PETROGRAPHY, GEOCHEMISTRY AND DEPOSITIONAL MODEL OF ISPIKAN CONGLOMERATE, MAKRAN ACCRETIONARY PRISM, SOUTHWEST PAKISTAN

The age and origin of the Ispikan Conglomerate of Markran Accretionary Prism (MAP) has been much debated during past several decades. A systematic and integrated sedimentologic, petrographic, and geochemical study of Ispikan Conglomerate has been carried out to deduce clues for the reconstruction of the paleotectonic and paleogeographic history of Makran Continental Margin. The Ispikan Conglomerate is a single, small (~2Km long, <1km wide, and 250m thick) but a key lithological unit of MAP. Very thin to massive beds, poor sorting, random fabric, and poor grading are the common sedimentologic features at the outcrop. Petrographically the unit is composed of mostly reworked, medium to coarse grained pebbly sandstone, siltstone and discontinuous lenses of matrix- and clast-supported conglomerate. The sandstone is composed of angular to sub-angular, poorly sorted, immature sand grains whereas Q₇₂F₁₃L₁₅represents average detrital modes. Pebbles are dominantly composed of metamorphic and acidic igneous rocks with minor proportion of limestone and mudstone.

Geochemically the sandstone samples fall in quartz-intermediate class, indicating tectonically active continental margin on or adjacent to an active plate boundary. Commonly used geochemical discriminants such as TiO₂, AI₂O₃/SiO₂, K₂O/Na₂O and Al₂O₃/(CaO+Na₂O), all plotted against Fe₂O₃+MgO also suggest that the sediments were deposited in the vicinity of an active continental margin. The Nb and Zr/Th and Ba/Y values fall relatively closer to a continental island arc setting. High K/Rb ratio indicates the derivation or detritus from acid and intermediate sources. Some geochemical discriminants indicate supply of detritus from two different provenance.

We propose that Ispikan Conglomerate is an olistostrome unit that was deposited after a localized submarine mass flow triggered by slope failure. Such olistostrome units are commonly associated with the active margins where progressive dismantling and erosion of overthrusted nappes of the orogenic front take place as part of the process. After the placement of Ispikan Conglomerate on the deeper marl facies of the Eocene Wakai Limestone, thick piles of flysch (Khojak Formation) kept accumulating as accretionary prism sediment.