The report of Bam map

Category Earthquake
Group Seismotectonic & Seismology
Published At 14 December 2009


The Bam map sheet lies in the Kerman province of southeast Iran


The Bam map sheet lies in the Kerman province of southeast Iran. The limits of the sheet are given by longitudes 58 .00 - 58 .30 E and latitude 29 .00 - 29 .30 N. The sheet is dominated by a northwest-southeast trending range of mountains, which extends from northwest corner of the sheet to the region north of Bam reaching a maximum elevation of 2424 m (Kuh-e Kafut). The southwestern corner of the sheet covers a small section of the Jebal-e-Barez mountain range. The drainages from this range and also from the southwestern side of the main range feed into the main northwest-southeast trending drainage system occupying the broad valley between the two ranges. The northeast side of the main range drains in a northeasterly direction towards the Dasht-e-Lut and to the east into a playa in the adjoining map sheet. There are few active streams or springs in the area. In the southwest corner of the sheet, there is an active stream at the village of Daruz. Within the main range, the only active stream noted was in the very northwest of the sheet. The mainstream bed immediately north of Bam contains running water but this is fed by a qanat, as are other streams supplying Bam and its satellite villages. The major qanat systems feeding Bam are supplied from the southwest side of the main range Barevat to the southeast of Bam is fed by a qanat system supplied from the Jebal-e Barez range. There are very few migratory camps, due to the extreme aridness of even the main range. Citrus fruits and dates arc the main produce of the region. The Bam area is part of the Lut-e-Zangi Ahmad desert region and has temperatures up to 50 c in summer. In spring, high winds causing dust storms can, and did hinder fieldwork. Winter temperatures range from below freezing to 15.20 c . Precipitation is low, but storms causing flash floods occur occasionally. Access to the sheet is by means of the Kerman-Bam-lranshahr asphalted highway. Motorable access within the sheet is good, landrover being the principle means of traversing. REVIEW OF FORMATION The oldest rock units of the Bam sheet is a limestone unit (K I ) occurring as an inlier surrounded by Tertiary volcanics. It also occurs as several inselbergs to the southwest of the main volcanic range. Fossil evidence dates this unit as Cenomanian. The unit, chronologically, above K I exist only in the southwest corner of the sheet, south of Daruz separated from other units by the broad valley trending northwest-southeast. This unit consists of siltstones, marls, limestones and calcareous sandstones (Kml), and is paleontologically dated as Senonian, therefore making it unconformable on the Cenomanian limestone. A conglomerate unit of Paleocene age, the Kerman conglomerate (Pe c k ), is exposed both in the southwest corner and on the southwest flank of the main range. In the southwest corner it is above the Senonain deposits and in the main range, it is above the Cenomanian limestone. Although the contacts have not been seen, this relationship suggests on unconformity. In the southwest corner, above the Kerman conglomerate and Senonian limestone, and resting on them with angular unconformity is a highly colored tuff sequence (E vt ). These tuffs are probably connected with volcanic activity related to the Jebal-e-Barez extrusives and cannot be equated stratigraphically with the main range of the bam sheet. The Eocene volcanic sequence forming the main mountain range in this area lies unconformably upon the Kerman conglomerate. The Eocene volcanic sequence can be divided into three groups of units. Each group consists of an association of lavas, ash-flow tuffs and volcanosediments in which lateral facies changes are conspicuous. The distribution of these volcanic groups illustrates some of the main structural features of this area. The oldest Eocene volcanics occur in the northwestern part of the map sheet. In this area, they outcrop along the southwestern side of the range and are exposed also in the center as a result of a major transcurrent fault. Trending approximately north-south. The middle volcanic group forms a large part of the central and northwestern thirds of the main range. This group is noticeably unconformable on the lowest group except in the extreme northwestern corner or the map sheet where there is approximate conformity. In the latter area, the middle group lens out strongly towards the northwest and becomes very thin. The highest group outcrops to the north, northeast and southeast of the main part of the range. In the north and east of the map sheet, this group laps onto the middle group with strong unconformity. In the east, the youngest group plunges to the southeast beneath Neogene and Quaternary sediments. The Lower group consists of three different lithological units: well-bedded volcanoclastic sediments (E vId ), mainly boulder conglomerates, detrital breccias, sandstones and siltstones; lavas interbedded with boulder conglomerates and breccias (E vId ); and well-bedded ash-flow ruffs associated with detrital breccias and conglomerates (E vIt ). These; three units alternate both laterally and vertically within this group. The sediments are clastic and generally coarse to very coarse grained. There are some local developments of very coarse boulder conglomerates with clasts of lava up to 2m in diameter. Some of the conglomerates and breccias contain clasts of Mesozoic limestone up to 4Ocm in diameter in addition to the volcanic debris. The latter is occasionally highly chloritised, apparently prior to its incorporation into the sediment. The finer sediments, which include sandstones, siltstones and rare mudstones, are typically purple-red in color. They are usually feldspathic; some of the siltstones and mudstones are calcareous. In a few localities, the sediments are interbedded with thin ash. Flow tuffs; lenses of lava, too small to be mapped separately, also occur. The lavas (E vId ) are most common in the lower part of the section. They are commonly. But not exclusively, associated with coarse volcanoclastics. The lavas are dominantly dacites but subordinate andesites and rhyolites are also found. Alteration is widespread and locally severe, plagioclase being replaced by sericite and mafic minerals being replaced by opaque minerals and sericite. Limonite staining is common. Some of the lavas were partly or wholly brecciaed during flow. Other breccias interbedded with the lavas are detrital and are associated with boulder conglomerates. Some of these coarse clastic sediments are lahar deposits. The Pyroclastics (E vIt ) are of ash.flow origin and petrographically are mainly crystal lithic tuffs, vitric crystal tuffs and vitric-lithic tuffs. They are well.bedded and have intercalations of detrital breccias and conglomerates. The tuffs are generally dacitic or rhyodacitic in composition and are frequently altered. limonitisation has occurred locally. Welding in the tuffs is poorly to moderately develop. In many specimens, the original matrix of glass shards has devitrified to granular felsitic material, though their pyroclastic nature is usually still evident. The coarse volcanoclastics associated with the tuffs are similar to those described above (E vIs) but contain some clasts of indurated tuff in addition. The middle group (E v2d) consists of an alternation of two massive units: lavas (E v2d) and ash-flow tuffs (E v2t ). In neither unit is bedding well developed. The lavas are mainly dacites with some rhyolites and minor andesites. A slight to moderate alteration is widespread, with phenocrysts replaced by seritite or opaque minerals or both. Some lava is partly chloritised. In highly altered areas, the lavas are completely bleached and show as pale tones on aerial photographs. Both volcanic and detrital breccias occur locally and there are also some rare sands tones. There is no defmite evidence for eruptive craters within this unit. However, the lavas are thickest, and contain least intercalated detrital material, near the center of the map sheet, suggesting a possible source in this area.

tags: KERMAN