GEOPHYSICAL MAPS AND PETROPHYSICAL DATA OF MOZAMBIQUE

Transcription

1 Geophysical maps and petrophysical data of Mozambique GTK Consortium Geological Surveys in Mozambique , edited by Yrjö Pekkala, Tapio Lehto & Hannu Mäkitie, 65 80, 2008 GEOPHYSICAL MAPS AND PETROPHYSICAL DATA OF MOZAMBIQUE by Ruotoistenmäki, T Geophysical maps and petrophysical data of Mozambique,, 65 80, 15 figures. The airborne geophysics that covers the GTK Consortium Project area comprise of four data sets: 1) airborne geophysical data compiled by Hunting Geology & Geophysics and covering mainly the Tete Province, 2) aeromagnetic data of southern Mozambique, digitized by Getech, 3) airborne geophysical grids of Fugro and 4) gravity data compiled by Getech. The Hunting data were available as contour maps and as digital grids comprising total magnetic field, potassium, thorium, uranium channels and total count. The grids were useful for a quick assessment on how to exploit the data in map production and for geological field work. The new Fugro line data including K, eth, eu, TC and total magnetic field channels, were received late in the Project (September 2003), but were quickly registered on the MOZNET datum and gridded on 200 meter cells using a minimum curvature gridding algorithm. Various geophysical maps were prepared of this data to support the lithostratigraphic mapping. Key words (GeoRef Thesaurus AGI): geophysical surveys, airborne methods, petrophysics, geophysical maps, Mozambique. Geological Survey of Finland (GTK), P.O. Box 96, FI Espoo, Finland tapio.ruotoistenmaki@gtk.fi INTRODUCTION In this report a short introduction is given to the geophysical and petrophysical data and maps that were collected and prepared during the Mozambique LOT 3 mapping project carried out by the Geological Survey of Finland in The maps were prepared as reference and working maps to support geologists in lithological mapping and their format is therefore relatively informal. They were made in Geosoft and ArcView-tiff formats ( geocoded ) and submitted to the National Directorate of Geology (DNG) on a separate CD. The maps are presented as colour maps or as hill- shaded versions that better show local variations and lineations by emphasizing the gradients of the anomalies. Moreover, some maps have been connected to lithological maps in order to illustrate the link between lithological characteristics and corresponding geophysical patterns (Fig. 1). Some visually observed features are also emphasized on the maps. Moreover, a magnetic profile interpretation crossing the Karoo group is presented. During the field work the geologists measured petrophysical parameters for ca. 300 samples whose susceptibility-density diagram will be discussed. 65

2 GRAVITY DATA AND MAPS The available gravity grid covers only the southern part of Mozambique. The records on land area have mainly been collected along roads and thus the data are locally relatively sparse (white areas). In some sea areas the measurements are denser. The gravity map is given in Fig. 2, and combined with the lithology in Fig. 3. From the maps it can be seen that the mafic Karoo group rocks in (A) and (B) are correlated with gravity highs. The gentle flanks of the anomalies also suggest that these rocks are dipping below the lighter sedimentary cover (C), southeast in (A) and northeast in (B). Moreover, the maps demonstrate that in off-shore areas (D) the gravity is high due to the mafic rocks of sea floor and shallow upper mantle. The gravity high at (E) indicates that there are high density rocks below the less dense sedimentary. These high density rocks can possibly be connected with ancient marine basalts underthrusting the western coast of Mozambique, thus being potential for oil or gas exploration. MAGNETIC DATA AND MAPS The magnetic grid used for this work mainly covers the western part of Mozambique. In general, the grid density of magnetic data is higher, than that of gravity data and they are more sensitive to shallow sources, thus being more useful for local-scale mapping compared to gravity data. The magnetic map of the western Mozambique is presented in Fig. 4 and combined with lithology in Fig. 5. The mafic Karoo rocks in (A) and (B) are connected with long linear anomaly zones, possibly representing ancient (Gondwana?) rift zones; i.e. mafic sills intruded in extensional fractures. Again, it can be seen that these rocks are continuing below the lighter sedimentary cover (C), southeast in (A) and northeast in (B). The dark linear anomaly close to the western border is apparently due to an electric power line or railroad. These maps clearly demonstrate that the anomaly (A) is a distinct block border separating the low-anomaly sedimentary blocks in (C) from the strongly folded, more magnetic area NW of anomaly (A). The magnetic map in Fig. 4 indicates that there might be some circular and semicircular anomalies due to (intrusive?) sources below the sedimentary cover, such as the one south of (A). Some examples of these circular structures are shown in the more detailed magnetic map in Fig. 6. In Fig. 7 a schematic magnetic interpretation is given across the linear anomaly group of Karoo rocks and the circular magnetic anomaly on the Phanerozoic sedimentary area. From the figure it can be seen that the interpretation supports the dipping of Karoo group rocks below the sedimentary cover. Moreover, the depth to the upper surface of the circular minimum source appears to be ca. 4 km. 66

3 Geophysical maps and petrophysical data of Mozambique Fig. 1. Simplified geology of Mozambique compiled by GTK Consortium geologists. The rectangular coordinates are given in metres; i.e. one coordinate grid square is 100 x 100 kilometres. 67

4 Fig. 2. Gravity map of the southern part of Mozambique. Anomalies (A) (E) are discussed in more detail in the text. 68

5 Geophysical maps and petrophysical data of Mozambique Fig. 3. Gravity map of the southern part of Mozambique combined with lithology. See text for details. 69

6 Fig. 4. Magnetic map of the western part of Mozambique illuminated from the NE and NW. See text for details. 70

7 Geophysical maps and petrophysical data of Mozambique Fig. 5. Magnetic map of the western part of Mozambique in grey tones combined with lithology. See text for details. 71

8 Fig. 6. Detail of the magnetic map showing some possible circular structures that may be due to intrusive sources. The coordinate grid square is 100x100 kilometres. Fig. 7. Magnetic interpretation across the contact zone from Karoo to the Phanerozoic sedimentary area. 72

9 RADIOMETRIC DATA AND MAPS Geophysical maps and petrophysical data of Mozambique In general, the radiometric data are noisy and strongly dampened by overburden, especially in wetland areas; over lakes, radiometric anomalies cannot be detected at all. However, apparently due to the relatively dry overburden in Mozambique, the data appear to be of high quality and useful for regional-scale interpretation and lithological classification. The radiometric data measured in the area consist of 3+1 components: potassium, thorium, uranium and total radiation. In Fig. 8 the radiometric maps of potassium and thorium are presented and in Fig. 9 are maps for total radiation and uranium. It can be seen that particularly the Archaean Proterozoic rocks in the NW corner of the maps are very anomalous. Moreover, the rocks of the Karoo group are connected with higher radiometric anomalies. It is interesting to note that potassium radiation is especially highly increased in the riverbeds and del- tas (due to mica containing clays?), as can be seen in the combination map of potassium radiation and the topographic map in Fig. 10. The high radiation fingers in the map separated from the anomalies of the main riverbeds of Limpopo River in the Gaza area (indicated by G in the figure) apparently show the maximum extent of flooding of the river. The radiometric ternary map, showing U-Th-K anomalies with a colour composition in Fig. 11 and its combination with lithology in Fig. 12, correlate very well with lithology, especially in the Arhaean Proterozoic block in the NW corner of the map area and with Karoo group rocks. Moreover, it gives indications of soil and rock variations in the Phanerozoic sedimentary block. It must be remembered, however, that the good correlation of radiometric maps with the bedrock map is at least partly due to their usage in drafting those maps. Fig. 8. Radiometric maps: Potassium and thorium. Red: high radiation level; blue-white: low radiation level. 73

10 Fig. 9. Radiometric maps: Total radiation and uranium Fig. 10. Topographic map of central Mozambique connected with the potassium radiation map. (G) = Limpopo River flood delta. 74

11 Geophysical maps and petrophysical data of Mozambique Fig. 11. Radiometric ternary map. 75

12 76 Fig. 12. Radiometric ternary map combined with lithology..

13 Geophysical maps and petrophysical data of Mozambique EXAMPLES OF LOCAL SCALE ANOMALIES The aerogeophysical data are described in more detail in reports by Schetselaar et al. (2004) and Westerhof (2005). The gravity data are described in a short report by GETECH (2003). The details of the data are not discussed here. In Fig. 13 more detailed versions of magnetic, radiometric ternary, gravity and lithological maps of the Archaean-Proterozoic Phanerozoic block boundary are presented. The lithological variations along the rifted zone x1- x2 are clearly visible on all maps. The gravity anomaly, in particular, does not appear to exactly follow the surface geology, which indicates deeper density variations in the area. Again, in these maps, the applicability of ternary radiometric data for delineating lithological variations is emphasized, such as the plutonite intrusions west and north from x2. These intrusions (Archaean plutonic rocks) are also connected with the gravity minimum, which indicates that they are relatively felsic or altered. The circular magnetic minimum structure at x3 (interpreted in Fig. 7) is connected with a larger gravity minimum zone. However, the surface geology appears to give no indications of the source of this feature, which means that the source of this anomaly lies below the sedimentary rocks, as is also interpreted above. Fig. 13. Detailed versions of magnetic, radiometric ternary, gravity and lithological maps of the Archaean-Proterozoic Phanerozoic block boundary. 77

14 Fig. 14. Detailed versions of magnetic, radiometric ternary, gravity and lithological maps in the southern part of the study area. In Fig. 14, corresponding map versions of the southern part of the study area are given. From the maps it is evident that the linear magnetic anomalies due to Karoo rocks at x4 continue below the sedimentary cover to the NE. This can be seen, for example, in the continuation of the corresponding gravity anomaly to the NE. Again, the circular mag- netic anomaly at x5 correlates with a larger gravity minimum zone, but not with surface geology in the lithological and radiometric maps, which indicates that the anomaly source is below the sedimentary cover. Moreover, in this area the radiometric ternary map also clearly shows the variations in bedrock and sedimentary cover. PETROPHYSICAL DATA During the geological mapping, density and magnetic susceptibility were measured for ca. 300 samples collected in the northern part of the LOT 3 area. Their susceptibility density diagram is presented in Fig. 15. In the diagram the samples can be classified into three main groups: 78

15 Geophysical maps and petrophysical data of Mozambique Fig. 15. Susceptibility density diagram of samples collected in the northern part of LOT3 area. 1) 2) Group A represents paramagnetic rocks where susceptibility is mainly due to mafic silicates. Sub-group A1 represents light-coloured felsic rocks (e.g. quartzites and mica schists) and A2 dark-coloured mafic rocks (e.g. basalts). Group B mainly consists of altered low-density paramagnetic rocks, e.g. slates and siltstones. 3) Group C samples are ferrimagnetic rocks where group C1 represents magnetic lightcoloured felsic rocks and group C2 magnetic dark-coloured mafic rocks (e.g. diabases and gabbros). SUMMARY The geophysical maps of Mozambique provide a valuable tool for mapping the bedrock and soil cover in areas where the coverage of roads is sparse and thus many locations are difficult to access. The radiometric maps particular appear to be exceptionally informative due to the relatively thin cover of vegetation and low number of bogs and lakes. Moreover, the combination of gravimetric and magnetic maps is useful for detecting major lithological and structural variations, including those below the Phanerozoic sedimentary cover. In coastal areas, the continuation of the seafloor even below the Phanerozoic sedimentary cover can be followed. These areas could possibly have potential for oil and gas occurrences. The petrophysical susceptibility density classification gives indications of major geophysical rock groups in the area and they could in the future be used for preliminary interpretation of geophysical (i.e. lithological) variations, structures and blocks. The need for more extensive petrophysical sampling covering the main lithological units in Mozambique is apparent. These data would provide the key for detailed analysis of the high quality aerogeophysical data that are already available from Mozambique. This work would not necessarily require high quality laboratories. For a good start, a geologist who can reliably define the rock types, equipped with GPS, a digital susceptibility meter and a good digital scale (and bucket of water) would be enough. 79

16 REFERENCES GETECH Southern Mozambique gravity and magnetic data. Data and descriptions. CD in National Directorate of Geology, Maputo. Schetselaar, E., Barritt, S. & Reinink, G Mineral resources management capacity building project, Mozambique. Component 2: Geological Infrastructure Development Project, Geological mapping (LOT 2/3). Report on the geoscience data compilation, phase II. Westerhof, A. B Remote sensing techniques in map production. Report in: GTK Workshop in National Directorate of Geology, Maputo, September 8 12,