Mapping Of Volcanic Series Rock Units Using Landsat Thematic Mapper Imagery,
Troodos Ophiolite Complex, Cyprus
K.T.U.S.De Silva
Geological Survey And Mines Bureau
No;4, Galle Road, Dehiwala Sri Lanka
Abstract
The island of Cyprus hosts one of the best-preserved ophiolite complexes in the world, known as the Troodos Ophiolite Complex. Litho logically, it comprises a displaced slab of altered ultra basic and basic plutonic igneous rocks capped by intermediate and basic lava flows. Its unique dormal structure permits one to study the complete ophiolite stratigraphy without different. Out of the complete ophiolite sequence, the volcanic series rocks units, namely, Upper Pillow Lava (UPL), Lower Pillow lava (LPL) and basal Group (BG) are difficult to map in the field due to their close similarities.
In the research, several advanced image processing techniques were applied and tested their effectiveness to the three sub scene which were selected from a TM image in the North and eastern part of the Troodos massif, comprising the volcanic stratigraphy. Out of these, images produced by Saturation Enhancement, Decor relation Stretching and Selective data ad well as with the field observations. It is important to mention that in the previously published geological maps (maps published up to 1979), most of the UPL unit in the Delikipo area in the estern part of the Troodos massif had been mapped erroneously as LPL rocks. Later in the early eighties, during the map updating programme. This has been corrected. With the use of the above mentioned image processing techniques, this can easily be confirmed.
The selective PCA using three bands is a new method which was created for enhancing geological formations in this study. The method uses only there of the possible six Landsat TM bands for mapping iron (bands1,3&5) and hydroxyl (bands 1,5 & 7 )alteration, readily apparent in TM imagery. Careful examination of the eigen vector loadings allow the identification of the correct Principal Component needed to map these alteration types. Final demarcation of the volcanic stratigraphy was based on the difference in hydroxyl and iron oxide contents in each rock unit.
Introduction
The Troodos massif of the Island of Cyprus hosts one of the best preserved and most thoroughly studied ophiolite complexes in the world. Troodos has received a special attention among geological community throughout the world due to its complete and non disruptive sequence tougher with a displaced slab of altered ultra basic and basic platonic complex, stratigrephically overlain successively by a sheeted dike complex, extrusive sequence and pelagic sediments (Moore & Vine1971; Gase 1980). Due to subsequent erosion, the original upward succession of ophiolite stratigraphy was arranged in an outward succession from centrally exposed platonic rocks to sheeted dike complex and peripheral pillow lavas.
Out of the complete ophiolite sequence, the volcanic series rock unit, especially the UPL and LPL are very difficult to differentiate in the field due to their close similarities. Complete discrimination of the two lava rock units is necessary as massive sulphide ore bodies that have long been exploited in Troodos massif, occur at their contact zone. In this research, applying remote sensing techniques together with field verfication studies carried out a study of the pillow lava series rock units.
Geology of Troodos Pillow Lava Series
The volcanic sequence, also known as the pillow lava series, is extending as an irregular incomplete ring around the periphery of the massif in a belt of metamorphosed pillow lavas and dikes, that from the uppermost 1.5-2 km of the ophiolite complex. On the basis of colour, mineralogy and abundance of dikes, the volcanic sequence is divided into three namely, Upper Pillow Lavas (UPL), Lower Pillow Lavas (LPL) and Basal Group (BG) (Wilson, 1959).
Upper Pillow Lavas (UPL)
Rocks of the UPL occupy a relatively narrow strip in the outer periphery of the ophiolite sequence, having hummocky topography. Overstepped in places by the sedimentary rocks, the outcrops of this division averages nearly one kilometer in width in the western part and gradually widens toward the east. Pathologically the UPL succession consist of prophyritic olivine basalts, limburgite and olivine free basalts.
Throughout the study area, UPL are well exposed as no vegetation cover is noted on these rocks, except for occasional pine trees and thorny bushes. However, although the pillowed nature of the lavas can be seen easily on majority of surfaces, fresh specimens are
hard to obtain as the rocks have been extensively weathered, decomposed and shattered into small fragments when struck with a hammer.
The UPL are characterized by well formed in abundance and intrusive such as dikes and sills are almost absent. The pillows vary in size and shape. The size of the pillow range from a few centimeters across by around 20 cm in diameter to 50 centimeters across by nearly 2 meters in diameter. Their shapes vary from spheroidal to ovoidal, the latter being the prominent.
Although, the lavas themselves are pale gary in colour, their inter pillow material gives a pink cast to their outcrops, in many places: The inter pillow spaces are occupied by iron stained calcite, altered glass, jasper and zeolites and their pinkish colour is due to sea floor oxidation. Some of its pillows are as cracked and the fractures are filled with pinkish colour inter pillow material and are seen as veins. In some areas, the pillow exhibit a dark appearance which resembles the Lower Pillow Lavas.
The UPL are resting on an undulating surface of LPL and they are immediately overlaid by the umbers. However, when the umbers are absent, they are sandwiched between underlying LPL and overlying rocks of Lefkara Formation.
