2. STUDY AREA
The Innere Ölgrube rock glacier (also known as Ölgrube Süd; “inner” or
“southern” as the valley head is towards south) is an active rock
glacier located about 1.5 km southeast of the Gepatsch hut in a small,
west-facing tributary valley of the Kauner Valley in the western Ötztal
Alps of North Tyrol (Austria) at an average elevation of 2582 m a.s.l.
with an areal extent of ~0.24 km² (Figure 1). The rock
glacier catchment area measures ~1.83 km² and the
bedrock as well as the moraine and talus deposits consist of
orthogneiss, paragneiss and mica schists of the Ötztal-Stubai
Metamorphic Complex (Hoinkes & Thöni, 1993). Two small cirque glaciers
(“western Hintere Ölgruben Ferner” and the “Hintere Ölgruben
Ferner”) covering an area of ~0.16 km² are (still)
present within the catchment (Buckel & Otto, 2018). A small creek
evolving from glacier melt water infiltrates a few tens of meters
downstream of the lower glacier. Another (very likely the same) creek
appears further down below a moraine ridge, which infiltrates at the
rooting zone of the rock glacier. The rock glacier is composed of two
tongue-shaped lobes with a length of 896 m and a width of 334 m; the
northern, larger lobe is composed of orthogneiss debris, the smaller
southern lobe of paragneiss and mica schists. Transverse and
longitudinal furrows and ridges characterize the distinct surface
morphology. The rooting zone is at an elevation of 2727 m and the active
front that is up to 70 m high with a steep gradient of 40 – 45° is
located at an elevation of 2394 m (Berger et al., 2004).
[Insert Figure 1]
At the base of the steep front several springs are developed (Figure 1c;
details see Berger et al., 2004; Hausmann et al., 2012; Krainer &
Mostler 2002, 2006; Krainer et al., 2007). The rock glacier is active.
During the period 2002 – 2005 flow velocities of up to 2.5 m/year were
recorded near the front (Krainer & Mostler, 2006). More recent analysis
of Groh and Blöthe (2019) based on image tracking are consistent with
these observations, indicating a maximum of 1.7 m/year for the period
2003 – 2015. The internal structure of the rock glacier was determined
by geophysical methods (ground penetrating radar, seismics and
gravimetry; Hausmann et al., 2012). The rock glacier is composed of an
active layer with a thickness of 4 – 6 m which is underlain by the
permafrost body (20 – 30 m). Ice-content in the permafrost body ranges
from 40 to 60 % (Hausmann et al., 2012). Between the permafrost body
and the bedrock an ice-free sediment layer is present, which is 10 – 15
m thick. This unfrozen sediment layer is interpreted to represent a
shallow groundwater storage responsible for base flow even during winter
(Wagner et al., 2020a).