FLUVIAL
SYSTEMS’ RESPONSE TO A RIFT MARGIN TECTONICS - AN EXAMPLE FROM MAKHTESH
RAMON AREA, SOUTHERN ISRAEL
Ram BEN-DAVID[1],
Yehuda EYAL[2],
Ezra ZILBERMAN[3],
Dan BOWMAN[4]
Key
words: peneplain, morphotectonic activity, Dead Sea Rift Valley, incision,
Late Miocene, Holocene, Kurnub and Judea Group, erosion cirque
ABSTRACT
Makhtesh
Ramon is the largest erosional cirque in the Negev Desert (southern Israel).
It is a feather-shaped drainage system incised along the Ramon monocline.
The Ramon structure, the adjacent Neqarot syncline to the south, and the
Ramon fault between them, are part of the Cretaceous Syrian Arc Fold Belt.
Detailed observations of Makhtesh Ramon and the Nahal Neqarot drainage
systems indicate that their geomorphic evolution is largely a response
to variations in tectonism of the Dead Sea Rift and its western shoulder,
located about 15 km east of the study area. A datum plane for reconstructing
the fluvial evolution is provided by remnants of clast sediments (Miocene
Hazeva Fm.) deposited on a peneplain that extended over the studied area
during the Oligocene. Remnants of the peneplain are presently located on
the shoulders of Makhtesh Ramon at various elevations ranging from +1000
to +450. Based on the datum line reconstruction and cross-sections, it
is shown that the peneplain that truncated the Ramon structure has tilted
eastward 0.7% since the Pliocene (post Hazeva Formation), while the Neqarot
syncline, south of the Ramon syncline, has tilted 1.2%.
The
elliptical exposure of the friable Lower Cretaceous sandstone at the center
of the truncated Ramon structure enabled the development of a new drainage
system towards the rift valley via the lower Neqarot-Meishar drainage system.
This new tributary, the embryonic Makhtesh Ramon flowing to the northeast,
captured the streams that previously drained to the Mediterranean Sea.
Fluvial gaps incised along the southeastern Ramon cliffs and abundant alluvial
fan relict (AFR) near Nahal Ramon that contain sediments from the Neqarot
syncline attest to the inward flows from the south into the Makhtesh. Nevertheless,
a water divide must have existed in the Neqarot syncline between the systems
flowing from the south and those that drained the syncline eastward via
Nahal Neqarot. Remnants of Plio-Pleistocene Arava conglomerate on the eastern
end of the Neqarot syncline contain clasts that are exposed inside Makhtesh
Ramon and in the syncline, indicating a U turn shape of the streams, from
the Neqarot block, into the Makhtesh through the gaps, flowing eastward
and exiting southeast via the Sha’ar Ramon gap. During the Mid to Late
Pleistocene, Nahal Neqarot started to retreat westward as a result of further
down-dropping of the Neqarot block, capturing the last inward Ramon stream
and isolating the Ramon cirque from the southern drainage systems. This
capture was facilitated by the down-tilting of the tectonic block east
of the Meizad-Neqarot fault line probably in the Late Pleistocene.
[1]
ROVED
Geological Consultant, PO Box 98, Lion 99835 and The Hebrew University,
Jerusalem, Israel.
[2]
Department of Geology and Mineralogy, Ben-Gurion University, Beer-Sheva,
Israel.
[3]
Geological Survey of Israel, 30 Malchei-Israel st., Jerusalem, Israel.
[4]
Department of Geography, Ben-Gurion University, Beer-Sheva, Israel.
