The Devonian ofWestern Karakorum (Pakistan)

Abstract

Devonian rocks crop out in several thrust sheets in the sedimentary belt of North-Western Karakorum, both to the south and to the north of the Reshun Fault. Gently metamorphosed Devonian dolostones and volcanics are also present in the Tash Kupruk Zone north of the thrust sheet system. The most complete succession is found in the Karambar Thrust sheet where dolostones and recrystallized limestones (Vandanil Formation) lie above the black shales and siltstones of the Baroghil Group. The age of the Vandanil Formation is poorly defined, approximating latest Silurian or earliest Devonian at its base and proven to be Pragian in the 4th of its 5 lithozones. The overlying Chilmarabad Formation is divided into two members. The lower member is a mixed siliciclastic–carbonate package, present in most southern and western thrust sheets. The upper member is ubiquitous; it consists of dolostones, often stromatolitic.

The calcareous part of the TashKupruk Zone consists of similar facies. The carbonate flats of the Chilmarabad Formation display a trend towards emersion towards the top, and are overlain, with regional unconformity, by the basal unit of the Shogrām Formation. The base of the latter consists of arenite and fine conglomerate (deposited in an alluvial setting) overlain by alternating marine bioclastic limestones and litharenites with one or two coral bafflestones in the middle to upper part. The age of the lowermost part of the Shogrām Formation is Givetian, extending through the Frasnian and seemingly into the Famennian, but the last is poorly documented (contrasting with the classic Shogrām and Kurāgh sections of Chitral); it reflects a return to fine terrigenous input. 

The inferred palaeogeography accords with the Northern Karakorum having been part of the Gondwana margin during the Devonian. Awide, mostly calcareous platform, characterised extensive areas of the Karakorum, Central Pamir, Badakhshan and, in a minor way, Central Afghanistan (Helmand Block). The sandstone petrography suggests that clastics polluting the carbonates originated from erosion of a pre-existing sedimentary cover. During theGivetian a first rifting episode, possibly echoing the opening of an ocean to the east, affected the whole area, with volcanic outpourings in the rifts, while eroding shoulders fed the basins, though never extending as deep as the crystalline basement. A minor volcanic input is also recorded. The tectonic pulse almost ceased during the Frasnian, gradually resuming towards the end of the Devonian.