VOLCANOGENIC SEDIMENTATION IN THE SECRET PASS CANYON VOLCANIC CENTER, NORTHWESTERN ARIZONA

The Secret Pass Canyon volcanic center (SPCVC) is an area displaying extensional settings in NW Arizona. It comprises a relatively coherent sequence of Miocene-aged intermediate (~18.5 Ma) to very felsic (~17.5 Ma) volcanic and shallow intrusive rocks. There exists a ~1 m.y. hiatus between the emplacement of the intermediate and felsic units, which is seemingly represented by a package of volcanogenic sediments (VS). Here we present our results from a study characterizing the VS; because this sedimentary package potentially reflects a majority of the time represented at the SPCVC, an understanding of the VS is critical for discerning the history of this area.

The VS ranges from ~50 to >100 m thick and consists of brown, poorly- to well-sorted sediments. Rhythmically bedded, fining upward layers (~10 cm to ≥1 m thick) of silt to sand-sized particles characterize the VS. These beds are dominated by angular to sub-rounded plagioclase with lesser amounts of andesite lithics and fresh biotite, though some coarsening upward sequences do occur; these layers mostly appear laterally consistent across the entire SPCVC. Locally interspersed within the VS are poorly-sorted conglomeratic layers with andesite cobbles. These layers are typically ~1 m thick and are not laterally extensive across the SPCVC.

The VS unconformably overlies andesite, but the nature of this contact is unresolved. Some VS sections show layers of andesite interbedded with the sediments, but it is not clear if the andesite represents sills or flows. Thin sections of the VS-andesite contact show possible recrystallization of VS grains. The top of the VS section, however, shows a gradual transition to plinian-style deposits.

Based on the laterally extensive rhythmic beds, we interpret the VS to represent a lacustrine environment that existed within the SPCVC. Previous intermediate magmatism undoubtedly provided the source materials for the VS where conglomeratic layers likely reflect lahar deposits derived from local volcanic edifices. Intermediate magmatism appears to have continued during VS deposition, and the occurrence of plinian deposits seems to mark a new material source for the lacustrine environment.