Ductile Shear Zone Structures

A deformation till at Gliedenberg, Dammer Berge, Germany (Fig. 8.12) is a clear illustration of how subglacial shearing of sediments may produce a laminated till and ultimately a massive diamicton within just 100 m. This process is similar to the one described by Hart and Boulton (1991). The Dammer Berge (Van der Wateren, 1987, 1995) are a thrust moraine formed during the Drenthe advance of the Saalian glaciation. They were overridden when the ice expanded to the Saalian limit, 100 km further to the south. The till is a mixture derived from the four Tertiary and

Ice flow

Nappe

Ice flow

Nappe

10 m Compressed foresets
Ductile Shear Zone

Figure 8.12 Deformation till at Gliedenberg, Dammer Berge, Germany (Van der Wateren, 1995). The Dammer Berge is a thrust moraine from the Saalian Drenthe advance and which had been overridden when the ice expanded 100 km further to the south. The till is a mixture derived from the four Tertiary and Pleistocene formations which comprise the thrust moraine. A nappe containing these sediments is exposed 50 m upstream (north) from the till outcrop. They are folded and extruded in a diapiric structure, in front of the nappe, cored with Tertiary clay (black). A) Schematic diagram showing deformation till and its source sediments at Gliedenberg, Dammer Berge thrust moraine, Germany (Van der Wateren (1995). Subglacial shearing of the source sediments produced a laminated diamict with numerous boudins and detached intrafolial folds. The lamination therefore is not a sedimentary layering but a transposed foliation, the product of glacitectonic deformation. Numbers refer to caption of Fig. 8.12b. B) The deformation till is composed of: 1 = Tertiary glauconitic clay, 2 = white Lower Pleistocene fluvial sand, 3 = pink Middle Pleistocene fluvial sand, 4 = green glauconitic glacifluvial sand (mixture of Pleistocene and Tertiary sediments), 5 = brown diamict.

Figure 8.12 Deformation till at Gliedenberg, Dammer Berge, Germany (Van der Wateren, 1995). The Dammer Berge is a thrust moraine from the Saalian Drenthe advance and which had been overridden when the ice expanded 100 km further to the south. The till is a mixture derived from the four Tertiary and Pleistocene formations which comprise the thrust moraine. A nappe containing these sediments is exposed 50 m upstream (north) from the till outcrop. They are folded and extruded in a diapiric structure, in front of the nappe, cored with Tertiary clay (black). A) Schematic diagram showing deformation till and its source sediments at Gliedenberg, Dammer Berge thrust moraine, Germany (Van der Wateren (1995). Subglacial shearing of the source sediments produced a laminated diamict with numerous boudins and detached intrafolial folds. The lamination therefore is not a sedimentary layering but a transposed foliation, the product of glacitectonic deformation. Numbers refer to caption of Fig. 8.12b. B) The deformation till is composed of: 1 = Tertiary glauconitic clay, 2 = white Lower Pleistocene fluvial sand, 3 = pink Middle Pleistocene fluvial sand, 4 = green glauconitic glacifluvial sand (mixture of Pleistocene and Tertiary sediments), 5 = brown diamict.

Pleistocene formations, which build the thrust moraine. A thrust sheet containing these sediments is exposed 50 m upstream (north) from the till outcrop. As a result of loading by this structure, the sediments are folded and extruded in front of it in a diapiric structure, cored with Tertiary clay (Fig. 8.12a).

The structures downstream of this are the product of overprinting of the upright and steeply dipping folds and diapirs by intense horizontal simple shear. Shearing of the source sediments produced a laminated diamict with lenses, discontinuous layers, boudins and detached intrafolial folds in which the original lithology can still be distinguished (Fig. 8.12b). The lamination clearly is not a sedimentary layering but a transposed foliation, the product of glacitectonic deformation. The individual components of the deformation till are Tertiary glauconitic clay, Lower Pleistocene white quartz-rich fluvial sand, Middle Pleistocene pink Buntsandstein-rich fluvial sand, green glauconite-rich glacifluvial sand (mixture of Pleistocene and Tertiary sediments) and a brown diamict.

Only a few tens of metres downstream the sediment changes aspect from a laminated diamict to a macroscopically massive till in a manner described by Boulton (1996a) as 'dissolving' of the sorted sediment bodies into the unsorted matrix. On the microscale, the matrix appears less homogeneous, as it consists of clay-rich and sandy laminae. The structural symmetry of the S, C and C' cleavages indicates sinistral shearing, consistent with the macrostructural geometry. Small boudins of muddy diamict and strongly oriented clay rims around rotated sand grains indicate very high finite shear strain, the sediment lenses and boudins forming 'islands of low strain' (Passchier and Trouw, 1996) in a high-strain matrix. At high finite strains (related to the transport distance within the till) they are only visible at high magnifications in thin sections (Van der Wateren, 1995, 2002a; Van der Wateren et al, 2000). The example of the Gliedenberg till demonstrates that homogenization may occur within a distance of the order of 100 m.

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Post a comment