13.2.2017

Science Blog: Erosional sedimentary boundary divides clayey sediments for urban planning

Teksti:
Antti Ojala, Senior Scientist
Joonas Virtasalo, Senior Scientist

GTK’s Senior Scientists Antti Ojala and Joonas Virtasalo tell in this blog text about their recent studies where they demonstrated that erosional sedimentary boundary divides clayey sediments for urban planning.  

Clayey sediments that range up to more than 20 m in thinkness in the Helsinki capital region vary consirably in their character and composition. Gray-colored clayey sediment are often truncated at the top by an undulating erosional surface (unconformity), which is covered by a silt to sand layer a few centimetres thick that has a gradual upper contact to darker and Fe-monosuphide-containing brackish water mud.

Ongoing urbanization has caused profound changes in land use and construction in larger cities, where underground planning and the use of unconsolidated sediments with poorer ground conditions has increased. The expansion of cities to locations where superficial sediments are more challenging for construction purposes has increased the need for comprehensive understanding of the engineering geological properties and 3D geometry of fine-grained (clayey) superficial deposits. As a consequence, the geological characteristics of superficial deposits have a substantial role in the development of urban areas, and the need for detailed maps for specific purposes, such as construction suitability assessments, has increased.

In a recent study, Ojala et al. (in press) used sedimentology, ground geophysics and geotechnical measurements to demonstrate that in many clayey areas in the City of Espoo, postglacial lacustrine silty clay is truncated at the top by an undulating erosional surface (unconformity). This is covered by a silt to sand layer a few centimetres thick that has a gradual bioturbated upper contact to greenish-grey brackish water mud. Evidently, this unconformity is of high engineering geological significance, because the overlying organic-rich brackish water mud has a poorer bearing capacity and higher abundance of sulphide minerals that form sulphuric acid upon oxidation (potential acid sulphate soils). “This unconformity was formed at around 7000–8000 years ago, when the glacioeustatic rise of the global ocean level resulted in marine flooding and a short-lived (Litorina Sea) transgression in southern Finland,” explains Antti Ojala of the Geological Survey of Finland (GTK). “The detection of this erosional boundary using sedimentology allows us to separate these two different types of fine-grained material with different engineering geological properties,” Joonas Virtasalo continues.

In their study, Ojala et al. (in press) further demonstrated that this unconformity is spatially well recognised and provides a key stratigraphic marker for wider and more accurate comparison between sites with fine-grained deposits in the Helsinki metropolitan area, and potentially in other urban areas in the northern Baltic Sea region. The unconformity is traceable by various profiling techniques, which improves the 3D mapping and visualization of sediment units for the benefit of urban land-use planning and construction. The study also enhanced the applicability of unconformities (allostratigraphy) to subdivide late- and postglacial sediments into stratigraphic units. The allostratigraphic approach has benefits compared to conventionally used lithostratigraphy, which classifies sediments based on their visual appearance, as the appearance varies over short distances in glacial and glacially-influenced strata.

About the publication

Published in the Bulletin of Engineering Geology and the Environment, the article entitled “An allostratigraphic approach to subdivide fine-grained sediments for urban planning” is the result of several joint projects between GTK and the City of Espoo during 2005–2015, including clayey stratigraphic studies in the Suurpelto, Perkkaa, Mustalahti and Rastaala areas of Espoo.

The publication is available at http://link.springer.com/article/10.1007/s10064-016-0981-4

See also

 

  1. Ojala AEK (2007) Espoon Äijänpellon savikon stratigrafia ja geokemialliset piirteet. Geological Survey of Finland, Espoo, Open file Report P22.4/2007/26, 10 pp. [in Finnish] (Translated title: Sequence stratigraphy and geochemical characteristics of the Äijänpelto site in Espoo)
  2. Ojala AEK (2009) Perkkaan ja Mustalahden alueiden hienorakeisten maalajien kerrosjärjestys ja ominaisuudet. Geological Survey of Finland, Espoo, Open file Report P22.4/2009/58, 24 pp. [in Finnish] (Translated title: Characteristics and stratigraphy of fine-grained deposits at the Perkkaa and Mustalahti sites in Espoo)
  3. Ojala AEK (2011) Construction suitability and 3D architecture of the fine-grained deposits in southern Finland—examples from Espoo. In: Nenonen K, Nurmi P (ed) Geoscience for Society—125th anniversary volume, Geological Survey of Finland, Espoo, Special Paper 49:205–212
  4. Ojala AEK, Palmu J-P (2007) Sedimentological characteristics of Late-Weichselian–Holocene deposits of the Suurpelto area in Espoo, southern Finland. Geological Survey of Finland, Espoo, Special Paper 46:147–156
  5. Ojala AEK, Ikävalko O, Palmu J-P, Vanhala H, Valjus T, Suppala I, Salminen R, Lintinen P, Huotari T (2007) Espoon Suurpellon alueen maaperän ominaispiirteet. Geological Survey of Finland, Espoo, Open file Report P22.4/2007/39, 51 pp. [in Finnish] (Translated title: Characteristics of Quaternary deposits at the Suurpelto construction site
Antti Ojala

Teksti: Antti Ojala

Dr. Antti E.K. Ojala from the Geological Survey of Finland participated the research team and was funded by the Academy of Finland (QUAL-project, grant #259343). Antti is a geologist with a wide experience on aquatic sedimentation processes, micro-structure sedimentology and dating of sediments with varves, radiometric and palaeomagnetic methods. He has led and participated in several international research projects related to aquatic environments and climate change. His recent QUAL project deals with quantitative Holocene climate variability in Southern Svalbard.

Contact:
Antti E.K. Ojala
Geological Survey of Finland
email: antti.ojala@gtk.fi
tel: +358 40 8489796

Joonas Virtasalo

Teksti: Joonas Virtasalo

Dr. Joonas Virtasalo is a marine geologist specializing in marine and glacial sedimentology, stratigraphy, (isotope) geochemistry and seismoacoustic sub-bottom profiling. He enjoys working at different spatial scales, ranging from petrographic thin section analysis to basin-scale stratigraphic studies. He also has professional experience in various geochronological techniques.