Sedimentology of Gravelly Alluvial Fans on the Western Margin of Late Triassic, Ordos Basin
摘要: 晚三叠世鄂尔多斯西缘冲积扇沉积主要由碎屑流、颗粒流、片流及河道砾岩等组成。文中讨论了各类砾岩的沉积学特征,同时指出,不同成因的砾岩,其粒度分布曲线形态及某些粒度参数亦不同.据此本文提出了根据-系列新的粒度分布曲线形态和粒度参数Mdf,Rgs和Mdf/Rgs等甄别冲积扇中不同类型砾岩的设想。Abstract: The alluvial fans on the western margin of Late Triassic Ordos Basin can be divided into three parts; upper, middle and lower. Upper fans mainly consist of conglomerates which genetically include debris flow, grain flow, sheetflood flow, mud flow and braided stream deposits. Debris flow deposits are mainly composed of clasts. Most of the conglomerates are matrix supported and poorly sorted. With the increase of distance from source area to depositional sites, maximum clast size are reduced, differences among clast size are minimized, ratio of clast to matrix are increased and the rocks are changed gradually from matrix supported to clast supported. Grain flow deposits are identified by their matrix lacking of features. Clasts in these deposits were cemented by iron-oxides, clay minerals or other materials which had precipitated around clasts as thin film through the processes of diagenesis. Sheetflood flow deposits occur as thinly bedded sandstones or gravelly sandstones. And most of their thickness are about tens of centimeters. Braided stream conglomerates are typical for cross beddings and scours on their bases. Mud flow deposits are characterized by a small percentage of clasts and a large percentage of mud. Long axes of some clasts in mud flow deposits may be perpendicular to beddings. Distribution frequency curves of clast size of the conglomerates mentioned above can be classified obviously into three groups, which are correspondent to debris or grain flow, braided stream and sheedflood flow deposits respectively. Curves for debris or grain flow deposits are characterized by multiple peaks showing lack of sorting. Curves of braided stream conglomerates possess a peak near the finest end and gently wave in coarse part, showing poor sorting. Curves of sheetflood flow deposits rise from coarsest end to finest end monotonously, representing quick waning down of the flood. In addition to the shape differences of the curves, the Maximum Distribution Frequency (Mdf) and the Respondent Gravel Size (Rgs) for each of the three deposits are also different. For debris or grain flow deposits, mean Mdf, Rgs and Mdf/ Rgs are 20. 1.5 and 13 respectively, for braided steam ones, they are 30. 1.2 and 23 and for sheetflood flow ones 38. 0.7 and 54. These parameters have their genetic significance. The lower of the values of Mds / Rgs, the higher cohesiveness or transportation energy the flow would have. The authors believe that it is very hopeful to use these curve patterns and parameters as tools in identifying environments of gravelly alluvial fan deposits.
 1. Bagnoid R A , 1954, Erpcnments on a gravlty-free dispersion of large solid spheres m a Newoman fluid utnder shear.Proc.Roy.Soc.Landon.ser.A225. p,49-63
2. Collnzson J. D，1978. Alluvial sediments. IN: H. G. Reading(ed.)Sedimentary Environments and Factes.Blackweil scientific Publications. p.15 -79
3. Gloppen T. G. et al.，1981、The deposits, internal 'structure and geometary in six alluvial fan-fan. delta bodies (Devonian-Norway)-a study in the significance of bedding sequence in conglomerates.lN: F.G.Ethridge et al (eds).Recent and .Ancient Nonmarine Depositonal Emironments Models for ExpIoraton. Tulsa. Okalahoma.USA.p.49-70
4. Hooke R-Le.B .I1967. Processes on and-reglon alluvial fan J Ceol,、V 75.P438-460.
5. Kochel R.C and Johnson R.A 1984.geomorphology and sedimentology of humld-tcmpcrate alluvial fans Cen tral Virginla I N E H Koster and R.J Steel leds Sedimentulogy of Gravels and Conglomerates. p109-122.
6. Middleton G.V 1970.Evpetmental studtes related to tiysh sedlmentation IN .J Lajole ed flysh Sedlmentology in North America p253-272.
7. Nemec W .and Steel R.J，1984. Alluwal and coglomerates:ther significant features and some comments on grwelly mas,-flow depoW s IN E.H.E8. Rupke N.A., 1978 Deep elastic seas.In H G Readtne(cd)，Sedtmentaw Environments and Fades Biackwell Scientitiu Publcations. Calgary Alberta Canadap372-415
- 文章访问数: 373
- HTML全文浏览量: 0
- PDF下载量: 388
- 被引次数: 0