Overdeepenings in the Swiss plateau: U-shaped geometries underlain by inner gorges

IF 1.8 2区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
Dimitri Bandou, Fritz Schlunegger, Edi Kissling, Urs Marti, Regina Reber, Jonathan Pfander
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Abstract

We investigated the mechanisms leading to the formation of tunnel valleys in the Swiss foreland near Bern. We proceeded through producing 3D maps of the bedrock topography based on drillhole information and a new gravimetric survey combined with modelling. In this context, the combination of information about the densities of the sedimentary fill and of the bedrock, together with published borehole data and the results of gravity surveys along 11 profiles across the valleys, served as input for the application of our 3D gravity modelling software referred to as PRISMA. This ultimately allowed us to model the gravity effect of the Quaternary fill of the overdeepenings and to produce cross-sectional geometries of these troughs. The results show that 2–3 km upstream of the city of Bern, the overdeepenings are approximately 3 km wide. They are characterized by steep to oversteepened lateral flanks and a wide flat base, which we consider as a U-shaped cross-sectional geometry. There, the maximum residual gravity anomaly ranges between − 3 to − 4 mGal for the Aare valley, which is the main overdeepening of the region. Modelling shows that this corresponds to a depression, which reaches a depth of c. 300 m a.s.l. Farther downstream approaching Bern, the erosional trough narrows by c. 1 km, and the base gets shallower by c. 100 m as revealed by drillings. This is supported by the results of our gravity surveys, which disclose a lower maximum gravity effect of c. − 0.8 to − 1.3 mGal. Interestingly, in the Bern city area, these shallow troughs with maximum gravity anomalies ranging from − 1.4 to − 1.8 mGal are underlain by one or multiple inner gorges, which are at least 100 m deep (based on drilling information) and only a few tens of meters wide (disclosed by gravity modelling). At the downstream end of the Bern area, we observe that the trough widens from 2 km at the northern border of Bern to c. 4 km approximately 2 km farther downstream, while the bottom still reaches c. 300 to 200 m a.s.l. Our gravity survey implies that this change is associated with an increase in the maximum residual anomaly, reaching values of − 2.5 mGal. Interestingly, the overdeepening’s cross-sectional geometry in this area has steeply dipping flanks converging to a narrow base, which we consider as V-shaped. We attribute this shape to erosion by water either underneath or at the snout of a glacier, forming a gorge. This narrow bedrock depression was subsequently widened by glacial carving. In this context, strong glacial erosion upstream of the Bern area appears to have overprinted these traces. In contrast, beneath the city of Bern and farther downstream these V-shaped features have been preserved. Available chronological data suggest that the formation of this gorge occurred prior to MIS 8 and possibly during the aftermath of one of the largest glaciations when large fluxes of meltwater resulted in the fluvial carving into the bedrock.
瑞士高原的过深:u形几何形状的内部峡谷
我们研究了伯尔尼附近瑞士前陆隧道山谷形成的机制。我们根据钻孔信息和新的重力测量与建模相结合,制作了基岩地形的3D地图。在这种情况下,关于沉积充填体和基岩密度的信息,连同已公布的钻孔数据和沿山谷11条剖面的重力调查结果,作为我们的3D重力建模软件PRISMA应用程序的输入。这最终使我们能够模拟第四纪过深充填的重力效应,并产生这些槽的横截面几何形状。结果表明,在伯尔尼市上游2 ~ 3 km处,过深区宽度约为3 km。它们的特点是陡峭到过度陡峭的侧翼和宽阔的平坦基础,我们认为这是一个u形的横截面几何形状。其中,Aare山谷的最大残余重力异常在−3 ~−4 mGal之间,是该地区的主要过深区。模拟表明,这对应于一个凹陷,其深度为每年约300米。再往下游接近伯尔尼时,侵蚀槽变窄约1公里,钻孔显示,基底变浅约100米。我们的重力调查结果支持了这一点,该结果显示,最大重力效应较低,为−0.8至−1.3 mGal。有趣的是,在伯尔尼市区,这些最大重力异常范围为- 1.4至- 1.8 mGal的浅槽下方有一个或多个内峡谷,这些内峡谷至少有100米深(根据钻探信息),只有几十米宽(由重力模型揭示)。在伯尔尼地区的下游端,我们观察到槽从伯尔尼北部边界的2 km变宽到下游约2 km的c. 4 km,而底部仍然达到c. 300至200 m a.s.l。我们的重力测量表明,这种变化与最大残余异常的增加有关,达到- 2.5 mGal。有趣的是,该区域的过深剖面几何形状是陡峭倾斜的侧翼向狭窄的底部收敛,我们认为这是v形的。我们将这种形状归因于冰川下方或冰川口处的水侵蚀,形成了一个峡谷。这个狭窄的基岩洼地后来因冰川雕刻而变宽。在这种情况下,伯尔尼地区上游强烈的冰川侵蚀似乎覆盖了这些痕迹。相比之下,在伯尔尼的地下和更远的下游,这些v形的特征被保存了下来。现有的年代数据表明,这个峡谷的形成发生在MIS 8之前,可能是在一次最大的冰川作用之后,当时大量的融水导致河流进入基岩。
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来源期刊
Swiss Journal of Geosciences
Swiss Journal of Geosciences 地学-地质学
CiteScore
4.50
自引率
12.90%
发文量
21
审稿时长
>12 weeks
期刊介绍: The Swiss Journal of Geosciences publishes original research and review articles, with a particular focus on the evolution of the Tethys realm and the Alpine/Himalayan orogen. By consolidating the former Eclogae Geologicae Helvetiae and Swiss Bulletin of Mineralogy and Petrology, this international journal covers all disciplines of the solid Earth Sciences, including their practical applications. The journal gives preference to articles that are of wide interest to the international research community, while at the same time recognising the importance of documenting high-quality geoscientific data in a regional context, including the occasional publication of maps.
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