Rb-Sr Dating of Illite Diagenesis: ABSTRACT

Abstract

In the Woodford Shale (Upper Devonian), apparent Rb-Sr ages decrease as clay grain size decreases, which in turn correlates with increasing abundance of diagenetic illite. Analyses of the fine-clay size fraction (< 0.2µ) from widely spaced wells in the Delaware basin of west Texas, plot on a single isochron indicating an age of 300 ± 4 m.y. (Middle Pennsylvanian). At this time the Woodford was buried only 200 to 600 m; consequently diagenesis must have been triggered by a circumstance other than deep burial. Possibly diagenesis was accomplished by hydrothermal fluids moving toward the craton out of the Ouachita geosyncline, which at that time was experiencing horizontal compression. These fluids may have been responsible for petroleum migration and lead-zinc ineralization. In the Frio Formation (Oligocene) of the Texas Gulf Coast, samples of fine clay-size material (< 0.06µ) from the 3 to 5 km depth interval in a single well also provide a well-defined isochron corresponding to 21.6 ± 2.2 m.y. Burial here was possibly so rapid that transformation of smectite to illite approximated an episodic event over the entire depth interval. Alternatively, because the sediment is geopressured, the age might record the time of geopressure development which was accompanied by a rapid rise in temperature. Clay diagenesis at the surface is illustrated by a paleosoil developed on Pennsylvanian shale in the Llano uplift of central Texas. The paleosoil was buried by Cretaceous basal conglomerate and records the time of marine transgression 119 ± 3 m.y. ago. Constituents of the shale were degraded by soil-forming processes which erased previous isotopic memory, then reconstituted by coming in contact with marine water. This field relation offers a new way to date directly a time of sedimentary deposition. End_of_Article - Last_Page 610------------