Tertiary non-marine limestone within the strata of the Sullivan Buttes volcanic field, Yavapai County, Arizona: A petrologic and diagenetic investigation

Abstract

The Oligocene fluvial-volcanic sequence in the Prescott area of central Arizona, USA, contains a thin and laterally limited bed of non-marine limestone. The limestone is characterized by an abundance of opaline diatoms, opalized charophytes, calcified reeds, and cyanobacteria encrustations. Limestone deposition occurred in a small pond of less than a few hundred meters width that formed as the result of damming of a small local stream by a volcanic event. The duration of the pond and limestone deposition was brief and was terminated by burial under volcaniclastic debris. Chemical weathering of coeval volcaniclastic debris was integral both to the formation of the limestone and its syndepositional diagenesis by providing a source for calcium, magnesium, and silica. Biogenicrelated consumption of calcium beyond its rate of re-supply resulted in the elevation of magnesium concentration relative to calcium (Mg/Ca) and the subsequent formation of fibrous calcite cements. Silica concentration seems to be connected to the presence of a series of burrows that provided a pathway from the silica source in the underlying volcaniclastic-rich fluvial sediments to the pond waters. Fluids passing from the sediment through the burrows resulted in the opalization of charophyte internodes and reproductive organs that had collected within the burrow. Silica supplied through the burrows to the pond resulted in diatom growth and their postmortem preservation. Blockage of the burrow system by the growth of cyanobacteria terminated the fluid connection between the sediment and pond water and the subsequent restriction on silica supply resulted in a loss of diatom preservation. The principal depositional phase of the pond was followed by a complex sequence of events that led to the formation of several geopedal fills and isopaceous cements, composed of both calcite and silica.