Download Influence Neotectonic Identified by Morphometric Data and

Influence Neotectonic Identified by Morphometric Data and Cartographic Products in the
Guruji’s River watershed (PB), Northeast of Brazil
Maria Emanuella Firmino Barbosa
Post graduate degree in geography- University Federal of Paraiba, Brazil
Laboratory of studies geological and environmental- LEGAM/UFPB
Tec. in Geoprocessamento - Inst. of Education, Science and Technology of the State of Paraiba
E-mail: [email protected]
Prof. Dr. Max Furrier
Department of Geosciences, University Federal of Paraiba. João Pessoa, Brazil
Laboratory of studies geological and environmental- LEGAM/UFPB
E-mail: [email protected]
Abstract
This study aims to point out evidence of neotectonics in Guruji’s river watershed through
morphometric analysis and cartographic products. The study area is located on the south coast of
Paraiba State, Northeast Brazil; and the watershed in question has asymmetric drainage with
tributaries more advantageous in the south portion. The methodology used for detection of
neotectonic influences consist in the analysis and interpretation of numerical data obtained in the
topographic maps and Digital Elevation Model (DEM) and its derivatives (Slope map and 3D
terrain model). The calculation used for the morphometric analysis neotectonic was Valley FloorValley Height (Vf) and the Hack Method – Slope Vs. Length (SL); and to investigate the slope of
all mountain slope that make up the watershed was made the Slope map, which allowed a more
comprehensive and accurate regressive erosion of drainage, incising fluvial and unevenness of
tectonic origin.
Keywords: neotectonics - morphometry - Barreiras Formation.
1. Introduction
The main objective of this study is to delimit the activities of neotectonic movements and
their relation to the drainage pattern, direction of watercourses and morphological features
developed. In the present work was conventionally used as an object of study the Guruji’s river
watershed, which is located on the south coast of Paraiba State, Northeast Brazil. The drainage
basin was chosen to be studied by the fact that it has irrefutable morphological evidence of the
occurrence of neotectonic movements.
The research aims to characterize neotectonic of
Guruji’s river watershed by morphometric calculations intended solely for the detection of
neotectonic and confection of cartographic products that will assist this assessment, such as: slope
map and 3D terrain model.
The study of drainage patterns is very important to show this kind of event, and hydrography
considered one of the most susceptible to changes caused by tectonic, responding immediately to
the processes deformative, even those of small scales and magnitudes [8] These characteristics
make the hydrography and, therefore, the watersheds appropriate elements to analysis of
neotectonic, which seek to determine areas subject to movement, allowing, the advance in
quantitative terms about these deformations.
The drainage network to be highly sensitive to transmission of the inputs trigger to
environmental changes [12] becomes the element of the landscape most vulnerable to any kind of
deformation, showing these processes in its drainage network, therefore is so important to analysis
of anomalies in terms of asymmetry, straightness, and presence of elbows, inflections, alignment of
confluences and even conditions capture of drainage, both in the area of the basin and in the
surroundings.
Studies related to drainage always possessed relevant function in geomorphology, and
analysis of the drainage network can lead to understanding and clarification of numerous issues
related to the tectonic and geomorphological configuration of the area, because the streams are
responsible for of the most morphogenetic processes active in sculpturing the landscape inland.
2. Study Area
The study area corresponds to Guruji’s river watershed, in the county of Conde, the south
coast of Paraiba State, Brazil. The watershed has a total area of 44.698 km ² (Figure 1).
The Guruji’s river watershed is composed of stream Estiva, Caboclo and Pau Ferro and
other bodies of water without designation that emptying into the north beach Jacumã. This basin has
peculiarities very expressive morphological and easily visible, for example, its asymmetrical
drainage pattern, with the right margin tributaries of more expressive than the left margin, and the
sharp inflection in the lower river Guruji’s course, a few meters of the coastline, whose direction
changes abruptly from W-E to S-N.
FIGURE 1: Location of Guruji’s river watershed.
3. Geological Characteristics
The study area is located, mostly, on sandy-clay sediments of poorly consolidated Barreiras
Group, a hedge residual platform capping several Brazilian marginal basins, including the
Pernambuco-Paraiba Basin, consisting by formations Maria Farinha , Gramame end Beberibe, the
first two carbonate formations, and the last, clastic (Figure 2).
According [1], deposition of sediments of the Barreiras Group was through fluvial channel
braided developed on alluvial fans. The facies of fluvial systems interlaced with sedimentary
deposits of varying size gravel, coarse sand and fine, has creamy yellow, with interbedded silty clay
microclast indicative of sedimentary environments calm as, for example, the alluvial plain. The
facies of fans consists of polymictic conglomerates cream red colored, with pebbles and subangular
grains of quartz and clay blocks reworked in tabular and lenticular bodies up to a meter thick,
intercalated with silty-clay, layer thinner.
About Grupo Barreiras, are generally developed coastal tablelands with flat tops, sometimes
uplifted, either lowered or tilted by the tectonic activity evident recently [5-6]. The headwaters
drainage of waterways that form the Guruji’s river watershed have high slopes and are very slotted
channels.
Maria Farinha Formation outcrop only in the lower course of river Guruji, nearness of the
coast line and forming a prominent hill that stand out in the landscape. This formation represents a
continuation of the sequence of limestone of Formation Gramame, being differentiated only by their
fossil content, which is considered of lower Paleocene-Eocene age [7].
FIGURE 2: Stratigraphic column of the Alhambra Sub-basin, one of the sub-basins that make up the PernambucoParaíba Basin [5]
4. Methodology
The work is based on measurement and quantification of morphological features found in
the area of study and analysis of results. The data can highlight the neotectonic activities in the
basin in question, and but quantify them and compare them with other areas studied, were
susceptible to the same activities.
The work consisted in measuring channels, the generation of cartographic products, analysis
of morphological features of the area and applying morphometric calculations for the investigation
of recent tectonics. Morphometric data of the basin were obtained from topographic maps of
Jacumã (SB. YC-25-III-3-NE) and Conde (SB. 25 - YC-III-3-NO), scale 1: 25,000 [10-11] with
equidistance of contour lines of 10 m. With these topographic maps was made a mosaic of
watershed and then be exported to the Software of GIS Spring 5.03, of could be the scanning image,
counting and measuring channels and the making of thematic maps.
Through the software Spring 5:03 was generated Digital Elevation Model (DEM) and was
make a slope map and 3D model or the terrain. The first displays the level slope of the basin, and
the second shows the altimetric representation of relief by the use of conventional color and also the
three dimensional effect of area of the watershed. The analysis of these cartographic products was
of fundamental importance in this work, because can observe several topographical differences,
different levels of carving, slope and the asymmetrical pattern of drainage basin. In literature, there
are several techniques for the detection and analysis of neotectonic activities. These methods show,
mathematically, the occurrence of these events through data obtained from the measurement of
fluvial channels, the gaps between tops and bottoms of valleys, the straightness of watercourses.
The results could display and evaluate the occurrence of neotectonic events in the area.
The morphometric indices used in this study was the Valley Floor – Valle Height Ratio
(Vf). This index was developed by Bull and McFadden [13], existing in the Brazilian literature, to
date, few records of its use in detecting intensity neotectonics.
According to Stewart and Hancock [9], this morphometric index is used to evaluate the
intensities of neotectonic activities of a region along the individual structural features. This
technique is used exclusively for fluvial and consists in measuring the height of the water divide of
the valley. Measurements are made by measuring the Eld (altitude of left divide) and Erd (Altitude
of right divide), Vfw (width of valley floor) and Esc (Altitude of stream channel) (Figure 3).
Figure 3: Equation math and measurement procedure to meet the morphometric index Vf (Valley floor- valley height
ratio).
Were chosen to implement this calculation four profiles of Guruji’s river watershed.
According to Wells et al. [13], active fronts valley has valleys with fomat of V, and low Vf. The
measurement starts with designing the profile within the watershed, covering the two water divide
on opposite sides with altimetric points listed (Figure 4).
FIGURE 4: Watershed catchment area with contour lines and spot elevations. The profiles plotted represent areas where
the information was extracted for the creation of Vf.
The methodological procedure for obtaining Vf was divided into four steps:
Step 1: We selected two watersheds with contour lines that are defined on opposite sides of the
watershed. In this work, the criterion for the choice of points was the wide gap between water
divide. Choose two water divide, by drawing a thread between these two points, which crosses the
river channel at right angles.
Step 2: Determine the Vfw (Widht of valley floor). It is measured in the topographic map, the
distance between the last two contour lines on each side of the river channel.
Step 3: The procedure for defining the Esc (Altitude of stream channel) is performed by extracting
the value of the last contour line before the river channel. The value of this contour is Esc.
Step 4: Inserting the values found in steps 1, 2 and 3 in the equation to obtain the values of Vf
(Valley floor - Valley height ratio).
Another morphometric index that stands out in international and Brazilian literature is
Hack's method, correspondent to index SL (Slope vs. Lenght), developed by Hack [3]. This index
was applied to the analysis of tectonic imprint in various locations, for example, in assessing the
behavior of the San Andreas Fault (California, USA). The Brazilian records exist to apply similar
techniques in the Amazon region [4], in the sedimentary basin of São Paulo [3] and the Peixe's river
watershed in São Paulo [2-3-4].
The SL index can be calculated for the entire length of a river, considering the amplitude
altimetric total, ie, the topographic difference (in meters) between the elevation of the headwater
and river mouth. The index SL can be applied in two different ways: on the entire drainage (SL total
or SLt) that provides a regional issued for large areas, or applied to segments of drainage (SLfollow
or SLf) that connotes more local. In this work we chose to calculate only the SL total of segments of
Guruji’s river watershed (Figure 5). This methodology was used in Brazil for Etchebehere [3],
where the author claims that this technique allows a quick assessment, effective and low cost on the
neotectonic framework of the region, serving as a model for regional implementation of inner
cratonic areas.
The application of two morphometric indices (Vf and SL) showed interesting results about
the recent tectonics in the basin and corroborate with morphological descriptions of neotectonic
character made in the area and adjacencies in previous surveys.
Likewise, this morphometric approach can add the other approaches, mainly, geodetic and
geophysical studies, which may contribute to the improvement of neotectonic analysis - in a line of
"convergence of evidence" - making the models and interpretations more consistent, especially for
intraplate regions such as the Brazilian territory.
FIGURE 5: Parameters used in calculating the index segment for drainage SL (interval between two consecutive
isoípsas). L = length of drainage between the headboard and isoípsa downstream; Δh = difference in altitude between
two subsequent isoípsas; ΔL = horizontal projection of segment length of drainage between two subsequent isoípsas [4].
The methodological procedure was divided into the following steps:
Step 1: was chosen size of the fluvial channel more expressive. In the case of Guruji's river
watershed were chosen the streams do Caboclo, Estiva and Pau Ferro and river Guruji (Figure 6).
Step 2: was measured with the help of the Spring software 5.03, the length of the river courses and
then tabulated the differences altitude (contour interval 10 meters) and horizontal length of each
segment of the drainage between two isoípsas followed. The dimensions of the bedside drainage
and the river mouth were estimated according to the value of the subsequent level curve.
Step 3: The values were entered into a spreadsheet and then were calculated the SLt index. The SLt
values for the river drainages of the Guruji also been tabulated for a comparison between the rates
found by Etchebehere [3] in the Rio do Peixe, São Paulo State (Table 2).
FIGURE 6: Selected Excerpts from the hydrographic basin of the river Guruji, for calculating the SLt.
5. Results
The results obtained in the Guruji's river watershed were compared with results obtained by
Wells et al. [13], who argue that the low values of Vf exhibit evidence of recent crustal movements
in the area. A fact that deserves attention is that all authors had not specify from which value
obtained is considered a low value. Therefore it is necessary to compare the values found in this
study with values from other areas surveyed. The values obtained by Wells et al. [13] for the coast
of Costa Rica are 6,25, 3,33, 0,79, 7,33, 3, 0,82 and 0,25. The values obtained for the Guruji's
river watershed , in the selected areas (Figure 4), were: 2,0407, 9,0909, 5,1489 and 5,1722, which
shows some results below those found in rivers of the west coast of Costa Rica, whose adjacency
has an active continental margin.
TABLE 1: Values for calculating the index Vf, and morphometric outcome.
Assessing the values found in the watershed, can be seen that the lowest value obtained was
the profile A - A ', with Vf of 2,0407. This value supports strongly the idea of the influence of
recent tectonics in this part of the basin, which also finds strong slots river and high river steepness
of the slopes, pointing to decreases of headwaters accelerated.
The morphometric index Slope vs. Lenght (SL), can point sensitive changes in the slope of a
river channel which, in turn, may be associated with the river mouths of tributaries flow expressive,
the different resistances to erosion hydraulic lithic substrate and / or the neotectonic activities in the
area. The rate increases where the river flows over rocks and decreases toughest courses where a
softer substrate. If you can eliminate the lithological factor or the possible presence of tributaries of
postage as causative agents of high index SL in a certain stretch of river, you may indicate the role
of neotectonic processes.
The indices found in the river basin Guruji (Table 2) are much higher than those found by
Etchebehere, [3] for the Fish River Basin, located in western Sao Paulo State, which sits
predominantly on sedimentary lithology. The role of neotectonic processes in the river basin Guruji
is also supported by the values found by applying such a morphometric index, since it is grounded
solely on the poorly consolidated sediments of the Barreiras Group and there are no issues of
significant tributaries of the caudal that could influence the high values obtained.
TABLE 2: Value Slope vs. Lenght (SL) for selected drainage basin of the Guruji’s river.
Among the values found with the largest SLt Caboclo is the stream of which is the portion of
the basin where there is a greater number of secondary channels that have strongly embedded in the
landscape. The highest altitude quotas are also located in this portion of the basin that can be
considered a knick points (point of direction change) along the drainages, representing anomalous
points to strong evidence of neotectonic influence.
Analyzing at the Slope map made it is observed that the largest steepness are the headwaters
of the drainage basin and the southern portion of the sub-basin of the stream of Pau Ferro. Most of
the watershed has slope around 0-12%, reflecting the predominantly tabular morphology of the
area. Beyond this value, one that stands out is the range of 12-30% slope, and small portions with
steep slopes reaching> 100% (45°), found predominantly in the southern basin. (Figure 7).
FIGURE 7: Slope map of the Guruji’s river watershed. Notice that the steepness are greater in the south.
One area important is sub-basin of the Pau Ferro's stream where rates reach slope, in a large
part, values between 45-100%. In all downslope of adjacent courses water, of the first order of this
sub-basin and the stream itself Pau Ferro strands exhibit high slopes to the encounter with the
stream of Caboclo. This result is indicative of accelerated retreat of headwater corroborates also
with the results obtained through calculations and morphometric Vf and SL (Figure 7).
Comparing the map of slope of the basin with the regional topographic map made by Furrier
[6], it is evident that the strong stream of cutouts Estiva, Caboclo and Pau Ferro, and the high slopes
found in this portion of the basin are closely related to Structural High Coqueirinho called by
Furrier [6], located south of the river basin Guruji.
Analyzing the 3D model produced is observed, with greater clarity, the nuances of relief
Guruji's River watershed , with sharp notches of the flow of first order, especially in the southern
portion, and the marked asymmetry of the basin with the right bank tributaries of more that slotted
the tributaries of the left bank (Figure 8). It is clear, too, the higher levels and dissected portion of
the south that are closely related to Structural High Coqueirinho. It's pretty clear that model the
structural control exerted in the watershed that directly involves shape and morphology.
FIGURE 8: Model three-dimensional (3D) of the Guruji's river watershed (projection parallel).
6. Conclusion
This study made use of GIS tools for identifying, analyzing and interpreting morphometric
and morphological of the Guruji's river watershed. The data from the morphometric analysis
showed that the basin has a strong carving, high degree of dissection of relief, especially in its
southern portion, and a marked asymmetry in its drainage pattern. The results with the application
of morphometric indices focused on the detection of neotectonic influences added to the
morphological description of the basin, a significant bearing on the neotectonics that can not be
denied on the nature of geomorphological studies in areas of the type passive continental margin,
principally those performed on the Brazilian Northeast.
There are several other studies of geological and geophysical nature that still must be made
to ratify and to quantify the influence of recent tectonics in the area. The results achieved in this
work, strictly geomorphological imprint, can open new fields of application and understanding on
the development and evolution of topography and its relation to recent tectonics in areas of passive
continental margin.
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