Sequence Stratigraphic Analysis of the Mixed Siliciclastic-Carbonate Sediments (Middle Jurassic) of the Patcham Island, Kachchh, Western India: An Ichnological Approach

Abstract

Abstract: The Middle Jurassic (Bajocian to Callovian) sequence of the Patcham Island, Kachchh, comprises mixed siliciclastic-carbonate sediments displaying transgressive-regressive cycles that record changes in the relative sea level condition. These mixed siliciclastic carbonate sediments are of varying composition representing six sedimentary facies namely, micritic sandstone, allochemic sandstone, sandy allochem limestone, sandy micrite, muddy micrite, and micritic mudrock along with the grey shale, allochemic limestone and ferruginous sandstone facies. These sedimentary facies are bioturbated with a total of 65 ichnospecies of 42 ichnogenera which are recurring throughout the sequence. The trace fossils are further grouped into nine trace fossil assemblages based on environmentally related traces based on diversity, frequency and preservational aspects (viz., Arenicolites, Asterosoma, Gyrochorte, Ophiomorpha, Planolites/Palaeophycus, Phycodes, Rhizocorallium, Skolithos and Thalassinoides assemblages). The sedimentary units (packages) coupled with ichnoassemblages reveal three transgressive system tracts (TST) and four regressive system tracts (RST) separated by flooding surfaces representing 3rd order transgressive-regressive (T-R) cycles bounded by regressive surfaces. In the T-R cycle, presence of the Rhizocorallium ichnoassemblage (TRC-II) and Arenicolites, Thalassinoides with Planolites-Palaeophycus assemblages (TRC-III) mark the flooding surfaces while the Skolithos assemblage with the Planolites-Palaeophycus assemblage (TRC-I), Arenicolites assemblage (TRC-II) and Planolites-Palaeophycus assemblage (TRC-III) mark the regressive surfaces. These soft ground ichnoassemblages mark the Skolithos and Cruziana ichnofacies conditions developed in progradational and retrogradational sequences and indicate cyclic changes in bathymetry within shoreface-offshore settings. An integrated sedimentological and ichnological approach has allowed the recognition of the internal organization of the sequence in absence of significant physical expressions of discontinuity surfaces at the T-R cycle scale.