The Eastern Sahara is devoid of surface drainage; this unusual characteristic distinguishes its morphology from that of most other desert regions where running water dominates landscape development. A map derived from SIR-A/B and LANDSAT images and the literature, shows the major presently known paleodrainages in the Eastern Sahara. This compilation permits consideration of the key questions: Where did the radar rivers come from and where did they go? Analysis of SIR-A data led McCauley et al. to suggest that the radar rivers, because of their southwestward trends, once flowed into the Chad basin. This key North African feature is a regional structural low formed in the Early Cretaceous in response to initial opening of the South Atlantic. The problem of the origin of headwaters for the radar rivers was less tractable. The idea that the source areas of the radar rivers might originally have been the same as those later captured by the Nile was proposed tentatively. A more extensive review of the Cenozoic tectonic history of North Africa reveals no reason now to suppose that the Central African tributaries of the present Nile were ever connected to the large alluvial valleys in southwestern Egypt and northwestern Sudan. formed in the Early Cretaceous in response to initial opening of the South Atlantic. The problem of the origin of headwaters for the radar rivers was less tractable. The idea that the source areas of the radar rivers might originally have been the same as those (The Ethiopian Highlands) later captured by the Nile was proposed tentatively. A more extensive review of the Cenozoic tectonic history of North Africa reveals no reason now to support that the Central African tributaries of the present Nile were ever connected to the large alluvial valleys in southwestern Egypt and northwestern Sudan

Breed, Carol S.

Mccauley, John F.

Schaber, Gerald G.

English

NASA Technical Reports Server

N87- 17139
THE MEGAGEOMORPHOLOGY OF THE RADAR RIVERS OF THE EASTERN SAHARA
John F. McCauley, Carol S. Breed, and Gerald G. Schaber
U.S. Geological Survey, Flagstaff, Arizona
I. SIGNIFICANCE OF THE RADAR RIVERS
The Eastern Sahara (Fig. i) is devoid of surface drainage -- this
unusual characteristic distinguishes its morphology from that of most other
desert regions where running water dominates landscape development (Peel,
1939). Cyclic aridity, which is responsible for the obscuring eolian blankets
and wind-erosion effects, had set in by the Pliocene-Pleistocene transition,
about 2 million years ago (Williams, 1982). SlR-A/B reveals, however, that
the "bull's-eye" of aridity in North Africa underwent a long prior history of
erosion by southwestward-trending Tertiary streams (McCauley et al., 1986;
Schaber et al., 1986).
Figure 2. derived from SIR-A/B and Landsat images and the litera-
ture, shows the major presently known paleodrainages in the Eastern Sahara.
Of particular importance in this compilation is the presence of the Abu Ballas
Divide. This structural arch contains Jurassic continental clastics that crop
out discontinuously along a topographically subdued prong and extend eastward
from the Gilf Kebir Plateau to the Dakhla-Kharga region, south and west of the
Limestone Plateau. Westward-flowing drainages lying to the south of this
post-Tethys divide would have been shepherded away from the Mediterranean to
take other routes to the sea or to intracontinental basins.
This compilation (Fig. 2) permits consideration of the key
questions: Where did the radar rivers come from and where did they go? The
detailed work done along the Nile Valley in the past three or more decades
(Said, 1981, 1982) is of limited assistance in this effort because the Nile is
chiefly a late Tertiary-Quaternary feature. The various rivers that once
occupied the Nile Trough are relatively young (late Miocene to Quaternary),
and they developed separately from the older (late Eocene) westward-trending
radar rivers. The oldest of the Niles, the Eonile (Said, 1981), is relevant
to the radar river problem because during the Messinian event it cut a canyon
from Cairo to Aswan that was longer and deeper than the Grand Canyon of the
Colorado River in Arizona. This deep southward incision by the Eonile in the
late Miocene created a new drainage barrier in Egypt that severely affected
the subsequent history of the radar rivers.
Analysis of SlR-A data led McCauley et al. (1982) to suggest that
the radar rivers, because of their southwestward trends, once flowed into the
Chad Basin. This key North African feature is a regional structural low
formed in the Early?oCretaceous in response to initial opening of the South
Atlantic (Browne and Fairhead, 1983). The problem of the origin of headwaters
for the radar rivers was less tractable. The idea that the source areas of
the radar rivers might originally have been the same as those (The Ethiopian
Highlands) later captured by the Nile was proposed tentatively. A more
extensive review of the Cenozoic tectonic history of North Africa reveals no
reason now to suppose that the Central African tributaries of the present Nile
were ever connected to the large alluvial valleys in southwestern Egypt and
northwestern Sudan.
25
https://ntrs.nasa.gov/search.jsp?R=19870007706 2020-03-20T13:03:09+00:00Z
New insights into the problem of determining the source areas of
the radar rivers are provided by Issawi (1983). An analysis of the relations
of wadis such as the Allaqi, the Qena, and those of the KomOmboarea, all
east of the present Nile, points to the Red Sea Hills in Egypt and Sudan as a
logical source area for the radar rivers. These valleys mark, in part, the
courses of old westward- and southward-flowing consequent streams. They are
the descendants of a more ancient drainage system that developed independently
of the successive rivers that later occupied the Nile Trough. These streams
and other westward-flowing consequent drainages, which came from the flanks of
the Red Sea uplift prior to integration of the Nile Trough, in either middle
or late Tertiary time must have run out onto the continental slope of a then
more simple African landscape. The radar rivers, when put into the context of
the plate tectonics of North Africa, prove to be the missing links to a prev-
iously unrecognized, now-defunct drainage system of continental proportions.
II. A TRANS-AFRICANDRAINAGESYSTEM
The unusual surficial characteristics of the Eastern Sahara,
particularly its present hyperaridity and unique eolian veneer, are products
of progressive Quaternary desiccation. The region is not a distinct geologic
entity. Its pre-Quaternary history is inseparable from the now-well-documented
sequence of Tertiary tectonic and volcanic events that shaped North Africa as
a whole. The more detailed picture of the history of the radar rivers than
possible in our earlier report (McCauleyet al., 1982) results from post-SIR-A
field work and a numberof recent key.papers on the tectonics of North Africa,
particularly the Central African Rift System (Berminghamet al., 1983).
Shortly after or simultaneously with the regression of the Eocene
Tethys Sea from southwestern Egypt, the Afro-Arabian (Red Sea) Rift began to
form, about 30 to 40 million years ago; the first movementsoccurred as early
as lower middle Eocene. A sequence of events involving doming, rifting, sedi-
mentation, magmatism,and tectonism then took place along the northeast edge
of Africa and profoundly affected this newly emergent landscape (Table i).
More than 2000 m of uplift of the crystalline basement and the Mesozoic and
early Tertiary sedimentary cover occurred in the Red Sea Hills region. A
number of large, roughly northward-trending troughs such as the Qenaand the
Nile, along with east-west structures like the Abu Ballas Divide, formed in
response to the opening of this new oceanic rift. The Red Sea Hills (Fig. I)
would have been the highest mountains in northeastern and central Africa
during Oligocene to middle Miocene time, because the great intraplate volcanic
complexes such as the Darfur and Tibesti had not yet developed. These
northward-trending Red Sea massifs sloped to the west, toward the older
Cretaceous sedimentary basins of north-central Africa, and the consequent
streams emerging from the Red Sea Hills were controlled by these regional
slopes (Fig. 3).
The Tertiary climate of Egypt, and by inference that of much of
North Africa was wet; a dense vegetation cover and moderate to intense chem-
ical weathering suggest warm temperatures. Discharge would have been substan-
tial off the continental slopes of the Eastern Sahara and the developing
Ethiopian-Afar volcanic complex. A family of westward-flowing, vigorous
branch streams that extended manyhundreds of kilometers downstreamfrom their
source areas must have developed. These streams are probably contemporaries
26
of the streams in northern and central Egypt that deposited the gravel spreads
of Oligocene age described by Said (1981, 1983) and Salem (1976). Unlike
those streams, however, the consequent streams from the Red Sea Hills would
have flowed not only over the marine sediments of the Limestone Plateau, but
in southern Egypt and Sudan they would have flowed around and beyond the
Tethys embaymentto run directly on older, pre-Tertiary surfaces. The radar
valleys in southwestern Egypt and northwestern Sudan discovered on SIR images
represent the middle reaches of this drainage system.
The earlier Trans-Saharan Seaway that existed between the Tethys
and the Atlantic in the Late Cretaceous, and which had closed by early
Tertiary time (Kogbe, 1980), may have acted as a palimpsest guidance system
for these new rivers as they sought the lowest ground of central North
Africa. The trunk streams are well expressed in northwestern Sudan, from
whence those trending to the southwest probably found their way into the
Bo_l_-Chad Basins south of Faya. Drainage patterns, however, are confused by
later tectonic and volcanic events in the area east of the Ennedi, between the
domal uplifts of the Tibesti and Darfur regions. Somedischarge into Chadmay
have been along the Mourdi Valley as previously suggested (McCauley et al.,
1982), but more likely paths are the Marhdogoum-Derbeligaps some480 km east
of Faya and the low regions south of the Ennedi (Figs. i, 2, and 3).
Grove and Warren (1968) point out that boreholes in the vicinity of
present Lake Chad show layers of sand alternating with thicker layers of clay,
indicating deposition under fluvio-lacustrine conditions. A hippopotamus jaw,
recovered from below 60 m, suggests that the deposits beneath the Paleo-Chad
Sea (Mega-Chad)and its shrunken Holocene remnant are of probable Tertiary age.
No evidence is reported of regional ponding of the putative
Tertiary drainages in the Chad Basin, as occurred when the Paleo-Chad Sea
formed during the height of the last major North African pluvial about I0,000
years ago. At this time, Mega-Chadwas about the size of the present Caspian
Sea; a reasonably complete shoreline can be traced around it at an elevation
of about 320 m. The waters of this great basin have risen as high as 380 to
400 m and the the Logone River, a tributary of the present lake, may have
acted as a spillway to the Benue. Even today, waters from the shrunken
remnant of Mega-Chadare reported to escape by way of a chain of small lakes
that lead to the Mayo Kebbi River, a tributary of the Benue (Grove, 1970).
The MayoKebbi is presently a small river occupying a large valley; it could
represent the main mid-Tertiary outlet to the Atlantic of our suggested
Trans-African drainage system (TADS).
The Bod_l_ depression now lies about 180 m below the present level
of Lake Chad, and the 500-km-long Bahr el Ghazal that connects them now
intermittently flows northeast. As described earlier, the drainages seen on
both SIR and Landsat images around the flanks of the Tibesti and Darfur
massifs are confused by multiple superposition patterns. Neither of these
situations poses any major constraints on the mid-Tertiary drainage system
proposed here because the Tibesti and Darfur, along with the intervening high
ground of the ennedi, represent an arm of domal uplift related to the Central
African Rift System of Browneand Fairhead (1983). The earliest lavas in the
27
Gebel Marra complex, which now forms the divide between the present Chad and
Nile Basins, have been dated provisionally at 13.5 m.y. (Berminghamet al.,
1983). The present-day topographic obstacles (domes, structural uplifts, and
depressions) did not exist in mid-Tertiary time. The Bahr el Ghazal may
represent the much-modified trace of one of the major trunk streams, or be
part of the master stream of the Trans-African system that traversed the Chad
Basin and flowed into the Atlantic by way of the Benueand the Niger (Fig. 3).
The BenueTrough, which is associated with the opening of the South
Atlantic (Burke et al., 1971) and which split the West African and Congo
Cratons, had closed by the Late Cretaceous. The Benue Trough has been
interpreted by Adighije (1981) as an earlier, abortive attempt to open western
Africa into an ocean similar to the later, more successful Red Sea Rift.
Regardless of this failure, the trough controls the path of the Benue River,
the largest tributary of the Niger River. By late Eocene time deltaic
sediments had begun to spread westward down the Benue Valley toward the
Atlantic and the present mouth of the Niger River, where they began to form a
delta that ultimately grew three times larger than the Nile Delta. This
trough, along with the other preconditioning elements described earlier, must
have determined the courses of the lower reaches of a continental drainage
system that extended some4500 km from the crest line of the Red Sea Hills to
the early Niger Delta (Fig. 3).
Other middle Tertiary branch streams that were originally part of
this Trans-African drainage system may have been present in central and
southern Sudan. Streams there might have flowed across or to the south of the
Darfur area before uplift; the downstream parts of such drainages would have
been controlled by the central African Rift System (particularly the
Ngaoundere and the Abu Gabra Troughs in Cameroon and Chad). These areas are,
however, beyond the scope of our present work.
The active life span of the Trans-African drainages described in
this scenario was at least 20 million years (Table i). The onset of middle
Miocene doming and later intracontinental volcanism in North Africa began to
disrupt the drainage system. Large segments would have remained active, but
how long the drainages might have remained integrated all the way to the
Atlantic is beyond speculation. At the end of the Miocene, an unusual set of
circumstances (drying up of the Mediterranean) created the Eonile, a vigorous
pirate stream in eastern Egypt. The headward (southward) incision of a deep
Eonile Canyon (Said, 1982) brought the newly formed, north-flowing Eonile into
direct conflict with the older, Atlantic-related Trans-African system. From
the coastal cliffs of the Cairo area to Aswan, and possibly as far south as
the second cataract of the present Nile, the Eonile beheaded the
still-operational parts of the Trans-African drainages from their headwaters
in the Red Sea Hills. This fatal blow left only the broad radar river valleys
and the scattered "gravel spreads" elsewhere in the Eastern Sahara as
testimony to the existence of a once-great transcontinental drainage system.
Temporary rejuvenations occurred during the Quaternary pluvials, when episodic
overbank flooding occurred on the terraces and small wadis (braided stream
complexes and local ponds) formed in the floors of many of the old alluvial
valleys. These ephemeral watercourses were utilized by successive groups of
early humans until almost the dawn of the historical period, when climatic
deterioration made the area virtually uninhabitable.
28
REFERENCES
R. F. Peel, "The Gilf Kebir", part 4, i__nnR. A. Bagnold, An expedition to the
Gilf Kebir and Uweinat, 1939: Geographical Journal, v. 93, no. 4, pp.
295-307, 1939.
M. A. J. Williams, "Quaternary environments in northern Africa", in Williams,
M.A.J. and Adamson, D.A. (eds.), A Rand between two Niles: Balkema,
Rotterdam, pp. 13-22, 1982.
Rushdi Said, The geological evolution of the River Nile: Springer-Verlag,
New York, 1981, 151 pp.
Rushdi Said, The geological evolution of the River Nile in Egypt: Zeitschrift
f_r Geomorphologie, v. 26, no. 3, pp. 305-314, 1982.
J. F. McCauley, G. G. Schaber, C. S. Breed, M. J. Grolier, C. V. Haynes, B.
Issawi, C. Elachi, and R. Blom, "Subsurface valleys and geoarchaeology of
the Eastern Sahara revealed by Shuttle Radar": Science, v. 218, no. 4576,
pp. 1004-1020, 1982.
S. E. Browne and J. D. Fairhead, "Gravity study of the Central African Rift
System: a model of continental disruption, v. i. The Ngaoundere and Abu
Gabra Rifts": Tectonophysics, v. 94, pp. 187-203, 1983.
Bahay Issawi, "Ancient rivers of the eastern Egyptian desert": Episodes, v.
1983, no. 2, pp. 3-6, 1983.
P. M. Bermingham, J. D. Fairhead, and G. W. Stuart, "Gravity study of the Cen-
tral African Rift System: a model of continental disruption, 2. The
Darfur domal uplift and associated Cainozoic volcanism": Tectonophysics,
v. 94, pp. 205-222, 1983.
Rushdi Said, "Proposed classification of the Quaternary of Egypt": Journal of
African Earth Sciences, v. i, no. I, pp. 41-45, 1983.
Rafik Salem, "Evolution of Eocene-Miocene sedimentation patterns in parts of
northern Egypt": American Association of Petroleum Geologists Bulletin,
v. 60, no. I, pp. 34-64, 1976.
C. A. Kogbe, "The Trans-Saharan Seaway during the Cretaceous", i__nM. J. Salem,
and M. T. Busrewil, (eds.), The Geology of Libya, v. I, Academic Press,
1980, pp. 91-95.
A. T. Grove and Andrew Warren, "Quaternary landforms and climate on the south
side of the Sahara": Geographic Review, v. 134, pt. 2, pp. 194-208, 1968.
A. T. Grove, "Rise and fall of Lake Chad": Geographical Magazine, v. 42, no.
6, pp. 432-439, 1970.
Kevin Burke, T. F. Dessauvagie, and A. J. Whiteman, "Opening of the Gulf of
Guinea and geological history of the Benue Depression and Niger Delta":
Nature, v. 233, no. 37, pp. 51-55, 1971.
29
C. I. Adighije, "A gravity interpretation of the BenueTrough, Nigeria": Tec-
tonophysics, v. 79, pp. 109-128, 1981.
J. F. McCauley, C. S. Breed, G. G. Schaber, W. P. McHugh, B. Issawi, C.V.
Haynes, M. J. Grolier, and A. E1 Kilani, "Paleodrainages of the Eastern
Sahara -- the Radar Rivers revisited (SIR-A/SIR-B implications for a
mid-Tertiary trans-African drainage system)," IEEE Trans. GRS, v. GE-24,
no. 4, pp. 624-6_8, 1986.
G. G. Schaber, J. F. McCauley, C. S. Breed, G. R. Olhoeft, "Shuttle Imaging
Radar: physical controls on signal penetration and subsurface scattering
in the Eastern Sahara," IEEE Trans. GRS, v. GE-24, no. 4, pp. 603-623,
1986.
F. W. B. Van Eysinga, Geological time table, 3rd Edition, Elsavier Scientific
Publishing Co., Amsterdam, Netherlands, 1975.
30
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Figure I. High altitude (Gemini Xl) photograph shows the presently hyperarid
Eastern Sahara west of the Nile River, and the Red Sea Hills,
former headwaters of TADS, to the east. Piracy by the headward-
incising Eonile cut off the headwaters of TADS, which once flowed
across this now barren, sand-mantled region. SlR-A swath, on
which TADS segments were first detected, indicated by dashed line.
33
Mediterranean Sea
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Peninsula
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Figure 2. Map of Eastern Sahara, showing existing drainages with conventional
symbols, and reconstructed paleodrainages with dotted lines.
Paleodrainage north, south, and west of Kufra, Libya, from recent
literature. Other reconstructions based on analysis of SIR-A,
SIR-B and Landsat imagery, and field investigations. Large arrows
show major drainage gradients inferred for mid-Tertiary time.
34
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35


The megageomorphology of the radar rivers of the eastern Sahara

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