PALEOENVIRONMENTAL SIGNIFICANCE OF FE-MG CLAYS IN SANDSTONES

Late Jurassic sandstones of the shallow marine Sundance Formation contain authigenic glauconite and chlorite minerals. Glauconite grains occur as laminae along planar and large- and small-scale cross-beds, indicating reworking and deposition as clasts in tidally-influenced regimes. By contrast, chlorite minerals, which are dominated by mixed-layer 7 Å/14Å Fe-rich chlorites (berthierine/chlorite: Be/Ch), predominantly occur as rosette-like pore-fillings. Localized corrensite (R1-ordered chlorite/smectite) and discrete chlorite, which were detected in uppermost Sundance sandstones, occur as honeycomb-like pore-linings. These textures indicate in situ authigenic formation of chlorite minerals in conditions that are interpreted as shallow marine for Be/Ch and evaporitic for the corrensite-chlorite. Regression of the Sundance Sea led to an influx of oxidizing terrestrial groundwater into uppermost Sundance sands, causing dissolution of precursor shallow marine clay (i.e. odinite and/or Be/Ch) and subsequent formation of corrensite and/or precursor saponite, a trioctahedral smectite that forms in evaporitic and aeolian environments.

These findings emphasize the significance of chlorite minerals as paleoenvironmental indicators in situations were other indicators (e.g. fossils, sedimentary structures) are absent or not diagnostic (e.g. Precambrian sandstones). Be/Ch represents diagenetic evolution of the shallow marine verdine facies, and more specifically the Fe-clay odinite, a common component of tropical and subtropical Holocene marine sands. Corrensite in sandstones most likely reflects maturation of early diagenetic saponite formed in evaporitic environments.

The wide range (5 to 28%) in % 7Å berthierine (Be) layers in the mixed-layer Be/Ch is consistent with disequilibrium common to low-grade diagenetic systems (e.g. <120C). The ratio of Fe/(Fe+Mg) exerts a strong influence on % Be layers, where specimens containing the highest Fe/(Fe+Mg) ratios (>0.8) also contain the greatest % Be layers. The Be-Ch is the Ibb polytype, and the corrensite-chlorite is disordered IIb chlorite, consistent with a low-grade diagenetic environment.