TECTONIC INTERPRETATION AND PALEOGEOGRAPHY

The paleogeographic setting of the Himalayan foreland during deposition of the Chitarwata Formation at Zinda Pir Dome was the product of both regional tectonic events related to India-Asia collision and eustatic sea level changes. The Indus Fan provides an aggregate record of sedimentation that must be addressed in discussions of paleodrainage of the Himalayas following the India-Asia collision. Analysis of multichannel seismic profiles and deep sea cores of the Indus Fan by Clift et al. (2001b) confirm that: 1) the Indus Fan and Indus River system are at least middle Eocene in age, 2) about 35% of the fan is comprised of Paleogene sediments signifying there was large topographic relief in southern Asia at that time, and 3) a rapid increase in sedimentation occurred during the middle Miocene. The Indus Fan also records a significant Oligocene-Early Miocene plate reorganization that impacted and influenced the uplift of the Owen Ridge (Mountain and Prell 1990) and Murray Ridge (at about 23 Ma) along the Owen Fracture Zone. This important tectonic event produced a southeastern deflection of sediments to the Indus Fan (Mountain and Prell 1990; Edwards et al. 2000; Clift et al. 2001b). It is also likely that this tectonic event had an as yet unascertained role in controlling the character of late Oligocene and early Miocene foreland drainage and sedimentation through its impact on plate interactions (e.g., Afghan Plate and Katawaz Block) along the Chaman fault zone.

The westward path of the Indus River during the Oligocene and Early Miocene was structurally controlled by the suture zone (Clift et al. 2001a). River capture leading to the Indus River’s current configuration would happen later through the development of the Nanga Parbat syntaxis and its component structures: the Pamir indent, Karakorum fault and Nanga Parbat uplift (Brookfield 1998). The Katawaz Basin was the chief path for Indus river fluvio-deltaic deposition that also fed the Indus Fan from the early Eocene through roughly the early Miocene (Qayyum et al. 2001). We name and assign the Oligocene-early Miocene expression of the Katawaz Fan to the Shaigalu Fan, after the younger Shaigalu member in the Khojak Formation to distinguish it from the earlier phase of Eocene sedimentation expressed by Qayyum et al. (2001) as the Katawaz Delta and Khojak Fan system.

Northwest and west of the Zinda Pir area was a highland formed of the remnant Paleogene fore-arc complex comprised of melange and ophiolites (Brookfield 1998; Qayyum et al. 2001). Presumably the Katawaz Block to the east of the Chaman Fault Zone underlying the modern Sulaiman Range (Haq and Davis 1997) in combination with the Afghan Block to the west played an important structural role in both the character of these highlands and the style of Indus River sedimentation. To the north, a forebulge was implicitly present (Garzanti et al. 1996; Pivnik and Wells 1996) between the Zinda Pir area and the suture zone. Against this backdrop of plate interactions the Oligocene and early Miocene was characterized by marked shifts in eustatic sea level. The early Oligocene witnessed a rise to 200 m above current sea level at 34-30 Ma with a corresponding and precipitous drop of over 150 m at 30 Ma (Haq et al. 1987). During the remainder of the Oligocene and into the early Miocene, sea level generally trended higher with several oscillations including a cycle at the early Miocene (24 Ma) and then again at about 20 Ma (Haq et al. 1987). Although a direct correlation between these eustatic fluctuations and Chitarwata Formation facies is not yet possible, the sequence of paleoenvironments of the Chitarwata reflects the net effect of the trend towards higher sea level moderated by regional plate flexure and sediment influx.

Our schematic paleogeographic reconstruction of the Chitarwata Formation is provided in Figure 4. With its dominant southeastward drainage, the coastal environments of the Chitarwata Formation received contributions from the highlands to the northwest and west as well as the forebulge to the north. Direct evidence of sediment contribution by proximal highlands is found in the upper member of the Chitarwata Formation where reworked and redeposited Eocene Nummulites are common in shell lags. If this scenario is accurate, there should be little if any contribution of recycled orogenic detritus from the suture zone since the Indus River is assumed restricted to the Katawaz Basin at that time. The Chitarwata coastal system would persist in this configuration for several million years only to be overrun by Indus River sediments at about 19.5 Ma (Lindsay et al. this issue) during a momentous shift to the current foreland basin marked stratigraphically by deposition of the Vihowa Formation.