A recently reported inverse correlation between boron and organic carbon in rocks suggests that development of illite-organic complexes during deposition may inhibit boron absorption. The validity of boron as a paleosalinity indicator consequently has been questioned. Analogous studies of illitic clay fractions from the late Visean Lower Yoredale Formation of England revealed no significant correlation between organic carbon and boron. Thus, the validity of boron as a salinity indicator is not affected by organic matter associated with clay fractions. If the reported correlation between boron and organic carbon in rocks is valid, environments which favor organic accumulation could suppress illite authigenesis or inhibit boron absorption. The already-known association betw en boron and salinity suggests that the first interpretation is more probable.

Equivalent boron, previously regarded as the best index of paleosalinity, depends on the boron-K₂O ratio and K₂O concentration in pure illite. However, in previous investigations the K₂O concentration in clay fractions was assumed to be the concentration in pure illite. Such a procedure leads to increasing error with increasing organic contamination of clay fractions. Therefore, revised equivalent boron values were calculated for the Lower Yoredale Formation by estimating the organic fraction from carbon analyses and recalculating K₂O concentrations in terms of pure illite. These revised values correlate more closely with other geological information and improve discrimination between salinity environments.

Organic carbon associated with illite also may be a useful environmental indicator because it correlates closely with facies and distance from shore. It follows that studies of boron, carbon, and potassium in illitic clay fractions could aid environmental discrimination in exploration for offshore stratigraphic traps.

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