ABSTRACT

Reflectance spectroscopy, a useful analytical tool available to geologists for over 100 years, has not been widely used in petrologic or stratigraphic studies of sedimentary rocks. We are routinely using laboratory spectral reflectance data covering the 0.4 to 12.0 µm wavelength interval in an investigation of the stratigraphic and structural evolution of the Wind River and Bighorn basin areas, Wyoming.

Two types of laboratory spectral reflectance measurements were made on 192 weathered outcrop, bedrock, and core samples: 1) hemispherical reflectance, from 0.4 to 2.5 µm (229 spectra) and 2) biconical reflectance, from 8.0 to 12.0 µm (84 spectra). Results show stratigraphic variations in the spectral properties of sedimentary rocks as encountered in the field. Sample mineralogy was determined using X-ray diffraction and interpretation of spectra. The mineralogy, lithology and stratigraphic distribution of samples are diverse and representative of Phanerozoic siliciclastic and carbonate strata encountered in foreland sedimentary basins.

Spectra were sorted into 36 distinct groups, according to the presence or absence of 68 visible and infrared reflectance features. Many spectral features can be assigned to specific molecular phenomena and allow rapid determination of specific minerals. Minerals detectable spectrally include: gypsum, kaolinite, smectite, analcime, goethite, jarosite, calcite, dolomite, and feldspar.

Results are contributing to our understanding of the depositional history of the Wind River and Bighorn basin areas; more importantly, results suggest that laboratory, field and remote reflectance spectroscopy have more general geologic applicability in studies of sedimentary rocks. Reflectance spectroscopy can augment conventional laboratory and field methods for petrologic analysis, correlation, interpretation of depositional environments, and construction of facies models.