Lithofacies and Palynostratigraphy of Some Cretaceous and Paleocene Rocks, Surghar and Salt Range Coal Fields, Northern Pakistan

By Peter D. Warwick, Shahid Javed, S. Tahir A. Mashhadi, Tariq Shakoor, Asrar M. Khan, and A. Latif Khan

LITHOFACIES OF THE HANGU FORMATION

The Hangu Formation of the Surghar Range is remarkably similar lithologically to the underlying Lumshiwal Formation, and on first inspection one would believe that the two units are part of the same stratigraphic sequence. Like the Lumshiwal, the Hangu is composed mostly of sandstone, with minor amounts of mudstone and claystone, carbonaceous shale, coal beds, and a few intercalations of limestone. Because the two formations are so similar, the same lithofacies terminology is used for both. Two major lithofacies are identified in both the Lumshiwal and Hangu Formations--(1) one type of sandstone (sandstone lithofacies A) and (2) a combined facies consisting of mudstone, claystone, carbonaceous shale, and coal beds (combined lithofacies). There is also a minor carbonate lithofacies that is composed of arenaceous limestone. Detailed descriptions of the various lithologies composing the formation are provided in the measured sections (appendix 1).

Although the two formations are similar, there are some differences. Unlike the Lumshiwal, lithofacies B is absent from the Hangu Formation. Instead, the Hangu is dominated by sandstone lithofacies A, which accounts for approximately two-thirds of the total formation; the combined lithofacies accounts for approximately one-third of the formation (figure 4). Intense burrowing is an overwhelming characteristic of Hangu sandstone lithofacies A, and so the primary bedding features are generally obscured. Burrowed and sometimes rooted mudstone interbeds, generally <1 m thick, are commonly gradational with the sandstone lithofacies A. In some places, the Hangu sandstone lithofacies is quartzose, such as at section 8 (appendix I; figure 4), and small crossbeds (sets <1 m thick) are common. The crossbeds are not as large as those observed in the sandstone lithofacies B of the Lumshiwal; therefore, all sandstone in the Hangu Formation is included within sandstone lithofacies A. Grain size is commonly fine to medium, but in the quartzose zones (figure 4), grain size is normally graded from coarse at the base to fine at the top of the unit. Glauconite is less common than in the Lumshiwal, and dispersed carbonaceous material is abundant.

The combined lithofacies of the Hangu Formation, as in the Lumshiwal Formation, contains mudstone, claystone, carbonaceous shale, and coal beds. The primary difference between the two formations is that the Hangu contains the thick (commonly <2-m-thick), widespread Makarwal coal bed. Below the Makarwal coal bed is a laterally discontinuous, iron-stained, lateritic, pyrite-rich, and contorted mudstone deposit (figure 3C). The red mudstone is not always present, as indicated in those sections measured in the northernmost part of the Surghar Range (sections 8-10, appendix I). In these places, the Makarwal coal bed overlies gray, rooted mudstone. No age for the red mudstones has been obtained, and so the deposits have been included in the Hangu Formation, with the Makarwal coal bed. In some places, a pisolitic structure is developed within the claystone (section 6, appendix 1). In section 14, at Nammal Pass, in the Salt Range, a thick (>7-m-thick), bauxitic claystone near the base of the Hangu is probably correlative to the red mudstone zone of the Surghar Range. The red mudstone deposits are similar to those described in the eastern part of the Salt Range by Warwick and Shakoor (in press) and probably represent paleosol horizons. Danilchik and Shah (1987), Shah (1984), and Whitney and others (1990) described these red deposits as laterites.

The combined lithofacies in the upper part of the Hangu Formation is calcareous and contains terrestrial plant and marine animal fossils. These units are gradational with the minor carbonate lithofacies and with the overlying Lockhart Limestone. The carbonate lithofacies of the Hangu is less arenaceous than that found in the Lumshiwal and is nodular and muddy (sections 4, 8, 12, appendix I; fig.4).

COAL CHARACTERISTICS OF THE HANGU FORMATION

Danilchick and Shah (1987) estimated that the remaining minable coal reserves of the Hangu Formation of the Surghar Range are about 16.6 million tons. These reserves are contained in a single coal bed, the Makarwal coal bed, which dips to the west at about 30\xb0 and ranges between 0.6 and 3.0 m in thickness (average, 1.2 m). Near the middle of the formation in the southern part of the range, a thin coal bed (<0.6 m thick) is locally referred to as the upper coal bed and is not part of this reserve estimate. Five in-mine sections measured in the Surghar Range (sections 2, 5, 7, 9, 13, appendix I) describe the physical characteristics of the Makarwal coal bed. The coal bed is commonly bright, sometimes banded, resinous, pyritic, and cleated. Gypsum occurs along some of the cleats. Sandy or muddy partings or stringers are common. Roof rock ranges from carbonaceous shale to mudstone or sandstone that is rooted or burrowed. Floor rock includes lateritic mudstone, rooted claystone, mudstone, and fine-grained sandstone that is commonly pyritic and carbonaceous.

Ghaznavi (1988) described the megascopic and petrographic characteristics of the Makarwal coal bed. He reported that the coal bed was resin rich and variable in thickness. Petrographic study of three Makarwal coal-bed samples from the Surghar Range indicates that they are vitrinite rich; their average vitrinite content is 71.8 percent. Inertinite macerals averaged 18.4 percent, and liptinite macerals averaged 9.8 percent.

Landis and others (1971) and Danilchik and Shah (1987) described and collected eight mine samples and one mine-run composite sample from the Makarwal coal bed of the Surghar Range. During the present study, one coal sample from the upper coal bed and six from the Makarwal coal bed were collected for chemical and physical characterization. In addition, M.I. Ghaznavi collected four samples of the Makarwal coal bed. Descriptions of the coal samples are summarized in table 2, and the results of coal-sample analyses are summarized in table 3 and collated in appendix II.

As reported by Landis and others (1971) and Danilchik and Shah (1987), the apparent rank of the Makarwal coal bed ranges from high-volatile B to high-volatile C bituminous. Averaged, nonweighted results from the samples described in Landis and others (1971) and the present study indicate that some of the averaged, as-received characteristics of the Makarwal and upper beds are (1) moisture content is 5.4 percent; (2) ash yield is 12.5 percent; (3) total sulfur content is 5.0 percent; and (4) calorific value is 11034 Btu/lb (table 3).

Four coal samples of the Makarwal coal bed were analyzed for various major, minor, and trace elements. The results of these tests are presented in appendix II (table II-3). The arithmetic means and standard deviations of these analyses are presented in table 3. For general comparison, data for selected elements from the medium-volatile Upper Freeport coal bed of Pennsylvania are also given (table 3). The Upper Freeport data set was selected for comparison with the Makarwal coal bed because both beds are bituminous and the chemical characteristics of the Upper Freeport coal bed are well documented (Cecil and others, 1981).

Trace elements such as arsenic (As) and selenium (Se) are sometimes environmental pollutants if their concentrations are greater than those found in the Earth's crust (NRC, 1980). The arithmetic mean for As in the Makarwal coal bed samples is 5.88 ppm and does not appear to pose a threat. This concentration is less than the mean of samples from the Upper Freeport coal bed (24 ppm) and is comparable to the overall range of As concentrations for all Western United States coals (0.34-9.8 ppm; Gluskoter and others, 1977). Selenium, however, is a different story. The mean for Se concentration for all United States coals is 4.1 ppm (Swanson and others, 1976). The mean for Se concentrations for Western United States coals is 1.4 ppm (Gluskoter and others, 1977) and for the Upper Freeport coal bed is 2.96 ppm (table 3). For the Surghar Range, the mean Se concentration is 13.4 ppm. Warwick and others (1990) found Se concentrations in coal samples from the Salt Range to average 11.48 ppm. Large-scale use of coal having very high concentrations of Se can cause unacceptable concentrations of Se from fly ash to accumulate in Pakistan's semiarid environment. Therefore, any plans to use Surghar Range coalsfor electric-powerplant feedstocks must take the concentration of Se into consideration.

LATERAL AND VERTICAL VARIATIONS OF THE LITHOFACIES OF THE HANGU FORMATION

The thickness of the Hangu Formation ranges from 4.88 m at the Chichali Pass section in the northern part of the Surghar Range (section 11, appendix I; figure 4) to 49.23 m at the Charles mine section (section 4, appendix I; figure 4) in the southern part of the range. The Charles mine section contains the thickest part of the formation and the greatest amount of sandstone (figure 4). Along strike, the thickness of the sandstone gradually decreases northward and rapidly decreases southward. In the northern part of the study area at the Chichali Pass section (section 11, appendix I), no sandstone is found in the Hangu, but farther east, in the western part of the Salt Range, sandstone beds in the Hangu are more than 10 m thick (Nammal Pass, section 14, appendix I).

The significant differences between the Hangu and the Lumshiwal Formations are that the Hangu contains a minable coal bed in its lower part and that the upper part of the Hangu becomes less sandy and grades into the overlying Lockhart Limestone. This sequence is the reverse of the sequence observed in the Lumshiwal Formation. The lower part of the Lumshiwal is finer grained and more calcareous than the upper part of the Lumshiwal.

DEPOSITIONAL ENVIRONMENTS OF THE HANGU FORMATION

Danilchik and Shah (1987, p. 18) suggested that the lower part of the Hangu Formation of the Surghar Range was "wholly of terrestrial origin," and that the upper part was transitional with the overlying marine Lockhart Limestone. Although the definition of depositional environments of the Lumshiwal and Hangu Formations is preliminary in this study, the most probable environment of deposition for the lower part of the Hangu is similar to that of the upper part of the Lumshiwal Formation--shallow marine and deltaic. This environment of deposition is suggested by the abundance of burrowing found in the sandstone of the Hangu, the frequent mudstone intercalations in the formation, and the presence of marine fossils in the Hangu (Davies and Pinfold, 1937; Haque, 1956). Frederiksen and others (in press) and Khan (appendix III) reported that rock samples from the Hangu of the Surghar Range contain pollen of the brackish-water palm of the genus Spinizonocolpites.

The Makarwal coal bed is unique because it represents a transition of environments of deposition from those initially associated with subaerial exposure and lateritic paleosol development to those associated with mire development and subsequent marine and deltaic environments in the upper part of the Hangu. A rise in relative ground-water base level may have triggered mire formation to spread over lateritic paleosol deposits that formed on weathered paleosurfaces of the Lumshiwal Formation. As the relative base level rose, the mires were flooded by shallow-marine water and buried by clastic deposits probably derived from local Paleocene deltas. Only brief periods of peat accumulation occurred after the formation of the Makarwal coal bed. The thin, laterally discontinuous carbonaceous shale and coal beds in the lower and middle parts of the Hangu probably represent short-lived periods of peat accumulation in the Paleocene coastal area. Warwick and Shakoor (in press) have shown that, during the deposition of the Paleocene Patala Formation in the Salt Range, about 75 km southeast of the Surghar Range, the shoreline was roughly oriented north-south and the Tethys Sea lay generally to the west.

In terms of sequence stratigraphy, the disconformity between the upper part of the Lumshiwal Formation and the base of the Hangu (figure 3C) is a possible sequence boundary. The lower part of the Hangu Formation of the Surghar Range, above the Makarwal coal, probably contains several marine-flooding surfaces, as suggested by the upward decreasing abundance of terrestrially related lithologies (such as coal beds) and the increasing presence of limestone beds having foraminifera. The flooding surfaces cannot be mapped without detailed local and regional stratigraphic control, and it may be difficult to distinguish eustatic flooding surfaces from flooding surfaces caused by shifts in the local subsidence rate or autocyclic environmental shifting. The transgressive deposits of the Hangu Formation in the Surghar Range appear, however, to be associated with the Paleocene transgressive-regressive sequences preserved in the Salt Range and across northern Pakistan (Warwick and Wardlaw, 1992). Additional fieldwork is needed to address these problems.

As in the Lumshiwal, local contemporaneous subsidence rates driven by tectonics probably influenced the deposition of the Hangu as much as, or even more than, eustatic or climatic variations influenced the deposition. As noted by Danilchik and Shah (1987) and as evident on the cross section of the area (figure 4), the Hangu thins rapidly to the north. Similar variations of formation thicknesses have been described by Fatmi and Haydri (1986) for Mesozoic sedimentary rocks southeast of the Surghar Range in the Salt Range. These differences in formation thicknesses may be attributed to local or regional basement faults that may have been active during Lumshiwal and Hangu deposition. Warwick and Shakoor (in press) and Drewes and others (in press) suggest that offsets of basement rocks in the Salt Range and Potwar Plateau, east and northeast of the Surghar Range, influenced thickness of the Paleocene formations in those areas. Influence from basement faults, which also probably affected Lumshiwal deposition, is suggested for the Hangu Formation. Likewise, tectonic controls, probably combined with eustatic and climatic controls, influenced the development of the laterites in the Surghar Range. Shah (1984) also argued for a dominate tectonic influence on various other lateritic deposits at different stratigraphic positions across Pakistan.

As pointed out by McCabe (1991), tectonics play an important role in peat accumulation. The Paleocene paleopeat of the Makarwal coal bed accumulated very close to a disconformable surface that was exposed or experienced nondeposition for approximately 30 million years during the Late Cretaceous. The reasons for the formation of this erosion surface are debatable, but the depositional influences of eustacy and climate change on these rocks were probably overprinted by tectonic forces. A relatively high base level is theorized for the upper part of the Hangu and is indicated by the increasing abundance of marine rocks above the Makarwal coal bed. Tectonic mechanisms, such as active basement faults, probably influenced sediment supply and subsidence rates and caused the rapid change in lithologies and formation thicknesses observed in the Surghar Range.

AGE OF THE HANGU FORMATION

Studies of marine fossils collected from the Hangu Formation (or the Dhak Pass beds, as it was previously known) by Davies and Pinfold (1937) and Haque (1956) have indicated a Paleocene age for the formation. Köthe (1988), on the basis of a regional study of nannofossils and dinoflagellates, gave the Hangu Formation a middle to late Paleocene age. These age ranges are confirmed by the results from pollen samples collected during this study (table 1; appendix III).

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