Some ongoing involvement in regional geomorphologic research in South America is described. Because of association with LARS at Purdue University, there has been engagement, vicarious or adivsory, in projects which led to LANDSAT 1-2 mapping of the natural resources of Bolivia (1:8,000,000 scale), and preparation of a geographic information system which mapped the general hydrology, geology, soils, and vegetation of Ecuador (1:4,000,000 scale). Currently we are involved more specifically in geological-geomorphological mapping of the Venezuelan portion of the Guiana shield, and because of manuscript limitations only questions pertinent to this region are posed in the ensuing discussion

Melhorn, W. N.

English

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SPACE IMAGERY AND SOME GEOMORPHOLOGICAL PROBLEMS
OF THE GUIANA SHIELD, S.A.
Wilton N. Melhorn
Purdue University
Introduction
Like most academic geomorphologists, I know relatively little about
planetary geomorphology, nor am I especially qualified to engage in
constructive "megathink" about global geomorphology, though I have been guilty
of some original considerations about the reality of ancient cyclic erosion
surfaces in the global framework (Melhorn and Edgar, 1975).
For purposes of this workshop, I therefore will describe some ongoing
involvement in regional geomorphologic research in South America. Because of
association with LARS at Purdue University, there has been engagement,
vicarious or advisory, in projects which led to Landsat 1-2 mapping of the
natural resources of Bolivia (1:8,000,000 scale), and preparation of a
geographic information system which mapped the general hydrology, geology,
soils, and vegetation of Ecuador (1:4,000,000 scale). Currently we are
involved more specifically in geological-geomorphological mapping of the
Venezuelan portion of the Guiana Shield, and because of manuscript limitations
only questions pertinent to this region are posed in the ensuing discussion.
The study of Venezuelan Guiana has evolved in the last few years from
preliminary study of Landsat imagery (Fig. I) into more localized and detailed
geomorphologic analysis of the Roraima portion of the Guiana Shield, using a
semi-controlled radar imagery mosaic (Fig. 2) and 4,000 black-and-white aerial
photos, both available only at 1:250,000 scale. Gradually emerging is a
geomorphological-geological map, at the same scale, intended for use in
regional planning and economic development of a thinly populated, wild,
inaccessible region of rich natural resource potential extremely important to
a developing nation. As expected, each step forward has led to new questions
about the geomorphic history and evolution of the region.
Regional Setting
Some geological background is prerequisite to questioning and analysis.
The Guiana Shield is of subcontinental magnitude. It consists of ancient
3.6-3.8 b.y. old, intensively deformed and metamorphosed Archean rocks.
Scattered across the Shield are remnant outliers of the Roraima Group, a
sedimentary sequence as much as 3300 m thick that lies unconformably on the
older basement; presumably the Roraima is of Proterozoic age on the basis of
radiometric dates obtained on dolerite dikes and sills which crosscut the
Roraima in part. Apparently the Roraima formerly was a great prism of intra-
cratonic sediments nested on and within the broad, bowl-shaped older pre-
Cambrian basin. However, long-term erosion has removed most of this cover,
leaving outliers marked by a series of near-vertical scarps, and a series of
magnificent step-like cuestas, whose dip slopes incline 4°-6 ° inward towards
the center of the Proterozoic depobasin. Because of lithologic variations,
sloping stripped surfaces abound and are the most striking landform element.
Gansser(1954) suggested that the Roraima formerly covered as much as 475,000
km2 and extended from Surinam to the Sierra Macarena in Colombia. Present
46
https://ntrs.nasa.gov/search.jsp?R=19850024055 2020-03-20T18:10:29+00:00Z
Fig. 1 - Examples of Landsat images, a) Chiguao River headwaters.
Auyantepu{ to the right, b) Detail of same area, showing
sandstone "bridge" between Guiquinima and Auyantepui'.
47
Fig. 2 - An interpretation of radar images of Amazonas Territory
(from Yanez, 1973). Approx. area: I00,000 km2.
48
exposure is limited to about 75,000 km2, mostly in southeastern Venezuela, and
this is the area on which discussion focuses.
The Roraima Group divides upward into three formations -- Canaima,
Guaiquinima, and Auyantepui -- each with a rather distinctive geomorphological
signature or surface expression. The lower and upper formations are poorly
cemented quartzose or feldspathic sandstones, arkoses, conglomeratic sand-
stones, and conglomerate lenses, all apparently deposited in a fluvio-deltaic
environment. The Canaima is poorly exposed under dense rain forest and
complete stratigraphic sections are few. The Auyantepui is a massive cliff-
former, with nearly vertical 500 m high scarps that culminate in flat-topped
or bowl-shaped, broad synclinal folds of the table mountains (tepufs) that
•
rise precipitously above the Orlnoco-Caronl lowlands (Fig. 3). Carbonate
content of the rocks based on soil analysis is less than 4%, yet summits of
some tepuis (Duida, Guaiquinima) are marked by 300 m deep, 2 km long, fluted
karren "pseudokarst" shafts (cimas) developed in massive but poorly indurated
quartzitic rocks. Local shale members apparently weather to hydromorphic
clays and silts, which in the past were redeposited in alluvial flood plains,
but do not constitute much of the sediment transported today.
The Guaiquinima is stratigraphically distinct because of the presence of
volcanic tuffs, intercalated with hard beds of red or green jasper, and
associated with intrusive dolerite or hornfel dikes or sills. Rb/Sr dating
has yielded dates that range from 2100 m.y. to 1500 m.y., and are the only
clue to relative age. Dikes where exposed surficially weather to positive
relief elements of the landscape.
Geomorphology
The geomorphology of Venezuelan Guiana is an almost classic case of
landform evolution controlled by lithology and structure. Although there is
some evidence of post-Roraima deformation into gentle, regional-scale
anteclise-syneclise folds, and minor reactivation of basement block-faulting,
nothing indicates that the region has been other than a stable craton of
non-deposition for the last 1500 m.y. This requires speculation about the
rate and magnitude of the long-term geomor_hic evolution of the region. In
the present climate, scarp retreat in tepuls clearly progresses rapidly.
Surfaces are fresh and lack case-hardening, and large blocks weighing hundreds
of tons are known to "peel" from tepul-scarps with great regularity. Further-
more, radiocarbon dates from organics beneath a 15 m thick sand-gravel overbur-
den in alluvial placer workings yield ages of only 3,000-10,000 yrs., indicat-
ing an enormous rate of denudation and flux during the Holocene, yet streams
today are clear-flowing and sediment transport is minimal even during peak
discharge. Furthermore, if stripping of the shield had occurred at the same
magnitude and rate for 1500 m.y., clearly no Roraima rocks should remain!
Unfortunately, we also have no hard evidence or even a "feel" about climates,
landforms, and erosion rates for the greater span of Phanerozoie time.
Other evidence of changing climate comes from extensive alluvial fans,
such as those near the Indian mission at Kamarata in the Chiguao River valley
southeast of Auyantepui (Fig. 4). These densely jungle-covered surfaces, as
much as 6 km broad, clearly did not form under the present climate or cover.
Elsewhere, as near Kavanayen Mission, are flat-topped sloping surfaces which
may be true pediments rather than the ubiquitous, stripped surfaces character-
istic elsewhere. Are all these, along with broad alluvial floodplains such as
mark the Kukenan River valley farther east, relicts of a Pleistocene, or
earlier, semiarid to arid climatic regime? The concept of global climatic
49
Fig. 3 - Geomorphological map of Auyantepu{, 1 - Angel waterfall.
2 - Churun River. 3 - Carrao River. 4 - Akanan River.
5 - Kamarata. 6 - Probable sill of basic rocks. 7 - Scarps,J
100-150mhigh, on Auyantepul. 8 - Pseudokarst landforms.9 -
Kukurital River flood-plain. I0 - Alluvial fans and apron
near Kamarata. Hachured borders are major scarps around¢
Auyantepul.
5O
Fig. 4 - Alluvial fans west of Kamarata. Hachured borders show
scarps; stipples are alluvial fans.
51
change and an arid episode at low latitudes has been proposed, on equally
tenuous evidence, by Damuth and Fairbridge (1970), Tricart (1974), and Wallace
et al (1960).
The region is located about 4° to 6° North, 59° to 64° West in the
moisture-laden subtropics. There is no pronounced dry season, and annual
rainfall on higher tepuis may exceed 5000 mm. More than three-quarters of the
region is tropical rain forest (bosque), the remainder tropical grassland and
brush savanna (selva). The latter apparently is natural in part (as on some
positive-relief dikes), but much is clearly anthropic, owing to clearing and
burning. Once vegetation is removed and a thin surface ferricrete-silcrete
layer breached, "instant badlands" form, carving canyons 30 m deep in a few
years. These rapid changes in landscape need repititive monitoring regionally
to determine the variations in hydrology, erosion, and sediment flux that
result from changing landuse in a region which has considerable mineral, soil,
and water resources but is very fragile under human impacts. For example,
under enhanced development sediment flux to streams may rapidly increase and
substantially impair the hydrologic regime. The world's highest waterfall
(Salto Angel) with direct free-fall in excess of I000 m is fed by a stream
only 4 km long that originates in rain forest atop Auyantepuf. Removal of
this forest assuredly would decrease or greatly vary flow, and inasmuch as the
region has important hydroelectric potential, any cumulation of flow changes
is an important economic consideration. Likewise, what would happen to
agriculturally attractive floodplains, such as along Kukenan and Kukurital
rivers, under strong flooding and regular overbank discharge and renewed
deposition of coarse-grained clastic debris, an event that does not now occur?
Role of Space Imagery
The foregoing recital lists only a few examples of how geomorphic change
has and continues to modify a major region of South America. But how does all
this relate to the value of sequential acquisition and analysis of high-
altitude and space imagery? Population explosion in South America inevitably
is leading to the opening of virgin interior lands; thus continuous monitoring
of landscape change clearly is of great interest. Space imagery, perhaps even
for several decades, accompanied by repetitive radar imaging and high-altitude
color-IR photography will tell much about such important matters as the gross
magnitude of denudation and erosion, actual rates and mode of scarp retreat,
the role of sediment flux and flooding, and continuing vegetative change under
the present climatic regime. The resolutional capability of the new Large
Format Camera System seems to offer much potential for monitoring important
site-specific changes and problems.
In a purely scientific sense, sensor products may help identify sites of
reasonable access that can be studied to provide better documentary evidence
of an earlier arid climate regional pedimentation episode or cycle and
long-term changes in landscape evolution.
52
References Cited
Damuth, J.E., and R.W. Fairbridge, 1970, Equatorial Atlantic deep-sea arkosic
sands and ice-age aridity in tropical South America: Geol. Soc. America
Bull., v. 81, p. 189-206.
Gansser, A., 1954, The Guiana Shield (South America), geological observations:
Eclogae Geol. Helv., v. 47, no. I, p. 77-112.
Melhorn, W.N., and D.E. Edgar, 1975, The case for episodic, continental-scale
erosional surfaces, a tentative model: in Theories of Landform Develop-
ment, Chap. 13, SUNY-Binghamton, p. 243-276.
Tricart, J., 1974, Existencia de periodes seches au Quaternaire en Amazonie
et dans les regions voisines: Rev. Geomorphol. Dynam., v. 23, no. 4,
p. 145-158.
Wallace, S., M. Ewing, and B.C. Heezen, 1960, Evidence for an abrupt change
in climate close to II,000 years ago: Amer. Jour. sci., v. 258, p. 429-
448.
Y_nez P., G., 1973, SLAR Interpretacion, una interpretacion de la tectonica
del Territorio Federal Amazonas (Venezuela): M.S. thesis, Univ. de
Oriente, Venezuela, 116 p.
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Space imagery and some geomorphological problems of the Guiana Shield, South America

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