GEOLOGICAL SETTING

The marine Mesozic rocks of Kutch range in age from the Bajocian to Aptian and were deposited in a shallow, shelf environment (Biswas 1977; Bose at al. 1988; Fürsich and Oschmann 1993). The thick stack of these Mesozoic sediments has been subdivided into four major formations; Patcham, Chari, Katrol and Bhuj (Rajnath 1932, Mitra et al. 1979), in ascending order, which in turn are subdivided into several members.

The ammonites occur in the Umia Member of the Bhuj Formation of Late Tithonian age (Mitra et al. 1979; Shome et al. 2004; Shome et al. 2005). The Umia Member at Lakhapar and other areas (Figure 1.1) yields a marine assemblage, which was previously assigned to various ages by different workers. Waagen (1875), Rajnath (1932), Spath (1931) and Krishna (1984) advocated a Tithonian age for the Umia Member based on ammonites. There is consensus amongst paleontologists that the upper part of the Umia Member is characterized by Late Tithonian fossils. Fossiliferous units of the Umia Member can be divided into two broad ammonite assemblages. The lower assemblage includes species of Micracanthoceras Spath, 1925, Aulacosphinctes Uhlig, 1910 and Virgatosphinctes denseplicatus (Waagen) group. The assemblage occurs in distinct coarse arenaceous facies. The upper assemblage includes older ammonite taxa along many new ammonite genera (Shome et al. 2004; Shome et al. 2005), which are Tithopeltoceras Arkel, 1953, Corongoceras Spath, 1925, Durangites Burckhardt, 1912, Himalayites Uhlig in Boehm, 1904 and Blanfordiceras Cossmann, 1907. Here, the characteristic lithology is alternating oolitic sandstone and shale. Umiaites is present in both the assemblages. Four bands of oolitic sandstone have been recognized locally, and ammonites are present in all except the topmost one (Figure 1.2). The newly discovered taxa indicate unambiguous Late Tithonian age, but some of them differ from zonal index fossils in Europe (Tavera 1985; Tavera et al. 1986). For example, Micracanthoceras is found to have ranged between the Microcanthum Zone and the Durangites Zone in Spain, which marks the latest Tithonian (Olóriz and Tavera 1983; Tavera et al. 1986). Tithopeltoceras is restricted to the Transitorius Zone. Corongoceras has distinct stratigraphic distributions in different areas. Tavera (1985) and Tavera et al. (1986) described "Corongoceras" from the Upper Tithonian Transitorius Zone to the lowermost part of the Lower Berriasian Jacobi Zone in southern Spain. However, the Lower Berriasian record of "Corongoceras" is dubious (Enay personal commun. 2005). Durangites is the zonal index for the top of the Tithonian. The presence of Umiaites in both assemblages suggests its stratigraphic distribution throughout the Late Tithonian.

Umia Member sedimentary facies are characterized by repeated cycles of several heterolithic facies such as coarse sandstone and alternating oolitic sandstone and shale. They have been interpreted as multiple transgression-regression couplets (Bose et al. 1988). Fürsich and Pandey (2003) investigated sequence stratigraphy in the Umia Member and recognized the oolitic horizon, which contains time-averaged ammonite assemblage, as the maximum flooding zone. Ammonite shells have variable preservational quality; many of them are internal molds or highly corroded. Umiaites specimens also show variable preservation. The associated bivalves occur with a high degree of articulation. According to Fürsich and Pandey (2003), shells accumulated below storm wave-base during slow sedimentation and taphonomic condensation subsequently took place.

All the Tithonian ammonites of Kutch including both endemic and cosmopolitan genera disappeared at this level, precisely within the second oolitic horizon. Elsewhere these genera also died along with many other Tithonian genera. Bardhan et al. (1989; 2007) correlated this regional extinction event with the global mass extinction episode coincident with the Jurassic-Cretaceous boundary (Raup and Sepkoski 1984, but see Hallam and Wignall (1997) for an opposing view).