Analysis of the Spectral Contrast between Fossil-bearing and Fossil-depleted Limestone from the Kachchh Region, NW India Using Reflectance and Emittance Spectroscopy Data

Abstract

ABSTRACT We analyzed the reflectance and emittance spectra of representative samples of mega and microfossil-bearing limestones and identified the mineralogical controls, which may have led to the preservation of mega and microfossils in the limestone within the spectral domain of 400-2500 nm and 8000-14000 nm. In this regard, we collected and analyzed the reflectance and emittance spectra of fossil-bearing and fossil-depleted limestone samples using respective reflectance and emittance spectra of their dominant constituent minerals. Reflectance spectra of mega and microfossil-bearing limestones have prominent absorption features diagnostic to iron and clay minerals, while emittance spectra of these rocks confirm the presence of silica in the fossil-bearing limestone samples. Spectral features of iron minerals are identified based on the presence of absorption minima at 480 nm and 900 nm, while clay minerals have features at 2200 nm. Presence of silica was identified with emissivity minima at 9000 nm. Laboratory spectral observations are substantiated by field observation and mineralogical studies. Fossil-depleted limestone samples, on the other hand, are characterized by the absence of iron oxide, clay and silica. Spectral contrast between fossil-bearing and the fossil-depleted limestone have allowed us to infer certain mineralogical controls essential for fossil preservation and the role of iron, clay minerals in preserving fossils is discussed in detail. Further, spectral analysis in visible, near infrared, short wave infrared and thermal infrared electromagnetic domain may provide rapid and non-destructive mineralogical assessment of fossil-bearing and fossil-depleted limestone.