Characterization by Cone Penetration Tests of the decalcified Zandvliet Sand (Lillo Formation, North Belgium)

IF 1.2 4区 地球科学 Q2 GEOLOGY
J. Deckers, J. Verhaegen, I. Vergauwen
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引用次数: 0

Abstract

The sandy Zandvliet Member represents a particular, decalcified facies in the top of the Pliocene Lillo Formation in northern Belgium. Based on the correlation with nearby boreholes at the type locality of the Zandvliet Member, we were able to characterize this unit on Cone Penetration Tests. Compared to the underlying Merksem Member, the Zandvliet Member generally shows markedly lower cone resistance values. Since besides the decalcification, the Zandvliet Member is lithologically nearly identical to the underlying Merksem Member, the lower cone resistance values in the Zandvliet Member compared to the Merksem Member can only be the result of the decalcification of the Zandvliet Member. Indeed, the partly decalcified top of the Merksem Member also gives similar cone resistance values as the Zandvliet Member. Decalcification of the Eocene Brussel Sand in central Belgium is also known to have resulted in lower cone resistance values. Our Cone Penetration Test interpretations show that the thickness of the Zandvliet Member strongly varies across short distances (>10 m across 1 km). As the Zandvliet Member thickens, the underlying Merksem Member thins and vice versa. This trend is not in line with that of the under- and overlying strata, i.e. intraformational, nor with the depositional environment of these units. The thickness changes of the Zandvliet Member therefore purely reflect changes in depth of the post-depositional decalcification into the original shell-bearing sand (i.e. original Merksem Member). This confirms the existing hypothesis that the Zandvliet Member actually represents the decalcified part of the Merksem Member. The anomalous heavy mineralogy of the Zandvliet Member compared to the other members of the Lillo Formation cannot be readily explained by the acid chemical weathering which caused the decalcification. This may rather be related to a change in the primary heavy mineral signal of the upper part of the Merksem Member and equivalent Zandvliet Member compared to the underlying sequences of the Lillo Formation. The reason for the post-depositional decalcification could be similar to the Pleistocene changes in soil acidity invoked for decalcification of time-equivalent Red Crag sand in England.
脱钙化Zandvliet砂(比利时北部Lillo地层)锥体穿透试验表征
砂质Zandvliet段代表了比利时北部上新世Lillo组顶部的一个特殊的脱钙化相。根据与Zandvliet成员类型位置附近钻孔的相关性,我们能够在锥贯入测试中对该单元进行表征。与底层Merksem成员相比,Zandvliet成员通常显示出明显较低的锥体阻力值。由于除了脱钙作用外,Zandvliet段在岩性上与Merksem段几乎相同,因此Zandvliet段与Merksem段相比的锥体阻力值较低只能是Zandvliet段脱钙作用的结果。事实上,Merksem成员部分脱钙的顶部也提供了与Zandvliet成员相似的锥体阻力值。比利时中部始新世布鲁塞尔砂的脱钙化也导致了较低的锥体阻力值。我们的锥体穿透测试解释表明,Zandvliet成员的厚度在短距离内变化很大(1公里>10米)。随着Zandvliet成员变厚,底层Merksem成员变薄,反之亦然。这一趋势与下覆地层(即地层内)不一致,也与这些单元的沉积环境不一致。因此,Zandvliet段的厚度变化纯粹反映了沉积后脱钙作用进入原始含壳砂(即原始Merksem段)的深度变化。这证实了现有的假设,即Zandvliet成员实际上代表了Merksem成员的脱钙部分。与Lillo组其他段相比,Zandvliet段的异常重矿物学不能用引起脱钙化的酸性化学风化来解释。这可能与Merksem段上部和等效Zandvliet段的原生重矿物信号与Lillo组下伏层序的变化有关。沉积后脱钙的原因可能与英国时间等效红岩砂脱钙所引起的更新世土壤酸度变化相似。
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来源期刊
Geologica Belgica
Geologica Belgica 地学-地质学
CiteScore
4.70
自引率
27.80%
发文量
8
审稿时长
>12 weeks
期刊介绍: Geologica Belgica is a Belgian journal that welcomes papers concerning all aspects of the earth sciences, with a particular emphasis on the regional geology of Belgium, North West Europe and central Africa. Papers not dedicated to the geology of Belgium, North West Europe and central Africa are only accepted when one of the authors is linked to a Belgian University or Institution. Thematic issues are highly appreciated. In this case, guest editors take in charge the selection of the manuscripts and the subject of the papers can be enlarged. The journal is in open access. Submitted manuscripts should be concise, presenting material not previously published. The journal also encourages the publication of papers from Belgian junior authors. Short letters are accepted. Papers written in English are preferred. Each mansucript will be reviewed by at least two reviewers.
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