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 LUMSHIWAL FORMATION

The general description of the Lumshiwal Formation by Danilchik and Shah (1987) provides a review of the previous literature and presents a good description of the basic characteristics of the unit. Detailed descriptions of the various lithologies composing the formation are provided in the measured sections (appendix 1). Three major lithofacies were identified in the Lumshiwal Formation: two types of sandstone (sandstone lithofacies A and B) and a combined facies consisting of mudstone, claystone, carbonaceous shale, and coal beds (combined lithofacies). There is also a minor carbonate lithofacies composed of arenaceous limestone.

The Lumshiwal Formation is dominated by the two sandstone lithofacies. These lithofacies generally are defined on the basis of grain size, clay content, and depositional bedding characteristics. Sandstone lithofacies A is composed of pale-yellowish-brown, yellowish-gray, and olive-gray, fine- to coarse-grained sandstone that is commonly burrowed, sometimes intensely. Bedding types include massive or parallel, and bed thicknesses are commonly greater than 1 m. Small, <0.5-m-thick, tabular crossbed sets may be present. Carbonaceous debris is common along the bedding planes. Ironstone nodules several centimeters in diameter are commonly found throughout the lithofacies. Red-iron-oxide stains are commonly present along fractures, and glauconite and carbonate cements are common, especially in the lower part of the Lumshiwal. Red-iron-oxide staining of the sandstone lithofacies is more common in the upper part of the formation. Quartz content of lithofacies A ranges from 80 to 90 percent, based on hand sample observations. Lithofacies A may grade vertically or laterally into the combined lithofacies and, in places, may be interbedded with the combined lithofacies. Sandstone lithofacies A commonly coarsens upward in grain size and is most common in the lower part of the Lumshiwal Formation (figure 4).

Sandstone lithofacies B generally resembles lithofacies A. Lithofacies B differs, however, by containing fewer claystone interbeds and much more medium- to coarse-grained sandstone characterized by large tabular and trough crossbedding. The individual crossbed sets are >1 m thick. Broad, low-angle crossbedding is rare. Lithofacies B is composed of individual units as much as 10 m thick that are separated by basal scours that extend laterally across the outcrop for many tens of meters. Individual beds tend to thicken upward (figure 3A). Although individual beds within lithofacies B commonly have a scoured base, the base of lithofacies B is transitional with lithofacies A or the combined lithofacies. Sandstone constituting lithofacies B is very quartzose in hand specimen; quartz content ranges from 80 to 90 percent. Coarse- to granule-size sandstone and pebbly bands up to 0.5 m thick are commonly interbedded within lithofacies B. The pebbles are composed exclusively of quartz (figure 4). Red-iron-oxide staining is very common in lithofacies B, especially in the uppermost part of the formation.

The combined lithofacies consists of mudstone, claystone, carbonaceous shale, and coal beds. This lithofacies generally is restricted to the upper part of the Lumshiwal Formation (figure 4). The color of the mudstone ranges from medium-reddish brown, to reddish brown, to black, and the mudstone is commonly carbonaceous, burrowed, and (or) rooted. Mudstone also occurs as thin (<0.5-m-thick) interbeds within the sandstone lithofacies in the upper half of the formation. In the uppermost part of the formation, the mudstone is interbedded and gradational with thin claystone, carbonaceous shale, and coal beds that are <0.3 m thick and laterally discontinuous. The claystone is carbonaceous, is often rooted, and commonly comprises underclay that is gradational with the carbonaceous shale and coal beds.

The minor carbonate lithofacies is rare in the Lumshiwal Formation. A single limestone bed (unit 6 in section 3, appendix 1; figure 4), which is 1.22 m thick, is the only occurrence in these measured sections. This facies consists of arenaceous limestone that is medium-light to light gray and glauconitic. Quartz grains and a few ironstone concretions occur within the carbonate matrix.

LATERAL AND VERTICAL VARIATIONS OF LITHOFACIES OF THE LUMSHIWAL FORMATION

In the Surghar Range, the Lumshiwal Formation generally consists of sandstone lithofacies A units overlain by stacked, coarsening-upward lithofacies B sandstone units that are monolithologic for as much as 85 m (section 4, Charles mine, appendix I; figure 4). Lithofacies B sandstone decreases in abundance toward the north and toward the south, as is illustrated on the cross section (figure 4). The combined lithofacies, which contains a few thin coal beds, is generally discontinuous and is restricted to the upper part of the formation (figure 4).

As noted by Danilchik and Shah (1987), the Lumshiwal Formation in the Surghar Range decreases in thickness from south to north. The thickest part of the formation is at the PMDC central tunnel (section 5 is between sections 4 and 6, on figure 4), where it is greater than 150 m thick. The thickness of the formation decreases northward to about 64 m at Chichali Pass (section 11, appendix I; figure 4). In the western part of the Salt Range at the Nammal Pass (section 14, appendix I), the unit is very thin, if present at all. At Nammal Pass, the rocks below the Hangu Formation, constituting the Jurassic Datta Formation and possibly the Lumshiwal Formation, are indistinguishable. Danilchik and Shah (1987) also observed this identification problem.

DEPOSITIONAL ENVIRONMENTS OF THE LUMSHIWAL FORMATION

Danilchik and Shah (1987) suggested that the Lumshiwal Formation of the Surghar Range was deposited under terrestrial conditions, presumably because of the scattered coal and carbonaceous beds found in the upper part of the formation. Although the identifications of depositional environments are preliminary in the present study, the most gross probable environments of deposition for the Lumshiwal Formation are shallow marine in the lower part of the formation and deltaic in the upper part of the formation. This interpretation is based on the predominance of lithofacies A sandstone in the lower part of the formation. Lithofacies A is transitional with the underlying marine Chichali Formation, which is rich in belemnites and coiled ammonites (Fatmi, 1972; Danilchik and Shah, 1987) and Early Cretaceous marine fossils from the lower part of the Lumshiwal described by Fatmi (1972). The lithofacies A sandstone, which has a clay matrix with glauconite, intense burrowing in places, generally thick bedding, and increasing grain size upward, was probably deposited in shallow-marine, prodeltaic environments. The glauconite probably represents mineralized fecal pellets of burrowing organisms. Characteristics similar to those described for the lower part of the formation also have been attributed to shallow-marine and deltaic depositional sequences in numerous other rock records (Galloway and Hobday, 1983).

The upper part of the Lumshiwal Formation, dominated by lithofacies B sandstone, is more complex than the lower part. The mix of primary bedding structures and various grain sizes present, the scattered occurrence of carbonaceous and coaly beds, and the abundance of quartzose sandstone indicate that a dynamic mix of depositional environments was probably responsible for the character of this unit. The large lithofacies B sandstone bodies probably resulted from an array of deltaic and nearshore marine processes that gave way to the development of small peat mires. The abundant quartzose sandstone may represent deposits that were enriched in their quartz content by reworking associated with nearshore processes. Frederiksen (1992) suggested a brackish-to-marine environment of deposition for the upper part of the Lumshiwal on the basis of the presence of the acritarch Veryhachium sp. Petrographic study of the major Lumshiwal rock types, detailed outcrop study of primary bedding structures, and three-dimensional stratigraphic control are needed for a better definition of the relative depositional environments of the formation.

Local tectonic subsidence contemporaneous with deposition most probably played a role in defining the character of the Lumshiwal Formation. Danilchik and Shah (1987) noted that the formation thins to the north. This thinning is evident on the cross section of the Surghar Range (figure 4). The thicker part of the Lumshiwal, composed primarily of sandstone lithofacies B, indicates that the southern part of the range had a relative subsidence rate that was greater than that of the northern part of the range. McDougall and Khan (1990) have shown that a major, modern strike-slip fault extends along the western part of the Salt Range. These authors also point out that the strike-slip fault overlies a deep (5 km) basement ridge that probably is of basement-fault origin. Movement may have occurred along these or similar basement features during the Cretaceous, thus influencing the distribution of depocenters, and so may have contributed to the variation in Lumshiwal Formation thickness. Indeed, during the Late Cretaceous, the area that is now the Surghar Range ceased to be a depocenter, became subaerial, and experienced erosion or nondeposition. This change is evidenced by the lack of Upper Cretaceous strata in the area. Such patterns in deposition may have been structurally influenced.

Without a regional stratigraphic data set that can be used to map sequence boundaries and parasequence packages, fitting the observed changes in Lumshiwal stratigraphy to Mesozoic sea-level curves such as those discussed by Haq and others (1988) is difficult. Undoubtably, sea-level and climatic changes affected the deposition of the Lumshiwal. The disconformable surface at the top of the Lumshiwal may be a sequence boundary, but the lateral variations of stratigraphic thickness of the Lumshiwal appear to be influenced primarily by the tectonics of the area.

AGE OF THE LUMSHIWAL FORMATION

The disconformable relation between the Lumshiwal and Hangu Formations is quite enigmatic, because the contact is very difficult to define. Wynne (1880) first described the rocks of the Trans Indus Mountains and classified some of the rocks as Cretaceous in age. Gee (1945) later named some of these rocks the Lumshiwal Formation. Fatmi (1972), on the basis of fossils including Gryphaea and Hibolithes collected from the basal 3 m of the Lumshiwal Formation in Baroch Nala (section 6, appendix I), suggested that the lower part of the formation was probably Aptian(?) to middle Albian in age. The pollen samples collected from the upper part of the Lumshiwal during the current study and described by Khan (appendix III), Frederiksen (1992), and Frederiksen and others (in press) indicate a broader age range for the Lumshiwal--from Late Jurassic to Early Cretaceous. One sample (NF89P-3, table 1; section 1, appendix I), however, collected and described by Frederiksen and others (in press), correlates with the age given by Fatmi (1972). Dates obtained from this sample suggest that the upper part of the Lumshiwal is Aptian to Albian(?) in age. The location of dated pollen samples collected from the Surghar Range are plotted on the cross section (figure 4), and the position of the difficult-to-define disconformable surface is approximately marked on the cross section.

Although Danilchick and Shah (1987) describe an Aptian to Albian(?) age for the Lumshiwal in their text, it is not clear why figures 3 and 5 of their report show the age of the formation to be Late Cretaceous. If the age of all the Lumshiwal is Aptian to Albian(?), then there is a substantial age difference (roughly 30 million years) between the upper part of the Lumshiwal and the overlying Paleocene Hangu Formation. Danilchik and Shah (1987, p. 18) addressed the situation as follows: ``In the Trans-Indus Mountains the [Lumshiwal] formation apparently disconformably underlies, but is lithologically gradational with, a fossiliferous formation known to be of Paleocene age.'' Presumably the lithologically gradational nature of the Lumshiwal and Hangu contact led Danilchick and Shah to suggest that the upper part of the Lumshiwal may be Late Cretaceous in age.

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