Geology topics

W. C. McIntosh

Publications and source records attributed to W. C. McIntosh.

6 recordsLinked to original sources

USGS2007

Miocene-Pliocene ice-volcano interactions at monogenetic volcanoes near Hobbs Coast, Marie Byrd Land, Antarctica

Paleoenvironmental reconstructions and 40 Ar/ 39 Ar geochronology of seven eroded monogenetic volcanoes near the Hobbs Coast, Marie Byrd Land, West Antarctica provide proxy records of WAIS paleo-ice-levels in Miocene-Pliocene times. Interpretations, based on lithofacies analysis, indicate whether the volcanoes erupted below, near, or above the level of the ice sheet. Our interpretations differ significantly from previous interpretations as they highlight the abundant evidence for ice-volcano interactions at emergent paleoenvironments but limited evidence of higher-than-present syn-eruptive ice-levels. Evidence for subglacial volcanic paleoenvironments is limited to Kennel Peak, a ~8 Ma volcano where a pillow lava sequence extending 25 m above current ice level overlies an inferred glacial till and unconformity. A major complication in the Hobbs Coast region is that the volcanism occurred on interfluves between regions of fast-flowing ice. Such a setting precludes establishing precise regional paleo-ice-levels although the presence or absence of ice at times of eruptions can be inferred.

Open-File Report↗

USGS2005

Timing and development of the Heise volcanic field, Snake River Plain, Idaho, western USA

The Snake River Plain (SRP) developed over the last 16 Ma as a bimodal volcanic province in response to the southwest movement of the North American plate over a fixed melting anomaly. Volcanism along the SRP is dominated by eruptions of explosive high-silica rhyolites and represents some of the largest eruptions known. Basaltic eruptions represent the final stages of volcanism, forming a thin cap above voluminous rhyolitic deposits. Volcanism progressed, generally from west to east, along the plain episodically in successive volcanic fields comprised of nested caldera complexes with major caldera-forming eruptions within a particular field separated by ca. 0.5-1 Ma, similar to, and in continuation with, the present-day Yellowstone Plateau volcanic field. Passage of the North American plate over the melting anomaly at a particular point in time and space was accompanied by uplift, regional tectonism, massive explosive eruptions, and caldera subsidence, and followed by basaltic volcanism and general subsidence. The Heise volcan ic field in the eastern SRP, Idaho, represents an adjacent and slightly older field immediately to the southwest of the Yellowstone Plateau volcanic field. Five large-volume (>0.5 km3) rhyolitic ignimbrites constitute a time-stratigraphic framework of late Miocene to early Pliocene volcanism for the study region. Field relations and high-precision 40Ar/39Ar age determinations establish that four of these regional ignimbrites were erupted from the Heise volcanic field and form the framework of the Heise Group. These are the Blacktail Creek Tuff (6.62 ?? 0.03 Ma), Walcott Tuff (6.27 ?? 0.04 Ma), Conant Creek Tuff (5.51 ?? 0.13 Ma), and Kilgore Tuff (4.45 ?? 0.05 Ma; all errors reported at ?? 2??). The fifth widespread ignimbrite in the regions is the Arbon Valley Tuff Member of the Starlight Formation (10.21 ?? 0.03 Ma), which erupted from a caldera source outside of the Heise volcanic field. These results establish the Conant Creek Tuff as a distinct and widespread ignimbrite in the Heise volcanic field, eliminating former confusion resulting from previous discordant K/Ar and fission-track dates. New 40Ar/39Ar determinations, when combined wi th geochemical, lithologic geophysical, and field data, define the volcanic and tectonic history of the Heise volcanic field and surrounding areas. Volcanic units erupted from the Heise volcanic field also provide temporal control for tectonic events associated with late Cenozoic extension in the Snake Range and with uplift of the Teton Range, Wyoming. In the Snake Range, movement of large (???0.10 km3) slide blocks of Mississippian limestone exposed 50 km to the east of the Heise field occurred between 6.3 and 5.5 Ma and may have been catastrophically triggered by the caldera eruption of the 5.51 ?? 0.13-Ma Conant Creek Tuff. This slide block movement of ???300 vertical meters indicates that the Snake Range had significant relief by at least 5.5 Ma. In Jackson Hole, the distribution of outflow facies of the 4.45 ?? 0.05-Ma Kilgore caldera in the Heise volcanic field on the eastern SRP indicates that the northern Teton Range was not a significant topographic feature at this time. ?? 2005 Geological Society of America.

Geological Society of America Bulletin↗

USGS2004

40Ar/39Ar dating of the eruptive history of Mount Erebus, Antarctica: Summit flows, tephra, and caldera collapse

Eruptive activity has occurred in the summit region of Mount Erebus over the last 95 ky, and has included numerous lava flows and small explosive eruptions, at least one plinian eruption, and at least one and probably two caldera-forming events. Furnace and laser step-heating 40Ar/39Ar ages have been determined for 16 summit lava flows and three englacial tephra layers erupted from Mount Erebus. The summit region is composed of at least one or possibly two superimposed calderas that have been filled by post-caldera lava flows ranging in age from 17 ?? 8 to 1 ?? 5 ka. Dated pre-caldera summit flows display two age populations at 95 ?? 9 to 76 ?? 4 ka and 27 ?? 3 to 21 ??4 ka of samples with tephriphonolite and phonolite compositions, respectively. A caldera-collapse event occurred between 25 and 11 ka. An older caldera-collapse event is likely to have occurred between 80 and 24 ka. Two englacial tephra layers from the flanks of Mount Erebus have been dated at 71 ?? 5 and 15 ?? 4 ka. These layers stratigraphically bracket 14 undated tephra layers, and predate 19 undated tephra layers, indicating that small-scale explosive activity has occurred throughout the late Pleistocene and Holocene eruptive history of Mount Erebus. A distal, englacial plinian-fall tephra sample has an age of 39 ?? 6 ka and may have been associated with the older of the two caldera-collapse events. A shift in magma composition from tephriphonolite to phonolite occurred at around 36 ka. ?? Springer-Verlag 2004.

Bulletin of Volcanology↗

USGS1997

Igneous evolution of a complex laccolith-caldera, the Solitario, Trans-Pecos Texas: Implications for calderas and subjacent plutons

The Solitario is a large, combination laccolith and caldera (herein termed “laccocaldera”), with a 16-km-diameter dome over which developed a 6×2 km caldera. This laccocaldera underwent a complex sequence of predoming sill, laccolith, and dike intrusion and concurrent volcanism; doming with emplacement of a main laccolith; ash-flow eruption and caldera collapse; intracaldera sedimentation and volcanism; and late intrusion. Detailed geologic mapping and 40 Ar/ 39 Ar dating reveal that the Solitario evolved over an interval of approximately 1 m.y. in three distinct pulses at 36.0, 35.4, and 35.0 Ma. The size, duration, and episodicity of Solitario magmatism are more typical of large ash-flow calderas than of most previously described laccoliths.

Texas↗

USGS1994

40Ar/39Ar chronology and volcanology of silicic volcanism in the Davis Mountains, Trans-Pecos Texas

Field studies and 40 Ar/ 39 Ar dating reveal that silicic volcanism in the Davis Mountains part of the Trans-Pecos Texas volcanic field occurred in six episodes at 0.3 m.y. intervals between 36.8 and 35.3 Ma. Additionally, two groups of silicic intrusions were emplaced at 34.6 and 32.8 Ma. This episodicity is similar to that determined for volcanic fields dominated by ash-flow tuffs, yet voluminous, extensive silicic lavas are considerably more abundant than tuffs in the Davis Mountains, by number of flows and by volume. The preponderance of voluminous silicic lavas over tuffs most likely reflects low water contents and high temperatures of the alkalic and commonly peralkaline Davis Mountains magmas. The earliest episode, at 36.8 Ma, included a widespread and voluminous (possibly >1,000 km 3 ) suite of rhyolite and quartz trachyte lavas, several rhyolite domes, and a strongly rheomorphic, peralkaline ash-flow tuff erupted from a caldera in the northern Davis Mountains. The lava suite extends well beyond the Davis Mountains. Silicic lavas of all episodes probably erupted from widespread, fissure vents. The 36.5 Ma episode consisted of rhyolite to quartz trachyte lavas, also extensive and voluminous (∼200 km 3 ). The 36.3 Ma episode consisted of rhyolite to trachyte tuffs and lavas erupted from a central vent volcano in the southern Davis Mountains. The 35.9 Ma episode consisted of a single, moderately large (∼50 km 3 ) rhyolite lava and a small-volume ash-flow tuff erupted from a caldera in the western Davis Mountains. Rocks emplaced during the 35.6 Ma episode were also rhyolites, including an enigmatic rock that may be strongly rheomorphic ash-flow tuff or a combination of tuff and lava, followed by definite lavas. The 35.3 Ma episode consisted of two ash-flow tuffs, one of which is strongly rheomorphic, and additional voluminous rhyolite lavas (∼120 km 3 ). The rheomorphic tuff erupted from a caldera in the southwestern Davis Mountains. The source of the other tuff is probably in the western Davis Mountains. Intermediate and mafic rocks are minor, except around the southeastern flank of the Davis Mountains, where basalt is abundant. Mafic lavas erupted only during gaps in the silicic activity and on the flanks of the Davis Mountains. Nevertheless, basaltic magma probably drove the silicic magmatism, either by differentiation or by crustal melting, and was present throughout the time of Davis Mountains activity but could not penetrate the low-density silicic magma chambers until they cooled and solidified. The time required for cooling and solidification appears to be 0.1-0.2 m.y.

Texas↗

USGS1992

Calibration of the latest Eocene-Oligocene geomagnetic polarity time scale using 40 Ar/ 39 Ar dated ignimbrites

A discontinuous record of late Eocene-Oligocene geomagnetic polarity has been determined using high-precision (±<0.15 m.y.) 40 Ar/ 39 Ar sanidine dating and a paleomagnetic study of 37-27 Ma ignimbrites in New Mexico, Colorado, and Texas. This record provides age control for several geomagnetic polarity reversals that occurred during three periods of intense ignimbrite volcanism: 36.8-33.5 Ma, 32.7-31.4 Ma, 29.1-26.9 Ma. The relative timing of these polarity reversals permits four possible correlations with the geomagnetic polarity time scale (GPTS). The preferred correlation yields calibration ages for Chron C10R (28.0-29.0 Ma) and Chron C13R (34.4-33.1 Ma) that indicate an Eocene-Oligocene boundary age near 33.4 Ma, some 0.3-0.6 m.y. younger than boundary ages indicated by other recently proposed GPTS calibrations based on terrestrial and marine sedimentary sequences.

Geology↗