Evolution of the Western Valles Caldera Complex, New Mexico: Evidence from intracaldera sandstones, breccias, and surge deposits

Abstract

Scientific core drilling in the Pleistocene Valles caldera complex (encompassing the Valles (1.13 Ma) and coaxial Toledo (1.50 Ma) calderas) of north central New Mexico has provided new insight into the origins of sandstones, breccias, and pyroclastic surge deposits interbedded with the thick intracaldera ignimbrite sequence. These rocks were previously interpreted from geothermal drill cuttings as dominantly fluvial in origin. As such, representing significant erosional intervals, they formed much of the basis for subdividing the intracaldera ignimbrite sequence (up to 2000 m in apparent thickness where drilled) into four major units: the lower tuffs (> 1.50 Ma); the Otowi (1.50 Ma) and Tshirege (1.13 Ma) members of the Bandelier Tuff; and a new unit, the upper tuffs, believed to be post‐Bandelier in age (<1.13 Ma). All but the upper tuffs correspond to mapped outflow facies ignimbrite sheets. However, Continental Scientific Drilling Program core holes VC‐2A (completed in 1986) and VC‐2B (completed in 1988), in the Sulphur Springs area of the Valles caldera, have provided continuous cores, revealing for the first time that some intracaldera rocks previously thought to be exclusively clastic actually have multiple origins. Some of these rocks are probably pyroclastic surge deposits; others could be lithic‐rich breccias of various origins incorporated nearly instantaneously in ignimbrites during ash flow eruption and concomitant caldera collapse. These new findings demonstrate the value of continuous core for subsurface characterization and correlation of complex intracaldera lithologies; they also necessitate revision of Nielson and Hulen's (1984) cuttings‐based intracaldera stratigraphic framework. For example, the hematitic S2 “sandstone” was initially interpreted as marking an erosional interval between the Tshirege Member of the Bandelier Tuff and the overlying, petrographically similar upper tuffs. Core from VC‐2A and VC‐2B, however, shows that the S2 cuttings could also represent disaggregated, Permian red bed‐rich, lithic lag breccias or caldera collapse mesobreccias. If this is the case, then most or all of the upper tuffs are actually uppermost Tshirege Member ignimbrites. In similar fashion and upon review of previously applied correlation criteria the “lower tuffs” of the western Valles caldera complex could represent both genuine pre‐Bandelier ignimbrites and those of the lowermost Otowi Member. The core, however, shows that in the Sulphur Springs subsurface the lower tuffs are separated from overlying ignimbrite sheets by prominent erosional and eruptive breaks; they appear to be slightly more mafic than the overlying tuffs and host distinctive pumice lapilli. At this site the lower tuffs almost certainly predate the Otowi Member and are probably correlative with the outflow facies San Diego Canyon ignimbrites (1.78 Ma). Cores from VC‐2A and VC‐2B support earlier interpretation of the S3 “sandstone” as a major marker horizon separating the intracaldera Otowi and Tshirege members of the Bandelier Tuff...

Distinct stores and the routing of water in the deep critical zone of a snow-dominated volcanic catchment

“…The influence of the hydrogeologic environment (i.e., geology, topography, and climate) on the groundwater flow system of a given geographic region has long been accepted as the theoretical framework used to conceptualize groundwater flow. Building on the conceptual model of Toth (1970) and the understanding that one part of the framework informs our knowledge of the other, several studies have focused on topography (Beven and Kirkby, 1979;Woods et al, 1997;Hutchinson and Moore, 2000;Kirchner et al, 2001;McGuire et al, 2005) and rock type (Farvolden, 1963;Freeze, 1972;Kelson and Wells, 1989;Mwakalila et al, 2002) as controls of groundwater flow systems. However, subsurface heterogeneities, which can be abundant and are challenging to identify, can give rise to complex, localized groundwater stores, whose contribution to streamflow can be very difficult to discern.…”

Isotopic and trace element characteristics of rhyolites from the Valles Caldera, New Mexico. Final technical report

“…In VC-2B this interval has epiclastic sandstone, breccia and a iithic-rich layer, The uppermost sandstone bed was probably deposited immediately prior to the tJBT, as evidenced by eruption timing and lack of a ',_oilhorizon. It has an ashy appearance, especially near the base, and Hulen et al ( 1991) recognize accretionary htpilli in its lower part, although they were not recognized in this work. The horizon is certainly a sedinaent, a feldspathic litharenite, and not a pyroclastic deposit.…”

“…Welding persists to within 1 to 2 m of the base of the UBT where it rests on the $3 sandstone. Hulen et al (1991) have given details of this unusual contact, suggesting that the sandstone was wet when the ignimbrite was deposited. Excess water may have promoted welding to the very base by lowering the effective viscosity of the glass shards, and this would be consistent with deposition into shallow water.…”

“…Generation of this sediment could have included reworking early UBT phreatomagmatic deposits to yield the accretionary lapilli noted in the base of the $3 beds by Hulen et al (1991), but not found in our $3 samples.…”

“…$3 beds (355.7 t_,_ 361,7 m) 1166-1186 + 1186-1248' A six meter thick ashy, crystal-rich sandstone ix the unit below the base of the UBT; the contact is conformable. We follow Hulen et al (1991) in terming these the $3 beds. In VC-2B this interval has epiclastic sandstone, breccia and a iithic-rich layer, The uppermost sandstone bed was probably deposited immediately prior to the tJBT, as evidenced by eruption timing and lack of a ',_oilhorizon.…”