JOHNSON, EDWARD A., U.S. Geological Survey, Denver, CO; PETER D. WARWICK, U.S. Geological Survey, Reston, VA; STEPHEN B. ROBERTS, U.S. Geological Survey, Denver, CO; and INTIZAR H. KHAN, Geological Survey of Pakistan, Quetta, Pakistan

Limestone-Pebble Conglomerate Facies of the Eocene Ghazij Formation, Balochistan, Pakistan: Evidence for Collison-Related Tectonism on the Northwestern Margin of the Indian Plate

The abrupt appearance of conglomerates in the otherwise finer grained paralic to marine Ghazij Formation (Eocene) in northeastern Balochistan, Pakistan, indicates tectonism associated with crustal collision. These poorly sorted limestone-pebble conglomerates consist of gray well-rounded pebbles and cobbles of limestone and subordinate chert in a matrix of lithic sandstone, and the facies is present in two widely spaced depocenters. In the Harboi Hills, outcrops locally show stratification and normal grading, and the conglomerate is commonly clast supported; some boulders are present. Channel-form bodies 2 in thick, some having accretionary surfaces, comprise amalgamated sequences that also contain cross-stratified sandstone lenses. This 439-m-thick wedge-shaped unit is present in the pper part of the Ghazij and grades eastward into paralic

deposits within several kilometers. In the Sor Range, 137 km north of the Harboi Hills, outcrops locally show faint stratification, normal grading, and imbrication. The conglomerate is commonly matrix supported and contains cross-stratified sandstone lenses. Channel-form bodies 1 in thick and scoured surfaces are locally present. The unit is at the top of the middle part of the Ghazij, is 6-39 in thick, has a sharp top and erosional base, and is traceable for 12 km. To the southeast, the unit apparently pinches out into paralic deposits.

High relief associated with sudden uplift west of the study area caused rapid erosion that provided molasse to a mountain-front foreland basin. An irregular belt of alluvial fans formed along the eastern margin of the uplift, and the two sites in this study represent separate depositional lobes of this system. The sequence in the Harboi Hills was deposited in a more proximal position than the tabular body observed in the Sor Range. These conglomerates provide evidence of tectonism caused by the collision of the northward-drifting Indian plate with undefined landmasses that existed marginal to the Eurasian supercontinent.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.