Faunal Assemblage and Sedimentological Characteristics of Carbonate Sequence of Hanging Wall of Main Boundary Thrust (MBT), Shah Allaah Ditta, Islamabad, Pakistan (P-0955) (MFN 5521)

Abstract

The Margalla hill ranges are part of the lesser Himalayas located at north of Islamabad, the Capital territory of Pakistan. The present study is focused on the biostratigraphy, microfacies, depositional environment and diagenetic analysis of the highly deformed carbonate sequence in the hanging wall of Main Boundary Thrust, located at the Shah Allah Ditta Section, Islamabad. The sequence includes Samana Suk Formation, Lockhart Limestone and Margalla Hill Limestone of Jurassic, Paleocene and Early Eocene age, respectively. The Margalla Hill Limestone is rich in a wide variety of faunal assemblage including larger benthic forams, planktic forams, echinoderms, mollusks and decycladacean algae. Various species identified includes Nummulitesmammillatus,Nummulitesatacicus, Rotaliatrochodiformis, Assilinasubspinosa,Assilinalaminosa,Lockhartia conditi,Lockhartia tipperi and Decastroporellatergestina. On the basis of faunal assemblages, Late Paleocene to Early Eocene age is assigned. Three microfacies are recognized; 1) Foraminiferal wackestone microfacies, inferred the deposition in a low energy, restricted circulation with high salinity in a lagoonal environment of the inner shelf, 2) Larger Benthic Foraminifera wacke-packstone microfacies, inferred to be deposited in relatively offshore middle shelf area and 3) Bioclastic mudstone microfacies, the occurrence of planktonic foraminifera and high abundance of micrite matrix inferred to form in deep and inner shelf environment. The diagenetic alteration of these facies includes micritization, microfractures, neomorphism (microspar, aragonite-calcite transformation) and compaction (pressure dissolution fabric, disorientation of bioclast). The Lockhart Limestone is well exposed and a section of 100 meters’ thickness is sampled for paleontological studies. Various species of larger forams including age diagnostic Miscellanea miscella, Lockhartia haimei were recorded along with Milliolids, Sakessaria sp., Ranikothaliasindensis, Daviesinalanghami, Discocyclina sp., Operculina sp.as well as Gastropods, Decycladacean algae, and Echinoderms were also observed. On iii the basis of recorded fauna, the Upper Paleocene age of Lockhart Limestone in this area is confirmed. Four microfacies are recognized as; 1) Algal foraminiferal wacke-packstone microfacies, based on restricted faunal assemblage, this microfacies is interpreted to have been deposited in a shallow subtidal lagoonal environment of the inner ramp, 2) Benthic foraminiferal wacke-packstone microfacies, the diverse fauna displayed by this microfacies include foraminifera, echinoderms, and rare decycladacean algae along with micrite matrix is evident for deposition under low energy conditions, inferred to be deposited in relatively deeper, middle to outer ramp area, 3) Diverse benthic foraminiferal wackestone-packstone microfacies, the gradual upward decrease in decycladacean algae and increase in diversity of benthic foraminifera and other skeletal allochems along with lime mud matrix indicates low energy, no turbulence and calm conditions of the middle ramp and 4) Planktic-benthic mixed foraminiferal wackestone-packstone microfacies, dominated by benthic and planktic foraminiferal assemblage with subordinate, brachiopod fragment, gastropods and decycladacean algae and is interpreted to represent deposition in outer ramp conditions. The diagenetic alteration of these facies includes micritization, microfaults, neomorphism (microspar, aragonite-calcite transformation) and compaction (pressure dissolution fabric, disorientation of bioclast). The Samana Suk Formation is a well bedded limestone, recognized in the field by shelly limestone fragments composed of gastropods and pelecypods, micritic beds, sandy limestone and the oolitic limestone. These features are indicative towards a shallow shelf deposits. There were two microfacies recognized; 1) Bioclastic-Peloidalgrainstone microfacies, indicating agitated origin characterized by well-rounded and well sorted grains, particularly the peloids in substantial amount indicates that the facies have been deposited on inner ramp beaches, shoals and tidal channels, 2) Dolomicrite microfacies comprised of dolomite and micrite with the absence of allochems indicated by the completely dolomitized fabric. The processesof microbial micritization, compaction (including pressure solution, disorientation of bioclast), neomorphism, dolomitization, and fracturing by tectonics are clear manifestation of the diagenetic processes that occurred in the formation.