Flow depth estimates and avulsion behaviour in alluvial stratigraphy (Willwood Formation, Bighorn Basin, Wyoming, USA)

IF 1.9 3区 地球科学 Q1 GEOLOGY Depositional Record Pub Date : 2023-07-17 DOI:10.1002/dep2.243
Brady Z. Foreman, Grace M. Sutherland, Delaney J. Todd, Kristopher D. Phillips, Anthony Semeraro
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Abstract

The size and geometry of river channels play a central role in sediment transport and the character of deposition within alluvial basins across spatiotemporal scales spanning the initiation of grain movement to the filling of accommodation generated by subsidence. This study compares several different approaches to estimating palaeoflow depths from fluvial deposits in the early Palaeogene Willwood Formation of north-west Wyoming, USA. Fluvial story heights (n = 60) and mud plug thicknesses (n = 13) are statistically indistinguishable from one another and yield palaeoflow depth estimates of 4 to 6 m. The vertical relief on bar clinoforms (n = 112) yields smaller flow depths, by a factor of ca 0.3, with the exception that the largest bar clinoforms match story heights and mud plug estimates. This observation is consistent with modern river data sets that indicate unit bar clinoforms do not capture the reach-mean bank-full flow depths except in rare circumstances. Future studies should use story heights (i.e. compound bar deposits) and mud plugs to estimate bank-full flow depths in alluvial strata. Additionally, the thickness of multi-storied fluvial sandbodies (n = 102) and overbank cycles composed of paired crevasse splay and palaeosol deposits (n = 45) were compared. The two depositional units display statistically indistinguishable mean and median values. Building upon previous depositional models, these observations suggest basin rivers aggraded approximately one flow depth prior to major avulsion. This avulsion process generated widespread crevasse splay deposition across the floodplain. Once the main river channel stem was reestablished, overbank flooding and palaeosol development dominated floodplain settings. The depositional model implies river aggradation autogenically generated topography in the basin that was effectively filled during the subsequent avulsion. This constitutes a meso-timescale (103–104 years) compensational pattern driven by morphodynamics that may account for the high completeness of fossil and palaeoclimate records recovered from the basin.

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冲积地层中的流动深度估算和崩解行为(Willwood组,Bighorn盆地,美国怀俄明州)
河道的大小和几何形状对冲积盆地的泥沙输运和沉积特征起着重要的作用,跨越了时空尺度,从泥沙运动的开始到沉降产生的调节空间的填充。本研究比较了几种估算美国怀俄明州西北部早古近纪威尔伍德组河流沉积古流体深度的不同方法。河流层高(n = 60)和泥塞厚度(n = 13)在统计上彼此无法区分,并产生4至6米的古水流深度估计。斜面棒状岩石(n = 112)的垂直起伏产生较小的流动深度,约为0.3倍,但最大的斜面棒状岩石与层高和泥塞估算值相匹配。这一观察结果与现代河流数据集一致,这些数据集表明,除非在极少数情况下,单位沙洲斜形不能捕获河段平均满岸流深度。未来的研究应该使用层高(即复合坝状沉积物)和泥塞来估计冲积层的满岸流深度。此外,还比较了多层河流砂体(n = 102)和由成对裂缝张开和古土壤沉积物组成的河岸旋回(n = 45)的厚度。这两个沉积单元的平均值和中位数在统计上难以区分。根据以前的沉积模型,这些观察结果表明,在主要的崩裂之前,盆地河流大约积聚了一个水流深度。这一撕脱作用在漫滩上产生了广泛的裂缝状沉积。一旦主河道被重建,河岸上的洪水和古土壤的发育主导了洪泛平原的环境。该沉积模式表明,在随后的崩解过程中,河流淤积自生地形被有效填充。这构成了一个由形态动力学驱动的中时间尺度(103-104年)补偿模式,这可能解释了从盆地恢复的化石和古气候记录的高度完整性。
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CiteScore
4.10
自引率
16.70%
发文量
42
审稿时长
16 weeks
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