Microfacies and Diagenetic Analysis of Lockhart Limestone, Shah Alla Ditta Area Islamabad, Pakistan Diagenesis
Diagenesis encompasses all the processes affecting car- bonate rocks until the realm of incipient metamorphism at elevated temperature and pressure (Tucker and Wright, 1990). The diagenetic processes characteristic of the Lockhart Limestone are responsible for the development of typical features diagnostic of those regimes. These processes include compaction, transformation of aragonite to more stable low-Mg sparry calcite, and pressure solution.
Conclusions
Three microfacies are recognized in outcrops of Shah Alla Ditta area:
i. Foraminiferal Wackestone/Biomicrite Microfacies, deposited during sea level stillstand.
ii. Algal Foraminiferal Wackestone Microfacies. Laid down during sea level fall.
iii. Mixed Bioclastic Mudstone Microfacies representing sea level rise.
These three microfacies represent inner shelf, near shore to inner-shelf and inner to middle shelf environments of deposition.
Acknowledegments
I am greatly thankful and highly indebted to Mr. Mumtaz Ali Khan, Lecturer, Department of Earth and Environmental Sciences, Bahria University Islamabad, who greatly assisted me with collecting, producing and arranging the data for this work. I am also grateful to all the associate editors of AIPG who reviewed my article. Last but not least I am grateful to Dorothy Combs, Membership Services AIPG, without whose help and valuable guidance this would not have been completed.
References
Figure 4c. Photomicrograph displaying (A) fossil replacement by mud and (B) &(C)calcite vein and (q) quartz crystal in mixed bioclast mud stone facies.
observed in these sections. Mostly the calcite veins found in this mocrofacies replace bioclasts. Veins are parallel to subparal- lel, and also cross-cut each other. This type of cross-cutting is evidence of a tectonically highly fractured zone. (Figure 4c).
Discussion
The microfacies identification and interpretation shows that the Lockhart Limestone represents deposition in near-shore, inner to middle shelf environments, (Figure 5).
Sea level changes control the distribution of microfacies, which reveal the onset of shallow marine conditions. During sea level still-stand the Foraminiferal Wackestone/Biomicrite Microfacies was deposited. Times of gradual sea level rise are associated with the deposition of Mixed Bioclastic Mudstone Microfacies. The Algal Foram Wackestone in the middle of the Lockhart Limestone indicates sea level fall and inner shelf subtidal settings.
Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks. W. E. Ham, ed., Mem. Am. Ass.
Petrol.Geol.1, p.108-121.
Embry, A.F., and Klovan, J.E., 1971, A Late Devonian reef tract on northeastern Banks Island, NWT: Canadian Petroleum Geology Bulletin, v. 19, p.730-781.
Flügel, E., 2004. Microfacies of carbonate rocks, analysis, inter- pretation and application. Springer Verlag, Berlin. 976 p.
Folk, R.L, 1959. Practical petrographic classification of limestone. Bull. Am. Ass. Petrol. Geol. 43, p. 1-38.
Jadoon, I.A.K., 1992. Thin-skinned tectonics on continent/ocean transitional crust, Sulaiman Range, Pakistan. Unpublished Ph.D. thesis. Geology Dept. Oregon State Univ, USA. p.23-30.
Tucker, M.E and Wright, V.P., 1990. Carbonate sedimentology, Blackwell ScientificPublications Oxford London, p. 23-25.
Wilson, J.L., 1975. Carbonate Facies in Geologic History, Springer- Verlag, Berlin, p. 471.
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