Damage Zone Origin of Teufelsmauer
Electronic Supplementary Material
Teufelsmauer and the Subhercynian Cretaceous Basin
The SCB is one of the many sedimentary Cretaceous basins in western and central Europe filled with several hundreds of meters of clastic and carbonate sediments. Throughout its eastern extent (Fig. 1a), the surface geology is dominated by Lower (Hauterrivian to Barremian, ~130-
120 Ma) and Upper Cretaceous sandstones, where the Upper Cretaceous units can be found as an up to 3000 m thick succession from Turonian (~90 Ma) to Campanian (~83 Ma). The rocks that form the different segments of the Teufelsmauer occur within different layers of that sequence, including the Hauterrivian to Barremian Neocomian Sandstone , the Coniacian Involutus
Sandstone (~88 Ma), and the Upper Santonian Heidelberg Sandstone (~84 Ma). Despite their age difference, they show similar grain sizes (~250 µm in diameter) and porosities (~15-20 %) and are of similar texture and composition, classifying them as well sorted, well rounded, porous quartz arenites.
These sandstones were affected by folding and faulting (Fig. 1b) during the uplift of the
Harz Mountains along the Northern Harz Mountains Border Fault, herein referred to as Harz border fault. The uplift of the Harz Mountains was originally regarded to be of simple block tectonic character (e.g. Mohr, 1978). However, the complexity of the geology within the SCB and especially along the northern rim of the Harz Mountains indicated a more sophisticated structural history of the area. This motivated a number of studies to better characterize and describe the nature of the Harz border fault. Flick (1986) suggested that the Northern Harz Mountains Border
Fault was as nappe-like structure thrusting deeper units over the SCB. Other studies described the
Harz border fault as a highly segmented fault system reactivated multiple times with changing
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Damage Zone Origin of Teufelsmauer character (Stackebrandt, 1983, 1986; Tröger, 1995) or found evidence for strike-slip motion and interpreted the Harz Mountains as push-up structure due to strike-slip tectonics (Wrede, 1988,
2008; König and Wrede, 1994).
Recently, the thrusting character of the Harz border fault as a frontal fault (Voigt et al.,
2004; Voigt and Eynatten, 2006) has been revitalized. Based on the progressively unconformable deposition of the Upper Cretaceous sequence in the SCB, the fault was found to be continuously active during Late Cretaceous time and is interpreted to have caused a fault-propagation anticline in the Mesozoic basinal units to form above the fault tip (Voigt et al., 2004). At least four unconformities in SCB occur within Santonian to Campanian units (Voigt and Eynatten, 2004;
2006; Voigt et al., 2004), constraining the timing of fault activity and pointing out the synkinematic deposition of the Santonian and Campanian units during the Harz uplift.
Due to this folding and faulting, the eastern SCB is subdivided into two sub-basins (e.g.
Stackebrandt, 1986), the Blankenburg and Halberstadt Basins (Fig. 1). The sub-basin closer to the
Harz block, the Blankenburg Syncline (Fig. 1), was affected most by the tectonic processes. The northern flank of the Blankenburg Syncline is moderately inclined, whereas the southern flank is dipping vertically and is, in places, overturned (Fig. 1b). At both flanks of the Blankenburg
Syncline, narrow ridges of highly consolidated and erosionally resistant sandstone form the prominent towers of the Teufelsmauer (Fig. 2a), whereas the horizontal parts of the same units, found in the center of the Blankenburg Synline, are largely unconsolidated.
The most prominent of all segments of the Teufelsmauer is an up to 30 m high and 3 to 4 m thick layer (Fig. 2a) that shows an extraordinarily strong cementation so that it is frequently described as a quartzite or as quartzite-like in the German literature (Schulschenk, 1930;
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Damage Zone Origin of Teufelsmauer
Trusheim, 1941; Tiwari and Roy, 1974; Knappe and Tröger, 1988; Voigt and Eynatten, 2008).
The cementation is caused by quartz overgrowths (Tiwari and Roy, 1974; Voigt and Eynatten,
2006), resulting in a macroscopic and microscopic crystalline (Fig. 2c) rather than sedimentary texture as compared to the directly adjacent sandstones (Fig. 2b, d). The quartz overgrowths are suggested to originate from silica-enriched artesian waters, which migrated through the most permeable layers from the center of the Blankenburg Basin into both flanks (Voigt and Eynatten,
2006).
Interestingly, all segments of the Teufelsmauer contain well-organized fracture networks with systematic geometries. These fractures networks were variously described as deformed quartz dikes (Bankwitz et al., 1988); reticulated or net-like ribs, grid formations, hole-weathering and honeycomb structure (Tiwari and Roy, 1974); quartz-filled cracks (Tiwari and Roy, 1974;
Stackebrandt, 1986; Bankwitz et al., 1988; Franzke, 1990); vein-like infiltrations of quartz
(Stackebrandt and Franzke, 1989); shear plane systems (Stackebrandt, 1986; Franzke, 1990;
Patzelt, 2003) or silicified shear structures (Stackebrandt and Franzke, 1985). None of these studies, however, related the fractures of the different Teufelsmauer segments to each other nor was the origin of the fractures considered in conjunction with the deformation of the porous host rock.
Cemented sandstone or quartzite?
The most pronounced towers of the Teufelsmauer (Fig. 2a) show relatively thin deformation bands, because this compositionally pure arenitic horizon of the Heidelberg
Sandstone, in which the bands grew, was affected early on by a widespread and unusually strong
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Damage Zone Origin of Teufelsmauer cementation. The crystalline texture (Fig. 2c) of the rocks there has them frequently referred to as a quartzite (e.g. Schulschenk, 1930; Trusheim, 1941; Tiwari and Roy, 1974; Knappe and Tröger,
1988; Voigt and Eynatten, 2008). The term quartzite, a loanword originating from the German term Quarzit , is commonly defined as metamorphosed sandstone, where the metamorphism is caused by either heat and/or pressure from tectonic processes and leads to the change of shape of the quartz grains by recrystallization (e.g. Neuendorf et al., 2005).
The term orthoquartzite was introduced for quartzose sandstones that underwent heavy cementation or recrystallization leading to interlocked grains, where cementation only occurs due to infiltration or pressure (Krynine, 1941; 1945; 1948; Ireland, 1974; Pettijohn et al., 1987) without having been subjected to the dynamic processes associated with metamorphism (Krynine,
1941). Cementation processes during structural diagenesis (Laubach et al., 2010) can lead to textures of quartz arenites and other quartzose sandstones (e.g. Lander et al., 2008; 2009; Lander and Bonnell, 2010) consistent with an orthoquartzite .
Whether one could describe these rock towers as quartzites seems a highly subjective matter, since these rocks meet both the criteria for orthoquartzites and cemented sandstones. The rocks have not lost their sedimentary texture, despite the observed heavy cementation. On the other hand, however, an origin of the heavy cementation induced by tectonic compression would classify the rocks as orthoquartzites . Cementation due to tectonic compression is likely, since the cementation is in places accompanied by grain bulging recrystallization (Fig. 8b), undulose extinction of grains (Fig. 8) and pressure solution (Fig. 5f), all evidence for increased pressure during the cementation process. The observed microstructural mechanisms, also frequently associated with low-grade metamorphism, indicate increased pressures and induce a change of
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Damage Zone Origin of Teufelsmauer shape and size of the quartz grains. Therefore, we suggest that the term quartzite is justified and appropriate to apply to the heavily cemented rocks of the Teufelsmauer. Such a tectonically induced quartzitification of sandstones is also in agreement with mechanisms of structural diagenesis (Laubach et al., 2010) observed along other prominent fault damage zones in sandstones, such as the Moab Fault, Utah (Eichhubl et al., 2009), where, similar to the sandstones of the SCB, fault-controlled diagenesis lead to iron oxide accumulations and abundant quartz overgrowth precipitation.
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