Table 3 Mineralogical analysis of spectral feature of different macrofossils.

Gastropod has weaker spectral feature at 900–1000 nm wavelength (ii in Fig. 6a,d). This indicates the presence of Fe–OH but as trace. Other spectral features are similar with the features identified in other macrofossils (Fig. 6a,d). This is supported by other mineralogical data. Spectral signatures of minimal diagenetic alteration are present

The spectral feature with the wavelength of absorption minima at 1030–1200 nm (ii-a Fig. 6a, ii in Fig. 6e) has wider range and this is may be due to the presence of siderite. However, XRD and other mineralogical data confirm the presence of siderite as traces. Spectral signature indicates that the fossil shells have minimal imprint of digenetic compaction and recrystallization. Other primary features (i, iv, vi in Fig. 6e) are prominent

The cephalopod has narrow spectral feature with absorption minima at 1010 nm; it is indicative of the presence of Fe-hydroxides. It shows inconspicuous 2300 nm Al–OH vibrational feature (Fig. 6c). No such clay minerals are observed by other geochemical analysis. Spectral signatures indicate the shells are free of diagenetic alteration

Brachiopod has asymmetric spectral feature at 900–1000 nm (ii Fig. 6a,b). This indicates the presence of Fe–OH. Al–OH vibrational feature (V) is more prominent in brachiopod than cephalopod. Presence of surficial clay mineral is observed by geochemical analysis. Spectral signatures of minimal diagenetic alteration with primary shell features are present