, debris flow). Based on sedimentary characters, the intraclastic debris folws in the prodelta can be subdivided into sandy debris flow, silty debris flow, carbonate debris flow and argillaceous debris flow. Based on fabric and lithologic characters, the rhythm of the preceding three debris flows can be divided into five division: floating mud-block rudite division, mixed seriate rudite division, parallel fabric division, flow laminae division and pillow lump division. The rhythm of argillaceous debris flow sediments can be divided into three division: sliding sand lump and mud-block division, rhyotaxitic structure division and pillow lump division. The floating mud-block rudite division and the mixed seriate rudite division(sliding sandy lump and mudb-lock division) are just the rigid raft(plug flow) in the division of rheology, and the parallel fabric division and flow laminae division(rhyotaxitic structure division) are just the laminar flow zone. The formation of these debris flows are caused by the fast depositional rate and the unstability the sediments in the high-constructive delta area. The river mouth bar and the far bar with sediments of higher specific gravity prograded and deposited on soft argillaceous sediments with lower specific gravity deposited originally in a prodelta. Such a unstability of gravity resulted in slide which evolved to debris flow. There are necessary conditions for the formation of debris flow in the high-constructive delta. The oil exploration in Dongying Basin indicated that the debris flow clastic rock body distributed broadly in the gentle slope area of prodelta. The sandy debris flow sediments, with lower mud content, higher porosity and permeability, high pressure(in the abnormal high pressure zone), appeared to be very well in oil prospect, and to be an important object for explorating occult oil reservoir. -
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