DefinePK

Research Objective

To examine and analyze the outcrop characteristics, petrography, microfacies, and depositional settings of the Chorgali Formation at the Bhattian and Gharaga sections in the Southern Hazara Basin, Pakistan.

Methodology

The study involved detailed examination and sampling of two stratigraphic sections of the Chorgali Formation. Outcrop data and petrographic analyses were conducted. Thin sections and polished surfaces of marls and limestones were prepared for petrographic analysis using a polarizing microscope. Softer marl and shale samples were treated for fossil separation and studied under a stereo-zoom binocular microscope. Microfacies were established based on faunal content, distribution, and detrital constituents, following established parameters. Carbonate rock classification was done using Dunham's (1962) method. Field logs were compiled using graphic software.

Methodology Flowchart

                        graph TD
    A[Field Study & Sampling] --> B[Stratigraphic Section Measurement];
    B --> C[Petrographic Analysis of Thin Sections];
    C --> D[Fossil Separation and Study];
    D --> E[Microfacies Identification];
    E --> F[Lithofacies Analysis];
    F --> G[Paleoecological Interpretation];
    G --> H[Depositional Environment Reconstruction];
    H --> I[Conclusion];                    

Discussion

The presence of various benthic and planktonic foraminifera, ostracods, and other fauna suggests shallow marine to slightly restricted depositional environments for the Early Eocene Chorgali Formation. The study supports previous findings of ramp settings for Eocene strata in the NW Himalayas. The analysis of microfacies indicates a shallowing upward sequence for the Chorgali Formation, with minor deepening and shallowing cycles due to transgression and regression of the shoreline.

Key Findings

Five microfacies were recognized: Nummulites-Lockhartia wackestone to packstone (MF-1), Nummulites- Assilina wackestone to packstone (MF-2), Ostracods-Miliolids packstone (MF-3), Marls Microfacies (MF-4), and Calcareous shale Microfacies (MF-5). The Chorgali Formation was deposited on mid-ramp settings, with some deposition in attached to partially restricted lagoonal areas of the inner ramp and the proximal part of the outer ramp. The formation consists of limestone, marls, and calcareous shale, with marly and argillaceous limestone in the lower and upper parts, respectively.

Conclusion

The Chorgali Formation in the study area exhibits a gradational conformable contact with the underlying Margalla Hill limestone and a gradational intercalated contact with the overlying Kuldana Formation. The formation comprises limestone, marls, and calcareous shale, with three main lithofacies: marly limestone, calcareous shale, and argillaceous limestone. The presence of well-preserved benthic foraminifera confirms an Early Eocene age. The depositional environments suggest ramp settings with an overall shallowing upward sequence.

Fact Check

1. Age of Chorgali Formation: The study assigns an Early Eocene age to the Chorgali Formation based on recovered larger benthic foraminifera.
2. Microfacies Count: Five distinct microfacies were identified and described in the study.
3. Depositional Setting: The study concludes that the Chorgali Formation was deposited on ramp settings.

Mind Map