Abstract Oligo-Miocene volcaniclastic sedimentation in northern New Mexico occurred before and during initiation of the Rio Grande rift. Reconstruction of dispersal patterns and paleogeography of extensive volcaniclastic aprons, using geochemical, paleocurrent, textural and petrofacies analyses, places important constraints on models for the tectonomagmatic initiation and evolution of the rift. K-Ar ages and chemical analyses of volcanic clasts from conglomerate provide details concerning three primary volcanic centers: 1) San Juan Mountains (29–27 Ma, high-K andesite and rhyodacite); 2) Latir volcanic field (28–25 Ma, high-K andesite and rhyolite); and 3) the previously unrecognized Servilleta Plaza center (23–22 Ma, Iatite, high-K andesite and rhyodacite), which may have been a southern extension of the Latir field. These three volcanic centers provided detritus to the following units, respectively: I) Esquibel Member of Los Pinos Formation, upper Abiquiu Formation, middle Picuris Formation, and Bishops Lodge Member of Tesuque Formation; 2) Cordito Member of Los Pinos Formation and uppermost Abiquiu Formation; and 3) Chama-EI Rito Member of Tesuque Formation and upper Picuris Formation. Correlation of these units is based on both conglomerate and sandstone petrofacies. Detailed point counting of volcanic lithic-grain varieties (vitric, granular, seriate, microlitic and lathwork) is especially useful in discriminating petrofacies. The Esquibel petrofacies has abundant plagioclase, seriate lithics and dense minerals, with low quartz. The Cordito petrofacies has abundant sanidine, quartz and vitric-to-granular lithics, with few dense minerals. The Plaza petrofacies is similar to the Esquibel, but with higher quartz and microlitic lithics, and lower total feldspar and dense minerals. Use of petrofacies simplifies the chaos of stratigraphic nomenclature and allows regional correlations of poorly exposed units. Paleocurrent and textural data from these volcaniclastic petrosomes define volcaniclastic aprons derived from the above volcanic centers. The coordinated use of clast ages and petrology to define source characteristics, sandstone petrofacies to define petrosomes, and paleocurrent and textural analyses to define dispersal patterns provides a powerful method for reconstructing ancient volcaniclastic systems.

Abstract Volcanic archipelagos are a source of numerous on- and offshore geohazards, including explosive eruptions and potentially tsunamigenic large-scale flank collapses. Fogo Island in the southern Cape Verdes is one of the most active volcanoes in the world, making it both prone to collapse (as evidenced by the c. 73 ka Monte Amarelo volcanic flank collapse), and a source of widely distributed tephra and volcanic material. The offshore distribution of the Monte Amarelo debris avalanche deposits and the surrounding volcaniclastic apron were previously mapped using only medium-resolution bathymetric data. Here, using recently acquired, higher-resolution acoustic data, we revisit Fogo's flank collapse and find evidence suggesting that the deposition of hummocky volcanic debris originating from the failed eastern flank most likely triggered the contemporaneous, multi-phase failure of pre-existing seafloor sediments. Additionally, we identify, for the first time, multiple mass-transport deposits in the southern part of the volcaniclastic apron of Fogo and Santiago based on the presence of acoustically chaotic deposits in parametric echo sounder data and volcaniclastic turbiditic sands in recovered cores. These preliminary findings indicate a long and complex history of instability on the southern slopes of Fogo and suggest that Fogo may have experienced multiple flank collapses.

Abstract Cenozoic nonmarine sedimentation within and adjacent to the Cascade volcanic arc has produced a varied depositional record. Eocene to early Miocene volcaniclastics accumulated to at least 3 km thick in a longitudinal trough coincident with the arc. Increasing constructional and tectonic relief in the Cascades during the Neogene resulted in development of volcaniclastic aprons adjacent to the arc that grade sourceward into volcanic sections. Quaternary sediments are largely restricted to terraces within valleys near the volcanoes. Neogene and Quaternary aggradation represents the impact of large eruption-related sediment loads on fluvial systems. There are three notable departures from existing fluvial facies models exhibited by these deposits. First, facies representing processes intermediate between more familiar dilute stream flow and viscous debris flow are important components of these sequences but have not been adequately described in the literature on fluvial processes. These hyperconcentrated flood-flow deposits are frequently produced by dilution of debris flows in stream channels, and their recognition is critical to successful evaluation of volcaniclastic sequences. Second, the volcaniclastic aprons and terraces contain considerable thicknesses of debris-flow and flood deposits for which existing models would suggest arid alluvial fan settings, but lateral grain-size variation, paleocurrent data, and paleogeographic considerations fail to support the presence of alluvial fans. Debris flows and floods in volcanic areas are more voluminous and cover much larger areas than alluvial fan counterparts and are not restricted to fan setting or arid climate. Third, aggradation occurs during short periods when eruption-produced sediment loads choke streams with debris and is separated by longer periods of nondeposition and incision. Thus, little or no record of inter-eruption sedimentation occurs and biases evaluation of fluvial style.

Stratigraphically equivalent sections within the Aycross and lower Wapiti Formations (middle Eocene) in the east-central Absaroka Range of northwest Wyoming locally are intensely deformed in a wide variety of structural styles. Deformation zones are primarily restricted to a well-defined, largely volcaniclastic interval within these two units of the lower Absaroka Volcanic Supergroup (AVS). Unlike the Mesozoic and Paleozoic formations that constitute the major displaced masses of the Heart Mountain, Reef Creek, and South Fork detachment faults in the northern Absaroka Range, the pre-volcanic strata (lower Eocene and older units) in the east-central Absaroka Range appear to have acted as basement during deformation. The rootless character of deformation, and the lack of involvement of younger middle to upper Eocene AVS units, prompted previous workers to propose middle Eocene detachment faulting as the principal process of deformation in the east-central Absaroka Range. However, no source area can be identified for the deformed intervals previously considered "allochthonous," nor has any persistent detachment horizon been discovered. In addition, the structures present represent an exceptionally broad range of structural styles whose geometries imply deformation prior to pervasive lithification, and whose attitudes suggest a nearly random kinematic pattern. Furthermore, the recognition of large-scale liquefaction phenomena in close association with most other deformational features throughout the study area suggests that the volcanic-volcaniclastic pile was more profoundly affected by in situ disturbances than by extensive detachment faulting in this area. Apparent lateral transport within local domains is generally limited, although well-developed sheath folds imply locally high shear strains ( ⩽ ≥10). Where subhorizontal mass transport is documented, deformation probably began in response to local instabilities generated by liquefaction at depth. Folding may have proceeded in places at creep rates for decades or centuries after initial liquefaction. Stratigraphic relations within the AVS in the east-central Absaroka Range are complex, but reveal a pattern of south-southeastward migration of intrusive/extrusive activity throughout middle to late Eocene time. The distal taper of the volcanic-volcaniclastic apron is revealed by southeastward thinning of the Wapiti/Aycross interval as a whole, and individual informally recognized volcanic (Twuf unit) and dominantly epiclastic (Twmb unit) members within this interval. I propose that the deformed basal AVS units in the study area may be the relatively distal stratigraphic equivalents of the Cathedral Cliffs Formation—the basal unit of the AVS in its more proximal northerly reaches. This tentative correlation suggests a genetic link between the spatially and stylistically distinct phenomena of liquefaction-related deformation and Heart Mountain faulting, both of which may stem from a common external high-energy stimulus, probably large-magnitude seismic activity.