- Email: [email protected]
Quaternary lakes of eastern Brazil
Photogrammetria - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
Q U A T E R N A R Y LAKES OF E A S T E R N B R A Z I L R. PFLUG Geological-Palaeontological Institute, University of Heidelberg, Heidelberg (Germany)
(Received March 4, 1968)
SUMMARY In the middle course of the Rio D6ce, an eastern Brazilian river approximately 700 km long, occur hundreds of dammed valley lakes. These lakes are restricted to a hilly terrain which accompanies the river in a width of more or less 20 km. The lakes, whose water levels now stand about 20 m above the Rio D6ce, were formed during a semi-arid cycle. According to a preliminary C-14 determination (14,160 ± 500 years) this cycle corresponds to the last ice age of the Northern Hemisphere. The process of formation of these lakes is deducable from the interpretation of aerial photographs. REGIONAL SETTING There exist no geomorphological or geological studies dealing with the area described in this paper. Therefore a short introduction into the regional development of the Rio D6ce valley system will be given first. The Rio D6ce valley, in its middle course (Fig.l), has obtained its major outline through a process of pediplanization during a probably semi-humid climate. By this process a wide plain was formed having a low relief and considerable extension. At present, it extends for about 10 km on both sides of the Rio D6ce and then abuts against mountain ranges which rise about 750 m above the plain. Erosion was performed through sheetwash on the plain and vertical retreat of the adjacent mountain slopes. After this pediplanization, an incision period of the river system started, probably due to a lowering of the base level of erosion of the lower Rio D6ce. This was accompanied by downcutting of about 70 m of the Rio D6ce, which then occupied a valley more or less 1 km wide. At the same time the former pediplain was dissected by a dendritic system of tributary valleys. These valleys attained a profile which is typical of tropical regions: that is, they developed a very low gradient, only the uppermost parts of the valley having steeper inclinations. The valley slopes, though, are rather steep. In consequence of this dissection, the former pediplain was dissolved into irregular small hills, which now rise about 50-70 m above the present river. They are entirely covered by thick red soils, which were formed during the formation of the tributary valley system. Following this erosional semi-humid cycle, there occurred a change in the Photogrammetria, 24 (1969) 29 35
30
R. PFLUG
behaviour of the river. It started to accumulate coarse clastics, which resulted in the formation of a thick terrace deposit. The river bed was elevated at least 20 m by these accumulations. The same happened in all valleys, which originated in the higher mountains of the hinterland. In contrast, all smaller valleys, which came out of the former pediplain, did not furnish any detritus. Therefore, they were cut 1,5 °
/,j2 °
3 9 ° w . Gr.
/ 18° S
I\D \: I J~,~
20°f
1
c,
.....
Coronlt
J
"CO
~ :.
I
i
20°
<:
I
°
i 1,5 °
42 °
22°
=0k= 39 °
F i g . l . K e y m a p of the region studied along the course of the Rio D6ce.
off at their entrance into the main valleys by the deposition of the accumulation terrace, and thus allowed the formation of the present lakes. The peculiar shape of the tributary valleys permitted a considerable length of the lakes, which may attain several kilometres. The material of the accumulation terrace is very rich in fresh feldspars and micas thus indicating prevailing mechanical decomposition in the hinterland, whereas the pronounced accumulation argues for a decrease in rainfall. Therefore one has to admit a semiarid climate for the formation of these features. Carbonaceous matter within the accumulation terrace was dated by Dr. K. O. Mtinnich and gave a preliminary C-14 age of 14.160 -+- 500 years. This age corresponds to the late stage of the last ice age in the Northern Hemisphere. After the accumulation period, the present semi-humid tropical conditions developed. The increase in rainfall resulted in a renewed incision of the Rio D6ce system. The main valley now lies 20 m below the upper level of the accumulation terrace. The dammed valleys were filled with permanent lakes, almost up to the level of the accumulation terrace, thus standing now about 20 m above the river. Photogrammetria, 24 (1969) 2 9 - 3 5
QUATERNARY LAKESOF EASTERN BRAZIL
31
A more detailed account of the valley development is presently in press (PFLUG, 1968). The multicyclical development of the Rio D6ce gave rise to a peculiar landscape. Above the river lies the very flat surface of the accumulation terrace. Once being on this plain, one soon discovers one of the hundreds of these lakes, which now occupy the dammed valleys. Some tens of metres higher one sees the rest of the former pediplain, forming a smoothly undulating surface. In the background, the mountains rise from this surface. The entire region is still largely covered by a tropical rain forest, or, where this has been cut down, by tall elephant grass. Therefore, field observations are rather limited. This can be overcome by the use of aerial photographs. The preceding scheme of valley development was principally deduced from photo interpretation. In the following, two typical examples will be described. PHOTO INTERPRETATIONOF DAMMED VALLEY LAKES The area shown in Fig.2 exhibits the dissected topography of the former pediplain. The local relief is about 70 m. The region is covered almost entirely by virgin tropical rain forest, which makes ground access very difficult. Embedded into the hilly topography are a number of lakes, which stand out with dark gray tones, due to the very clear lake water. The Rio D6ce in the lowermost part of the area gives a white gloss, caused by the heavy load of suspended matter. Within the area, there are several very flat zones, which are formed by the surface of the accumulation terrace of the Rio D6ce. This surface is best developed in the uppermost part, where, in places, it is more than a km wide. No important drainage system has yet developed on this surface. There occur some small meandering creeks, which, in most places, are difficult to observe through the forest cover. Some of the creeks discharge into the lakes, and, in one case (about 1 km above the centre of the photograph), a creek has even built up a small delta into a lake. On the surface of the accumulation terrace, one can distinguish the course of an abandoned river bed. It shows its presence through differences in the vegetation. This bed is between 200 and 300 m wide and has not developed a thick forest cover, indicating swampy conditions. On both sides of the accumulation terrace lies a variety of lakes, one of them with a length of more than 5 km. These lakes are obviously filling former tributary valleys. The water level of the lakes is slightly below the surface of the accumulation terrace, thus indicating that the accumulation was more rapid in the main valley than in the tributaries. The distribution of the accumulation terrace in the lower part of the picture is less widespread. It occurs in small patches on both sides of the river, the surface being about 20 m above the waterlevel of the river. Besides, it occupies a small valley, which diverges from the present river valley. In this region, the accumuPhotogrammetria, 24 (1969) 29-35
32
R. PFLUG
lation terrace has also dammed the tributaries and caused the formation of lakes. However, almost all these lakes are dry now, indicating that their primary depth has not been as great as in the other lakes of the area.
..
~
.~."
.
...,.-
1 km I
::: 2
~4
~
J
6
Fig.2. A e r i a l p h o t o g r a p h a n d g e o m o r p h o l o g i c a l sketch of t h e Rio D 6 c e valley, 25 k m s o u t h o f C o r o n e l F a b r i c i a n o , M i n a s Gerais. 1 a c c u m u l a t i o n terrace; 2 = erosion terrace; 3 = river; 4 = a b a n d o n e d river bed; 5 =: d r y valley; 6 = lake; 7 = d r y lake.
The course of the Rio D6ce is very irregular. The stream widens and narrows considerably and in two places there are rapids, where the river cuts into bedrock. The distribution of the accumulation terrace in the area of Fig.2 allows the deduction of several steps in the development of the river. First the Rio D6ce
Photogrammetria, 24
(1969) 2 9 - 3 5
QUATERNARYLAKESOF EASTERN BRAZIL
33
occupied the wide valley in the upper part of the photograph. Due to the heavy accumulation in this area, the river uplifted its bed until the water spilled over a local watershed and entered into a tributary valley. This happened outside the shown area. The valley which now took over the river was narrower than the original valley. This can be seen in the lower left corner, where the river enters into the photograph. Because of its narrowness, the newly formed accumulations could not occupy such wide areas as in the former main valley. Again, all the tributaries were closed off by accumulations. Once again a local watershed was reached by the river deposits and the waters spilled over into a neighbouring valley (lower right corner of Fig.2). This latter valley is still occupied by the present river.
2O 40 20 0 0 20 ~0
40 40
20 40
").!(:) 1 i::...:..)!i~.,::-2
~
60
60 40
3
o
40
60
60
so, o r e
// 7- 2 4
~
40
~2o
.5
Fig 3. Aerial photograph and geomorphological sketch of the Rio D6ce valley, 15 km east of Coronel Fabriciano, Minas Gerais (construction of the form lines by P. Cantarel). 1 = accumulation terrace; 2 -- erosion terrace; 3 = dry lake; 4 = abandoned channel; 5 = form lines above the river (in m).
Photogrammetria, 24 (1969) 29-35
34
R. PFLUG
As the present river flows well below the surface of the accumulation terrace, one can deduce that now accumulation has stopped, and that the river is eroding into its own sediments or into bedrock. The area of Fig.3 gives a more detailed view of the conditions in the immediate vicinity of the Rio DSce. Besides, this region has been deforested already, so that it was possible to construct topographic form lines with the aid of a ZeissStereotop. The valley development is similar to the situation described from Fig.2. Before the formation of the accumulation terrace, the river flowed from the middle of the left side to the upper right corner. This former main valley was filled by the coarse sediments of the accumulation terrace. During the accumulation the river spilled over into a tributary, located slightly above the middle of the right side. Besides, all the other smaller valleys coming in from the accompanying hilly terrain were cut off by the sedimentation in the main valley. By this process, five lakes were formed in the area of Fig.3, of which only one is still preserved. Two former lakes are easily discerned on the photographs as dry lake beds, lying slightly below the surface of the terrace. The dams, which closed off the remaining two lakes (lower and upper left corner), were cut through by small gorges, so that the lake water could flow out. The sedimentary cone which was built up into the mouth of the tributary is still well preserved in the contours of the form lines. The depth of the former lake in the lower left corner must have been between 10-15 m. At present, the Rio DSce is eroding, the water level now being about 20 m below the surface of the accumulation terrace. During the downcutting, several levels of erosional terraces were formed. Where the river still remains in its former main valley, it has not yet cut completely through its own sediments, as is the case in the main part of Fig.3. Where the river leaves the area on the right hand of Fig.3, it follows a former tributary. There, the terrace deposits have been removed entirely. Here, the river must cut into crystalline basement rocks, which causes the river to narrow considerably. CONCLUSION Similar phenomena have been observed along 80 km of the River D6ce in a width of generally 20 kin. Areas outside this region have not been visited by the author. Dammed valley lakes have been mentioned by MAULL (1924) from the lower Rio DSce. Semi-arid climatic conditions corresponding to ice ages of the Northern Hemisphere have lately been observed in Brazil by several investigators (p.e., BIGARELLA and AB'SABER, 1964; BIGARELLA and MOUSINHO, 1966; MABESOONE, 1966). These results throw some doubts on the existence of pluvials in the lower latitudes, which supposedly correspond to the ice ages of higher latitudes. In the Rio D6ce area, the climate definitely was much dryer during the last ice age than nowadays.
Photogramrnetria, 24 (1969) 29-35
QUATERNARY LAKESOF EASTERN BRAZIL
35
ACKNOWLEDGEMENTS This study was made possible through a grant by the Deutsche Forschungsgemeinschaft. The preliminary C-14 age was kindly furnished by Dr. K. O. Miinnich (2. Physikalisches Institut der Universit~it Heidelberg). Further material is intended to be dated. The permission to reproduce aerial photographs is gratefully acknowledged to Servi~os Aerofotogrammetricos/Cruzeiro Do Sul S. A., Rio de Janeiro, Brazil. REFERENCES BIGARELLA,J. J. and AB'SABER,A. N., 1964. Pal~iogeographische und pal~ioklimatische Aspekte des KSnozoikums in Si,idbrasilien. Z. Geomorphol., 8: 286-312. BIGARELLA,J. J. and MOUSINHO,M. R., 1966. Slope development in southeastern and southern Brazil. Z. Geomorphol., 10: 150-160. MABESOONE, J. M., 1966, Relief of northeastern Brazil and its correlated sediments. Z. Geomorphol., 10: 419-443. MAULL, O., 1924. Die geomorphologischen Grundzfige Mittelbrasiliens. Z. Ges. Erdkunde, 161-197. PFLUG, R., 1968. Das Erberschfittungsrelief des Rio D6ce, Ostbrasilien. Z. Geomorphol., in press.
Photograrnmetria, 24 (1969) 29-35
Q U A T E R N A R Y LAKES OF E A S T E R N B R A Z I L R. PFLUG Geological-Palaeontological Institute, University of Heidelberg, Heidelberg (Germany)
(Received March 4, 1968)
SUMMARY In the middle course of the Rio D6ce, an eastern Brazilian river approximately 700 km long, occur hundreds of dammed valley lakes. These lakes are restricted to a hilly terrain which accompanies the river in a width of more or less 20 km. The lakes, whose water levels now stand about 20 m above the Rio D6ce, were formed during a semi-arid cycle. According to a preliminary C-14 determination (14,160 ± 500 years) this cycle corresponds to the last ice age of the Northern Hemisphere. The process of formation of these lakes is deducable from the interpretation of aerial photographs. REGIONAL SETTING There exist no geomorphological or geological studies dealing with the area described in this paper. Therefore a short introduction into the regional development of the Rio D6ce valley system will be given first. The Rio D6ce valley, in its middle course (Fig.l), has obtained its major outline through a process of pediplanization during a probably semi-humid climate. By this process a wide plain was formed having a low relief and considerable extension. At present, it extends for about 10 km on both sides of the Rio D6ce and then abuts against mountain ranges which rise about 750 m above the plain. Erosion was performed through sheetwash on the plain and vertical retreat of the adjacent mountain slopes. After this pediplanization, an incision period of the river system started, probably due to a lowering of the base level of erosion of the lower Rio D6ce. This was accompanied by downcutting of about 70 m of the Rio D6ce, which then occupied a valley more or less 1 km wide. At the same time the former pediplain was dissected by a dendritic system of tributary valleys. These valleys attained a profile which is typical of tropical regions: that is, they developed a very low gradient, only the uppermost parts of the valley having steeper inclinations. The valley slopes, though, are rather steep. In consequence of this dissection, the former pediplain was dissolved into irregular small hills, which now rise about 50-70 m above the present river. They are entirely covered by thick red soils, which were formed during the formation of the tributary valley system. Following this erosional semi-humid cycle, there occurred a change in the Photogrammetria, 24 (1969) 29 35
30
R. PFLUG
behaviour of the river. It started to accumulate coarse clastics, which resulted in the formation of a thick terrace deposit. The river bed was elevated at least 20 m by these accumulations. The same happened in all valleys, which originated in the higher mountains of the hinterland. In contrast, all smaller valleys, which came out of the former pediplain, did not furnish any detritus. Therefore, they were cut 1,5 °
/,j2 °
3 9 ° w . Gr.
/ 18° S
I\D \: I J~,~
20°f
1
c,
.....
Coronlt
J
"CO
~ :.
I
i
20°
<:
I
°
i 1,5 °
42 °
22°
=0k= 39 °
F i g . l . K e y m a p of the region studied along the course of the Rio D6ce.
off at their entrance into the main valleys by the deposition of the accumulation terrace, and thus allowed the formation of the present lakes. The peculiar shape of the tributary valleys permitted a considerable length of the lakes, which may attain several kilometres. The material of the accumulation terrace is very rich in fresh feldspars and micas thus indicating prevailing mechanical decomposition in the hinterland, whereas the pronounced accumulation argues for a decrease in rainfall. Therefore one has to admit a semiarid climate for the formation of these features. Carbonaceous matter within the accumulation terrace was dated by Dr. K. O. Mtinnich and gave a preliminary C-14 age of 14.160 -+- 500 years. This age corresponds to the late stage of the last ice age in the Northern Hemisphere. After the accumulation period, the present semi-humid tropical conditions developed. The increase in rainfall resulted in a renewed incision of the Rio D6ce system. The main valley now lies 20 m below the upper level of the accumulation terrace. The dammed valleys were filled with permanent lakes, almost up to the level of the accumulation terrace, thus standing now about 20 m above the river. Photogrammetria, 24 (1969) 2 9 - 3 5
QUATERNARY LAKESOF EASTERN BRAZIL
31
A more detailed account of the valley development is presently in press (PFLUG, 1968). The multicyclical development of the Rio D6ce gave rise to a peculiar landscape. Above the river lies the very flat surface of the accumulation terrace. Once being on this plain, one soon discovers one of the hundreds of these lakes, which now occupy the dammed valleys. Some tens of metres higher one sees the rest of the former pediplain, forming a smoothly undulating surface. In the background, the mountains rise from this surface. The entire region is still largely covered by a tropical rain forest, or, where this has been cut down, by tall elephant grass. Therefore, field observations are rather limited. This can be overcome by the use of aerial photographs. The preceding scheme of valley development was principally deduced from photo interpretation. In the following, two typical examples will be described. PHOTO INTERPRETATIONOF DAMMED VALLEY LAKES The area shown in Fig.2 exhibits the dissected topography of the former pediplain. The local relief is about 70 m. The region is covered almost entirely by virgin tropical rain forest, which makes ground access very difficult. Embedded into the hilly topography are a number of lakes, which stand out with dark gray tones, due to the very clear lake water. The Rio D6ce in the lowermost part of the area gives a white gloss, caused by the heavy load of suspended matter. Within the area, there are several very flat zones, which are formed by the surface of the accumulation terrace of the Rio D6ce. This surface is best developed in the uppermost part, where, in places, it is more than a km wide. No important drainage system has yet developed on this surface. There occur some small meandering creeks, which, in most places, are difficult to observe through the forest cover. Some of the creeks discharge into the lakes, and, in one case (about 1 km above the centre of the photograph), a creek has even built up a small delta into a lake. On the surface of the accumulation terrace, one can distinguish the course of an abandoned river bed. It shows its presence through differences in the vegetation. This bed is between 200 and 300 m wide and has not developed a thick forest cover, indicating swampy conditions. On both sides of the accumulation terrace lies a variety of lakes, one of them with a length of more than 5 km. These lakes are obviously filling former tributary valleys. The water level of the lakes is slightly below the surface of the accumulation terrace, thus indicating that the accumulation was more rapid in the main valley than in the tributaries. The distribution of the accumulation terrace in the lower part of the picture is less widespread. It occurs in small patches on both sides of the river, the surface being about 20 m above the waterlevel of the river. Besides, it occupies a small valley, which diverges from the present river valley. In this region, the accumuPhotogrammetria, 24 (1969) 29-35
32
R. PFLUG
lation terrace has also dammed the tributaries and caused the formation of lakes. However, almost all these lakes are dry now, indicating that their primary depth has not been as great as in the other lakes of the area.
..
~
.~."
.
...,.-
1 km I
::: 2
~4
~
J
6
Fig.2. A e r i a l p h o t o g r a p h a n d g e o m o r p h o l o g i c a l sketch of t h e Rio D 6 c e valley, 25 k m s o u t h o f C o r o n e l F a b r i c i a n o , M i n a s Gerais. 1 a c c u m u l a t i o n terrace; 2 = erosion terrace; 3 = river; 4 = a b a n d o n e d river bed; 5 =: d r y valley; 6 = lake; 7 = d r y lake.
The course of the Rio D6ce is very irregular. The stream widens and narrows considerably and in two places there are rapids, where the river cuts into bedrock. The distribution of the accumulation terrace in the area of Fig.2 allows the deduction of several steps in the development of the river. First the Rio D6ce
Photogrammetria, 24
(1969) 2 9 - 3 5
QUATERNARYLAKESOF EASTERN BRAZIL
33
occupied the wide valley in the upper part of the photograph. Due to the heavy accumulation in this area, the river uplifted its bed until the water spilled over a local watershed and entered into a tributary valley. This happened outside the shown area. The valley which now took over the river was narrower than the original valley. This can be seen in the lower left corner, where the river enters into the photograph. Because of its narrowness, the newly formed accumulations could not occupy such wide areas as in the former main valley. Again, all the tributaries were closed off by accumulations. Once again a local watershed was reached by the river deposits and the waters spilled over into a neighbouring valley (lower right corner of Fig.2). This latter valley is still occupied by the present river.
2O 40 20 0 0 20 ~0
40 40
20 40
").!(:) 1 i::...:..)!i~.,::-2
~
60
60 40
3
o
40
60
60
so, o r e
// 7- 2 4
~
40
~2o
.5
Fig 3. Aerial photograph and geomorphological sketch of the Rio D6ce valley, 15 km east of Coronel Fabriciano, Minas Gerais (construction of the form lines by P. Cantarel). 1 = accumulation terrace; 2 -- erosion terrace; 3 = dry lake; 4 = abandoned channel; 5 = form lines above the river (in m).
Photogrammetria, 24 (1969) 29-35
34
R. PFLUG
As the present river flows well below the surface of the accumulation terrace, one can deduce that now accumulation has stopped, and that the river is eroding into its own sediments or into bedrock. The area of Fig.3 gives a more detailed view of the conditions in the immediate vicinity of the Rio DSce. Besides, this region has been deforested already, so that it was possible to construct topographic form lines with the aid of a ZeissStereotop. The valley development is similar to the situation described from Fig.2. Before the formation of the accumulation terrace, the river flowed from the middle of the left side to the upper right corner. This former main valley was filled by the coarse sediments of the accumulation terrace. During the accumulation the river spilled over into a tributary, located slightly above the middle of the right side. Besides, all the other smaller valleys coming in from the accompanying hilly terrain were cut off by the sedimentation in the main valley. By this process, five lakes were formed in the area of Fig.3, of which only one is still preserved. Two former lakes are easily discerned on the photographs as dry lake beds, lying slightly below the surface of the terrace. The dams, which closed off the remaining two lakes (lower and upper left corner), were cut through by small gorges, so that the lake water could flow out. The sedimentary cone which was built up into the mouth of the tributary is still well preserved in the contours of the form lines. The depth of the former lake in the lower left corner must have been between 10-15 m. At present, the Rio DSce is eroding, the water level now being about 20 m below the surface of the accumulation terrace. During the downcutting, several levels of erosional terraces were formed. Where the river still remains in its former main valley, it has not yet cut completely through its own sediments, as is the case in the main part of Fig.3. Where the river leaves the area on the right hand of Fig.3, it follows a former tributary. There, the terrace deposits have been removed entirely. Here, the river must cut into crystalline basement rocks, which causes the river to narrow considerably. CONCLUSION Similar phenomena have been observed along 80 km of the River D6ce in a width of generally 20 kin. Areas outside this region have not been visited by the author. Dammed valley lakes have been mentioned by MAULL (1924) from the lower Rio DSce. Semi-arid climatic conditions corresponding to ice ages of the Northern Hemisphere have lately been observed in Brazil by several investigators (p.e., BIGARELLA and AB'SABER, 1964; BIGARELLA and MOUSINHO, 1966; MABESOONE, 1966). These results throw some doubts on the existence of pluvials in the lower latitudes, which supposedly correspond to the ice ages of higher latitudes. In the Rio D6ce area, the climate definitely was much dryer during the last ice age than nowadays.
Photogramrnetria, 24 (1969) 29-35
QUATERNARY LAKESOF EASTERN BRAZIL
35
ACKNOWLEDGEMENTS This study was made possible through a grant by the Deutsche Forschungsgemeinschaft. The preliminary C-14 age was kindly furnished by Dr. K. O. Miinnich (2. Physikalisches Institut der Universit~it Heidelberg). Further material is intended to be dated. The permission to reproduce aerial photographs is gratefully acknowledged to Servi~os Aerofotogrammetricos/Cruzeiro Do Sul S. A., Rio de Janeiro, Brazil. REFERENCES BIGARELLA,J. J. and AB'SABER,A. N., 1964. Pal~iogeographische und pal~ioklimatische Aspekte des KSnozoikums in Si,idbrasilien. Z. Geomorphol., 8: 286-312. BIGARELLA,J. J. and MOUSINHO,M. R., 1966. Slope development in southeastern and southern Brazil. Z. Geomorphol., 10: 150-160. MABESOONE, J. M., 1966, Relief of northeastern Brazil and its correlated sediments. Z. Geomorphol., 10: 419-443. MAULL, O., 1924. Die geomorphologischen Grundzfige Mittelbrasiliens. Z. Ges. Erdkunde, 161-197. PFLUG, R., 1968. Das Erberschfittungsrelief des Rio D6ce, Ostbrasilien. Z. Geomorphol., in press.
Photograrnmetria, 24 (1969) 29-35