Judith Baker

Palaeomagnetic dating of the earliest continental Himalayan foredeep sediments: Implications for Himalayan evolution

Abstract Between the time of the India-Eurasia collision (50–45 Ma) [1] and the climax of crustal shortening and thrust stacking in the Himalaya when the Main Central Thrust (MCT) was active (21 Ma) [1,2] there is a ca. 30 My gap about which little is known. This paper aims to shed light on this period by dating the initiation of major erosion from the rising Himalaya and the probable start of uplift, a significant event in the orogens history. This was achieved by accurately dating, for the first time, the Dagshai Formation sediments of northern India, which are interpreted as early Himalayan foreland basin deposits that record initial large-scale erosion of the orogen [3]. Oriented hand samples were collected from six sites and analysed, using palaeomagnetic techniques. Both polarities are represented and the remanence passes a fold test. Fitting the measured palaeolatitude to that expected for the Indian plate dates the Dagshai Formation at 35.5 Ma ± 6.7 Ma, and this is taken as the time when the embryonic Himalaya began to be strongly eroded and regionally uplifted.

Constraints on the nature and thickness of sedimentary fill and underlying basement rocks in Bowser and Sustut basins, north-central British Columbia from analysis of potential field data

Abstract Qualitative and quantitative interpretations of potential field data, constrained by rock property measurements, are presented for an area in north-central British Columbia encompassing the Bowser and Sustut basins. Rock property measurements demonstrate that the Bowser Lake Group and underlying Hazelton Group have an identical mean density limiting the capacity of gravity to differentiate the two. The finding is important for the design of effective hydrocarbon exploration programs in the area. However, significant contrasts in magnetic susceptibility between basin fill and underlying basement rocks facilitate the use of magnetic data to map first-order variations in the thickness of fill in each of the basins. Results show a relatively uniform fill thickness in Sustut Basin, on the order of 2.5–3 km. In contrast, considerable topography on the basement interface beneath Bowser Basin is identified with variations in the thickness of overlying sedimentary rocks ranging from less than 2 km to more than 6 km. Resolving this topography is an important advance in understanding the hydrocarbon potential of the basin. A large (approximately 50 km × 60 km × 3.5 km) buried intrusion beneath the northeast part of Bowser Basin can account for an observed magnetic anomaly and explain the high coalification gradients and localized high maturation levels of the overlying sedimentary rocks. Neither of the latter can be adequately explained by the estimated burial depths. At least one regionally extensive (>150 km long) fault that cuts basement of Bowser Basin is delineated. The fault may have facilitated migration of hydrocarbons sourced in the basement into Bowser Basin. Correlation of magnetic data with published isotopic age dates indicates that Maitland volcanism spanned five polarity reversals during a 0.8 Ma period, and it facilitates a reduction in the errors associated with published isotopic ages.