Paleoenvironmental analysis of diagenetically and metamorphically-altered strata: Methods and interpretations
Paleoenvironmental analysis has, thus far, been restricted to areas where diagenetic and metamorphic influences have been minimal, since modifications resulting from elevated temperatures and pressures can homogenize and overprint environmental signatures. However, results from an analysis of geochemistry and bulk and clay mineralogy in the Pennsylvanian-age, Buck Mountain underclay and the Lykens Valley underclay, Anthracite region, Northeastern Pennsylvania, suggest that the metamorphic overprint did not completely obliterate the original mineralogical composition, thus allowing paleoenvironmental interpretation. The Buck Mountain underclay is less than 2 meters thick; yet, within the bed, the vertical distribution of feldspars such as albite, of clay minerals--chlorite, kaolinite and illite--and the metamorphic mineral pyrophyllite, differ considerably. The Lykens Valley underclay occurs approximately 140 meters down section and is 0.5 meters thick. Geochemical, petrographic and mineralogical analyses, indicate that the Buck Mountain underclay formed during a successional transition from flood plain lake to peat-accumulating swamp and that the Lykens Valley underclay is a well-formed, upland soil profile.
The Lykens Valley paleosol profile shows evidence of rooting at depth and is geologically the oldest example of an Ultisol yet identified and provides evidence that an active and complex, forest-type, ecosystem occupied tropical uplands at a time when coal formation dominated the adjacent lowland environments. The formation of this soil during the Lower Pennsylvanian demonstrates that plant communities had complex rooting systems that were capable of collecting nutrients and surviving in a base-poor and well-drained environment at a much earlier time than was previously thought. Within the upper portion of the paleosol profile, base cations are depleted. However, potassium is enriched. This potassium enrichment coincides with organic enrichment. This research presents evidence for the role of organic matter in the illitization of kaolinite and proposes that this enrichment is the reflection of the biological process of nutrient uptake and sequestration in plant tissues. Thus, illite in the Lykens Valley paleosol is a fossil reflection of Paleozoic nutrient uptake.