REMANENT MAGNETIZATION OF OOLITIC IRONSTONE BEDS, HAZARA AREA, LESSERHIMALAYAN THRUST ZONE, NORTHERN PAKISTAN - ITS ACQUISITION, TIMING, AND PALEOENVIRONMENTAL IMPLICATIONS

Chamosite-hematite type oolitic ironstone is distributed in the Cretac eous-Tertiary (K-T) boundary zone of the Hazara area, Lesser Himalayan thrust zone, in the northwestern margin of the Indo-Pakistani subcont inent. A total of 52 oriented samples were collected from 6 beds in tw o open pit mines, Nathia Gali and Bagnotar. Ferromagnetic properties o f the oolitic ironstone are dominated by fine-grained hematite that po ssesses a characteristic remanent magnetization (ChRM). The ChRM is a chemical remanent magnetization (CRM) acquired by the hematite which f ormed from Fe-rich initial materials (iron hydroxides) during early di agenesis of the ironstone bed. The direction of the remanent magnetiza tion indicates that the magnetization was acquired in an equatorial re gion (average inclination = 2 degrees, paleolatitude = 1 degrees N), w hich is consistent with the paleoposition of the Indian subcontinent d uring early Paleocene time, as estimated by previous data. The initial Fe-rich sediments were deposited under anoxic conditions, probably in a non-marine or brackish environment. During early diagenesis, after development of oolitic textures, the paleoenvironment became arid, whe re the CRM was acquired through the conversion of amorphous hydroxides or goethite to fine-grained hematite. The paleomagnetic results revea l complicated local rotational movements of oolitic ironstone blocks a bout vertical axes, which might be a result of intense thrusting withi n the terrane after early Tertiary collision between India and Asia.