Jökull


Jökull - 01.01.2012, Page 170

Jökull - 01.01.2012, Page 170
Í. Ö. Benediktsson described the sediment distribution in the marginal zone of the 1890 surge and the architecture and sed- imentary composition of the end moraine, and con- cluded that the moraine was an inseparable part of a marginal sedimentary wedge that was formed during the last few days of the surge. Despite their detailed descriptions, the structural evolution of the actual end- moraine ridge remains to be described. The aim of this paper is therefore to describe the glaciotectonic architecture and the structural evolution of the 1890 end moraine where it consists of fine-grained sedi- ments in Kringilsárrani in the central forefield of Brú- arjökull (Figure 1), and thereby linking the moraine directly into the ice-marginal landsystem presented by Benediktsson et al. (2008). SETTING Brúarjökull is a surge-type outlet of the northern Vatnajökull ice cap in Iceland (Figure 1). It descends from c. 1500 to 600 m a.s.l. and terminates with an approximately 55 km long ice margin (Björnsson et al., 1998). Historical surges occurred in 1625, ∼1730, 1775?, 1810, 1890, and 1963-64, giving a surge cycle of 80–100 years, whereof the active surge phase dura- tion is only about 3 months (Eythorsson, 1963, 1964; Thorarinsson, 1964, 1969; Björnsson et al., 2003). During the last two surges, the glacier advanced 10 and 9 km, respectively, in Kringilsárrani in the cen- tral glacier forefield, with maximum ice-flow veloci- ties of at least 120 m/day (Kjerúlf, 1962; Thorarins- son, 1964, 1969; Guðmundsson et al., 1996). During the surges in 1810 and 1890, Brúarjökull advanced further than during previous surge cycles overriding and deforming a sediment sequence of loess, peat and tephra (Benediktsson et al., 2008). The area of Kringilsárrani, in which the central forefield occurs, is a triangular area bounded by the glacier to the south, and the glacial rivers Jökulsá á Dal and Kringilsá to the east and west, respectively (Figure 1). The Brúarjökull forefield is glacially streamlined with a 6–7 m thick sediment sequence overlying basaltic bedrock. The most prominent landforms of the forefield are end-moraine ridges, ice-cored landforms and ice-free hummocky moraine, crevasse- fill ridges, eskers, concertina eskers, and flutings (Evans and Rea, 1999, 2003; Kjær et al., 2006, 2008; Schomacker et al., 2006; Evans et al., 2007; Schomacker and Kjær, 2007; Benediktsson et al., 2008, 2009) (Figure 1). At present, there is negligible ice movement in the marginal 1–2 km of Brúarjökull and the snout is rapidly retreating and downwasting (Kjær et al., 2008). METHODS The sedimentology and glaciotectonic architecture of the 1890 end moraine in Kringilsárrani were investi- gated in four natural cross-sections that were cleaned and enlarged by hand. Sediment lithologies and struc- tures were documented on the basis of the data chart by Krüger and Kjær (1999) and deformation struc- tures were described according to the terminology of Twiss and Moores (1992) and Evans and Benn (2004). The glaciotectonic architecture was mapped at a scale of 1:20 and structural elements, such as strike and dip of primary bedding and fault planes and plunge and direction of fold axes, were plotted and statistically analysed in a Schmidt equal-area net with the Spheri- Stat 2.2 software. Line and area balancing were applied to the four cross-sections to calculate the horizontal shortening of the sedimentary strata and the depth to the dé- collement plane (Marshak and Mitra, 1988; Bene- diktsson et al., 2010). By tracing tephra marker hori- zons through the sections and measuring the horizon- tal (shortening) distance which they occupy, the dif- ference (∆L) between their length in the undeformed (Lu) and deformed (Ld ) states could be determined. Then the total shortening is described as: s = Ld–Lu / Lu. By calculating the total area of the deformed section (A), the décollement depth can be estimated by: h = A / ∆L. Then it is assumed that the area of the body subjected to stress remained constant before (Au) and after (Ad) deformation. The tephra marker horizons were visually traced through each section on the basis of their general appearance and properties (particularly grain size and colour) and stratigraphi- cal position. It should be noted that tracing the tephra markers through the sections often requires inferences as to where and how different marker segments con- nect. The inferences made are conservative and there- 168 JÖKULL No. 62, 2012
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