ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

IRINA E. P AVLOVSKAYA
The influence of neotectonic processes on the
river valleys morphology in Belarus
Institute of Geological Sciences, Academy of Sciences of Belarus,
Zhodinskaya str., 7, Minsk 220141, Belarus
Belarus lies within the area of glacial accumulative relief.
The thickness of the Quaternary cover amounts 80-100m
and more on the most part of the region. Despite the concealing
effect of the glacial morphogenesis an influence of
fault and block neotectonics on the rivers network pattern
and changes of river channels gradients can be distinctly
traced.
Three areas distinguished by a degree of endogenic effect
on the recent rivers are recognized. The first area covers
territories with the Quaternary deposits thickness of 140­
160 m and more.Tills dominate among the Quaternary accumulations.
River valleys are mainly of the Late Pleistocene
age. The river network has a dendritic pattern, slightly
depending on fracture zones distribution. 36,2 % of the total
number of river profiles deformations are caused by
fault tectonics. 80-100 m thickness of the Quaternary cover
prevalence of the glaciofluvial deposits and the Middle
Pleistocene age of river valleys are characteristic of the second
area. The river network predominently follows the
submeridianal fault zones . That caused the subparallel
orientation of the main river and its tributaries as well as
the curving of lower sections of the main river tributaries
valleys at the right angle in places of their crossing of fault
zones. The anomalies of river channels, conditioned by
fault block tectonics makes 72.80/0. The third area is characterised
by insignificant thickness of the Quaternary cover
(20-40 m). The glaciofluvial and alluvial accumulations
prevail among the Quaternary deposits here. The river valleys
formation dates to the Early and Middle Pleistocene .
The most distinct dependence of channel profiles deformations
on neotectonically active fault zones orientation is
displayed in this area. The structurally caused anomalies
make 82.1 % of the total number of profile anomalies.
Besides changes of profiles and configurations of valleys,
the neotectonic influence may be revealed in alteration of
terraces width. Within the blocks of prevailing neotectonic
subsidence a widenning of the flood-planes, the first and
second fluvial terraces is registered. Neotectonic rising of
separate parts of the large river basins provokes the terraces
narrowing. In some cases terraces princh out and a valley
acquires the canyon shape. This morphological transformations
are accompanied by changes of the alluvium
facies (perstrative and instrative alluvium of the flood-plain
and meander facies prevail in subsiding locations, konstrative
channel alluvium - in rising terrains).
Analysis of belorussian river systems configuration and
morphology has allowed to establish that the degree of
neotectonic processes reflection in morphological features
of river valleys depends on thickness of the Quaternary deposits,
prevailing genetic type of the Quaternary accumula-
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tions and the age of relief. Minimal amount of neotectonic
deformations of the drainage network is observed within
the area of Late Pleistocene relief with thick glacial deposits
and till dominating among them.
FRANK I. PAZZAGLIA1 & THOMAS W. GARDNER 2
Late Cenozoic large-scale landscape evolution of the
U.S. Atlantic passive margin
1Department of Earth and Planetary Sciences, University of New Mexico,
Albuquerque, NM, 87131 U.S.A.
2 Department of Geology, Trinity University, San Antonio,
TX, 78212 U.S.A.
The u. S. Atlantic passive margin has been fertile ground
for the development of long-term landform evolution models.
In this paper, we review large-scale landscape evolution
models of the Appalachians to critically reevaluate the
origin of Appalachian drainage, understand the role of late
Cenozoic flexural isostatic deformation, and provide insights
into the question of why North America is one of the
few continents not widely recognized to have a Great
Escarpment along its rifted margin.
Post-rift denudation of the Appalachian mountains over
the past 180 m.y. has left few stratigraphic and geomorphic
clues in the Appalachian landscape, but is well-preserved
in the sediments of the Atlantic offshore basins. We propose
that long-term denudation of the continent and offshore
sediment deposition drives flexural isostatic deformation
of the margin which we constrain with late Cenozoic
fluvial terraces correlated to dated Coastal Plain marine
deposits, and known volumes of sediment in offshore
basins. Two simple geodynamic models, a one-dimensional
line load model and two-dimensional distributed point
load model are constructed and parameterized with results
from previous, published studies. Model results strongly
suggest that: (1) the primary late Cenozoic deformational
response of the u.s. Atlantic margin has been flexural subsidence,
(2) the Fall Zone is the landward geomorphic expression
of a flexural hinge, and (3) nearly 100 m of post­
20 Ma flexurally driven rock uplift has occurred west of
the Fall Zone.
Post-rift flexural subsidence of the middle Atlantic margin
produces a short, steep fall to base level for east-flowing
streams. Steep fluvial gradients, coupled with the narrow
outcrop width of resistant Piedmont and Blue Ridge rocktypes,
favored rapid growth and westward extension of
Atlantic drainages and the concomitant dissection of any
rift-generated Great Escarpment. Along the middle Atlantic
margin, the drainage divide has migrated westward to
the Allegheny Plateau where it forms the highly-embayed,
east-facing Allegheny escarpment which typically has less
than 500 m of relief. In contrast, the relatively subdued
flexural depression of the southern Atlantic margin produ-