SANDSTONE PETROGRAPHY AND GEOCHEMISTRY AS INDICATORS OF SOURCE AREA FOR THE OLIGOCENE BINGA FORMATION (KHOJAK FORMATION), MAKRAN FLYSCH BELT, PAKISTAN

The medium to coarse grained, sub-angular to sub-rounded, moderately sorted sandstone samples (N=19) of Binga formation (now part of Khojak Formation) of Makran Flysch Belt, were analyzed for mineralogical and chemical characteristics. The results were used to report, for the first time, the (whole rock) geochemical and mineralogical composition of the Binga formation and were compared with NASC, PAAS, GLOSS, Upper Continental Crust and Indus Fan sediments. Provenance and paleoweathering conditions are inferred.

Quartz (66.1%), SRF (14.4%), K-feldspar (13.5%), MRF (3.2%) and subordinate amounts of plagioclase, VRF, and clays dominate the mineralogy of the Binga formation. Using geochemical parameters (log SiO2/Al2O3 and log Fe2O3/K2O), the Binga samples are classified as litharenite ( n=8) and wacke (n=6). Few samples (n=4) plot in Fe-sand, and one sample in Sublitharenite field. Geochemical results show that Al2O3 is positively correlated with K2O (r=0.94),Th (r=0.91), TiO2 (r=0.83), Nb (0.83) and U (r=0.71), whereas it is negatively correlated with Sr (r=-0.75). Similarly, positive correlation also exists between Hf and Zr (r=0.999), Nb and Ta (r=0.99), TiO2 and Nb (r=0.97), K2O and Rb (r=0.97), U and Th (r=0.93), CaO and Sr (r=0.91), Rb and Cs (r=0.90), and SiO2 and Ba (r=0.72). Barium is negatively correlated with Sr (r= -0.66) whereas Al2O3 and Ba show week positive correlation (r= 0.46). The comparison of the geochemistry of Binga sediments with that of UCC, PAAS, NASC, GLOSS, and Indus Fan sediments, revealed that the Binga sediment are deficient in K, Nb, Ta, Rb, Ba, U, and Th. The chemical indices of alteration (CIA) indicate fluctuations in the weathering intensity and suggestive of episodic uplift in the source area.

In one of the discriminant function diagrams, most of the Binga samples (n=17) plot in the quartzose sedimentary provenance, whereas in the other diagram the same samples plot in three different fields of provenance; intermediate igneous (n= 9), quartzose sedimentary (n=7), and felsic igneous (n=3). In the tectonic discrimination diagram using Al2O3/(CaO+Na2O) vs Fe2O3+MgO, the Binga samples plot in Continental Island Arc (n=10), Active Continental Margin (n=6), and the Oceanic Island Arc (n=4) fields.