NEW PERSPECTIVES ON THE EMPLACEMENT MECHANISMS INVOLVED IN DIATREME FORMATION IN THE NORTHEASTERN NAVAJO VOLCANIC FIELD, SOUTHWESTERN COLORADO

Previous models for the emplacement and formation of Oligocene diatreme-dike complexes in the Navajo volcanic field (NVF) have required the interaction of ultramafic to mafic, potassic mantle magmas with groundwater reservoirs to generate explosive subsurface-surface eruptions in these systems. Recent detailed field studies on a series of northeast-trending diatreme-dike complexes exposed in the northeastern Navajo volcanic field near Mesa Verde National Park document subsurface eruptions that were dominated by lateral transport of breccia and ash, and emplacement of several generations of related lamprophyre dikes. In these systems, elliptical bodies of tuff and tuff breccia formed during and prior to the emplacement of northeast and east-west trending dike sets. Bedding in these breccia deposits dip between 25° to 60° degrees away from a central core of dikes, as opposed to the vertical to near vertical bedding found in other diatreme complexes in the NVF (e.g., Ship Rock). Tuff deposits in the diatremes preserve graded bedding, cross lamination, and scour surfaces that we interpret as evidence for rapid lateral transport of fragmented material. In addition, bedding surfaces in Late Cretaceous sedimentary units in the region have localized lateral breccia eruptions, as indicated by interbedded tuff breccia and sandstone.

Our working hypothesis for the formation of the Mesa Verde diatreme-dike complexes, is that the diatreme centers were initiated by decompressive gas release and “budding” as gas pressures in the magma systems exceeded lithostatic pressure. Magma transported and emplaced on northeast fractures systems acted as catalysts to gas-rich, subsurface eruptions that created the diatreme “buds” and aprons. In our model, these systems either represent a different mechanism of diatreme formation or perhaps a deeper and separate eruptive phase in the diatreme complex.