Microfacies and Diagenetic Analysis of Lockhart Limestone, Shah Alla Ditta Area Islamabad, Pakistan
The emphasis of the present study is on micro- facies analysis and the diagenetic history of the Lockhart limestone. Dunham (1962) is used for classification of rocks and Flügel (2004) is used for the identification of microfacies and inter- pretations. The standard microfacies scheme of Wilson (1975) is also used for the comparison of identified microfacies.
Geological Setting
Figure 2. Geological map of northern Pakistan showing major structure boundaries, the study area is indicated by diagonal ruling. (modified after Ghazanfar et al., 1990)
Geologically the study area is situated in the fault zone of the Main Boundary Thrust (MBT) and the Panjal thrust, (Figure 2). The overall structural grain of the area is NE-SW. The Main Boundary Thrust (MBT) is well exposed in this area and is located in the southern part of the study area, in Islamabad. It carries Jurassic Samanasuk Formation over Miocene Muree Formation at the surface. The strike of the fault is ENE to WSW. The average dip of the fault is 35 degrees to 40 degrees towards the north. The lithologies present in the area have a strong influence on the style of faulting. In the study area the exposed stratigraphy ranges from the Mesozoic to the Cenozoic era. The oldest for- mation is the Samanasuk Formation of Jurassic age. The Lockhart and Patala Formations are of Paleocene age. The Margalla Hill limestone is of Eocene age, and the Muree Formation of Miocene age is present in the area. The Lockhart Limestone has an unconformable contact with the underlying Jurassic Samanasuk while its upper contact is conformable with the Eocene Margalla Hill Limestone. The youngest formation exposed in the area is the Muree Formation (Figure 3).
Methodology and Data
Twenty samples were collected from the Shah Alla Ditta area. Detailed measured sections in the field were used to describe the facies and determine sample locations (Figure 5). Thin sec- tions of all samples were prepared. The Dunham (1962) and Embry and Klovan (1971) classifica- tions were used for the microfacies identification and interpretation.
Microfacies Analysis
On the basis of detailed petrographic and micro- palaeontological studies three types of microfacies were recognized with distinct texture, allochem types, fossil contents and sedimentary structures. The Lockhart limestone microfacies are, from the base of the formation upwards:
Lockhart-Microfacies 1: Foraminiferal Wackestone/ Biomicrite Microfacies.
Figure 3. A brief description of the stratigraphy involved in the Shah Alla Ditta area.
This microfacies is mainly characterized by skeletal allochems, which are the only allochemi- cal constituents marked by moderate diversity of organisms. These skeletal allochems include: larg-
www.aipg.org
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56
