Abstract:
The Middle Jurassic, Samana Suk Formation at Tauda China locality, Kurram
District is selected for research study. It is located at north-west, Kurram District of
Pakistan (33°32′69 ″N, 70°20’026” E). At study area, the Formation consists of thin to
thick bedded, light to dark grey and massive limestone with reddish grey sandstone. It
contains hard, oolitic, pisolitic limestone and has karstified topography (due to severe
dissolution). In some parts, the limestone is partially recrystallized. Non-oolitic
limestone is also present at the base of the Formation and some places limestone appear
laminated (due to presence of sand). Its upper contact is disconformable with Chichali
Formation and lower contact is not exposed in the study area. Brachiopod fossils are
found at the base of the Formation. The microfacies identified from Samana Suk
Formation include intraclastic ooidal grainstone, intraclastic bioclastic ooidal peloidal
grainstone, peloidal bioclastic grainstone, bioclastic ooidal grainstone and bioclastic
peloidal ooidal packstone, intraclastic peloidal ooidal wackestone and mudstone facies.
On the basis of reported microfacies ramp margin depositional environment has been
assigned to this Formation. The diagenetic fabrics observed in Samana Suk Formation
are compaction, cementation, dissolution and recrystallization. Diagenetic history
reveals prominent microbial activity during early marine diagenesis which caused
micritization of ooids and deformation of their internal structural. Early diagenetic
cement identified as fibrous and bladed type calcite cement. Shallow burial phase
resulted in the physical compaction and brittle deformation of bioclast and quartz grain.
Some pore filling calcite and recrystallized calcite are also the representative of this
phase. Deep burial stage involves chemical compaction in the form of stylolites and
stylolitic seams that are quite abundant in thin sections. While uplifting the rock to
surface is evident from tectonically induced fractures and dissolution of calcite grain.
Pyritization and cementations is the products of later stages of diagenesis. This
Formation is strongly affected by diagenetic processes which has significantly reduced
the pores and open fractures in the study area.
