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Sedimentological and geochemical analyses of gravity and piston cores retrieved from Lake Quinault, Washington, reveal an ~4000-year flood-dominated depositional record. Individual flood event layers are identified by combining core stratigraphy, sedimentology, and the ratio of incoherent to coherently scattered x-ray radiation ( inc/coh) from µXRF (x-ray fluorescence) core scans. The inc/coh time series is used as a proxy for sediment grain size and, in combination with radiocarbon-anchored core age–depth models, enables the reconstruction of late-Holocene hydrologic variability for the Quinault River catchment. Decadal to centennial variability in inc/coh is interpreted to reflect trends in ocean-atmosphere teleconnections favorable for the formation of land-falling atmospheric rivers along the Pacific Ocean flank of the Olympic Mountains. Such processes likely modulate the rate of flooding and may explain notable increases in the frequency of flood event layers observed during the periods 2350–2450 cal. yr BP and the most recent century (AD 1910–2010). Understanding past hydrologic variability has important implications for the landscape and ecosystem response of Olympic Mountain catchments to future climate warming.

Lake Crescent, a 180-m-deep, glacially carved lake located on the Olympic Peninsula in western Washington, USA, overlies the Lake Creek-Boundary Creek fault zone, a system of structures with at least 56 km of late Pleistocene to Holocene surface rupture. Investigation of the lake’s sediment, including a reflection seismic survey and analysis of piston cores, reveals evidence that the fault beneath the lake has ruptured four times in the past ∼7200 years, producing unusually thick deposits termed megaturbidites. The earthquakes triggered rockslides that entered the lake and caused displacement waves (lake tsunamis) and seiches, most recently ca. 3.1 ka. Seismic reflection results from beneath the depth of core penetration reveal at least two older post-glacial ruptures that are likely to have similarly affected the lake. The stratigraphy of Lake Crescent provides insight into the behavior of a fault system that partially accommodates regional clockwise rotation and contraction of the northern Cascadia forearc through oblique dextral shear, and highlights the potential for disruption to critical infrastructure, transportation corridors, and industry on the North Olympic Peninsula during future surface-rupturing earthquakes. Our results illustrate the potential synergism between lacustrine paleoseismology and fault-scarp trench investigations. More precise dating of strong earthquake shaking afforded by continuous accumulation of lake sediment improves earthquake histories based on trenched fault scarp exposures, which are commonly poorly dated.

Investigation of Lake Quinault in western Washington, including a reflection seismic survey, analysis of piston cores, and preliminary mapping in the steep, landslide-prone Quinault River catchment upstream of the lake, reveals evidence for three episodes of earthquake disturbance in the past 3000 yr. These earthquakes triggered failures on the lake’s underwater slopes and delta front, as well as subaerial landsliding, partial channel blockage, and forced fluvial sediment aggradation. The ages of the three Lake Quinault disturbance events overlap with those of coseismically subsided, coastal marsh soils nearby in southwest Washington that are interpreted to record ruptures of the Cascadia megathrust. Absent from Lake Quinault, however, are signals of obvious disturbance from five additional subduction earthquakes inferred to have occurred during the period of record. The lack of evidence for these events may reflect the limitations of the data set derived from the detrital, river-dominated lake stratigraphy but may also have bearing on debates about segmentation and the distribution of slip along the Cascadia subduction zone during prior earthquakes.

Source to sink sedimentary systems are important settings of carbon cycling, serving as sites of carbon transfer between terrestrial and marine reservoirs, and as the primary locations for organic carbon burial on Earth. The age and character of the carbon that is buried at the terminal ends of these systems reflects the sources and transformations of the organic carbon (OC) throughout their linked terrestrial and marine segments. Profound differences are observed between large passive and small active margin systems. Large passive margin systems are characterized by large floodplains and relatively broad shelves where OC has protracted exposure to oxidants. Rapid burial in prograding, subaqueous deltaic clinoforms or bypass to submarine fans, however, leads to high burial efficiency of terrestrial biospheric OC in some passive margin settings. The OC in small active margin systems, in contrast, follows relatively short pathways from headwaters to seabed. This rapid transit, facilitated by the important role of storm-driven transport in such settings, can lead to high OC burial efficiencies. The study of OC sources and transformations in contemporaneous source to sink sedimentary systems informs interpretations about the systems in which OC was buried in the geologic past, their stratigraphic records of environmental change, and their potential to produce petroleum resources.

A fundamental goal of the Earth Science community is to understand how perturbations on Earth's surface are preserved in the stratigraphic record. Recent Source to Sink (S2S) studies of the Waipaoa Sedimentary System (WSS), New Zealand, are synthesized herein to provide a holistic perspective of the processes that generate, transport and preserve sedimentary strata and organic carbon on the Waipaoa margin in the late Quaternary. Rapid uplift associated with subduction processes and weak sedimentary units have conspired to generate rapid rates of incision and erosion in the Waipaoa catchment since the Last Glacial Maximum (LGM). We show that although much of the sediment exported offshore during this time interval originated from valley excavation, a substantial portion emanated from hillslopes, mostly through deep-seated landslide and earthflow processes that were vigorous during periods of rapid fluvial incision just prior to the Pleistocene–Holocene transition. Lacustrine sediments deposited in naturally-dammed 7-ky-old Lake Tutira provide a record of Holocene environmental controls on upper catchment sedimentation in the WSS, with 1400 storms identified. Storm frequency is modulated by the waxing and waning of atmospheric teleconnections between the tropics and Antarctica. Furthermore, clear long-term changes in sediment yield are evident from the Lake Tutira record following human settlement as conversion to pasture is accompanied by a 3-fold increase in the long-term lake sediment accumulation rate. Whereas there is ample evidence that Waipaoa River flood deposits are routinely deposited offshore in the sheltered confines of Poverty Bay, over the longer term, waves and currents subsequently resuspend and transport these deposits both landward (sandy fraction) and seaward (finer fraction). Thus, the timing of sediment supply to areas of net sediment accumulation is more often driven by wave events that are not associated with river flooding. Therefore, we conclude that asynchronicity of river-sediment delivery and of wave resuspension in most instances precludes the direct preservation of flood events in the stratigraphic record of the Waipaoa Shelf. Over the longer term, the sediment package preserved on the shelf and slope since the LGM can be explained in large measure by sequence-stratigraphic models forced by varying sea level and ongoing tectonic deformation of the margin. As sea level rose, sediment supply to the slope was reduced by about a factor of 5 due to shelf trapping. Despite this reduction, turbidites are found at similar frequency throughout the LGM–Present, as the dominant trigger appears to be subduction earthquakes, with large ones having a return interval of about 200 ± 100 years. Sediment-budget exercises that consider both modern (river discharge versus centennial accumulation rates) and post-LGM (terrestrial production versus offshore isopachs) mass balances indicate that about half of the total sediment production from the Waipaoa escapes the study area. Moreover, a coupled sediment transport-hydrodynamic model and observations of textural trends on the shelf indicate that a large fraction of the sediment is carried outside the study area along the shelf to the northeast by the river plume or by combined current/wave activity. Therefore, we conclude that the WSS is an open system with sediment escape from the present day through the LGM. The organic matter associated with sediment as it moves from upland source to marine sink is a product of particle history, and provides a record of materials that have cycled over timescales of days to millions of years. The ubiquity of fossil Organic Carbon (OC) in both the terrestrial and marine realms of the Waipaoa attests both to the chronic nature of its source, crumbling mudstones further destabilized by land use, and its biogeochemical recalcitrance. Modern OC persists by virtue of its continual production along the S2S transit, and is buried and preserved within the adjacent marine depocenters. The Waipaoa contrasts with dispersal systems on wide, energetic shelves (e.g., the Amazon and Fly Rivers) where sediment is extensively refluxed in oxygenated overlying water resulting in the biogeochemical incineration of particulate OC. The Waipaoa, like other small mountainous rivers on active margins, exhibits a high riverine OC preservation efficiency (> 50%) in its marine depocenters because of the relatively rapid, event-driven accumulation of sediment.

Sedimentary processes on the inner Mekong Shelf were investigated by examining the characteristics of sediments sampled in gravity cores at 15 locations, including grain size, clay mineralogy, sediment accumulation rates, and the elemental and stable carbon isotopic composition of organic matter (atomic C/N ratios and delta C-13). Deltaic deposits exhibit contrasting characteristics along different sides of the delta plain (South China Sea, SCS hereafter, to the east and Gulf of Thailand, GOT hereafter, to the west) as well as on and off the subaqueous deltaic system. On one hand, cores recovered from the subaqueous delta in the SCS/GOT are consisted of poorly/well sorted sediments with similar/different clay mineral assemblage with/from Mekong sediments. Excess Pb-210 profiles, supported by C-14 chronologies, indicate either "non-steady" (SCS side) or "rapid accumulation" (GOT side) processes on the subaqueous delta. The delta C-13 and C/N ratio indicate a mixture of terrestrial and marine-sourced organic matter in the deltaic sediment. On the other hand, cores recovered from areas with no deltaic deposits or seaward of the subaqueous delta show excess Pb-210 profiles indicating "steady-state" accumulation with a greater proportion of marine-sourced organic matter. Core analysis's relevance with local depositional environment and previous acoustic profiling are discussed. (C) 2012 Elsevier Ltd. All rights reserved.

Holocene sediments buried on the continental shelf seaward of the Waipaoa River, northeastern New Zealand, preserve a stratigraphic record of terrestrial environmental change. The well characterized, long-term record of storms, volcanism, and human disturbance in this region provides an opportunity to examine how such changes are reflected in the character of organic carbon (OC) buried on the continental margin. Complimentary evidence obtained from analyses of the 14 C content and the elemental and stable carbon isotopic composition of different sedimentary fractions, including charcoal, wood, and clay-sized isolates, indicates that these perturbations led to mobilization of OC components with variable storage histories in the watershed. Charcoal transported in the modern Waipaoa River and buried in offshore depocenters includes a highly aged component that has apparently been slowly released from storage in soil or alluvial terraces. The charcoal fraction ages become dramatically younger in sediments deposited after the Taupo volcanic eruption (1717 cal yr BP) and then Polynesian settlement (ca. 700 cal yr BP), both signaling biomass burning. The mean age of woody plant fragments and clay-bound OC deposited on the shelf also varies over time, with older material having accumulated in the middle Holocene and since human occupation. Deposition of older-than-average wood fragments and clay-bound OC between about 5 and 3.6 kyr BP correlates with evidence for a period of increased storm frequency in the region and may reflect the enhanced delivery of aged soil and sedimentary rock mobilized from hill slopes via earthflows and/or deep seated landslides. Similarly, the deposition of older wood and clay-bound OC commensurate with the first anthropogenic disturbance ca. 700 years ago is consistent with accelerated mass wasting due to deforestation. At the same time, a change in the elemental and stable isotopic composition of bulk and clay-bound OC buried on the shelf may reflect increased marine primary productivity and/or mobilization of OC from deep levels in soil profiles. Deforestation of the Waipaoa headwaters by European settlers beginning in the middle 19th century is evinced by a sharper rise in the age of clay associated OC buried offshore. Today, deep gully incision into fractured sedimentary bedrock is a major source of sediment and kerogen to the river, and this process has left its mark on the age of sedimentary OC delivered to the adjacent margin.

The composition of particulate organic carbon in sedimentary systems can provide information concerning sources of materials and their history. The NSF-MARGINS Source to Sink study of the Waipaoa Sedimentary System in New Zealand provides a platform to calibrate the particulate organic carbon sedimentary record by linking chemical characteristics to terrestrial and marine sedimentary processes. This report is a preliminary evaluation of the available organic carbon concentration, elemental C/N ratio and C-isotopic composition data for the terrestrial portion of the system. Samples analyzed were rocks, soils, floodplain sediments and riverine suspended sediments. Measurements confirm previous conclusions that a portion of the riverine particulate organic carbon is derived from sedimentary rocks. The rock C (kerogen) concentration is relatively invariant (~ 0.25%) from rock source, to riverine and floodplain sediments. The non-rock C is derived from C3 vegetation in most of the watershed, the exception being the floodplain in which local C4 plant inputs are detected. The non-rock organic carbon has a mean 14C-age of approximately 1000 years, which reflects a mixture of modern and aged components. The presence of aged non-rock C, which is chronic in its supply to the river, indicates storage of particulates within the watershed. Remobilized colluvial and alluvial deposits are among the possible sources. Systematic differences are found between δ13C databases generated by various groups. While there may be natural explanations for these differences, methodological artifacts have to be considered. Standardization of methods that cross source-to-sink boundaries is recommended.

The rivers that drain active, collisional margins of the southwest Pacific deliver up to 35% of particulate organic carbon (POC) to the world ocean, and are a key component of the global organic carbon flux. However, knowledge of the fate of terrestrial POC in the ocean is both limited and necessary for quantifying terrestrial and coastal ocean carbon budgets. Here, the fate of terrestrial POC is determined off the high discharge, Waipaoa River (sediment yield 15 Mt y − 1 ) based on a transect of seven cores from the river floodplain to the adjacent continental shelf and slope. Total organic carbon (%TOC), δ 13 C, 14 C, C/N ratios and lipid biomarker compounds were used to determine biogeochemical characteristics of surface sediments from terrestrial source to marine sink, and how these characteristics vary with river discharge. Complementary to surface sediments, down-core characteristics of three multi-cores covering the shelf and slope regions were used to identify perturbations in sediment supply via major floods. The presence of flood deposits allows us to compare their OC characteristics with non-flood sediment, thereby helping address the question of how flood events in the river catchment affect the transfer and fate of terrestrial OC through the marine environment. Results from this study show that as surface sediments are physically and biologically processed across the continental margin, they gain a marine signature. Biomarker analyses of surface samples show decreases in terrigenous vascular plant sources with increasing distance offshore. Biomarkers also demonstrate that terrestrial OC is being transferred across the continental margin, with plant sterols, long-chain alcohols and long-chain fatty acids (biomarkers indicative of vascular plants) persisting as far offshore as the mid-continental slope. In contrast to ambient conditions represented by surface sediments, rapid delivery by floods allows for more complete transfer of terrestrial carbon to the marine environment. A 1–10 cm thick flood layer preserved from Cyclone Bola (March 1988) contains a significant amount of terrestrially-sourced OC which subsequently was rapidly buried by sediments delivered during less extreme conditions.

The burial of organic carbon (OC) on continental margins is strongly coupled to the supply and accumulation of inorganic mineral particles. It follows that carbon burial on the margins should be impacted by changes in riverine sediment delivery, yet these impacts have not been well documented. In this study, an ∼2000-year record of sedimentation and carbon burial on the continental shelf offshore from the Eel River in northern California was examined. The record reveals a 6–11-fold increase in the rate of sediment accumulation on the mid-shelf beginning about 1955, and a concomitant decrease in grain size and increase in flood-layer preservation. At the same time, the age of buried wood fragments abruptly decreased and their stable carbon isotopic composition became enriched in 13C. We argue that these changes can be explained largely as the result of altered land use in the Eel watershed during the past century and its impacts on shelf sediment dispersal processes. Sedimentary OC on the Eel shelf consists primarily of discrete wood fragments associated with coarse-silt- and sand-sized particles, and of organic matter strongly bound to clay-sized mineral grains. The clay fraction is a particularly sensitive recorder of environmental change in the Eel system. Above the 1955 horizon, the clay fraction shows an abrupt decrease in OC concentration and loading (OC content normalized to particle surface area) attendant with the increased accumulation rate. Kerogen carbon constitutes a relatively constant proportion of the clay-associated OC throughout the ∼2000-year record. Increases in mass wasting and input of bedrock material following the onset of intensive industrial logging in the Eel watershed may have resulted in a lower loading of terrestrial plant OC in the clay fraction deposited after 1955 as suggested by isotopic mass balance calculations. The Eel River is representative of small mountainous watersheds worldwide that deliver a major portion of the sediment and carbon flux to the margins and that have been strongly impacted by land-use change during the past century. Our results suggest that such changes leave a distinctive mark in both the sedimentological and geochemical records preserved offshore.

Deltas sequester nearly half of the organic carbon (OC) buried in the marine environment. The composition of the buried organic matter reflects both watershed and seabed processes. A conceptual model is presented that describes the evolution of particulate organic carbon (POC) as it travels from its terrestrial source to its burial at sea. Alterations to the POC occur primarily in bioactive reservoirs, such as soils and the surface mixed layer (SML) of the seabed, where new organic matter can be added and older material degraded. Bypassing or rapid passage through the reservoirs is a key parameter because it avoids change. The Eel River of northern California and the Amazon River systems illustrate the importance of reservoir transit time and storage in determining the character of POC delivered to the continental margin. The Eel exemplifies a bypass system. Mass-wasting processes on land deliver unaltered bedrock along with OC derived from extant vegetation directly to the river channel without significant storage in soils. Rapid burial on the shelf occurs as a result of flood events. As a consequence, the buried material appears to be a simple mixture of carbon derived from kerogen (bedrock C), and modern terrestrial and marine sources. This is predicted to be a characteristic of the many similar short rivers on active margins that supply >40% of the fluvial sediment to the world's ocean. Extensive storage and processing of OC in lowland soils is a characteristic of the large Amazon watershed. Upland POC compositions are either overprinted or replaced by lowland sources. Upon delivery to the shelf, over half of the riverine POC is lost as a result of residence in sediment layers that are periodically reworked over time scales of days to months. The addition of fresh reactive marine OC, exposure to oxygen, and the regeneration of metal oxidants during resuspension events fuel the oxidation of the riverine organic matter. The nature of the watershed-shelf processes likely produce a complex mixture of organics possessing a continuum of ages and reactivities. The model illustrates the need to develop tools to measure residence times of particles in the various reservoirs so that the behavior of POC can be calibrated as it moves through a sedimentary system. The ultimate goal is to be able to use the organic geochemistry of soils and sediments to quantitatively infer the history of processes that determine both the composition and amount of POC present in different depositional environments.

The cycle of organic carbon burial and exhumation moderates atmospheric chemistry and global climate over geologic timescales. The burial of organic carbon occurs predominantly at sea in association with clay-sized particles derived from the erosion of uplifted continental rocks. It follows that the history of the fine-grained particles on land may bear on the nature of the organic carbon buried. In this study, the evolution of clay-associated organic matter was followed from bedrock source to the seabed in the Eel River sedimentary system of northern California using natural abundance 13C and 14C tracers. Approximately half of the fine-grained organic carbon delivered to the shelf is derived from ancient sedimentary organic carbon found in the uplifted Mesozoic-Tertiary Franciscan Complex of the watershed. The short residence time of friable soils on steep hill slopes, coupled with rapid sediment accumulation rates on the shelf-slope, act to preserve the ancient organic carbon. A comparable quantity of modern organic carbon is added to particles in the watershed and on the shelf and slope. The bimodal mixture of ancient and modern C in soils and sediments may be characteristic of many short, mountainous rivers. If the Eel River chemistry is typical of such rivers, more than 40 Tg of ancient organic C may be delivered to the world’s oceans each year. A flux of that magnitude would have a significant influence on marine and global C-cycles.

Previous investigations of the factors governing organic carbon burial on continental margins have pointed toward the important, apparently protective association of carbon with mineral particles. These studies have also revealed dramatic transformations of carbon-particle relationships at the land-sea interface. Riverine particles in some settings lose a large portion of their loads of sorbed terrestrial carbon upon discharge to the ocean and gradually reload to similar levels with marine carbon. The Eel River in northern California and the adjacent continental shelf were selected as an ideal system to investigate the rates of these processes. The river is episodically subject to large floods, and the shelf stratigraphy preserves a record of the resultant large pulses of sediment and carbon input to the marine environment. Carbon isotopic, carbon to nitrogen, and carbon to surface area ratios of particles in flood deposits were expected to reflect the rapid unloading of terrestrial carbon from discharged particles, whereas nonflood sediments that have accumulated at slower rates on the shelf were expected to carry higher loads of marine carbon. Our results indicate, however, that particles on the Eel shelf have retained their loads of terrigenous carbon, and that a significant portion of the particle-sorbed carbon buried on the shelf is kerogen derived from the Mesozoic-Tertiary Franciscan Complex. We hypothesize that rates of uplift and mass wasting in the Eel watershed and rates of particle delivery to and burial on the continental shelf, are so rapid that kerogen is not completely oxidized and is recycled instead. The loading of carbon on clay-sized particles delivered to the shelf, moreover, is dependent on river discharge and may reflect the relative importance of different mass wasting processes during precipitation events of varying intensity. The Eel River system is likely to be representative of other small, mountainous rivers and indicates that processes on land may play an important role in governing the amount and character of carbon being buried on the continental margins.

The 1995 flood of the Eel River in northern California provided an opportunity to follow the short-term history of riverine particulate matter on a continental shelf. Particulate organic carbon, including both vascular plant debris and soil carbon, was utilized as a tracer of the distribution and modification of the shelf flood deposit. These components were sorted during initial emplacement of the deposit, both in the river plume and in the benthic boundary layer. During the 8 months following the flood, lateral transport, shallow burial, and bioturbation contributed to spatial changes in the amount and character of carbon in the flood deposit. Study of the deposit suggests that this and previous flood layers may be preserved in the shelf and upper slope stratigraphic record as clay-rich beds with relatively high carbon to nitrogen ratios and more negative δ13C values than background, non-flood sediment. Concentrations of vascular plant debris are locally characteristic of such layers, and may serve as distinctive stratigraphic markers. Investigation of organic carbon in the 1995 flood layer on the Eel River shelf provides insight into the fate of terrestrial particulate carbon on continental margins in general. Flooding of the Eel River, and of similar river systems draining mountainous terrains, introduces large quantities of terrestrial carbon to the marine environment and may favor the preservation of both terrestrial and marine carbon. The abundant woody vascular plant debris discharged during floods is especially likely to survive oxidation and sulfate reduction in the shallower zones of the seabed. This study reveals the dominant role that physical and biological processes may play in attenuating the terrestrial organic-carbon signal on shelves over monthly to yearly time scales.

A study was designed to investigate the relationship between depletion of benthic oxygen and 4th-order sea-level change within the Cretaceous Western Interior Seaway of North America. Three 4th-order sea-level change cycles were studied in strata belonging to the Tropic Shale and the Tununk Shale Member of the Mancos Shale in Utah. These sedimentary deposits accumulated within prodeltaic environments. In Utah, strata belonging to all three 4th-order cycles exhibit evidence for depletion of benthic oxygen at times of peak 4th-order transgression and development of condensed sections. The evidence includes decreased intensities of bioturbation, increased Cr Al , Ni Al , and Zn Al ratios, and decreased Mn Fe ratios. The data also suggest that depletion of benthic oxygen in the distal section preceeded that in the more proximal one and thus may reflect episodic expansion of an oxygen-depleted water mass(es) from the pelagic to the prodeltaic environment of the seaway. Evidence for subsequent increases in benthic oxygen level with the onset of 4th-order regressions is interpreted to reflect either aggradation of the seafloor above the oxygen-poor water due to sediment accumulation or contraction of the oxygen-depleted water mass from the prodeltaic part of the seaway, or a combination of the two. S/Organic C ratios of strata interpreted to have been deposited under conditions of depleted benthic oxygen are distinctly below 0.4 and thus were not useful for recognition of paleo-oxygen levels in the prodeltaic environment. This study highlights the importance of depletion of benthic oxygen as a mechanism for enhancing preservation of organic carbon in condensed sections on a hundred thousand year time scale. Our results underline the potential for source rock formation in similar prodeltaic deposits of other epicontinental seaways during periods of relatively high sea level and global depletion of benthic oxygen.

The Tropic Shale and correlative Tununk Shale Member of the Mancos Shale accumulated during Cenomanian-Turonian time, within prodeltaic environments near the western margin of the Western Interior Seaway of North America. Stratigraphical and sedimentological analysis has revealed a detailed history of relative sea level change in the thick, fine grained succession. The Tropic and Tununk shales were deposited during the Greenhorn second-order sea level cycle, over a time span of about 2–5 million years. In southern Utah, six depositional sequences are superposed upon the record of this long term sea level change. The sequences developed during third-order relative sea level cycles of hundreds of thousands of years duration and are composed of at least 37 parasequences, arranged in retrogradational, aggradational and progradational parasequence sets. The Tropic Shale and Tununk Shale Member accumulated just basinward of the axis of maximum subsidence of a foreland basin. Stratal geometries and facies distribution patterns in the succession indicate that in southern Utah the Greenhorn cycle was tectonically controlled. During the Greenhorn transgression and highstand, rapid rates of tectonic subsidence trapped terrigenous sediment to the west of the study area, in the more proximal foreland. At this time, hemipelagic facies accumulated at relatively slow rates in southern Utah and type 2 sequences developed during third-order sea level cycles. In contrast, during the Greenhorn regression rates of thrust-induced subsidence in the proximal foreland basin evidently slowed, and deltaic clinoforms prograded across the study area. At least one forced regression occurred in southern Utah at this time, and type 1 sequences developed. The formation of type 1 sequence boundaries in the upper part of the Tropic Shale and Tununk Shale Member points to episodes of base level fall and indicates that the six third-order sea level cycles recorded in the succession were not the result of changes in sediment supply alone. The third-order cycles may have been a consequence of episodic tectonism. The timing of these cycles, however, suggests that development of sequences and parasequences in the Tropic Shale and Tununk Shale Member may have been related to orbital forcing in the Milankovitch band. Glacioeustasy or climatically related fluctuations in the amount of groundwater stored on continents may explain these high frequency sea level changes.

The long-term goal of this research is to contribute toward our understanding of the processes by which strata are formed on continental margins. In particular, this research is aimed at elucidating the processes that control the distribution, characteristics, and modification of event layers on continental shelves.

Rhythmically alternating beds of limestone and shale in pelagic deposits of the Cenomanian-Turonian North American Western Interior Seaway have been widely attributed to climatic cyclicity in the Milankovitch band. Whether these rhythmic beds record cycles of terrigenous dilution or of productivity remains controversial. We present results of a study of early Turonian strata of the Tropic Shale and Tununk Member of the Mancos Shale in southern Utah, aimed at testing the dilution model in a prodeltaic environment within 130 km of the paleoshoreline. The study transect consisted of four localities spaced along a 110 km onshore-offshore transect. In each of the four sections, we studied a stratigraphic interval characterized by beds of limestone or marlstone that alternate with beds of marly shale or calcareous shale. Along the transect, content of quartz sand and coarse silt in each of the beds generally decreases and content of calcium carbonate increases. In each section, quartz sand and coarse silt content varies antithetically with calcium-carbonate content. These observations suggest that the limestone/marlstone beds accumulated during times when rates of terrigenous sediment transport to offshore sites were relatively slow. Conversely, the shaly beds were deposited during times of enhanced terrigenous sediment flux. These ob ervations support the hypothesis that limestone-shale bedding rhythms record wet and dry phases of climate cycles. In Utah, the limestone/marlstone beds contain smaller proportions of mixed-layer illite/smectite clays than do the shaly beds. Climatic cyclicity apparently led to cyclic variations in the proportions of clay minerals supplied to the prodeltaic environments, or to variation in the loci of accumulation of different clay minerals. Analysis of intensity of bioturbation and preservation of fecal pellets also indicates that the limestone/marlstone beds accumulated under more oxic benthic conditions than did the shaly beds. Detailed examination of the alternating beds of limestone/marlstone and shale in southern Utah indicates that the deposits are characterized by a distinctive succession of facies. These deposits are better interpreted as bedding cycles, composed of three gradational lithologies, than as bedding rhythms, composed of two alternating lithologies. Data on grain size and calcium-carbonate content suggest that the bedding cycles correspond to shoaling-upward parasequences that developed in response to small-scale changes in relative sea level. Our study suggests that climatically modulated changes in fresh-water discharge and terrigenous sediment supply were mainly responsible for these sea-level changes and for cyclic variations in benthic oxygenation in prodeltaic environments of the earl Turonian Western Interior Seaway.

In Phanerozoic fine-grained marine sedimentary deposits, preservation of physical lamination is commonly presumed to record anoxic benthic conditions. Lamination may also be preserved, however, in oxygen-rich settings where sediment influx is rapid and, as a consequence, high levels of turbidity and fluid substrates inhibit colonization by benthic organisms. Differentiation between these two situations is critical for accurate paleoenvironmental interpretations. The recognition of clinoforms, which are common in ancient fine-grained successions, is a key step in making this distinction. In the Tropic Shale and correlative Tununk Member of the Mancos Shale of southern Utah, clinoforms are preserved in two superposed depositional sequences, the fourth and fifth sequences in the succession. The geometry of the clinoforms indicates that foreset deposits accumulated relatively rapidly and bottomset deposits more slowly. In sequence 4, patterns of preservation of lamination in clinoforms resemble those in modern subaqueous deltas. These ancient clinoforms evidently prograded into oxygenated waters, and preservation of lamination was primarily controlled by the rate of sediment accumulation. In sequence 5, however, the oxygen level apparently controlled the preservation of lamination.

The conglomeratic sandstone of the Sandstone of Floras Lake (Miocene) is a superbly exposed example of a coarse-grained sequence deposited in a high-energy, storm-wave-dominated, shallow-marine environment. The features of the unit suggest deposition on a shoreface in proximity to a fluvial source. Distinguishing characteristics include amalgamated hummocky-stratified sandstone, low-angle cross-stratified conglomerate beds, lenticular conglomerate beds, tabular conglomerate beds, high-angle scours, trough cross-bedded sandstone, organic-rich beds, and burrows of several sizes. These features are interpreted in terms of their depths of deposition, generation by fair weather, storm, and/or flood processes, and as indicators of relative rates of sedimentation. The unit records complex interaction of tectonic, wave, fluvial and tidal processes. Relative depth changes indicated in the sequence probably reflect changes in the balances between sediment supply, basin subsidence, and eustatic sea-level changes. Wave processes are manifest in the presence of structures deposited both above and below fair-weather wave base. Deposition and reworking by chaotic seas associated with local storms and by long-period swell associated with distant storms may be indicated by the interbedding of amalgamated hummocky-stratified sandstone and conglomerate beds. The effects of the lateral migration of a fluvial source with time are reflected by the changing nature and apparent rates of supply of sediments deposited in the nearshore zone. Tidal processes are manifest largely as superimposed features on wave- and current-generated structures. Biogenic structures are an important clue to the rates and frequencies of all these processes. The conglomeratic sandstone was deposited on a high-energy, tectonically active continental margin, similar to the present setting of the southern Oregon coast. It was deposited at varying depths in a subsiding basin during a time of world-wide sea-level rise. The sequence provides an opportunity for the study of coarse-grained nearshore-marine sediments and the delineation of the interaction of various processes during deposition. Detailed sedimentologic and palaeoecologic analysis provides insight into the nature of storm and flood processes that are difficult to observe in the modern environment.

The Rio Dell Formation was deposited in a tectonically active area during a time of fluctuating eustatic sea-level. Detailed interpretation of Rio Dell shelf sediments, facilitated by comparison to the nearby, analogous, modern Eel River shelf, has proved a vital tool for decoding the record of these influences. Understanding of the nature and relative roles of processes on the modern and ancient shelf, including those associated with storms and with river floods, not only allows for detailed paleobathymetric reconstruction of the ancient sequence, but also for recognition of relatively subtle effects of syndepositional tectonism. Shelf sediments of the Rio Dell Formation comprise four depositional sequences bounded by thin, submarine condensed sections. The condensed sections formed during periods of rapid, probably largely eustatic, sea-level rise. The sequences each record progradation of the shelf, predominantly during sea-level highstands. Paleobathymetric reconstruction provides the basis for interpretation of the magnitude of local, relative sea-level changes within and between the sequences. Subsidence of the Rio Dell shelf apparently played a role in recorded local sea-level fluctuations, as did eustasy. Subsidence ("geohistory") analysis of the sequence indicates that the shelf was subsiding throughout its existence, due not only to sediment loading but also due to tectonism. The analysis points to a period of rapid shelf subsidence, accompanied by onland uplift, between about 1.3 and 1.2 Ma. An increase in subsidence rates after about 1.3 Ma apparently caused an increase in Rio Dell shelf gradient, which changed the balance between flood and storm processes on the middle shelf. Increased shelf inclination evidently resulted in restriction of storm-wave reworking to a narrower zone extending from the shoreline, whereas distance from shoreline to the shelf area most influenced by river-flood deposition remained unchanged. The resultant increased segregation of flood and storm effects caused a fining of background grain-size distributions at mid-shelf sites.