Sources of Intraformational Variation in the Major Element Chemistry of Sandstones: The Effects of Depositional Environment and Diagenesis

Suzanne Kairo

Intraformational variations in the major element composition of sandstones of the Minturn Formation (Pennsylvanian) at McCoy, Colorado reflect modification by depositional environment and diagenesis. In this study, geologic setting, provenance, burial history, and depositional conditions were constrained so that causes of compositional variation could be identified. Thin sections of 80 sandstones were point counted to determine abundances of detrital components. Relative abundances of cements and replacements, and degree of dissolution were visually estimated using standard petrographic microscopy and scanning electron microscopy. Inductively coupled plasma emission spectrometry was used to obtain element oxide abundances from whole rock powders of the same 80 sandstones long with nine samples representing crystalline basement sediment sources.

The composition of most Minturn sandstones is modified from that of representative crystalline source rocks by either enrichment of SiO₂ and Na₂O, or depletion of Al₂O₃ and K₂O. Differences in the elemental oxide ratios of interstratified deposits correlate with textural and mineralogical characteristics imposed by processes in the depositional environment. Depositional facies can be qualitatively discriminated by elemental ratios of Na₂O/K₂O, SiO₂/Al₂O₃, and (MgO+Fe₂O₃)/Al₂O₃. Differences reflect detrital quartz, feldspar, and mica abundances, as well as clay and mica content of grains and matrix. A diagenetic enrichment of Na resul ing from selective albitization overprints the sedimentary-sourced variations.

These results have implications for hydrocarbon exploration. If range and source of intraformational chemical variations in a formation are not understood or calibrated, ambiguous interpretations could result from applying chemical signatures to determine sedimentary provenance, model diagenesis, interpret logs, and correlate strata.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995