The ‘tunnel’ valleys of East Anglia and Fenland, UK: subaerial, not subglacial

IF 0.5 4区 地球科学 Q4 GEOLOGY Proceedings of the Yorkshire Geological Society Pub Date : 2018-08-31 DOI:10.1144/PYGS2017-009
Society R. Gallois
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Abstract

Integrated field surveys, continuously cored boreholes and seismic-reflection surveys in Fenland and the adjacent marine embayment of The Wash have revealed a network of concealed valleys. These are interpreted as a pre-glacial, cold-climate fluvial system that entered the North Sea via a gap in the Chalk escarpment at the mouth of The Wash. The pre-glacial Fenland valley system was overridden by an ice sheet during the Anglian Stage (Marine Isotope Stage (MIS) 12) when it became infilled with glacial deposits, mostly till and varved clay. East of the Chalk escarpment, a pre-glacial network of dipslope valleys drained directly to the North Sea. This was also overridden by Anglian ice and locally over-deepened, presumably by subglacial meltwater. The over-deepened parts have been referred to as tunnel valleys by various authors, but they can be more accurately described as glacially modified fluvial valleys. The analysis of high-resolution seismic-reflection profiles in the North Sea and adjacent areas has revealed complex anastomosing networks of tunnel valleys that were formed subglacially without a fluvial precursor or superimposed on possible fluvial systems that were glacially modified to the extent that they cannot now be confidently identified as fluvial in origin. In comparison, the concealed fluvial valleys of East Anglia and Fenland have been subject to minor glacial modifications, and are morphologically distinct from the North Sea examples. They should not be referred to as tunnel valleys. Prior to the Anglian glaciation, the rivers that flowed from the Midlands to the North Sea passed through one of two gaps in the Chalk escarpment, the Goring Gap in the south and a similar gap that is now concealed beneath the Wash. The field evidence does not support the presence of an additional gap in the intervening Bury St Edmunds area. A wide embayment to the north of this in the Thetford to Swaffham area in which the crest of the Chalk escarpment has been displaced eastwards by up to 20 km is an Anglian glaciation erosion feature.
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英国东安格利亚和芬兰的“隧道”山谷:在地面上,而不是在冰川下
综合野外调查、连续取心钻孔和地震反射调查在芬兰和邻近的海洋海湾发现了一个隐蔽的山谷网络。这些被解释为冰河时期前,寒冷气候的河流系统,通过沃什河口白垩悬崖的一个缺口进入北海。冰期前的芬兰河谷系统在盎格鲁阶段(海洋同位素阶段(MIS) 12)被冰盖覆盖,当时它充满了冰川沉积物,主要是粘土。在白垩悬崖以东,是一个由倾斜山谷组成的冰川前网络,直接流入北海。它也被盎格鲁冰覆盖,局部过度加深,可能是冰下融水造成的。过深的部分被不同的作者称为隧道谷,但它们可以更准确地描述为冰川修饰的河流谷。对北海及邻近地区高分辨率地震反射剖面的分析揭示了复杂的隧道山谷网络,这些隧道山谷是在冰川下形成的,没有河流的前体,或者叠加在可能的河流系统上,这些河流系统经过冰川的改造,现在不能自信地确定它们的起源是河流。相比之下,东安格利亚和芬兰的隐伏河流山谷受到了轻微的冰川改造,在形态上与北海的例子不同。它们不应该被称为隧道谷。在盎格鲁冰期之前,从中部地区流向北海的河流流经白垩悬崖的两个裂口之一,一个是南部的戈林裂口,另一个是现在隐藏在沃什山脉下面的类似裂口。现场证据不支持在伯里圣埃德蒙兹地区存在额外的缺口。在此以北的塞特福德至斯瓦弗姆地区有一个宽阔的海湾,白垩悬崖的顶部向东移动了20公里,这是盎格鲁冰川侵蚀的特征。
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来源期刊
CiteScore
2.00
自引率
30.00%
发文量
7
审稿时长
>12 weeks
期刊介绍: The Proceedings of the Yorkshire Geological Society (PYGS) has been published without a break since 1839. It is one of the leading journals of British geology. Each year two parts are issued containing original research papers on all aspects of geology. Traditionally the Proceedings has given particular attention to the geology of northern England and its neighbouring areas. The submission of papers on related topics but of a more general interest is encouraged. All papers are subjected to the full scrutiny of two independent referees.
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