ABSTRACT

Detailed field and laboratory analyses were made of 117 samples of Pierre Shale and their stratigraphic equivalents from midcontinental North America. Techniques used were hulk chemical analyses, quantitative X-ray diffraction, sodium bisulfate fusion, atomic absorption spectrophotometry, and petrographic studies. The objective was to determine whether the nonclay petrology of the formation could be used to interpret its petrolgenetic history.

Quartz is the major nonclay constituent, averaging 23% ( = 9%) of the samples. The percentage of quartz in nonbentonitic samples is highly correlated with distance from the nearest shoreline (P > 0.99) despite sporadic influxes of airborne volcanic ash from distant volcanic sources. Most quartz was derived from older sedimentary rocks to the west of the Pierre seaway.

The feldspar content averages 8.5% of the whole rock and 39% of the quartz plus feldspar fraction. Sixty percent of the feldspar is normative plagioclase. Mean An content is 8.2 mole percent (range 1-15%) and An content is positively correlated with the percentage of plagioclase. The ratio of plagioclase to potassium feldspars is nearly constant at 3/1 for all samples. There is no correlation between the percentage of feldspar and distance to the nearest shoreline. The majority of feldspar in the Pierre was derived from penecontemporaneous volcanic eruptions to the west.

The only major diagenetic change in the quartz and feldspar fraction was the conversion of biogenic silica to cristobalite and quartz. Only one sample contained authigenic feldspar.