ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

sis are considered. In the northern Calcareous Alps, topographic
elevations and morphology are strongly influenced
by the 'thick Upper Triassic carbonates (Hauptdolomit vs.
Dachsteinkalk). In the central zone of the mountain range,
relief and elevations are mainly determined by'differental
uplift of tectonic blocks in Neogene times. Paleogene apatite
fission track ages >30 Ma are characteristic of Austroalpine
basement areas east of the Tauern window, where
relatively smooth morphology in the peak regions represent
modified Oligocene-Miocene land surfaces. In contrast,
the Niedere Tauern have a rugged relief and higher elevations
and reveal Miocene apatite fission track ages. They
are part of a wide zone including also the areas west and
south of the Tauern window and the Tauern window itself,
where Miocene apatite fission track ages correspond with
high relief and topography.
The geomorphologic evolution since the' start of extrusion
tectonics around the Oligocene/Miocene boundary is reconstructed
from information attained (1) by apatite fission
track data in the mountain body as well as in conglomerates
and sandstones of the Neogene basin deposits, (2)
by provenance and grain size studies of clastic material in
these basins, (3) by sediment mass balances, and (4) by the
evaluation of the main tectonic lines active in Neogene times
and acting as guidelines for the drainage system. The
result is a reconstruction of the paleogeology and paleotopography
for time slices in the Late Oligocene and Miocene,
shown in maps on the basis of a palinspastic restoration
of the tectonic evolution. This restoration shows that the
Penninic Tauern window was essentially exhumed by tectonic
denudation and only to a minor extent (in the order
of 10-20 0/0) by erosion. The tectonic blocks presently positioned
to the west and east of the window were originally
coherent and were pulled apart for 160 km in the course of
extrusion tectonics, which means stretching of the entire
Eastern Alps in the order of 155% since the Oligocene.
The Northern Calcareous Alps (Nca), the Rhenodanubian
Flysch and the Subalpine Molasse experienced the same
amount of stretching. This means that the Eastern Alps attained
their elongated shape in E-W direction during Neogene
postcollisional tectonics. Our paleotopographic reconstruction
has been performed for three time slices:
1. Late Oligocene (ca. 29-23 Ma): In the area west of the
later Tauern window a mountainous relief already existed,
which is probably related to Oligocene uplift processes in
the Swiss Alps. The western Nca formed a mountain range
that dewatered into the molasse basin due north. The crystalline
area to its south delivered material in the molasse
basin in the Chiemsee area further east, because the drainage
was deviated by the paleo-Inn valley which followed a
prominent fault zone. The main water divide was situated
as far south as the Periadriatic magmatic belt, today positioned
south of the main divide. The area east of the later
T auern window was a hilly landscape formed by Paleozoic
rock sequences, which delivered eroded material due
north. This conglomeratic and sandy material was deposited
by braided rivers as a sediment sheet several hundred
metres thick on top of the central and eastern Nca, which
formed lowlands not much above sea level.
2. Early/Middle Miocene (ca. 18-15 Ma): In the western
part of the Eastern Alps the situation did not experience
much change. The area of the later Tauern window became
increasingly mountainous. Enhanced tectonic movements
caused fundamental changes in morphology and
drainage pattern east of the later Tauern window. Rivers
created a higher relief in the uplifting area and followed
the main tectonic lines. They became oriented towards the
east and deposited their load in the Styrian basin. A number
of short-lived intramontane basins formed as pullapart
structures along the main tectonic lines. The clastic deposits
on top of the Nca started to be eroded and redeposited
in the molasse basin further north.
3. Middle/Late Miocene (ca. 13-8 Ma): The Penninic contents
of the Tauern window was exhumed by that time.
Surface uplift governed the entire Eastern Alps. The paleo­
Inn river built a large fan system in the molasse basin NE
of the town of Salzburg. The central and eastern Nca started
their uplift history in pulses separated by periods of relative
quiescence. Around that time, the Eastern Alps start
to become a climatic divide.
AMOS FRUMKIN
Radiocarbon dating of a karst terrain exposure
Cave Research Section, Department of Geography,
The Hebrew University of Jerusalem, 91905, Israel
Mount Sedom salt diapir, Israel, appears to be unique in its
short time scale of landscape evolution, for it was exposed
above base level only during the Holocene. An extensive
salt karst system has developed during this short period.
Multi-level vadose caves were 14C dated using wood fragments
embedded in alluvial deposits. The oldest date of each
cave is used to constrain the age of the salt exposure.
The area exposed above base level has grown from a small
hill at 7000 yr B.P. to two ridges by 4000 yr B.P., which
combined recently to form the present elongated mountain.
The upper portion of the southeastern escarpment was the
first to rise above base level -7100 yr B.P. Caves in the surrounding
area indicate gradual landscape exposure around
this initial karstified area between 7000 and 4000 yr B.P.
The northern part of the mountain experienced a similar
exposure history, lagging some 3000 yr after the southern
part. This lag may be attributed to the narrow width of the
diapir in the north, which increases viscous drag at the
borders of the rising diapir.
The method used is different from other techniques of surface
exposure dating, such as in situ produced cosmogenic
isotopes and may be applicable to other karst landscapes.
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