Table 6 Brief descriptions and interpretation of facies types under the tide-dominated fluvio-tidal facies association (FA-4)

Facies 4A: Heterolithic sandstone–mudstone

2–10-cm-thick, fine- to medium-grained, moderately-sorted sandstone alternating with thick, massive to laminated mudstone; sandstone beds characterized by ripple cross-strata with mud-draped foresets and lateral accretion against frequent reactivation surfaces, sigmoidal cross-strata, bi-directional ripple cross-laminae, climbing ripples with sinuous, bifurcated crests, and broad-crested symmetrical wave ripples; mud-dominated units characterized by wavy to lenticular bedding; bed thickness 5–20 cm; sand lenses 2–4 cm in thickness, asymmetrical and discontinuous with internal mud-draped laminae become continuous up-section; thin interbedded mudstone and fine-grained, plane-laminated sandstone laminae also present, with sandstone laminae thickness 1–8 mm and mudstone laminae thickness 0.5–2 mm.

Mutually opposite cross-strata set indicate tidal fluctuations (Klein 1971; Boersma and Terwindt 1981; Elliott 1986); symmetrical ripples and combined flow ripples manifest interference of tides and waves; wavy and lenticular beddings indicate pronounced suspension fall-out over alternate traction current; alternation of laminated sandstone–mudstone indicates tidal inclined heterolithic strata (IHS) in low-energy tidal setting (Reineck and Wunderlich 1967, 1969; Reineck and Singh 1980; Choi et al. 2004; Choi 2010).

Facies 4B: Mud-clast-bearing conglomerate

Lenticular, sharp (erosional) based conglomerate with mud clasts; bed thickness 5–10 cm; Size of the mud clasts 0.2–3 cm with frequent bending; angular to subangular mud clasts show yellow to brown, faded violet, and white colour.

Scouring of mud from underlying unconsolidated mud bed by high-energy current/wave and rapid deposition without significant transportation (Bhattacharya and Bhattacharya 2012; Eide et al. 2016).

Facies 4C: Plane-laminated sandstone

Fine-grained plane-laminated sandstone beds (bed thickness of 2–25 cm) with intermittent thin, impersistent mudstone layers; soft-sediment deformation structures (SSDS) such as convolute laminae, load and flame structures.

Suspension deposit during slack phase (Reineck and Singh 1980); SSDS indicates density contrast among the litho-units during deposition (Jha et al. 2017).

Facies 4D: Bioturbated sandstone

Medium- to coarse-grained, crudely laminated sandstone with subangular grains; thickness 40 cm – 2.5 m, increasing in the west than in the east; laminae partly destroyed by bioturbation; diversity and intensity of bioturbation increase towards west; vertical and horizontal U-shaped, Y-shaped and J-shaped burrows of Thalassinoides and Ophiomorpha, with subordinate Planolites, Skolithos, Palaeophycus, Arenicolites and Rosselia; concentration of various shell fragments along with microscopic bivalve shells.

Intense bioturbation indicates nutrient-rich, oxygenated conditions during deposition; burrowing by Thalassinoides–Ophiomorpha dominated ichnoassemblage indicates a nearshore setting (Buatois and Mángano 2011).

Facies 4E: Green sandstone

Coarse-grained, well-sorted and rounded, massive, green sandstone; predominates in the western part of the study area; locally, small trough cross-strata present; facies thickness 0.4–2 m; glauconite abundant.

Deposition from to-and-fro movements of sand grains by shallow marine waves; glauconite also indicates shallow marine condition.