Abstract The Channeled Scabland of east-central Washington comprises a complex of anastomosing fluvial channels that were eroded by Pleistocene megaflooding into the basalt bedrock and overlying sediments of the Columbia Plateau and Columbia Basin regions of eastern Washington State, U.S.A. The cataclysmic flooding produced huge coulees (dry river courses), cataracts, streamlined loess hills, rock basins, butte-and-basin scabland, potholes, inner channels, broad gravel deposits, and immense gravel bars. Giant current ripples (fluvial dunes) developed in the coarse gravel bedload. In the 1920s, J Harlen Bretz established the cataclysmic flooding origin for the Channeled Scabland, and Joseph Thomas Pardee subsequently demonstrated that the megaflooding derived from the margins of the Cordilleran Ice Sheet, notably from ice-dammed glacial Lake Missoula, which had formed in western Montana and northern Idaho. More recent research, to be discussed on this field trip, has revealed the complexity of megaflooding and the details of its history. To understand the scabland one has to throw away textbook treatments of river work. —J. Hoover Mackin, as quoted in Bretz et al. (1956, p. 960)

Two Oligocene conglomeratic units, one primarily nonvolcaniclastic and the other volcaniclastic, are preserved on the west side of the Jemez Mountains beneath the 14 Ma to 40 ka lavas and tuffs of the Jemez Mountains volcanic field. Thickness changes in these conglomeratic units across major normal fault zones, particularly in the southwestern Jemez Mountains, suggest that the western margin of the Rio Grande rift was active in this area during Oligocene time. Furthermore, soft-sediment deformation and stratal thickening in the overlying Abiquiu Formation adjacent to the western boundary faults are indicative of syndepositional normal-fault activity during late Oligocene–early Miocene time. The primarily nonvolcaniclastic Oligocene conglomerate, which was derived from erosion of Proterozoic basement-cored Laramide highlands, is exposed in the northwestern Jemez Mountains, southern Tusas Mountains, and northern Sierra Nacimiento. This conglomerate, formerly called, in part, the lower member of the Abiquiu Formation, is herein assigned to the Ritito Conglomerate in the Jemez Mountains and Sierra Nacimiento. The clast content of the Ritito Conglomerate varies systematically from northeast to southwest, ranging from Proterozoic basement clasts with a few Cenozoic volcanic pebbles, to purely Proterozoic clasts, to a mix of Proterozoic basement and Paleozoic limestone clasts. Paleocurrent directions indicate flow mainly to the south. A stratigraphically equivalent volcaniclastic conglomerate is present along the Jemez fault zone in the southwestern Jemez Mountains. Here, thickness variations, paleocurrent indicators, and grain-size trends suggest north-directed flow, opposite that of the Ritito Conglomerate, implying the existence of a previously unrecognized Oligocene volcanic center buried beneath the northern Albuquerque Basin. We propose the name Gilman Conglomerate for this deposit. The distinct clast composition and restricted geographic nature of each conglomerate suggests the presence of two separate fluvial systems, one flowing south and the other flowing north, separated by a west-striking topographic barrier in the vicinity of Fenton Hill and the East Fork Jemez River in the western Jemez Mountains during Oligocene time. In contrast, the Upper Oligocene–Lower Miocene Abiquiu Formation overtopped this barrier and was deposited as far south as the southern Jemez Mountains. The Abiquiu Formation, which is derived mainly from the Latir volcanic field, commonly contains clasts of dacite lava and Amalia Tuff in the northern and southeastern Jemez Mountains, but conglomerates are rare in the southwestern Jemez Mountains.

We investigated a Plio-Pleistocene alluvial succession in the Albuquerque Basin of the Rio Grande rift in New Mexico using geomorphic, stratigraphic, sedimentologic, geochronologic, and magnetostratigraphic data. New 40 Ar/ 39 Ar age determinations and magnetic-polarity stratigraphy refine the ages of the synrift Santa Fe Group. The Pliocene Ceja Formation lies on the distal hanging-wall ramp across much of the Albuquerque Basin. The Ceja onlapped and buried a widespread, Upper Miocene erosional paleosurface by 3.0 Ma. Sediment accumulation rates in the Ceja Formation decreased after 3.0 Ma and the Ceja formed broad sheets of amalgamated channel deposits that prograded into the basin after ca. 2.6 Ma. Ceja deposition ceased shortly after 1.8 Ma, forming the Llano de Albuquerque surface. Deposition of the Sierra Ladrones Formation by the ancestral Rio Grande was focused near the eastern master fault system before piedmont deposits (Sierra Ladrones Formation) began prograding away from the border faults between 1.8 and 1.6 Ma. Widespread basin filling ceased when the Rio Grande began cutting its valley, shortly after 0.78 Ma. Although the Albuquerque Basin is tectonically active, the development of through-going drainage of the ancestral Rio Grande, burial of Miocene unconformities, and coarsening of upper Santa Fe Group synrift basin fill were likely driven by climatic changes. Valley incision was approximately coeval with increased northern- hemisphere climatic cyclicity and magnitude and was also likely related to climatic changes. Asynchronous progradation of coarse-grained, margin-sourced detritus may be a consequence of basin shape, where the basinward tilting of the hanging wall promoted extensive sediment bypass of coarse-grained, margin-sourced sediment across the basin.

ABSTRACT Aquifer heterogeneity at small scales (meters to tens of meters) can be characterized with hydrofacies. We investigate the feasibility of translating lithofacies into hydrofacies by testing the hypothesis that the permeability frequency distributions of different lithofacies are distinct. We mapped 11 lithofacies and performed more than 1800 in situ permeability measurements at an outcrop exposing poorly cemented, nonmarine, clastic sediment. The lithofacies represent both channel and interchannel deposits, are both ribbon-form and tabular, and vary in grain size from clay to sandy gravel. For each lithofacies permeability sample, we calculated variograms to define correlation lengths that were used to select spatially uncorrelated subsamples from each sample. The frequency distributions of permeability subsamples from the various lithofacies were compared using nonparametric statistical tests. The statistical tests generally support the claim that the lithofacies permeability distributions are distinct from one another.

Abstract Pre-Quaternary hillslope records provide a physical link between the source of sediment on hillslopes and the sedimentary sink of depositional basins. This guide complements a field excursion to localities in the Panaca and Table Mesa basins of southeast Nevada that are related to study of this unusual type of sedimentary deposit. Study of the sedimentology and stratigraphic context of this “ancient colluvium” provides new information on climate controls on sedimentation and landscape dynamics in this dry setting. Topics addressed herein include the stratigraphy, age, depositional environment, provenance, and paleontology of late-stage basin fill of the Muddy Creek and Panaca Formations; rock-type versus climate controls on sediment yield; and the landscape evolution of the region. Below we provide a background to the research problems addressed along with the general geologic setting and history, then follow with descriptions and research results associated with each of the two days of the field trip.

Abstract Volcanism provides a unique and locally abundant source of sediment to many of the earth's subaerial and subaqueous basins. In basin and tectonic analyses, volcanic sediments can be treated methodologically like nonvolcanic sediments, but the close association of tectonism and volcanism provides an added dimension to the analytical importance of volcaniclastic sediments. Volcanism occurs at plate margins and in some instances within plates ("hot spots"), and sediments within those environments have a strong, and in some cases, an exclusive component of volcaniclastic particles. The record of tectonism is commonly preserved only within fillings of sedimentary basins, and because volcanism is associated with activity at plate margins, knowledge about volcaniclastic sediments and rocks may be critical for interpreting plate activity and tectonic environments. Moreover, great volumes of clastic materials that rapidly flood sedimentary environments during active volcanism (e.g., 1980 eruption of Mount St. Helens, Lipman and Mullineaux, 1981) can result in disruption of river courses, rapid deposition along deltaic fronts, and therefore increase the number and volumes of turbidity currents in marine basins (Davies and others, 1979; Buesch, this volume; R. Smith, this volume), resulting in sedimentary responses not encountered in nonvolcanic areas where rates of sedimentation do not fluctuate so rapidly (Fig. 1). In the early 1960s the term volcaniclastic Fisher 1961 was quickly accepted into the general literature because the time was ripe for combining the fields of volcanology and sedimentology within a single system to describe sedimen tary facies affected or produced by volcanism The term volcaniclastic is

Abstract The geologic record shows possible connections between volcanism and climate change on various spatial and temporal scales, from documented seasonal and regional climatic effects of historic eruptions to the long-term impact of episodes of increased global volcanism/tectonism on atmospheric composition, climate, and life. The geologic record of volcanism, in the form of volcanic rocks, widespread ash layers, disseminated ash in sediments, and alteration products, provides information on the size, frequency, and location of past eruptions, the composition of the erupted magma, and amounts of climatically important volatiles released (CO 2 , SO 2 , HCl). These may be compared with climatic records from oxygen-isotope measurements, faunal/floral analyses, sea-level changes, and so forth. Recent eruptions and their climatic effects can be traced in proxy climate records such as polar ice cores, tree rings, lakes and bogs. Historic eruptions (up to about 50 km 3 of erupted magma, e.g., Tambora AD 1815) that produced widespread clouds of sulfuric acid aerosols are associated with small (<1°C) coolings on a hemispheric to global scale that last from 1 to 3 years after the eruptions. Coolings are accentuated at high latitudes. The climatic effects are generally proportional to the amount of sulfur-rich gases injected into the stratosphere, where they are converted into aerosols. Seasonal and regional effects, resulting from perturbations of atmospheric circulation patterns, may be considerably greater, for example, climate excursions during "the year without a summer" in AD 1816. Many prehistoric eruptions were much larger (>1,000 km 3 of magma, e.g., Toba 75 ka), released more dust and sulfur-rich aerosols (more than 10 3 megatons), and may have triggered significant global cooling. There is some evidence for correlation between climate cooling and enhanced volcanism on time scales from decades to 10 6 years, but problems in dating the episodes make it difficult to establish cause-and-effect relationships. Climate cooling for longer than the stratospheric residence time of the aerosols (a few years) requires that positive feedback mechanisms, such as snow/ice-albedo feedback, come into play, or that climatically significant eruptions in the past were more closely spaced in time than in recent centuries. On long time scales (10 7 –10 8 years), release of CO 2 through subduction-zone volcanism is a major control of climate. Intervals of rapid ocean-floor accretion (and subduction) correlate with times of increased volcanism, high sea levels and warm global climates. Accretion-rate data can be used in biogeochemical models to estimate paleoatmospheric CO 2 levels. Increased levels of atmospheric CO 2 and warm climate, so-called "Greenhouse Episodes", are predicted for those periods in the Phanerozoic showing evidence of enhanced volcanic/tectonic activity.

Abstract A remarkable variety of pyroclast types is produced in explosive volcanic eruptions, each reflecting the many factors controlling the eruption energy and eruption sequence, including viscosity, gas content, and phenocryst content of the magma. Fragmentation during a volcanic eruption and the resulting pyroclast characteristics can also be linked to external water affecting the eruption process. Volcanic ash characteristics can be used to infer the eruption type and emplacement processes.

Abstract Recent developments in sedimentology, diagenesis, and hydrocarbon exploration suggest that the recognition and interpretation of volcaniclastic sediments can significantly influence exploration methods, and the prediction of reservoir geometry and quality in volcaniclastic sequences. Volcaniclastic sediments are characterized by predictable changes in composition, texture, geometry and distribution, which can be used during both geologic and seismic interpretation. Interpretations based on volcaniclastic sediments help to better define the volcano-tectonic and paleogeographic setting that controls the deposition of associated siliciclastic and/or carbonate reservoirs. Volcaniclastics are also important to an understanding of the thermal history of a sedimentary basin and its deposits, and to evaluations of source-rock maturity and reservoir diagenesis. Seismic lithostratigraphic modeling and facies interpretations can be used to differentiate high-impedance volcaniclastic facies from associated siliciclastic deposits. In additon, laterally continuous "marker bed" pyroclastic-fall and large-volume ignimbrite deposits, characterized by continuous reflections, can be differentiated from discontinuous pyroclastic-flow deposits and lahars characterized by more discontinuous reflections. The characteristics of reservoirs in volcaniclastic sequences are controlled by the volcano-tectonic setting, eruptive mechanism, depositional system, composition, age, diagenesis, and thermal and burial history. Volcaniclastic lithofacies typically have predictable distribution patterns that control reservoir geometry. The reservoir quality of volcaniclastic sediments is controlled primarily by their diagenetic history, because volcaniclastics are composed largely of reactive and unstable minerals, with the potential for rapid and extensive changes during burial diagenesis. The abundance of unstable minerals commonly leads to the destruction of porosity by cemetation and compaction processes, but may also enhance porosity by grain dissolution. Successful efforts to find hydrocarbons in volcaniclastic deposits will depend on the coincidence of porosity preservation and generation processes, with the timing of hydrocarbon migration and entrapment.

Abstract Explosive volcanism at island arcs and convergent continental margins generates pyroclastic flows and surges that transport and deposit large quantities of pyroclastic material in areas adjacent to active volcanic centers. Pyroclastic flows are inferred to be relatively high-concentration, hot, gas-solid dispersions that flow in a nonturbulent or plug-flow manner. Their movement is typically controlled by the underlying topography, as reflected in the common valley-fill relationships of their deposits. Theoretical and experimental studies have confirmed the importance of partial fluidization in the transport of pyroclastic flows. En masse freezing is a common depositional mechanism for relatively low-energy pyroclastic flows that form high-aspect ratio ignimbrites. Highly energetic flows develop a variety of depositional regimes from various portions of the moving flow such as the head, body, and tail. The interaction of these depositional regimes with the topography over which a flow travels leads to marked contrasts in facies and stratigraphic relationships of the resulting deposits. Pyroclastic surges are low-concentration, gas-solid dispersions that flow in a turbulent fashion. As a result of their more expanded nature, surges are less influenced than flows by underlying topography and produce radially distributed deposits around volcanic centers. Surge deposits are commonly cross-stratified and form dunes, although they can also occur as massive or planar beds. Cross-stratified dune structures indicate that particle support is dominantly by turbulence of the interstitial gases and that grain concentration is relatively low. Particles are also moved along near the base of a surge by saltation and rolling in a traction layer, where high, lateral, shear velocities are developed. Deposition from surge clouds occurs when the velocity in the basal boundary layer or in the overlying turbulent eddies is less than that required to support grains of a certain size. Reduction in the surge velocity will cause material to be transferred from suspension transport to traction transport. The rate at which material is transferred between these regions of the surge cloud versus the rate at which material is deposited from the basal traction flow is critical in determining the facies of the resultant deposits. At low-particle concentrations, the beds should be dominated by cross-stratified units, whereas at higher particle concentrations, more massive types will form. A whole spectrum of facies relationships is likely to exist because of the great differences in the properties of the source materials at different volcanoes and the complicating effect of steam-to-water-phase transitions in "wet" versus "dry" types of surges.

Abstract Many volcano-hydrologic events produce high-discharge, sediment-laden flows originating on or near volcanoes, during or following eruptions. Water for some flows is derived from normal precipitation, commonly with enhanced runoff because of reduced infiltration into slopes mantled by pyroclastic debris and on which most vegetation has been destroyed. Flows triggered by eruptions may contain large volumes of water mobilized by eruption-induced snowmelt, discharge of crater lakes, or liquefaction of saturated debris-avalanche material. Sediment is available as unconsolidated pyroclastic or autoclastic material, principally of sand to fine-gravel size, that is thickest on the steep slopes of the volcano itself. Resulting flows generally possess the characteristics of debris flows or hyperconcentrated flows. Lahars have traditionally been defined as volcanic-debris flows and their deposits. Events and deposits called lahars, however, include a considerably wider range of flow phenomena. More recently, lahar has been applied as a general term for rapidly flowing sediment-laden mixtures of rock debris and water from a volcano. As such, a lahar may include one or more discrete flow processes. This definition emphasizes that volcano-hydrologic events are complex, encompassing a variety of rheological flow types and flow transformations. The complexity of lahars lies in the common occurrence of flow transformations, especially bulking and dilution. Initially dilute, high-discharge flows readily entrain loose material, particularly by streamed entrenchment, and may evolve into debris flows. Debris flows traveling within stream valleys are subject to dilution by mixing with water along the flow fronts, resulting in progressive downstream transformation into hyperconcentrated and dilute flows. Characterization of lahar-flow rheology is generally based on study of the resulting deposits. Relative to nonvolcanogenic-debris flows, those developed on volcanoes are typically mud poor and have much greater flow depths and discharges. In most cases, cohesion contributes little to sediment support, although the resulting deposits resemble those of nonvolcanogenic-debris flows. Hyperconcentrated flows, which are transitional between debris flows and more dilute stream flows, have been variously defined on the basis of sediment concentration, rheological properties, and depositional features. Lahars may be triggered by a variety of processes, including pyroclastic flows and debris avalanches. In some cases, it is possible to infer triggering mechanisms, as well as the extent of flow bulking or dilution, from study of lahar deposits.

Abstract At least 59 rain lahars have occurred on Mayon Volcano, on the Philippine island of Luzon, since its last eruption in 1984. In the southeastern sector of the volcano, where lahar activity has been greatest, we have evaluated the generation of these flows and their effects by measuring lahar-generating rainfall on the slopes during each main typhoon and rain season (September-December) since 1986 and by mapping the adjacent Mabinit and Matanag channels in detail since 1985. Sixteen debris flows occurred during the monitoring period. Each was triggered by a rainfall that lasted at least 1.4 hours, delivered a minimum of 40 mm of rain at an overall rate of 11 mm/h or more, and included at least one 10-min interval during which at least 10 mm fell. The empirical relationship between the threshold values of lahar-triggering rainfall duration (D) and intensity (I) is the power function I 27.3D -0.38 , a substantially higher threshold than one determined for debris flows world-wide. This higher threshold is due to the coarse, granular, and very porous volcaniclastic surface materials, which also render the role of antecedent rain insignificant in generating lahars. Comparable sediment-delivery systems are situated along the ESE (Basud) and SSE (Bonga) radii of the volcano. Each system is composed of a summit ravine with a fan of pyroclastic deposits at its base, and a pair of channels, one along each side of the fan. Only one of each channel pair continues to be a major lahar conduit. Basud Channel, the principal recipient of runoff and sediment from its ravine and fan of the Basud system, experienced the most frequent lahars in the first year after the eruption; however, its principal source of lahar sediment, an ash capping on Basud pyroclastic fan, was depleted very quickly, thus it has experienced debris flows only twice since 1986. Over the same period, 14 debris flows have occurred along Mabinit Channel because its delivery system has the largest catchment area in its sector and includes the largest ravine of the edifice. Bonga Ravine is deeply incised into a composite lava-tephra sequence; strata cropping out in the steep ravine sides avalanche frequently. Debris that collects along the axis of the ravine during the relatively dry months is mobilized into lahars by the first large storms of the typhoon season. Erosion and deposition by lahars keep the active portion of Mabinit Channel narrow and deep. The debris-flow phases of a lahar spread out upon entering a widening stretch of channel, and the thinner lateral portions stop and aggrade while the thicker centra) portions continue moving downchannel, leaving a narrowed channel. Waning-stage or subsequent hyperconcentrated and flood flows cut down through the new debris-flow deposits. A narrow, deep channel results, constricted between the vertical walls of new debris-flow terraces. The terraces also store material for subsequent lahars to erode and incorporate. A debris-flow fan at the end of Mabinit Channel, initially produced by unconfined debris flows during the 1984 eruption, has continued to evolve. The apex of the fan has twice been extended headward by avulsing debris flows, during Typhoons Saling in 1985 and Unsang in 1989. These events have added about 9 percent of area to the east side of the fan. When debris flows stop occurring along a channel such as Matanag Channel, it is quickly widened by laterally eroding floods and hyperconcentrated flows, which also deposit and thus aggrade the channel floor.

Abstract In New Zealand, volcanic-debris avalanches have originated mostly in water-rich source volcanoes. Most wet-debris avalanches produced three distinctive mappable lithofacies that occupy adjacent areas. The axial-A lithofacies is dominated by brecciated and stratified megaclasts that form a self-supporting framework. Relict stratigraphy from the source area is preserved locally both within and among megaclasts. Interclast matrix in this lithofacies is silty sand, and the associated surface of the deposit is characterized by closely spaced, large mounds (hummocks). The axial-B lithofacies, in contrast, is interclast-matrix rich, so that at many exposures brecciated clasts and megaclasts are surrounded by clasts and an interclast matrix of silty sand. Mounds on the surface of this lithofacies are lower and more widely spaced than on the surface of axial-A. The marginal lithofacies is distinguished by an allophane-rich, muddy or silty interclast matrix and a relatively planar upper surface. Clasts are derived from disaggregation of brecciated megaclasts and sediment eroded from underlying deposits. The lithofacies represent division of the debris-avalanche deposits into megaclast-rich (axial-A) and interclast matrix-rich (axial-B) parts and transformation of the avalanches into lahars (marginal lithofacies). Megaclasts preserving original volcanic stratigraphy in axial-A represent collapse and transport of coherent pieces of the edifice by the debris avalanches. Transformation into lahars occurred as the coarse avalanche debris came to rest and the finer portion continued to flow, enhanced by a secondary component eroded from underlying deposits. In general, flow across relatively featureless ring plains allowed multidirectional-lithofacies development, so that lobes of axial-A are surrounded by axial-B and marginal lithofacies both laterally and, where visible, distally. Variations in avalanche size and ring-plain physiography produced different lithofacies architectures in the New Zealand deposits. Large debris avalanches were little affected by physiography and spread out in fan-shaped sheets comprising near-concentric zones of each lithofacies. Smaller avalanches were confined at some point during flow, so that central parts of the deposit are shoestring diamictons of axial-B and the marginal lithofacies. The tripartite axial-A through axial Β and marginal lithofacies associations are present in proximal areas of these deposits. Distal areas are usually axial-B and marginal lithofacies.

Abstract Volcanic-debris-avalanche and related deposits form important parts of the sedimentary record in volcanic regions. A rigorously defined terminology and various sedimentary elements are used to describe the deposits. Particles are clasts (hard rocks) and debris-avalanche blocks (soft pieces of debris). Facies, which may be observed on the map scale or exposure scale, are block facies, consisting of debris-avalanche blocks, and mixed facies, a mixture of rock types. The hummocks of debris-avalanche deposits are classified into three types: type A, block facies hummocks with no mixed facies; type B, predominantly mixed facies hummocks; and type C., hummocks composed of debris-avalanche blocks resting in mixed facies. Fractures are common in the block facies on both macroscopic and microscopic scales. The fractures form from propagation of waves of compression and rarefaction that develop as the mass moves over rough topography during the initial sliding phase of the events. Lahars (volcanic mudflows and debris flows) are commonly associated with volcanic-debris avalanches. Lahars may form from the debris avalanche during emplacement or minutes to hours after emplacement, or from breakout of debris-avalanche-dammed lakes hours to years after emplacement.

Abstract Studies of the response of fluvial systems to volcanic eruptions generating voluminous pyroclastic material point to two fundamental conditions of landscape and sedimentation. The syneruption period is characterized by geologically instantaneous production of large volumes of volcaniclastic sediment, combined with enhanced and more variable runoff leading to sedimentation principally by high-sediment-load flood and debris-flow processes. The periods of high-volume syneruption sedimentation are short and separated by relatively longer inter-eruption periods when volcanism has little or no impact on the fluvial systems. During the inter-eruption periods, sediment delivery is greatly diminished and normal streamflow processes are dominant. Incision of deposits aggraded during the syneruption period is typical. The syneruption and inter-eruption deposits can often be distinguished on the basis of sediment composition (syneruption deposits show less lithologic diversity), sediment grain size (syneruption deposits tend to be rich in sand-size pyroclasts, whereas gravel-bedload facies are more abundant in inter-eruption deposits), and the distinctive large lateral extent of volcanism-related debris-flow and flood deposits. On broad alluvial-plain aprons (syn. ring plains) associated with continental-margin arcs, distinctive facies geometries exist. Syneruption deposits are laterally extensive sheets, whereas inter-eruption facies, if present, are typically much thinner and confined to valley systems incised into syneruption sheets. Aggradation of syneruption deposits is driven by volcanism. Aggradation of inter-eruption deposits is independent of volcanism and requires other allogenic driving forces, of which basin subsidence is probably most effective, to induce deposition of preservable thicknesses of material. The facies geometry, therefore, is variable depending on the extent to which sedimentation is in response to extrabasinal volcanism or intrabasinal subsidence.

Abstract At the end of the 1.8-ka eruption from Taupo Volcanic Center, New Zealand, the emplacement of a moderately voluminous, unwelded and highly pumiceous ignimbrite destroyed vegetation and drainage networks over ≈20,000 km 2 around the vent in the Lake Taupo basin. A recent study of derived sediments and landforms in an area on the southeast shore of the lake has revealed the style and magnitude of the landscape response to this cataclysmic event. In upland areas away from Lake Taupo the response took two forms. Where the ignimbrite overlies impermeable hardrock, distinctive landforms and a wide range of lithofacies indicate that debris flows, hyperconcentrated flows, flood-prone streams, temporary lakes, and braided and meandering rivers were common and generally occurred in the order given. Where the impermeable rock is absent, however, the range of deposits is limited to those from the first three processes. The differing associations suggest that where the hard rock formed a barrier to the downward movement of ground water, the large volumes of runoff moving through the devegetated landscape continued to flow at the land surface, facilitating the establishment of a wide range of depositional environments, until the re-establishment of a full vegetation cover some 15 to 20 years after the eruption reduced runoff by ≈30 percent. Where the ground water base was unimpeded in its downward movement through the relatively permeable ignimbrite, however, surface flow decreased substantially within a few years of the eruption. Near Lake Taupo, valleys were carved into the ignimbrite by rivers and streams whose waters refilled the lake basin, which had been nearly emptied by the eruption, at a rate of ≈10 m yr -1 . Blockage of the only outlet caused the lake to rise 34 m higher than its present level. Transgressive sequences of beach and nearshore sediments with some debris-flow deposits are now exposed on the resulting lake terrace and within those early valleys, some of which were partially re-excavated as the lake-level fell. The general lack of regressive sequences and a greatly reduced drainage density on the caldera-lake terrace and in upland areas away from the lake indicate both a decrease in sediment supply and runoff and a lake-level fall somewhat greater than 10 m yr -1 .

Abstract Emplacement of the widespread 18.5-Ma Peach Springs Tuff (PST) ignimbrite over a region of varied topography in southeastern California, western Arizona, and southern Nevada, caused adjustments in local drainage and depositional systems. Deposition of the post-PST sediment occurred shortly after, and probably in response to, the emplacement of the PST, where the sediment was deposited on nonwelded tuff or co-ignimbrite fallout tuff, and rills or channels were not eroded into the tuff. Only 12 of 32 exposures show a lack of erosion. The remaining exposures record a period of erosion with post-PST sediment deposited on partially to densely welded or vapor-phase-crystallized PST. At all 32 locations, pre- and post-PST sedimentary sections within 20 m stratigraphically of the PST are interpreted as representing alluvial-fan, braided trunk stream, and lacustrine-to-playa depositional environments. Pre- to post-PST sedimentary rocks do not change in a 1- to 2-km-wide, 500-m-relief valley; thus, local relief was more influential in controlling sedimentation than change in stream gradients caused by deposition of 30 m of PST. In and along the margins of 5- to 25-km-wide valleys, finer grained lithofacies were deposited in post-PST compared to pre-PST rocks. Locally, changes of nearly 90° occur in the flow direction from pre-PST alluvial-fan deposits to post-PST braided-trunk-stream deposits. At 21 of 32 locations, a fining- and thinning-upward sequence occurs between the pre- and post-PST section. The pre- to post-PST change in sedimentation is not simply a result of the time needed for alluvial fans to prograde over the tuff, but is a function of lowered stream gradients caused by deposition of 60 to 200 m of PST in broad valleys. Welding and vapor-phase crystallization in the PST formed erosionally resistant rock that slowed incision and promoted lateral cutting of streams into nonwelded tuff.

Abstract Repeated small eruptions of basaltic to intermediate magma produce monogenetic volcanic fields, which are common worldwide. The variety of volcanic landforms produced in such fields is controlled by whether or not water is present at or near the surface at the time of eruption. Basinal settings are typically "wetter," and hydrovolcanic eruptions produce tuff cones, maars and tuff rings instead of the scoria cone fields developed in dry, upland settings. Hydrovolcanic fields produce less lava and relatively more clastic debris than scoria cone fields and often contain large craters excavated by maar eruptions. Tephra eroded from volcanoes within hydrovolcanic fields tends to be reworked and redistributed within the basin in sheetform deposits and as crater-filling sequences, whereas the tephra eroded from scoria cone fields is largely carried from the fields in deeply incised streams skirting the fields' lava flows. The stratigraphic records of both groups of monogenetic fields differ from that of large, long-lived polygenetic volcanoes: the record of monogenetic volcanism is chiefly the result of long periods of inter-eruption erosion and sedimentation, whereas that of larger volcanoes is typically dominated by rapidly deposited units produced in immediate response to large eruptions.

Abstract A 7-km-thick basaltic sequence composed of 50 percent flows and 50 percent intercalated volcaniclastic rocks records emergence above sea level of a submarine composite volcano (Amisk Lake Volcano), partial resubmergence, and eventual re-emergence. Both emergence and resubmergence are marked by crudely bedded surf-zone or shore-facies deposits up to 300 m thick, comprising pillowed flows and palagonitized tuff, lapilli tuff, and tuff breccia produced by a combination of quenching as subaerial flows entered the sea and by littoral explosions. Subaerial units form the upper 900 m and up to 1 km of the lower part of the sequence. They are dominantly surge-and-fall tuff and fine lapilli tuff composed of non-amygdaloidal vitric granules and lesser amygdaloidal, in part scoriaceous, vitric ash, lithic basalt, and clinopyroxene and plagioclase crystals; accretionary and armored lapilli and tuff vesicles are common. Subaqueous units contain up to 40 percent intercalated fall and reworked tuff and fine lapilli tuff composed of particles similar to those in subaerial units; some deposits are turbidites. Both subaerial and subaqueous tuff and lapilli tuff were produced largely by long-lived or repeated subaerial phreatomagmatic explosive eruptions generated by rise of basaltic magma through the water-saturated coastal plain of a rapidly subsiding volcanic island; the degree of water-magma interaction was variable. The volume of explosively produced ash is orders of magnitude larger than that in modem volcanoes and reflects the rapid subsidence and consequent low-gradient volcano slopes.