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The phosphorite facies belonging to the Fatehgarh Formation, exposed in the rifted graben referred to as Barmer Basin of western Rajasthan, India. Microfacies analysis of the phosphorite facies indicate four major types viz: phosphatic sandstone, bone bed, bedded phosphorite and gastropod bed. Among these microfacies, the bone bed contains significant framework elements such as rich assemblage of fauna along with unusual magnetic objects.
The microvertebrate comprises well-preserved isolate bone, teeth, spines, and vertebrae. The assemblage represented of cf. Igdabatis sp., Semionotiformes indet., Lepisosteus indicus., Enchodontidae indet., Albuloidei indet., and Stephanodus sp. Beside, fragmentary pictoral and dorsal spines of Siluriformes indet. are also present ( Mathur et al., 2006).
The vertebrate comprises an isolated fragmentary dinosaurian reptilian femur, a small crocodilian cranium with isolated teeth and vertebrae and well-preserved turtle scute. A large number of dinosaurian and crocodilian coprolite and dung also form a major portion of recovered fossils. A Late Cretaceous (Maastrichian) age is suggested from the close faunistic affinities of Fatehgarh fauna with those from Lameta and intertrapian sequences of India. As such the phosphorite of the Barmer Basin represents a global Late Cretaceous phosphogenic event in the northwestern part of the India.
The bone bed also contains some unusual and significant magnetic objects viz: magnetic spherules, fine magnetic particles and microbracciated matrix (Mathur et al., 2005a; Mathur 2005b). To understand the nature of host rock and magnetic objects we used different tools such as conventional petrography, SEM, EDAX, ICPMS, and INAA, Mossbaure spectroscopy, X ray florescence and XRD. The results reveal that the phosphorite facies represent anomalies, biostratigraphically, mineralogically and geochemically. The results also indicate that the Magnetic objects represent a high temperature event either of volcanic or impact nature. These high temperature objects along with rich biota occur in a very thin, disrupted, desiccated and discontinuous layer in the top part of the bone bed indicate some cause and effect relationship that brought significant biological changes perhaps mortality enmasse of the Fatehgarh biota.
Refernces: Mathur, S. C., S. K. Mathur and R. S. Loyal. 2006. First Report of microvertebrate assemblage from Cretaceous Fatehgarh Formation. Geological Society of India. 67, 6.
Mathur, S.C., S.D. Gaur, R.S. Loyal, A. Tripathi and M.S. Sisodia. 2005a. Spherules in the Fatehgarh Formation (Cretaceous) of Barmer Basin, India. Gondwana Research. 8, 4, 579 - 584.
Mathur S.C., Gour, S.D., Loyal, R.S., Tripathi., A., Tripathi, R.P. and Ajay Gupta. 2005b. First report on magnetic spherules recovered from Maastrichtian bone bed, in the sedimentary sequence of Fatehgarh Formation, Barmer Basin, India. Current Science. 89, 7. 1259 - 1268.
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