Facies analysis and depositional environments of the early Eocene Naredi Formation (Nareda locality), Kutch, Western India

Abstract

The early Eocene Naredi Formation in the southwestern Kutch represents the initial marine sedimentation in Kutch area during Tertiary period. The 22 m thick succession of Naredi Formation, exposed along cliffs (N 23°34′36.8″, E 68°38′38.1″ and N 23°34′3.1″, E 68°39′7.8″) of the tributary of the Kakdi River in and around Nareda village, is dominantly composed of argillaceous member (also known as gypseous shale member) in the lower part and dominantly biochemically precipitated carbonate member, including newly identified algal and coral reef facies, in the upper part. A total of eight lithofacies have been identified based on sedimentological and micropaleontological attributes; those are alternate green- and brown shale facies, comprising the argillaceous member followed by bioclastic wackestone, Assilina packstone, organically bounded framestone (bioherm), clayey limestone, bioclastic packstone–wackestone alternation and ferruginous coralline limestone in ascending order, which constitute the upper carbonate member. The green as well as brown shale facies are splintery in nature and show horizontal interlamination with gypsum layers. Both these facies contain glauconites with circular to elliptical outline and radial fractures, thus suggesting deposition on a mid- to outer-shelf or restricted lagoonal depositional setting. The overlying horizontally bedded wackestone facies and sparitic packstone facies containing fossil shells of larger benthonic foraminifera, including Assilina spinosa, Assilina and Nummulites burdigalensis with some planktonic forms such as Globigerina and Globorotalia in association to rotalids, ostracods, bivalve and gastropod shells, whose presence depict their deposition in mid- to inner-shelf marine realm under normal seawater salinity. The abundance of peloids, calcispheres and limestone fragments are the major non-biogenic allochems in these facies. Replacement of the ostracods, bivalve and gastropod shells by sparry calcite is also the common feature. The overlying organically bounded algal framestone (bioherm) is characterized by slightly undulatory tabular form in the lower part to concentric undulatory mounds in the upper part showing reef growth at outcrop section whereas framework of curvi-radial concentric growth of carbonate layers under thin section indicates reef formation on shallow marine depositional setting in warm and clear tropical water under normal seawater salinity. The thinly bedded brownish white clayey limestone indicates deposition during deepening of the lagoon. The overlying bioclastic packstone–wackestone alternation facies studded with fragments of algal reef and stromatolitic limestone along with various fossil shells like bivalves, echinoids and gastropods, most of them are micritized, resembles its deposition under back-reef lagoonal environments during storm condition which is overlain by an intertidal coralline limestone deposit showing colonial growth and partial to complete replacement of some of the coral shells by ferruginous mineral along with secondary precipitation of the same within the pores during shallowing of the sea and/or due to late diagenetic changes. Based on shale-carbonate wackestone-packstone-reefal-coralline facies association along with the presence of some key minerals such as glauconite, pyrite, siderite and anhydrite and their genetic link to characteristic depositional milieu, authors have proposed fluctuating depositional environments from lagoonal-barrier ridge to lagoonal-tidal flat for the Naredi Formation.