The Kuldana Formation is a fossil-bearing geological formation of Lutetian (Early Eocene) age which crops out in northern Pakistan. The abundant fossil remains were deposited by rivers and estuaries crossing an arid to semi-arid environment, between several marine transgressions. Its fossil fauna is best known for the early cetaceans Indohyus, Pakicetus and Ambulocetus, that helped to shed a new light on the evolution of whales, but it also features a large number of early ungulates, rodents and primates.
History
Fossils from the Kuldana Formation have been studied since long before the name was established, with some of the earliest papers being those of Pilgrim, published in 1940. At that point in time, the sediments were known as the Chharat beds. The name Kuldana Formation as such was first coined by Mir Abdul Latif in 1970, prior to which the sediments had already been known as the Kuldana Beds and Kuldana Series, deriving its name from a nearby village. Around the same time, Meissner and colleagues studied similar outcrops near the village of Mami Khel, 200 km (120 mi) southwest of Kuldana, naming it the Mami Khel Clay. Although study began prior to the publication of Latif's work, this paper was not published until later. Several subsequent papers comparing the two generally agreed that they were synonyms and represented a single geological formation, that being the Kuldana Formation. A 1996 publication by Pivnik and Wells meanwhile used the name Mami Khel Formation and Maas et al. (2001) treated the two as distinct formations primarily based on their geographic separation through the Indus River while providing no other indicators in how the two differed. Other studies treating the two formations as distinct include Leinders (1999) and Thewissen (2001).
Geography
Outcrops of the Kuldana Formation are found in the form of isoclinal folds across northern Pakistan's Kohat plateau and Potwar plateau. The Formation stretches from Barbora and Mami Khel at the western edge of the Kohat plateau to the name-giving locality of Kuldana at the eastern end of the Potwar plateau. Between these points are a variety of other localities as well, such as Chorlakki, located within the eastern-most Kohat plateau, as well as Lammidhan and Ganda Kas (Kala Chitta) at the western end of the Potwar plateau.
Stratigraphy
While the stratigraphy within the Kuldana Formation is poorly understood, its relation to under- and overlying formations has been much better studied thanks to the succession being consistently visible across various localities. At Banda Daud Shah, the Kuldana Formation overlies the Ghazij Formation, the Shekhan Formation and the Jatta Gypsum. The Ghazij and Shekhan Formations also underlie the Kuldana Formation at Chrolakki while at Ganda Kas and Gali Jhagir the formation preceding the Kuldana was the Chorgali Formation. The Kohat Formation consistently overlies the Kuldana Formation.
Age
The age of the Kuldana Formation has been subject to repeated revision and multiple back and forths among researchers. Two early estimates, proposed by Cotter and Pilgrim respectively, suggested that the Kuldana Formation was either Ypresian (Early Eocene) or Lutetian (early Middle Eocene) in age, with subsequent authors generally following one of these hypothesis. After officially establishing the name Kuldana Formation, Latif suggested that the formation dates to the lower to middle Eocene on the basis of shallow benthic foramins, somewhat straddling the line between both the results of Cotter and Pilgrim. This would come to be the general result of subsequent papers as well, which often recovered an age within that general range. In 1983 Gingerich used the fact that the Kuldana Formation is preceded and followed by marine strata to compare it with then available maps of sea levels during the Eocene, arguing for a late Early Eocene age, once again falling into the convention established by prior work. Gingerich did however come to revise his 1983 age estimate in a paper published in 2003 that dealt with the stratigraphy observed across different localities in order to obtain a more reliable result. Another aim of this work was to provide counter arguments to some claims made around the turn of the century, when several papers claimed that the sediments at Mami Khel represented a distinct formation that dated to the early Early Eocene. To do so, foramins of underlying formations were used to establish a maximum age for the Kuldana strata. At Banda Daud Shah, Gingerich recovered a late Early Eocene to early Middle Eocene (P9 or P10) age based on the planktonic foraminifera of the underlying Shekhan Formation, with similar results being recovered based on shallow benthic foramins at Chorlakki, Ganda Kas and Gali Jhagir (all dated to SB12 to SB13). Gingerich further utilizes updated knowledge on global sea levels to find similar results. Unlike in 1983, Gingerich now recognized two distinct periods of low sea levels, one during the late Early Eocene and a second, longer-lasting period during the early Middle Eocene. Gingerich argues that, since only one terrestrial phase is observed within the stratigraphy of Eocene Pakistan, it is likely that the Kuldana Formation represents the longer period of low sea levels, placing it within the early Late Eocene.
Paleoenvironment
The Kuldana Formation featured several different environments across its time of deposition, with the oldest layers showing fluvial conditions. The strata overlying these sediments suggest that the environment shifted to a mix of freshwater and shallow marine habitats, which is supported by multiple lines of evidence. Evidence for freshwater biomes is present through isotopic analysis as well as the local fauna, for example the freshwater snail Planorbis, whereas the presence of sharks and pycnodontoid fish shows the presence of marine habitats. It is thought that the archaeocetes that inhabited the formation at the time stuck to freshwater biomes, even if some localities were evidently closer to the coast. This mix of fresh- and saltwater habitats was eventually followed by a marine transgression that characterizes the geology of the uppermost layers of the formation.
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