陆相盆地河—湖沉积源—汇系统收支分析

源—汇系统分析已成为沉积学领域的研究热点,其中关键参数定量恢复是源—汇系统分析的重要内容。以柴北缘鱼卡地区中侏罗世发育的陆相盆地源—汇系统为例,通过支点法进行源—汇系统收支定量分析。首先识别干流河道沉积,测量或计算河道尺寸,进行粒度分析；然后计算瞬时满岸水流量及沉积物流量,结合古气候与流域分析,对年均沉积物量进行计算,得出在给定地层持续时间内输送的沉积物量；再对沉积区的沉积物量进行测量统计,并与计算出的沉积物量进行对比,分析源—汇系统收支状况。对柴北缘鱼卡地区中侏罗统石门沟组下部沉积的源—汇系统收支定量分析显示,目标研究层段河流沉积的干流满岸深度在3.1~3.3 m,河道宽度为69~77 m,流经了较缓的坡度（0.000 204 6~0.000 217 8）,流速一般为1.046~1.048 m/s,搬运了中—细砂为主的沉积物。该时期流域面积约为3 209.8~3 781.6 km2,流域长度介于177.8~196.2 km,满岸水流量为239.9~286.2 m3/s,满岸推移载荷流量为0.043~0.048 m3/s,满岸悬浮载荷流量范围为0.083~0.094 m3/s。基于现代类似河流的对比研究,计算出鱼卡地区干流年均沉积物搬运量介于158 862.4~179 242.3 m3,在层序S2所持续的2.2 Ma共向沉积区输入349.5~394.3 km3的沉积物,与沉积区所统计的沉积体积（322 km3）大致相符。河流沉积物输入体积的高值约为支点下游沉积区统计体积的1.22倍,如果这一分析结果准确,则表明存在一定程度的沉积物遗失现象,研究区局限发育的重力流沉积可能是沉积物遗失的主要方式。建立的收支模型可进一步推广应用于陆相河湖沉积组合的源—汇系统分析。

Source-to-sink System Budget Analysis of Continental Fluvial-lacustrine Sedimentary Association: A case study from the Middle Jurassic in the northern Qaidam Basin

The current research focus on source-to-sink system analysis in sedimentology involves the quantitative reconstruction of key parameters. The present study adopted the fulcrum approach to quantitative budget analysis of the continental basin source-to-sink system in the Middle Jurassic northern Qaidam Basin, in particular the Yuqia area. The thickness and width of channels, paleoslope and grain size were directly measured or calculated from field outcrop, borehole and geophysical logging. Bankfull water discharge and sediment discharge in the channels were estimated using paleohydrologic equations giving the sediment volume transported in a given stratal duration. The sediment volume in the sedimentary basins was estimated separately （e.g., from stratigraphic isopach maps） to check that the mass of source and sink material on each side of the fulcrum were in balance. This indicates that the trunk river in the Yuqia area was 3.1-3.3 m deep and 69-77 m wide, carrying mainly fine- to medium-grain-size sand, and that it flowed over a low-gradient paleoslopes of 0.000 204 6-0.000 217 8. The drainage area was about 3 209.8-3 781.6 km2； drainage length was 177.8-196.2 km. The bankfull discharge of the trunk river was estimated to be 239.9-286.2 m3/s； the bedload sediment discharge estimated by the Chézy coefficient method ranged from 0.043 m3/s to 0.048 m3/s； the suspended-load sediment discharge from the van Rijn equations ranged from 0.083 m3/s to 0.094 m3/s. Annual total sediment load was estimated to be 158 862.4 m3 to 179 242.3 m3, including both bedload and suspended load. Within the 2.2 Ma duration of the S2 sequence, the river was estimated to have transported 349.5-394.3 km3 of sediment into the basin, which is consistent with the 322 km3 of sediment estimated for the sink area. The high input volume of sediment obtained from the paleohydrologic equation was about 1.22 times the statistical volume of the downstream sedimentary area of the fulcrum. If this is accurate, it implies that some sediment escaped, mainly by gravity flow deposits in the study area. The budget model established in this study may be applied in the analysis of other source-to-sink systems of continental fluvial-lacustrine sedimentary association.