Ed DeWitt

Structural, eruptive, and intrusive evolution of the Grizzly Peak caldera, Sawatch Range, Colorado

Volcanic and shallow intrusive features of the deeply dissected 34 Ma Grizzly Peak caldera in west-central Colorado record evolution of the magmatic center from pre-caldera through post-resurgent stages. Pre-caldera dikes, zones of hydrothermally altered rocks, and lava flows formed along a circular swarm of cone-sheet fractures around the site of the future caldera. Early, largely rhyolitic uppercrustal magmatism culminated in the caldera-forming eruption of the Grizzly Peak Tuff. Intracaldera tuff is zoned from high-silica rhyolite at the base to low-silica rhyolite at the eroded top and, further, contains dacite to mafic latite pumice lumps in two heterogeneous tuff layers in the upper third of the preserved section. Half of the erupted tuff ponded in the 17-by 23-km, >600-km 3 caldera, filling the asymmetric depression to a compacted thickness locally >2.7 km, including intercalated rock-avalanche megabreccias shed from ring-fault scarps. The asymmetric caldera has an inner ring-fracture zone that separates two structural segments that collapsed to different depths. Caldera fill buried collapse structures as they formed; the inner ring-fracture zone is a growth (or syn-depositional) fault in the single-cooling-unit tuff. Welded-tuff ring dikes are locally exposed at erosion levels below the caldera fill. These dikes are remnants of fissure vents in the outer ring-fracture zone. Following collapse, the Grizzly Peak caldera was uplifted, forming a complexly faulted resurgent dome. Resurgence resulted partly from emplacement of a composite granodiorite laccolith now exposed in the eroded core of the dome. A belt of mafic latite to rhyolite porphyry dikes and small stocks formed across the center of the domed caldera during the waning of the magmatic center. Latite intrusions in this suite represent the penetration of relatively mafic magma to the surface following solidification of the felsic subcaldera batholith.

SHRIMP U-Pb dating of recurrent Cryogenian and Late Cambrian–Early Ordovician alkalic magmatism in central Idaho: Implications for Rodinian rift tectonics

Composite alkalic plutonic suites and tuffaceous diamictite, although discontinuously exposed across central Idaho in roof pendants and inliers within the Idaho batholith and Challis volcanic-plutonic complex, define the >200-km-long northwest-aligned Big Creek-Beaverhead belt. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dates on these igneous rocks provide direct evidence for the orientation and location of the Neoproterozoic–Paleozoic western Laurentian rift margin in the northern U.S. Cordillera. Dating delimits two discrete magmatic pulses at ca. 665–650 Ma and 500–485 Ma at the western and eastern ends, respectively, of this belt. Together with the nearby 685 Ma volcanic rocks of the Edwardsburg Formation, there is a 200 Ma history of recurrent extensional magmatic pulses along the belt. A similar history of recurrent uplift is reflected in the stratigraphic record of the associated miogeoclinal and cratonal platform basins, suggesting that the Big Creek–Beaverhead belt originated as a border fault during continental rift events. The magmatic belt is paired with the recurrently emergent Lemhi Arch and narrow miogeoclinal facies belts and it lies inboard of a northwest-striking narrow zone of thinned continental crust. These features define a northeast-extending upper-plate extensional system between southeast Washington and southeast Idaho that formed a segment of the Neoproterozoic–Paleozoic miogeocline. This segment was flanked on the north by the St. Mary–Moyie transform zone (south of a narrow southern Canadian upper-plate margin) and on the south by the Snake River transfer zone (north of a broad Great Basin lower-plate margin). These are the central segments of a zigzag-shaped Cordilleran rift system of alternating northwest-striking extensional zones offset by northeast-striking transfers and transforms. The data substantiate polyphase rift and continental separation events that included (1) pre- and syn-Windermere rifting, (2) Windermere margin subsidence, (3) late Ediacaran-Cambrain rifting, and (4) well-developed late Ediacaran-Devonian passive margin subsidence and deposition. Timing and geometries support synchronous but opposing divergence along Cordilleran and Atlantic rifts with a junction in Southern California–Sonora.

Evidence and characteristics of hydrolytic disproportionation of organic matter during metasomatic processes

Abstract Petroleum-geochemical analyses of carbonaceous regionally metamorphosed rocks, carbonaceous rocks from ore deposits, and alkalic plutonic rocks from diverse settings, demonstrated the presence of very low to moderately low concentrations of solvent-extractable organic matter, this observation in spite of the fact that some of these rocks were exposed to extremely high metamorphic temperatures. Biomarker and δ13C analyses established that the extractable organic matter originated as sedimentary-derived hydrocarbons. However, the chemistry of the extractable bitumen has been fundamentally transformed from that found in sediment bitumen and oils. Asphaltenes and resins, as defined in the normal petroleum-geochemical sense, are completely missing. The principal aromatic hydrocarbons present in oils and sediment bitumens (especially the methylated naphthalenes) are either in highly reduced concentrations or are missing altogether. Instead, aromatic hydrocarbons typical of sediment bitumens and oils are very minor, and a number of unidentified compounds and oxygen-bearing compounds are dominant. Relatively high concentrations of alkylated benzenes are typical. The polar “resin” fraction, eluted during column chromatography, is the principal compound group, by weight, being composed of six to eight dominant peaks present in all samples, despite the great geologic diversity of the samples. These, and other, observations suggest that a strong drive towards equilibrium exists in the “bitumen.” Gas chromatograms of the saturated hydrocarbons commonly have a pronounced hump in both the n-paraffins and naphthenes, centered near the C19 to C26 carbon numbers, and a ubiquitous minimum in the n-paraffin distribution near n-C12 to n-C14. Multiple considerations dictate that the bitumen in the samples is indigenous and did not originate from either surficial field contamination or from laboratory procedures. Our observations are consistent with the hydrolytic disproportionation of organic matter (HDOM), in which water and organic matter, including hydrocarbons, easily exchange hydrogen or oxygen with one another under certain conditions (Helgeson et al., 1993) . The process appears to take place via well-known organic-chemical redox reaction pathways and is most evident in open-fluid systems. The conclusion that HDOM took place in the analyzed samples, thus producing the chemistry of the extractable bitumen, is supported by numerous previously published organic-geochemical studies of metamorphic, volcanic, plutonic, and ore-deposit-related rocks by other investigators. HDOM is suggested as an unrecognized geologic agent of fundamental importance. The process appears to control major chemical reactions in diverse geologic environments including, but not limited to, petroleum geology and geochemistry, regional metamorphism, and base- and precious-metal ore deposition.

Early Miocene mylonitization and detachment faulting, South Mountains, central Arizona

The South Mountains of central Arizona are one of the geologically simplest metamorphic core complexes of the North American Cordillera. An early Miocene age of mylonitization is indicated by crosscutting relationships between mylonitic fabric and a composite pluton dated at 22-25 Ma by Rb-Sr, U-Th-Pb, and K-Ar techniques. The kinematic agreement and close temporal association of mylonitization and detachment faulting support models in which the two processes are related to an evolving crustal shear zone that accommodated mid-Tertiary continental extension. 19 references, 3 figures, 2 tables.

Early Mesozoic uplift in west-central Arizona and southeastern California

Mesozoic rocks in west-central Arizona and southeastern California record an episode of uplift in Late Triassic to Middle Jurassic time. The main evidence for this uplift is an unconformity that is cut on Triassic to Proterozoic rocks and is overlain by a pre-160 Ma conglomerate containing clasts of Proterozoic crystalline rocks. The uplift is an important early Mesozoic paleogeographic feature and may be part of the long-sought source terrane for detritus in the Chinle Formation and other lower Mesozoic units of the Colorado Plateau.

Late Cretaceous stratigraphy, deformation, and intrusion in the Madison Range of southwestern Montana

Dating of orogenic rock units in the central part of the Madison Range shows that Laramide deformation was virtually completed by the end of the Cretaceous. Early Campanian K-Ar dates of about 79 m.y. were obtained from welded tuffs in the basal part of the Livingston Formation, a volcanic and volcaniclastic assemblage that is conformable with underlying Cretaceous clastic rocks and with the overlying Sphinx Conglomerate. No datable materials were obtained from the Sphinx, but both it and the Livingston were deformed by the Hilgard fault system, a series of thrust faults and associated folds which extend along the western side of the southern two-thirds of the range. This north-trending fault system represents the culmination of Laramide shortening within the range. K-Ar and 40 Ar/ 39 Ar dating of hornblende from dacitic laccolithic rocks that intruded the Hilgard fault system in the central part of the range indicates an approximate date of 68–69 m.y. B.P. for emplacement of the igneous suite. The dacite postdates movement along faults of the Hilgard fault system, and postdates the synorogenic Sphinx Conglomerate.

Using geochemistry to identify the source of groundwater to Montezuma Well, a natural spring in Central Arizona, USA: part 2

Montezuma Well is a natural spring located within a “sinkhole” in the desert environment of the Verde Valley in Central Arizona. It is managed by the National Park Service as part of Montezuma Castle National Monument. Because of increasing development of groundwater in the area, this research was undertaken to better understand the sources of groundwater to Montezuma Well. The use of well logs and geophysics provides details on the geology in the area around Montezuma Well. This includes characterizing the extent and position of a basalt dike that intruded a deep fracture zone. This low permeability barrier forces groundwater to the surface at the Montezuma Well “pool” with sufficient velocity to entrain sand-sized particles from underlying bedrock. Permeable fractures along and above the basalt dike provide conduits that carry deep sourced carbon dioxide to the surface, which can dissolve carbonate minerals along the transport path in response to the added carbon dioxide. At the ground surface, CO2 degasses, depositing travertine. Geologic cross sections, rock geochemistry, and semi-quantitative groundwater flow modeling provide a hydrogeologic framework that indicates groundwater flow through a karstic limestone at depth (Redwall Limestone) as the most significant source of groundwater to Montezuma Well. Additional groundwater flow from the overlying formations (Verde Formation and Permian Sandstones) is a possibility, but significant flow from these units is not indicated.