U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY GEOLOGIC MAP OF THE ISLAMABAD-RAWALPINDI AREA, PUNJAB, NORTHERN PAKISTAN by Van S. Williams1 and Mustafa K. Pasha2 and Iqbal M. Sheikh2 Open-File Report 99-47 Report prepared jointly by the U.S. Geological Survey and the Geological Survey of Pakistan under the auspices of the U.S. Agency for International Development This report is preliminary and has not been reviewed for conformity with the U.S. Geological Survey editorial standards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. AUTHOR AFFILIATIONS 1 U.S. Geological Survey 2 Geological Survey of Pakistan Base Credit: Base from Survey of Pakistan, Islamabad and Surroundings, 1985 Place names used in this report do not necessarily conform with names approved by the U.S. Board on Geographic Names Geology credit: Geology mapped and compiled in 1988-90 by I.M. Sheikh, M.K. Pasha, and V.S. Williams; with help from S. Qamar Reza, Kanwar Sabir Ali Khan, and Akram M. Bhatti. DESCRIPTION OF MAP UNITS Qal Stream-channel alluvium (Holocene)--Unconsolidated gravel, sand, and silt that is subject to stream transport each year. Poorly to moderately sorted and low-angle crossbedding. Generally without soil or vegetation. Forms low islands and bars within braided ard meandering stream channels. Maximum thickness about 3 m Qfp Flood-plain and fan alluvium (Holocene)--Moderately bedded and sorted, sand and gravel channel and debris-flow deposits overlain by thin veneer of sandy silt and clay fro~a overbank flooding and slope-wash deposition. Typically deposited adjacent to streams or in fan-shaped bodies at mouth of canyons or gullies. Maximum thickness beneath flood plains about 6 m and beneath fans about 20 m Qas Alluvium and windblown silt, undifferentiated (Holocene and upper Pleistocene)~Eolirn silt and stream-channel, flood-plain, terrace, and slope-wash alluvium intermixed in small areas that cannot be depicted separately at map scale. Such deposits typical'"7 occupy small depressions in the Margala Hills. Less than 10m thick Qp Potwar Clay (Holocene and Pleistocene)--Windblown silt and clay and subordinate amounts of alluvial gravel. Sediment is light brown to gray, very fine grained, hard, compact, and calcareous. Windblown sediment averages 71-74 percent silt-size and 15-16 percent clay-size material. Unit is 14-18 percent calcium carbonate. Mineral composition is predominantly quartz, but subordinate amounts of feldspar and clay minerals, such as kaolinite and illite, are present (Rendell, 1988, p. 392). The well- developed vertical partings and lack of bedding suggest that much of the sediment is atmospheric dust, but stratification of some of the sediment indicates partial reworkir 7 by surface wash and streams. Locally, silt is intercalated with crossbedded lenses of sand and gravel and with the Lei Conglomerate. Thermoluminescence ages of loers from the Rawat area range from 20 to 132 thousand years (ka) from near the surface to 11-m depth, and greater than 170 ka for more deeply buried loess beneath a gravel facies. Calculated accumulation rates range from 6 to 27 centimeters per thousand years (cm/1,000 yr)(Rendell, 1988, p. 393). The silt and clay beds are very erodibh; hence deep, steep-sided gullies and badlands are extensive. Unit subject to loss of bearing strength when wet. Thickness is highly varied, depending on relief of underlying unconformity. Exposed thickness 1-35 m. Similar deposits intercalated with Lei Conglomerate extend to a depth of 152 m (Ashraf and Hanif, 1980) Terrace alluvium (Holocene and Pleistocene) Gravel, clay, and silt locally cemented by calcium carbonate. Includes clast-supported boulders, cobbles, and pebbles of sedimentary rocks in a sandy and clayey matrix. These former stream-channel and flood-plain deposits no longer receive sediment because subsequent downcutting by streams has left them high above flood level. Repeated episodes of uplift or climate change and erosion have left terrace deposits at several levels. Unit resembles Lei Conglomerate but is younger and retains depositional form Qty Younger terrace alluvium (Holocene and upper Pleistocene)~Terrace alluvium who?e depositional surface is less than 5 m above modern flood level. Generally forms benches along sides of modern stream valleys. Thickness about 3 m Qto Older terrace alluvium (Pleistocene)--Terrace alluvium whose depositional surface is mere than 5 m above modem flood level. Generally preserved as discontinuous remnants of gravel capping ridges and flat-topped hills. Maximum thickness about 3 m Ql Lei Conglomerate (middle Pleistocene)~Carbonate-cemented cobble conglomerate consisting of 93 percent subangular limestone clasts intercalated with and grading laterally into weakly consolidated silt, sand, and clay. Clasts are 5 percent old^r Siwalik Group sedimentary rocks, 2 percent quartzite, and trace amount of igneous rocks (Gill, 1951; Raynolds, 1980). Generally flat lying, but locally folded and faulted. Overlies rocks of Siwalik and Rawalpindi Groups upon an angular unconformity. At Shahpur (lat 33° 30.8' N., long 73° 04' E.), fine overbank sedimerte are preserved beneath an erosional disconformity at base of conglomerate and abo^re angular unconformity. Volcanic ash from sediments older than die Lei Conglomerate and younger than the Soan Formation has been dated by the fission-track method at 1.6±0.18 Ma, representing a local maximum age for the Lei Conglomerate (Johnson and others, 1982). Interpreted as an alluvial basin-fill sequence of coarse, angular gravel derived from the uplifting Margala Hills to the north interbedded with funr sediment derived from sandstone and shale of the Rawalpindi Group and windblown silt. Deposited along the axis of the subsiding Soan syncline. Cemented conglomerate beds are resistant to erosion and form ledges and hills. Uncemented conglomerate beds are the most important ground-water aquifer in the area. Exposed thickness 106 m, but drill hole FC-12 southwest of Saidpur (grid G-3) penetrated more than 152 m of interbedded clay (72 percent) and gravel (28 percent) (Ashraf and Hanif, 1980). In this drill hole the average thickness of gravel beds is 6 m and of clay beds is 14 m Siwalik Group (Pleistocene? and Neogene) Nss Soan Formation (lower Pleistocene(?) and Pliocene)~Conglomerate and subordinate interbeds of sandstone, siltstone, and claystone. Conglomerate clasts range in size from pebbles to boulders and consist of about 80 percent rounded quartzite, about 10 percent fine-grained volcanic trap rock, and 10 percent metamorphic rocks combined with sedimentary rocks of the Siwalik Group. Clasts are cemented in a calcareous sandy matrix. Sandstone is greenish gray, coarse grained, and soft. Claystone is orange, brown, and pale pink and soft. Exposed thickness is 200-300 m. Upp^r contact beneath the Lei Conglomerate and younger sediments is an unconformity ohhr than 1.6±0.18 Ma (Johnson and others, 1982). Base locally rests on an unconformity dated by fission-track dating of volcanic ash at 1.9±0.4 Ma (Raynolds, 1980, p. 190) Nsd Dhok Pathan Formation (Pliocene and upper Miocene)--Sandstone and claystone containing lenses of conglomerate in upper part. Sandstone is light gray, fine to medium grained, medium bedded, and crossbedded. Claystone is orange red and chocolate brown, hard, and compact. Measured thickness is 500-825 m. Unit is overlain unconformably by the Soan Formation. Near type locality base was dated by fission-track dating at about 8.5 Ma (Johnson and others, 1985) Nsn Nagri Formation (upper Miocene)~Sandstone and subordinate claystone and conglomerate. Sandstone is gray, greenish gray, and brownish gray; medium to coarse grained; thick bedded; crossbedded; calcareous; and has a salt-and-pepper pattern that is produced by magnetite and ilmenite. Claystone is brown, reddish gray, and orange and is sandy or silty. Thickness is 500-900 m. Contact with overlying Dhok Pathan Formation is conformable. Near type locality base was dated by fission-track dating of volcanic ash at about 10.8 Ma (Johnson and others, 1985) Nsc Chinji Formation (upper and middle Miocene)~Claystone and sandstone. Claystone is brick red, friable, hard, and intercalated with sandstone. Sandstone is dark gray to brownish gray, medium to thick bedded, soft, and crossbedded. Thickness varies from 880 to 1,165 m. Upper contact with the Nagri Formation is conformable. Near type locality base was dated by fission-track dating of volcanic ash at about 14.3 Ma (Johnson and others, 1985) Rawalpindi Group (Miocene) Nrk Kamlial Formation (middle and lower Miocene)~Sandstone and claystone. Sandstone is purple, gray, and dark brick red; medium to coarse grained; thick bedded; micaceou-; jointed; and calcareous. Interbeds of hard purple claystone; some claystone beds are weathered and have yellow mottles. These weathered beds resemble conglomerate. Unit is distinguished from underlying Murree Formation by its spheroidal weathering. Measured thicknesses range from 1,200 to 1,600 m. Upper contact beneath the Chinji Formation is conformable. Near the village of Chinji, base was dated by fission-tracV dating of volcanic ash at about 18.3 Ma (Johnson and others, 1985) Nrm Murree Formation (lower Miocene)~Continental sandstone and claystone. Sandstone is reddish gray to purple gray, fine to medium grained, thick bedded, micaceou^, crossbedded, jointed, and calcareous. Claystone is purple to dark red and contairs mottled lenses of pseudoconglomerate. Epidote is common in sandstone of the Murrra Formation. Contact with overlying Kamlial Formation is conformable. Measured thickness ranges from 2,000 to 2,895 m in the area Cherat Group (lower Eocene) Peck Kuldana Formation Marine claystone, marl, limestone, and minor sandstone. Claystone is variegated in color and has gypsum intercalations. Marl is pale gray to brownish gray, thin to medium bedded, and contains fibrous gypsum. Limestone is white to very pale brown.