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The qualitative resemblance between terrestrial and submarine branched valley networks has led to speculation that common underlying processes control their formation. However, quantitative comparisons have been impeded by methodological limitations and coarse resolution in marine systems. We analyze channel concavity and steepness indices of 23 terrestrial and 29 submarine catchments to determine whether their profile morphologies are distinct. Statistical comparisons of these quantities demonstrate that concavity indices in submarine systems are, in general, lower than in subaerial systems, and that submarine tributaries are steeper than their associated mainstem. These differences may reflect distinct drainage formation mechanisms and dynamics of submarine sediment gravity flows as compared to overland flow processes.

The presence of abundant phyllosilicate minerals in Noachian (>3.7 Ga) rocks on Mars has been taken as evidence that liquid water was stable at or near the surface early in martian history. This study investigates some of these clay-rich strata exposed in crater rim and inverted terrain settings in the Mawrth Vallis region of Mars. In Muara crater the 200-m-thick, clay-rich Mawrth Vallis Group (MVG) is subdivided into five informal units numbered 1 (base) to 5 (top). Unit 1 consists of interbedded sedimentary and volcanic or volcaniclastic units showing weak Fe/Mg-smectite alteration deposited in a range of subaerial depositional settings. Above a major unconformity eroded on Unit 1, the dark-toned sediments of Unit 2 and lower Unit 3 are inferred to represent mainly wind-blown sand. These are widely interlayered with and draped by thin layers of light-toned sediment representing fine suspended-load aeolian silt and clay. These sediments show extensive Fe/Mg-smectite alteration, probably reflecting subaerial weathering. Upper Unit 3 and units 4 and 5 are composed of well-layered, fine-grained sediment dominated by Al-phyllosilicates, kaolinite, and hydrated silica. Deposition occurred in a large lake or arm of a martian sea. In the inverted terrain 100 km to the NE, Unit 4 shows very young slope failures suggesting that the clay-rich sediments today retain a significant component of water ice. The MVG provides evidence for the presence of large, persistent standing bodies of water on early Mars as well as a complex association of flanking shoreline, alluvial, and aeolian systems. Some of the clays, especially the Fe/Mg smectites in upper units 1 and 2 appear to have formed through subaerial weathering whereas the aluminosilicates, kaolinite, and hydrated silica of units 3, 4, and 5 formed mainly through alteration of fine sediment in subaqueous environments.

The 2011–2014 removal of two large dams on the Elwha River, Washington State, the largest dam removal yet completed globally, created extensive cutbank exposures of reservoir sediments, allowing the first characterization of the facies architecture of sediments through direct observation in reservoirs worldwide and providing an unparalleled opportunity to assess the relationship between environmental influences, such as changes in sediment supply, and their expression in the stratigraphic record. Using a combination of facies description from observation of 49 measured sections and >100 exposures and analysis of digital elevation models and historic aerial photographs, we delineated the characteristic depositional zones of each reservoir and mapped the evolution of the subaerial delta over the life span of the reservoir. Former Lake Mills, the younger, upstream reservoir, was characterized by a tripartite, subaerial Gilbert-style delta that prograded >1 km into the main reservoir from 1927 to 2011. Sediments were composed of coarse-grained topset beds, steeply dipping foreset beds, and a fine-grained, gently dipping prodelta. While individual event horizons were discernible in fine-grained sediments of former Lake Mills, their number and spacing did not correspond to known drawdown or flood events. Former Lake Aldwell, impounded from 1913 to 2011, was initially defined by the rapid progradation of a Gilbert-style, subaerial delta prior to the upstream completion of Glines Canyon Dam. However, the 1927 closure of Glines Canyon Dam upstream caused the delta to evolve to a fine-grained, mouth-bar–type delta indicative of low, finer-grained sediment. This evolution, combined with a previously unrecognized landslide deposit into the upper delta plain, suggests that understanding the exogenic influences on reservoir sedimentation is critical to interpretation and prediction of the sedimentation within individual systems.

Turbidity currents are the principal processes responsible for carving submarine canyons and maintaining them over geological time scales. The turbidity currents that maintain or “flush” submarine canyons are some of the most voluminous sediment transport events on Earth. Long-term controls on the frequency and triggers of canyon-flushing events are poorly understood in most canyon systems due to a paucity of long sedimentary records. Here, we analyzed a 160-m-long Ocean Drilling Program (ODP) core to determine the recurrence intervals of canyon-flushing events in the Nazaré Canyon over the last 1.8 m.y. We then investigated the role of global eustatic sea level in controlling the frequency and magnitude of these canyon-flushing events. Canyon-flushing turbidity currents that reach the Iberian Abyssal Plain had an average recurrence interval of 2770 yr over the last 1.8 m.y. Previous research has documented no effect of global eustatic sea level on the recurrence rate of canyon flushing. However, we find that sharp changes in global eustatic sea level during the mid-Pleistocene transition (1.2–0.9 Ma) were associated with more frequent canyon-flushing events. The change into high-amplitude, long-periodicity sea-level variability during the mid-Pleistocene transition may have remobilized large volumes of shelf sediment via subaerial weathering, and temporarily increased the frequency and magnitude of canyon-flushing turbidity currents. Turbidite recurrence intervals in the Iberian Abyssal Plain have a lognormal distribution, which is fundamentally different from the exponential distribution of recurrence intervals observed in other basin turbidite records. The lognormal distribution of turbidite recurrence intervals seen in the Iberian Abyssal Plain is demonstrated to result from the variable runout distance of turbidity currents, such that distal records are less complete, with possible influence from diverse sources or triggering mechanisms. The changing form of turbidite recurrence intervals at different locations down the depositional system is important because it ultimately determines the probability of turbidity current–related geohazards.

Frequent high-resolution measurements of topography at active vol­canoes can provide important information for assessing the distribution and rate of emplacement of volcanic deposits and their influence on hazard. At dome-building volcanoes, monitoring techniques such as LiDAR and photogrammetry often provide a limited view of the area affected by the eruption. Here, we show the ability of satellite radar observations to image the lava dome and pyroclastic density current deposits that resulted from 15 years of eruptive activity at Soufrière Hills Volcano, Montserrat, from 1995 to 2010. We present the first geodetic measurements of the complete subaerial deposition field on Montserrat, including the lava dome. Synthetic aperture radar observations from the Advanced Land Observation Satellite (ALOS) and TanDEM-X mission are used to map the distribution and magnitude of elevation changes. We estimate a net dense-rock equivalent volume increase of 108 ± 15M m3 of the lava dome and 300 ± 220M m3 of talus and subaerial pyroclastic density current deposits. We also show variations in deposit distribution during different phases of the eruption, with greatest on-land deposition to the south and west, from 1995 to 2005, and the thickest deposits to the west and north after 2005. We conclude by assessing the potential of using radar-derived topographic measurements as a tool for monitoring and hazard assessment during eruptions at dome-building volcanoes.

Early Pleistocene synrift deltas developed along the southern Corinth rift margin were deposited in a single, dominantly lacustrine depocenter and were subject to the same climate-related base-level and sediment supply cyclicity. Two synrift deltas, just 50 km apart, show markedly different sequence geometry and evolution related to their location along the evolving border fault. In the west, strongly aggradational fan deltas (>600 m thick; 2–4 km radius) deposited in the immediate hanging wall of the active border fault comprise stacked 30–100 m thick stratal units bounded by flooding surfaces. Each unit evolves from aggradational to progradational with no evidence for abrupt subaerial exposure or fluvial incision. In contrast, in the central rift, the border fault propagated upward into an already deep lacustrine environment, locating rift-margin deltas 15 km into the footwall. The deltas here have a radius of >9 km and comprise northward downstepping and offlapping units, 50–200 m thick, that unconformably overlie older synrift sediments and are themselves incised. The key factors driving the marked variation in sequence stratigraphic architecture are: (1) differential uplift and subsidence related to position with respect to the border fault system, and (2) inherited topography that influenced shoreline position and offshore bathymetry. Our work illustrates that stratal units and their bounding surfaces may have only local (

The evolution of roughness as a function of surface age was used to quantify weathering rates on rocky desert surfaces. Surface topography on eight late Quaternary alluvial terraces, which record the weathering of Holocene (5 ± 1 ka) boulder-strewn deposits into mature (87 ± 2 ka) desert pavements in the Negev desert of Israel, was measured with ground-based lidar. Roughness on each terrace was characterized with power spectral density (PSD) analysis, and changes in PSD as a function of length scale (λ ∼ 0.04–1.50 m) and surface age were used to estimate diminution/weathering rates of the surface rocks. We found PSD values that systematically increase as a power-law function of λ (roughness exponent of ∼2.0) and decrease as an inverse power-law function of surface age. This PSD evolution indicates a fragmentation rock weathering process driven by salt shattering throughout the 87 k.y. period examined. PSD analysis of the lidar data also revealed weathering rates that increase with rock size and decrease as an inverse power-law function of time, from initial values >20 mm/k.y. to

We generate a detailed ~30 Myr record of pCO2 spanning most of the Late Triassic (Carnian-Norian-Rhaetian) to earliest Jurassic (Hettangian), based on stable carbon isotope ratios of soil carbonate and preserved organic matter from paleosols in the eastern North American Newark rift basin. Atmospheric pCO2 was near 4500 ppm in the late Carnian, decreasing to below ~2000 ppm by the late Rhaetian just before the earliest Jurassic eruption of the Central Atlantic Magmatic Province, which triggered measurable pulses of CO2 outgassing. These data are consistent with published modeling results using the GEOCLIM model, which predict a decrease in pCO2 over the Late Triassic as a result of the progressive increase in continental area subject to the intense weathering regime of the tropical humid belt due to Pangea’s northward motion. The fi ner-scale pCO2 changes we observe may be dependent on the lithology introduced to the tropics, such as the dip to ~2000 ppm around 212 Ma and its rebound to ~4000 ppm at 209 Ma, which can be accomplished by introducing a more weatherable subaerial basaltic terrain. These observations indicate that the consumption of CO2 by continental silicate weathering can force long-term changes in pCO2 comparable to those driven by presumed changes in mantle degassing.

The Basal Complex of La Gomera (Canary Islands) has been analyzed with the aim of deciphering its structural evolution, spanning the middle to late Miocene. Detailed structural maps and cross sections, and the results of a systematic measurement of dike orientation and fault-slip data are presented in this work. The main structural features found in the Basal Complex of La Gomera are four dike sets and two large, normal faults (Guillama and Montana de Alcala faults). Several extensional episodes have been identified. The main deformation phase corresponds to a NNW-SSE– to NW-SE–directed extension that generated first a dense swarm of mafic dikes, and then a large-scale collapse of the Basal Complex with displacement along the Guillama and Montana de Alcala faults and associated rotation of large blocks of La Gomera basement. It is interpreted that the supposed staircase geometry of the Guillama fault is responsible for the observed arrangement of rotated dikes due to development of kilometric-scale fault-bend folds with rounded hinges. Deformation of the Basal Complex shows a complete geometric, kinematic, and chronologic consistency with the large volcanic flank collapses that affected the units belonging to the early growth stages of La Gomera subaerial shield volcano. The results of this work support the models that invoke the importance of the large-scale geodynamic setting on volcano destabilization. More attention should be paid to the structural and geophysical characteristics of volcano basements in order to better evaluate the danger of large, catastrophic volcanic landslide events.

The >800-m-thick, Oligocene Ohanapecosh Formation records voluminous sedimentation of volcanic clasts in the Ancestral Cascades arc (Washington State, USA). Most volcaniclastic beds are dominated by angular pumice clasts and fiamme of andesitic composition, now entirely devitrified and altered. All beds are laterally continuous and have uniform thickness; fine sandstone and mudstone beds have features typical of deposits from low-density turbidity currents and suspension settling. Erosion surfaces, cross-beds, and evidence of bi-directional oscillatory currents (i.e., wave ripples and swaley and hummocky cross-stratification) are almost entirely absent. We infer that the setting was subaqueous and below wave base.The abundance of angular pumice clasts, crystals and dense volcanic clasts, and the extreme thickness of several facies, suggest they were derived from magmatic volatile-driven explosive eruptions. The extremely thick beds are ungraded or weakly graded, and lack evidence of hot emplacement, suggesting deposition from subaqueous, water-supported, high-concentration volcaniclastic density currents. Some of the thickest beds contain coarse, rounded, dense clasts at their base and are interbedded with accretionary lapilli–bearing mudstone; these beds are interpreted to be deposits from subaqueous density currents fed by subaerial pyroclastic flows that crossed the shoreline. Shallow basaltic intrusions and mafic volcanic breccia composed of scoria lapilli indicate the presence of intra-basinal scoria cones that may have been partly subaerial.The range in facies in the Ohanapecosh Formation is typical of below-wave-base, continental (lacustrine) basins that form in proximity to active volcanic arcs, and includes eruption-fed and resedimented facies. Extreme instantaneous aggradation rates are related directly to explosive eruptions, and sediment pathways reflect the locations of active volcanoes, in contrast to conventional sedimentation processes acting in non-volcanic environments.

Modern and preserved coral reefs on islands exhibit a broad range of forms, from actively accreting fringing and barrier reefs to terraces preserved by drowning or subaerial exposure. Darwin’s canonical model of reef development proposes an evolutionary sequence of reef forms as a volcanic island ages and subsides, from fringing reef to lagoon-bounding barrier reef to atoll. Compiled data from modern systems show, however, that many islands do not follow this sequence, implying that reefs are shaped by more than island subsidence alone. We show that the diversity of modern reef morphology arises from the combined effects of island subsidence, coral growth, and glacial sea-level cycles. A model for the evolution of a reef elevation profile over the past 400 k.y. reveals that different combinations of reef accretion rate and island vertical motion produce a variety of forms that matches the observed distribution of modern reefs. This match occurs only if the model is driven by Pleistocene sea-level oscillations—few modern environments have the right combination of conditions to produce the Darwinian atoll progression.

Alteration minerals, assemblages, and textures were studied in a 175-m-thick volcanic sequence found between 759.32 and 584.19 m below seafloor within the 1285-m-long ANDRILL (Antarctic Geological Drilling project) McMurdo Ice Shelf core (MIS AND-1B). Three main alteration zones were identified through the application of different analytical methods (optical and scanning electron microscopy, electron microprobe, and X-ray diffraction). Alteration zoning is guided by the texture of the volcanic deposits, which is in turn determined by the eruptive style, transport mechanisms, and paleodepositional conditions. In particular, alteration reflects the evolution of paleodepositional conditions from submarine or shallow water to subaerial due to the growth of a nearby volcanic edifice. The general alteration trend is also influenced by the contribution of volcanogenic sediments derived from the reworking of silica-rich pyroclasts from earlier volcanic activity.

The Qaidam Basin is an internally drained basin located in the northeastern Tibetan Plateau. Presently, over 50% of the basin floor exposes thick (>1 km) sections of Pliocene–Quaternary strata that are deformed by folding and faulting. We investigated this nearly continuous Pliocene–Quaternary sedimentary record for the effects of global climate change and deformation on basin sedimentation. New detailed stratigraphic, magnetostratigraphic, and stable isotope (δ 18 O, δ 13 C) data from the Pliocene Shizigou and Pleistocene Qigequan Formations along the southwestern flank of an intrabasin fold within the north-central Qaidam Basin are presented here. Strata reveal climatically controlled, meter-scale parasequences within shallow-lacustrine, marginal-lacustrine, and deltaic lithofacies. Paleocurrents shift from eastward at the base to southwestward for the majority of the section, but they abruptly shift toward the south-southeast in the upper 400 m. Twenty-two magnetozones constrain deposition between 5.2 Ma and ca. 0.8 Ma and reveal that sedimentation rates were fairly constant (474 ± 34 m/m.y.) from 5.2 to 3.0 Ma, after which time rates abruptly decreased to 154 ± 40 mm/yr before increasing again to ∼750 m/m.y. since 1.2 Ma. The δ 18 O values shift from relatively constant values (avg. −6.8‰, range −9.6‰ to −4.5‰ relative to Vienna Peedee belemnite [VPDB]) to less negative values (avg. -1.2‰, range -1.2‰ to -2.7‰ VPDB) between 3.1 and 2.6 Ma and to extremely variable values (avg. –2.9‰, range −8.3‰ to 4.0‰) after ca. 2.6 Ma. The post–2.6 Ma extreme variability in stable isotopes reflects the same timeframe of enhanced climatic cyclicity associated with Northern Hemisphere glaciation. The δ 13 C values remain relatively constant (average −4.0‰, range −5.7‰ to -1.0‰) until ca. 0.9 Ma, when the values increase to -0.3‰ (range -1.0‰ to 1.5‰) VPDB. The appearance of growth strata at 3.0 Ma, shallow-water, evaporite deposition after 2.6 Ma, and the observation of paleoyardangs (buried, wind-sculpted landforms) within lake-marginal strata at 2.4 Ma imply that emergence of the adjacent anticline was followed by the shallowing and partitioning of the lake basin and subaerial exposure and erosion of marginal lake sediments. These data reflect a significant change to a more arid climate in the Qaidam Basin between ca. 3.1 Ma and 2.6 Ma, overlapping with the onset of significant Northern Hemisphere glaciations and basin-floor deformation. Lake-level cycles were on ∼230,000 yr frequency from 2.6 Ma to 1.2 Ma, before increasing to ∼12,000 yr frequency, suggesting increased aridity and broad, subaerial exposure of lake sediments after the Pliocene-Quaternary transition. Deformation of intrabasin sediments by at least 3.0 Ma caused uplift of the basin floor, which provided an environment rich in friable material for wind deflation of the Qaidam Basin, a likely source for sediments on the Chinese Loess Plateau and nutrients for the Pacific Ocean carried by westerly winds. By 2.6 Ma, deformation of the Qaidam Basin created closed depressions that facilitated evaporite sedimentation that continues today. Coeval intrabasin deformation, combined with increasing aridity after 3.1 Ma, thus controlled both deposition and erosion within the region.

U-Th and (U-Th)/He zircon geochronology redefines the timing of volcanic activity in the Salton Trough (Southern California, USA), the subaerial extension of the incipiently oceanic Gulf of California. U-series disequilibrium corrected (U-Th)/He zircon analyses for a granophyre ejecta clast from the Red Island rhyolite dome indicate an eruption age of 2480 ± 470 a (calendric dates between 0 and 940 Before Common Era, BCE; error at 95% confidence). This eruption age is supported by U-Th zircon crystallization ages for two obsidian-bearing lavas: Red Island (the host for the granophyre) and Obsidian Butte, a prehistoric quarry for obsidian that is widely distributed in southern California and northern Mexico archaeological sites. Lavas and granophyre display overlapping zircon crystallization age distributions that support field and compositional evidence that they are cogenetic and contemporaneous. The (U-Th)/He eruption age is younger and significantly more precise than previous ages for these volcanoes, and is the first indication that the eruption of obsidian flows coincided with human presence in the region. A late prehistoric eruption age agrees with the absence of the Obsidian Butte lithic source among early prehistoric cultural artifacts, previously attributed to submergence of the quarry location during hypothesized persistent flooding by ancient Lake Cahuilla.

This article uses measurements from a three-dimensional exposure of a sinuous submarine channel and its fill to document, for the first time, how flow properties and sedimentation differ between erosional and filling stages. Two units, recording this sequential evolution, are documented. Unit 1 records gravity flows that deepened and laterally migrated the channel, resulting in the deposition of point bars, with coarsening-upward profiles, on the inner part of channel bends. These gravity flows were mud-rich with a wide grain-size distribution. Flow heights exceeded the depth of the channel resulting in the deposition of levees. Strong secondary flow is evident with a helical pattern reversed to their subaerial counterparts. Strata in point bars and levees are inclined and deposited primarily by tractive processes. Unit 2 records gravity flows that filled the channel. These gravity flows were sand-rich with a relatively narrow grain-size distribution. Flow heights scaled to the depth of the channel, and they contain no evidence for secondary flow. Associated strata are horizontal and deposited primarily from suspension.

The global CO 2 discharge from subaerial volcanism has been estimated at ~300 Mt yr –1 . However, estimates of CO 2 emissions from volcanic lakes have not been considered. In order to improve this information, extensive research on CO 2 emissions of volcanic lakes worldwide has been performed. The observed normalized average CO 2 emission rates increase from alkaline (5.5 t km –2 d –1 ), to neutral (201.2 t km –2 d –1 ), to acid (614.2 t km –2 d –1 ) in volcanic lakes. Taking into account (1) normalized CO 2 emission rates, (2) the number of volcanic lakes in the world (~769), and (3) the fraction and average areas of the investigated alkaline, neutral, and acid volcanic lakes, the estimated global CO 2 emission from volcanic lakes is 117 ± 19 Mt yr –1 , with 94 ± 17 Mt yr –1 as deep-seated CO 2 . This study highlights the importance of a revision of the actual global CO 2 discharge from subaerial volcanism.

Explosive volcanism results in a wide range of volcaniclastic deposits in many of Earth9s subaerial and subaqueous environments. In this paper, we introduce a unique, shallow-marine volcaniclastic deposit from Jeju Island, Korea, for which the materials were transported to the water surface by pyroclastic clouds and then settled from the surface as they were entrained in the water. The deposition occurred under alternating currents and still waters, which is most plausibly attributed to tidal processes. Mud flasers or drapes intercalated in the deposit, which indicate periods of slack water during tidal cycles, suggest that the deposit accumulated in a very short period of a fortnight or a month, about a million times faster than the adjacent sedimentary strata. Because of the unusually high sedimentation rate, the volcaniclastic deposit could record the “usual” fair-weather processes in the depositional site at a resolution that is almost never provided by ordinary sedimentary deposits. This finding highlights the biases in Earth9s stratigraphic records and teaches us that volcanic deposits, commonly regarded as the products of catastrophic events, can in some cases record more faithfully the ordinary and usual processes that nonvolcanic deposits cannot.

Isostatic uplift of tectonically stable, passive margin lithosphere can preserve a record of paleo-shoreline position by elevating coastal geomorphic features above the influence of nearshore wave activity. Conversely, depositional ages and modern elevations of these features can provide valuable information about the uplift history of a region. We present a numerical model that combines sea-level oscillation, subaerial exposure, a precipitation-karstification function, and isostatic uplift to explore the dynamic geomorphic behavior of coastal carbonate landscapes over multiple sea-level cycles. The model is used to estimate ages of coastal highstand depositional features along the Atlantic coast of north Florida. Numerical simulations using current best estimates for Pleistocene sea-level and precipitation histories suggest ages for Trail Ridge (1.44 Ma), the Penholoway Terrace (408 ka), and the Talbot terrace (120 ka) that are in agreement with fossil evidence. In addition, model results indicate that the rate of karstification (void space creation or equivalent surface lowering rate) within the north Florida platform is ∼3.5 times that of previous estimates (1 m/11.2 k.y. vs. 1 m/38 k.y.), and uplift rate is ∼2 times as high as previously thought (0.047 mm/yr vs. 0.024 mm/yr). This process has implications for landscape evolution in other carbonate settings and may play an underappreciated role within the global carbon cycle.

The Phanerozoic9s longest-lived and most widespread glaciation, the late Paleozoic ice age, is undergoing a resurgence in interest. Long-held models of the timing, duration, and magnitude of glaciation are being reevaluated due to emerging evidence from former high latitudes, evidence that the late Paleozoic ice age was punctuated by long-lived glacial minima or possibly ice-free times. The history of the late Paleozoic ice age is archived within the biostratigraphically well-constrained, carbonate-dominated succession of Arrow Canyon, Nevada, United States. In this paleo-tropical succession, the distribution of lithofacies, flooding surfaces, and subaerial exposure horizons and their stacking into meter-scale cycles record a detailed climate history. The onset of this phase of glaciation during the middle Mississippian was followed by a dynamic evolution of glacioeustasy through the late Mississippian to late Pennsylvanian. Moderate- to high-amplitude glacioeustasy was likely interrupted by an earliest Pennsylvanian short-lived glacial minimum, but otherwise appears to have persisted through the middle Pennsylvanian. Upper Pennsylvanian strata record low- to moderate-amplitude relative sea-level changes, suggesting a long-lived interval of diminished ice volume. This proposed glacial minimum is coincident with a notable minimum in glaciogenic sedimentation near the former southern pole, aridification across paleo-tropical Pangea, and significant floral and faunal turnover, suggesting a link between tropical environmental change and high-latitude glaciation. These conclusions, however, are at odds with those traditionally inferred from Euramerican cyclothems, i.e., persistent high-amplitude glacioeustasy driven by a single, large ice sheet. Rather, the Arrow Canyon archive of varying depositional facies and cycle stacking patterns records major changes in the magnitude of short-term glacioeustasy. This finding contributes to recent and growing near- and far-field evidence for a more dynamic glaciation history than previously inferred from the classic Euramerican cyclothems.

Seafloor hydrothermal systems precipitate Cu, Zn, and Fe sulfides at and below black smoker vents on the seafloor; as a result, the metal concentrations in the vent fluids are minimum values. We sampled deep, unboiled liquids from the Reykjanes geothermal reservoir, Iceland, and measured the metal concentrations. This active, seawater-dominated system, situated on the Mid-Atlantic Ridge, is the subaerial equivalent to mid-ocean-ridge hydro thermal systems. The liquids, collected at 1350–1500 m depth and 284–295 °C, contain 154–2431 μ M Fe (9–140 ppm), 207–261 μ M Cu (14–17 ppm), 79–393 μ M Zn (5–27 ppm), 0.6–1.4 μ M Pb (120–290 ppb), 6–31 n M Au (1–6 ppb), and 250–960 n M Ag (28–107 ppb). Fluids discharged at surface from the same wells have orders of magnitude lower metal concentrations due to precipitation caused by boiling and vapor loss during depressurization. The concentrations of Cu, Zn, and Pb in the high-temperature reservoir liquids at Reykjanes are similar to those in the highest-temperature black smoker discharges, whereas Au and Ag concentrations are one to two orders of magnitude higher at Reykjanes; lower-temperature seafloor fluids have lower metal contents, suggesting subseafloor deposition before discharge. The Reykjanes heat flux of 130 MW requires a liquid flux of ~100 kg/s; over 104 yr, the minimum life of the system, 0.5 Mt each of Cu and Zn may have precipitated at depth.

Abundant microbial mats from the Mesoarchean Moodies Group (Barberton Greenstone Belt, South Africa, ca. 3.2 Ga) are densely interbedded with coarse-grained and gravelly sandstones in a nearly mud-free setting. They apparently grew in marginal marine and possibly subaerial coastal and tidal environments. Characteristic sedimentary structures include anastomosing bedding, gas or fluid escape structures, sand volcanoes, biomat doming, patchy silicified microstromatolites, and microbial sand-chip conglomerates. They indicate rapid growth of mechanically tough microbial mats, possibly aided by early seafloor silicification, in a high-energy, high-sedimentation-rate environment. The observations expand the knowledge of the habitat of Archean microbial mats and life on early Earth. If tidal environments were more widespread in the Archean than today, biomats in extensive intertidal settings may have contributed to immobilize large volumes of sand.

Models of Euramerican cyclothem deposition invoke orbitally driven glacioeustasy to explain widespread cyclic marine and nonmarine late Paleozoic sedimentary sequences. Base-level fluctuations of ~100+ m have been estimated for the deposition of mid-continent North American subpycnoclinal black shales, subaerial exposure relief of algal bioherms in the Sacramento Mountains, and Russian Platform carbonates. Similar to the Pleistocene, these glacioeustatic fluctuations are thought to be driven by variations in orbital parameters. To evaluate this hypothesis, a coupled general circulation model–ice sheet model was used to simulate the effects of both transient orbital changes and variable atmospheric p CO 2 concentrations on late Paleozoic continental ice sheets. In our model, large continental ice sheet inception is simulated at and below p CO 2 levels of 280 ppm. Model results predict that while changing orbital parameters results in dynamic ice sheet behavior, the maximum orbitally induced sea-level fluctuation is ~25 m. The model also demonstrates that the complete ablation of ice sheets formed at 280 ppm (~7.9 × 10 7 km 3 , sea-level change ~135 m) requires an increase in atmospheric p CO 2 to levels >2240 ppm. These results present a potential paradox: while our model is able to simulate widespread Gondwanan glaciation, it is unable to reproduce significant orbitally driven glacioeustatic fluctuations without very large magnitude carbon cycle perturbations. We discuss possible solutions to this paradox.

Explosive eruptions from subaerial arc volcanoes can have significant environmental impact because of the discharge of ash and volatiles directly into the atmosphere and oceans. The link between climate cooling and voluminous volcanic eruptions has remained speculative due to a lack of supporting evidence. A compilation of 2814 K-Ar and 40 Ar/ 39 Ar age determinations from four circum-Pacific arcs indicates that periods of high volcanic output (i.e., flare-ups) have been episodic and, in some cases, synchronous. Peak periods and subsequent lulls in arc magmatism over the past 50 m.y. have occurred coeval with major fluctuations in global climate, including the Eocene-Oligocene transition, one of the most prominent global climate reorganizations in the Cenozoic. Hundreds of intermediate to silicic eruptions occurred during an extremely vigorous period of circum-Pacific volcanism beginning in the late Eocene, which likely led to the production of sulfur aerosols in the stratosphere and fertilization of surface waters of the Pacific Ocean. We provide a mechanism that may have been partly responsible for the climatic preconditioning that must have preceded and ultimately promoted Antarctic ice sheet growth ~34 m.y. ago.

The alkenone organic paleotemperature proxy has been used with great success to generate sea-surface temperature (SST) records from diverse areas of the world ocean. To date, however, this methodology has been applied almost exclusively to unconsolidated marine sediments; the fidelity of alkenone preservation in lithified, uplifted marine sequences exposed on land has not been explicitly investigated. Such sedimentary sequences have been buried at depths of hundreds of meters to kilometers before uplift, and in addition have been exposed to oxic pore fluids and subaerial weathering upon ascent to the surface, any of which may cause alteration of biomarker compounds. In this study, we compare alkenone SST estimates from the Vrica land section in Crotone, Italy, with alkenone SST estimates derived from coeval marine sediments retrieved at Ocean Drilling Program Site 964, which is ~300 km offshore of the Vrica location. We find that SST records from the land and marine sections are well correlated, showing similar mean values, cyclicity, and amplitudes. Although results from different sedimentary sequences might depend on the lithologies, burial depths, and weathering histories of individual outcrops, this study suggests that it will be possible to reconstruct quantitative SST histories from marine strata exposed on land.

Three lithostratigraphic units have been distinguished in the volcanic succession of the basal complex of Fuerteventura Island. These units are, from bottom to top: the submarine volcanic group, the transitional volcanic group, and the subaerial volcanic group. These three groups record the submarine growth and emergence of the island. The volcanism is represented by ultra-alkaline and strongly alkaline igneous series. The igneous activity was due to the presence of an anomalous zone in the sublithospheric mantle, the low density of which also caused uplift of the Mesozoic oceanic crust. Two extensional phases and an intervening contractional phase developed coeval to the generation of the volcanic succession. The submarine volcanic group was deposited in the hanging wall basin of a large listric extensional detachment directed toward the SSW. The transitional volcanic group was syntectonic with respect to a late inversion of the listric detachment. Finally, the subaerial volcanic group resulted from a second episode of WNW extension. This study of the evolution of the basal complex of Fuerteventura serves as the basis for a tectonic model of submarine growth and emergence of volcanic islands.

The mid-Cretaceous was marked by emplacement of large igneous provinces (LIPs) that formed gigantic oceanic plateaus, affecting ecosystems on a global scale, with biota forced to face excess CO 2 resulting in climate and ocean perturbations. Volcanic phases of the Ontong Java Plateau (OJP) and the southern Kerguelen Plateau (SKP) are radiometrically dated and correlate with paleoenvironmental changes, suggesting causal links between LIPs and ecosystem responses. Aptian biocalcifi cation crises and recoveries are broadly coeval with C, Pb, and Os isotopic anomalies, trace metal infl uxes, global anoxia, and climate changes. Early Aptian greenhouse or supergreenhouse conditions were followed by prolonged cooling during the late Aptian, when OJP and SKP developed, respectively. Massive volcanism occurring at equatorial versus high paleolatitudes and submarine versus subaerial settings triggered very different climate responses but similar disruptions in the marine carbonate system. Excess CO 2 arguably induced episodic ocean acidifi cation that was detrimental to

Coarse rock debris exposed in the subaerial environment of northern Sweden displays the development of a wide variety of surface coatings. These include weathering rinds as well as a great diversity of geochemical coatings, including those dominated by Ca, Fe, Si, and Al. Each of these types of rock coatings displays a high degree of geochemical complexity. The weathering rinds exhibit both geochemical and morphologic changes to the parent rock cores, including extensive dissolution, oxidation, and hydration as well as disaggregation, fracturing, and cementation by secondary geochemical coatings. Chemical breakdown of the rock to form weathering rinds includes loss of K, Si, Mg, and Ca and buildup of Fe. Weathering rinds are fundamentally the result of dissolution, as no clay minerals were identified in the rinds. Rock coatings are distinguished from rinds by their clear accretionary nature. Despite the distinction made between weathering rinds and rock coatings, it is important to note that often the two are intimately related. The presence of rock coatings as well as weathering rinds on rock surfaces amplifies the important role played by geochemical processes in the Arctic alpine climate of northern Sweden in particular, as well as its significance in landscape evolution in such environments in general.

A positive carbon isotope (δ 1 3 C) excursion has been recognized in upper Kinderhookian and early Osagean carbonates in three sections in southeast Idaho and Nevada. The oldest δ 1 3 C peak (+7‰) is dated to the isostichaconodont zone, and a younger peak occurs in the typicus Zone. The shifts are recorded in a range of carbonate lithofacies representing various water depths along the shelf. Lithofacies sampled for δ 1 3 C and δ 1 8 O at the Samaria Mountain section in southeast Idaho record the shallowest-water conditions, indicated by cross-bedded skeletal and peloidal grainstones. The deepest water conditions are present in the Pahranagat Range section in eastern Nevada, which consists mainly of bioturbated lime mudstone and skeletal wackestone. The δ 1 3 C values from these widely separated sedimentary basins show a consistent trend that correlates with Early Mississippian curves generated from brachiopod calcite in western Europe and the Midcontinent of North America, as well as dolomites in Utah and Wyoming. δ 1 8 O values become more positive up section, generally paralleling the positive trend in δ 1 3 C during the late Kinderhookian. No subaerial exposure surfaces are recognized in the sections examined in southeast Idaho and Nevada, and at least the δ 1 3 C trends are interpreted as primary seawater fluctuations. Sea-level changes occurred near the beginning of the late Kinderhookian δ 1 3 C shift (early to middle parts of the isosticha Zone) and within the peak of the δ 1 3 C excursion (Kinderhookian-Osagean boundary), although tectonic changes associated with the Antler orogeny have likely modified the eustatic signature.

Trace elements have been used as provenance indicators of igneous rocks for many years, even though the conditions under which they remain immobile during subaerial weathering have not been thoroughly defined. We have used a novel geochemical and quantitative mineralogical data set of variably weathered basaltic rocks from northeast Ireland to investigate the immobility of various elements. Titanium and niobium seem to be the most immobile of all elements. Relative to these elements, zirconium is predominantly depleted but is also locally enriched. Yttrium displays a marked depletion from the early stages of weathering during the growth of clay minerals. Yttrium, zirconium, and their ratios with other elements should thus be used with caution as provenance indicators unless the rocks show no detectable signs of weathering.

The corrugated form of the Harcuvar, South Mountains, and Catalina metamorphic core complexes in Arizona reflects the shape of the middle Tertiary extensional detachment fault that projects over each complex. Corrugation axes are approximately parallel to the fault-displacement direction and to the footwall mylonitic lineation. The core complexes are locally incised by enigmatic, linear drainages that parallel corrugation axes and the inferred extension direction and are especially conspicuous on the crests of antiformal corrugations. These drainages have been attributed to erosional incision on a freshly denuded, planar, inclined fault ramp followed by folding that elevated and preserved some drainages on the crests of rising antiforms. According to this hypothesis, corrugations were produced by folding after subaerial exposure of detachment-fault footwalls. An alternative hypothesis, proposed here, is as follows. In a setting where preexisting drainages cross an active normal fault, each fault-slip event will cut each drainage into two segments separated by a freshly denuded fault ramp. The upper and lower drainage segments will remain hydraulically linked after each fault-slip event if the drainage in the hanging-wall block is incised, even if the stream is on the flank of an antiformal corrugation and there is a large component of strike-slip fault movement. Maintenance of hydraulic linkage during sequential fault-slip events will guide the lengthening stream down the fault ramp as the ramp is uncovered, and stream incision will form a progressively lengthening, extension-parallel, linear drainage segment. This mechanism for linear drainage genesis is compatible with corrugations as original irregularities of the detachment fault, and does not require folding after early to middle Miocene footwall exhumation. This is desirable because many drainages are incised into nonmylonitic crystalline footwall rocks that were probably not folded under low-temperature, surface conditions. An alternative hypothesis, that drainages were localized by small fault grooves as footwalls were uncovered, is not supported by analysis of a down-plunge fault projection for the southern Rincon Mountains that shows a linear drainage aligned with the crest of a small antiformal groove on the detachment fault, but this process could have been effective elsewhere. Lineation-parallel drainages now plunge gently southwestward on the southwest ends of antiformal corrugations in the South and Buckskin Mountains, but these drainages must have originally plunged northeastward if they formed by either of the two alternative processes proposed here. Footwall exhumation and incision by northeast-flowing streams was apparently followed by core-complex arching and drainage reversal.

The northern Voring volcanic margin was initiated by late Campanian–Paleocene rifting, culminating with massive breakup-related igneous activity near the Paleocene-Eocene transition. Paleocene uplift of the central rift zone led to subaerial erosion and deposition in a restricted basin west of the Fles fault complex. The western source area was active toward the end of Eocene time. In the east, the low-relief land surface underwent modest relative uplift, which led to the construction of an early Oligocene delta system on the Trondelag platform. This event was followed by a period of margin subsidence and modest sedimentation until late Pliocene time. Although Miocene and early Pliocene biosiliceous hemipelagic sediments dominate on the outer margin, the influx of ice-rafted detritus records the climatic deterioration and the establishment of glaciers in late Miocene and early Pliocene time. Since ca. 2.6 Ma, ongoing epeirogenic uplift of Fennoscandia and the onset of Northern Hemisphere glaciation increased the erosion potential. A huge prograding wedge of glacial sediments was constructed from ca. 2.6 to 1.0 Ma, when the glacial mode changed from moderate, relatively stable icecaps to distinct glacial-interglacial cycles. The wedge overlies the base of the late Pliocene horizon, which marks pronounced changes in lithology and physical sediment properties, and corresponds to a distinct, regional velocity inversion. The differential glacial sediment load over unconsolidated and mobile biosiliceous oozes may have caused abundant small-offset faulting and diapirism in the western Voring basin.

The rule that sedimentation rates decrease as the time interval increases has been examined by a procedure that avoids the mathematical problem of plotting a variable, time, against its inverse. The test confirms that the scaling trend has real physical meaning. In the time range of 10−1–108 yr, sedimentation rates change roughly proportionally to the inverse of the square root of time. This report proposes and evaluates the hypothesis that the growth potential of reefs and carbonate platforms follows the same scaling trend. The trend implies that a growth of 104 µm/yr sustained over 103 yr, an upper limit of reef growth in the Holocene, translates into 102 µm/yr sustained over 107 yr. Decrease of the growth potential with increasing time solves the paradox of drowned platforms if the decrease results largely from environmental factors and not simply from increase of the time fraction of subaerial exposure.

An ∼3.1-km-thick volcanic complex exposed in the southern Inyo Mountains, east-central California, records Jurassic subaerial depositional environments along the east flank of the Sierran arc. This complex, which we name the Inyo Mountains Volcanic Complex, is subdivided into lower, middle, and upper stratigraphic intervals. The 200–580-m-thick lower interval comprises predominantly epiclastic strata deposited on alluvial fans and adjacent river flood plains that were inclined northeast. Mafic lava flows and rare reworked tuff in this interval record the onset of Jurassic(?) volcanism in this part of the arc. The 300–700-m-thick middle interval is composed predominantly of intermediate to silicic lava flows and tuffs representing a major episode of volcanism ending at ca. 169 Ma that is contemporaneous with emplacement of numerous plutons in the region. The >2260-m-thick upper interval is composed of epiclastic strata with minor intercalations of volcanic rock. Most of this interval accumulated on low-gradient flood plains that hosted evaporative lakes and that were episodically invaded by alluvial fan complexes. Three new U-Pb age determinations constrain the lower half of the upper interval to have been deposited during the interval from ca. 169 Ma to 150 Ma. The uppermost part of the complex remains undated but probably accumulated prior to 140 Ma. The Inyo Mountains Volcanic Complex is part of a belt of volcanic complexes that are the easternmost preserved Jurassic complexes of the Sierran arc. These complexes share sufficient similarities to suggest that they represent a distinctive arc-flank depositional province significantly different from that represented by coeval volcanic complexes preserved in roof pendants farther west, closer to the magmatic axis of the arc. Similarities among arc-flank complexes include predominantly to exclusively subaerial settings, substantial (>30%) portions of epiclastic strata, and existence at times of north- to northeast-inclined paleoslopes. We infer on the basis of the varying types and amounts of volcanic rocks that whereas most complexes in the arc-flank province were rarely if ever proximal to major eruptive centers, complexes in two areas (White Mountains and eastern Mojave Desert) were at times located in or adjacent to such centers. These differences lead us to speculate that the east flank of the Jurassic arc consisted of eastward-projecting volcanic salients separated by arc recesses—typified by the Inyo Mountains area—in which epiclastic deposition was dominant.

Seven lithologic formations, ranging in age from Oligocene to Pleistocene, were recently penetrated by core holes in southernmost Florida. From bottom to top, they are the early Oligocene Suwannee Limestone; late-early Oligocene-to-Miocene Arcadia Formation, basal Hawthorn Group; late Miocene Peace River Formation, upper Hawthorn Group; newly proposed late Miocene-to-Pliocene Long Key and Stock Island Formations; and Pleistocene Key Largo and Miami Limestones. The rocks of the Suwannee Limestone form a third-order sequence. Although the entire thickness was not penetrated, 96 m of Suwannee core from one well contains at least 50 vertically stacked, exposure-capped limestone cycles, presumably related to rapid eustatic fluctuations while experiencing tropical to subtropical conditions. The Arcadia Formation is a composite sequence containing four high-frequency sequences composed of multiple vertically stacked carbonate cycles. Most cycles do not show evidence of subaerial exposure and were deposited under more temperate conditions, relative to the Suwannee Limestone. The Arcadia Formation in southernmost Florida is bounded by regional unconformities representing third-order sequence boundaries. Post-Arcadia transgression produced a major backstepping of sediment accumulation above the upper sequence boundary of the Arcadia Formation. The Peace River Formation, composed of diatomaceous mudstones, has been identified only beneath the Florida peninsula and is not present beneath the Florida Keys. Deposition occurred during marine transgressive to high-stand conditions and a local phosphatization event (recorded in northeast Florida). The transgression is possibly related to a global rise in sea level, which resulted in upwelling of relatively cooler, relatively nutrient-rich water masses onto the Florida Platform. It is proposed that the absence of Peace River sediments beneath the Keys is due to sediment bypass of the upper surface of the Arcadia, a result of sediment sweeping by an ancestral Florida current. During late Miocene to Pliocene time in the Florida Keys, siliciclastics of the Long Key Formation and fine-grained carbonates of the Stock Island Formation prograded toward the southern edge of the Florida Platform and downlapped onto the regional unconformity at the top of the Arcadia. Shallow-marine Pleistocene limestones (Key Largo and Miami Limestones), deposited during tropical to subtropical conditions, drape over accretionary successions of the Long Key and Stock Island Formations.

Hot (≥ 88-120 °C) and acidic (pH ≤ 2.1) hydrothermal fluids rich in sulfate were discovered venting in the DESMOS caldera (depth = 1926 m), eastern Manus back-arc basin, Bismarck Sea, surrounded by Papua New Guinea. The abundant sulfate (≥ 32.8 m M , higher than the seawater value of 28 m M ) with elemental sulfur deposition around the vents, and remarkably low δD(H 2 O) and δ 34 S(H 2 S) values (−8.1‰ and −5.6‰, respectively), are suggestive of the incorporation of a magmatic fluid and the disproportionation of the exsolved SO 2 from a magma body. The DESMOS fluid may be similar in origin to the acidic sulfate-chloride hot springs associated with subaerial volcanic activity. In contrast to the typical hydrothermal end member Mg concentration of 0, the DESMOS fluids are rich in Mg (46–52 m M ), probably because of Mg dissolution by acid attack upon magnesium silicate minerals.

Single-channel high-resolution seismic reflection data from the South Carolina lower Coastal Plain and continental shelf delineate no substantial deformation that could possibly be associated with the seismicity of the Charleston area. Correlation of the seismic data with available wells reveals seaward-dipping Paleocene through Quaternary depositional sequences representing carbonate and terrigenous sediments that were deposited in shallow to outer shelf paleoenvironments. Only middle Eocene and younger units are delineated on the lower Coastal Plain due to limited acoustic penetration. The seismic profiles also reveal thick Miocene and Pliocene progradational sediments within a broad north-northwest–south-southeast–trending depositional basin in the middle to outer shelf region. The variations in the thicknesses of the depocenters are attributed to an interrelationship of subaerial and marine processes, which are in turn governed by the shifting of the Gulf Stream, the Suwannee Channel, and sea level. The Cenozoic section in the study area indicates a stable structural framework except offshore Charleston where the Paleocene through Miocene reflectors are displaced by a northeast-southwest–trending monoclinal flexure downthrown to the southeast. The amount of displacement decreases from 20 m on the Paleocene surface to 10 m on the shallowest Miocene reflector seen 10 m below the sea floor. This flexure probably originated by reactivation of the deeply seated pre-Cretaceous Helena Banks fault revealed in multichannel profiles reported elsewhere.

Long-runout subaerial landslides (>50 km) are rare on Earth but are common features shaping Mars’ Valles Marineris troughs. In this study, we investigated the highly debated emplacement mechanisms of these Martian landslides by combining spectral and satellite-image analyses. Our results suggest that hydrated silicates played a decisive role in facilitating landslide transport by lubricating the basal sliding zone. This new understanding implies that clay minerals, generated as a result of water-rock interactions in the Noachian and Hesperian (4.1–3.3 Ga), exert a long-lasting influence on geomorphic processes that shape the surface of the planet.

The contact between the Hartselle Sandstone (Mississippian, lower Hombergian) and the underlying Monteagle Limestone (Mississippian, upper Gasperian) in central Tennessee and northeastern Alabama is a regional disconformity. A paleoweathering surface developed in the uppermost Monteagle is truncated by this disconformity, and only remnant pedogenic features are preserved. These paleosol remnants and associated meteoric diagenetic features are evidence for the subaerial exposure and weathering of the Monteagle Limestone prior to intense erosional scour accompanying deposition of the Hartselle Sandstone. Petrographic evidence for subaerial exposure and meteoric diagenesis includes extensive micritization, reddening, dissolution, and calcitization of allochem grains. Stable carbon and oxygen isotope compositions of whole-rock samples exhibit depth patterns consistent with subaerial exposure, pedogenesis, and meteoric diagenesis. Stable isotope compositions of nonferroan calcite spar occluding moldic pores are considerably depleted relative to Mississippian marine carbonate compositions, further evidence for subaerial exposure and meteoric diagenesis. The paleoweathering surface was modified after burial when reducing conditions developed, resulting in pyritization. Stabilization of high-Mg calcite grains and cements after burial provided a source of magnesium for localized replacement by ferroan dolomite. This disconformity is interpreted to be a third-order Vail/Exxon sequence boundary that marks a dramatic basinward shift in facies belts in the overlying Hartselle Sand-stone, the result of a sea-level fall.

The Scafell caldera, an Ordovician ensialic andesite-rhyolite caldera in the English Lake District, is probably the best-exposed "piecemeal" caldera so far documented. Intra-caldera volcanotectonic faults indicate whole-sale fragmentation of the caldera floor and were reactivated repeatedly during a major subaerial caldera-collapse episode. Such piece-meal collapse is in contrast to calderas in which the floor has undergone coherent piston-like ( en bloc ), downsag, or trapdoor-like subsidence. Mesobreccias, large slide blocks, complex growth-fault geometries, and abrupt local thickness and facies variations in the caldera-fill ignimbrites show that the caldera floor repeatedly developed intricate and rapidly changing topographies of high relief, owing to formation of close-spaced volcanotectonic faults and their frequent reactivation, including movements of reversed sense. At the onset of caldera collapse, volcanotectonic faulting and associated seismicity caused widespread (>260 km 2 ) and ubiquitous subaerial gravity sliding and spreading of an extensive, thick (>30 m), phreatoplinian ash layer. This is recorded as abundant low-angle soft-state slide surfaces, with compressional and extensional deformation off slipped material, including numerous close-spaced ramifying high-angle faults and zones of disaggregation. Particularly intense soft-state deformation is localized at steeply inclined faults that define numerous caldera-floor fault blocks, 0.1-2 km across, and at these faults, the style and geometry of the small-scale soft-state deformation show the sense of fault displacement. Intracaldera silicic ignimbrites overlie the disturbed ash layer and are cut by numerous steep zones of ductile (rheomorphic), brittle, and hybrid deformation structures that formed in response to faulting before the hot tuffs cooled. The deformation zones are characterized by rheomorphic folds, steep high-attenuation rheomorphic shear fabrics, and a facies termed "domain breccia" that is interpreted as fused rheomorphic breccia. The style of this hot-state deformation is variable and reflects differences in competence, strain rate, and timing of faulting relative to the emplacement and cooling history of each ignimbrite. The deformation zones separate relatively undeformed ignimbrite that underwent dip-slip displacements of as much as 400 m and angular rotations of as much as 20°. The zones can be traced downward into steeply dipping faults that cut caldera-floor rocks. Overlying lacustrine (caldera lake) sediments display slumps and a depositional architecture that record declining persistence of the fault activity. There is no evidence of largescale resurgent doming.

Rock-avalanche deposits can be significant components of aggradational sequences formed in dry climates. Features distinguishing such deposits from their closest counterparts, debris-flow deposits, reflect the higher energies and shear strengths associated with rock-avalanche lobes, and their tendency to generate their own matrix through pulverization associated with the failure event. Evaluation of sedimentary breccias in Oligocene-Miocene strata deposited under dry climatic conditions in west-central Arizona indicates that many are of rock-avalanche origin. Some are sufficiently large to have represented large rock avalanches (that is, sturzstroms) at the time of emplacement and display features consistent with descriptions of such lobes. Some rock-avalanche deposits interbedded with lacustrine sediments represent initially subaerial lobes that flowed into lakes. These bodies locally are intruded by substrate-derived injection structures and contaminated by lakebed mud; mud contamination was initially concentrated along the bases of lobes but affected a progressively greater proportion of the flows with increasing subaqueous runout. Contaminated portions of rock-avalanche lobes exhibit features consistent with decreased shear strength, and thoroughly contaminated lobes appear to have transformed into slow-moving, slurry-like flows that experienced internal cycling of debris.

Approximately 4,400 km of single-channel seismic reflection data were collected over the northern Hawaiian flexural moat between Hawaii and Kauai. These data include eight cross-moat profiles and three ∼600-km-long moat-parallel lines. The thick (>2 km) sedimentary section filling the Hawaiian flexural moat is composed primarily of the products of large-scale mass wasting from massive slope failures on the adjacent Hawaiian islands. The observed stratigraphy can be described by four main lithostratigraphic units: (1) a basal unit of relatively constant thickness that we interpret as pelagic sediment predating the formation of the flexural moat; (2) a thick wedge of lens-shaped units onlapping the flexural arch, each with highly chaotic internal reflectivity that we interpret as buried landslide deposits; (3) a sequence of highly reflective, continuous horizons that offlap the flexural arch and are tilted down toward the islands; and (4) a ponded unit confined to the deepest part of the moat representing the youngest sediments transported to the moat. This distinctive stratigraphy—onlap of the flexural arch in the lower moat section (landslide unit), offlap migrating back toward the islands in the upper moat section offlapping unit), and ponding of the youngest sediments in the deepest parts of the moat (ponded unit)—can be explained in terms of the competing effects of sediment influx to the moat from large-scale mass wasting and distributed subsidence due to the progressive loading of a purely elastic plate by each successive volcano. The role of large-scale mass wasting in the sedimentation history of the Hawaiian flexural moat is expected to characterize the moats of other large subaerial oceanic volcanoes.