Showing papers in "Journal of Sedimentary Research in 2006"

Width and Thickness of Fluvial Channel Bodies and Valley Fills in the Geological Record: A Literature Compilation and Classification

Abstract: The three-dimensional geometry of fluvial channel bodies and valley fills has received much less attention than their internal structure, despite the fact that many subsurface analyses draw upon the geometry of suitable fluvial analogues. Although channel-body geometry has been widely linked to base-level change and accommodation, few studies have evaluated the influence of local geomorphic controls. To remedy these deficiencies, we review the terminology for describing channel-body geometry, and present a literature dataset that represents more than 1500 bedrock and Quaternary fluvial bodies for which width (W) and thickness (T) are recorded. Twelve types of channel bodies and valley fills are distinguished based on their geomorphic setting, geometry, and internal structure, and log-log plots of W against T are presented for each type. Narrow and broad ribbons (W/T 1000, respectively) are distinguished. The dataset allows an informed selection of analogues for subsurface applications, and spreadsheets and graphs can be downloaded from a data repository. Mobile-channel belts are mainly the deposits of braided and low-sinuosity rivers, which may exceed 1 km in composite thickness and 1300 km in width. Their overwhelming dominance throughout geological time reflects their link to tectonic activity, exhumation events, and high sediment supply. Some deposits that rest on flat-lying bedrock unconformities cover areas > 70,000 km2. In contrast, meandering river bodies in the dataset are < 38 m thick and < 15 km wide, and the organized flow conditions necessary for their development may have been unusual. They do not appear to have built basin-scale deposits. Fixed channels and poorly channelized systems are divided into distributary systems (channels on megafans, deltas, and distal alluvial fans, and in crevasse systems and avulsion deposits), through-going rivers, and channels in eolian settings. Because width/maximum depth of many modern alluvial channels is between 5 and 15, these bodies probably record an initial aspect ratio followed by modest widening prior to filling or avulsion. The narrow form (W/T typically < 15) commonly reflects bank resistance and rapid filling, although some are associated with base-level rise. Exceptionally narrow bodies (W/T locally < 1) may additionally reflect unusually deep incision, compactional thickening, filling by mass-flow deposits, balanced aggradation of natural levees and channels, thawing of frozen substrates, and channel reoccupation. Valley fills rest on older bedrock or represent a brief hiatus within marine and alluvial successions. Many bedrock valley fills have W/T < 20 due to deep incision along tectonic lineaments and stacking along faults. Within marine and alluvial strata, upper Paleozoic valley fills appear larger than Mesozoic examples, possibly reflecting the influence of large glacioeustatic fluctuations in the Paleozoic. Valley fills in sub-glacial and proglacial settings are relatively narrow (W/T as low as 2.5) due to incision from catastrophic meltwater flows. The overlap in dimensions between channel bodies and valley fills, as identified by the original authors, suggests that many braided and meandering channel bodies in the rock record occupy paleovalleys. Modeling has emphasized the importance of avulsion frequency, sedimentation rate, and the ratio of channel belt and floodplain width in determining channel-body connectedness. Although these controls strongly influence mobile channel belts, they are less effective in fixed-channel systems, for which many database examples testify to the influence of local geomorphic factors that include bank strength and channel aggradation. The dataset contains few examples of highly connected suites of fixed-channel bodies, despite their abundance in many formations. Whereas accommodation is paramount for preservation, its influence is mediated through geomorphic factors, thus complicating inferences about base-level controls.

Terminal Distributary Channels and Delta Front Architecture of River-Dominated Delta Systems

Abstract: Using modern and ancient examples we show that river-dominated deltas formed in shallow basins have multiple coeval terminal distributary channels at different scales. Sediment dispersion through multiple terminal distributary channels results in an overall lobate shape of the river-dominated delta that is opposite to the digitate Mississippi type, but similar with deltas described as wave-dominated. The examples of deltas that we present show typical coarsening-upward delta-front facies successions but do not contain deep distributary channels, as have been routinely interpreted in many ancient deltas. We show that shallow-water river-dominated delta-front deposits are typically capped by small terminal distributary channels, the cross-sectional area of which represents a small fraction of the main fluvial "trunk" channel. Recognizing terminal distributary channels is critical in interpretation of river-dominated deltas. Terminal distributary channels are the most distal channelized features and can be both subaerial and subaqueous. Their dimensions vary between tens of meters to kilometers in width, with common values of 100–400 m and depths of 1–3 m, and are rarely incised. The orientation of the terminal distributary channels for the same system has a large variation, with values between 123° (Volga Delta) and 248° (Lena Delta). Terminal distributary channels are intimately associated with mouth-bar deposits and are infilled by aggradation and lateral or upstream migration of the mouth bars. Deposits of terminal distributary channels have characteristic sedimentary structures of unidirectional effluent flow but also show evidence of reworking by waves and tides.

Significance of Different Modes of Rhizolith Preservation to Interpreting Paleoenvironmental and Paleohydrologic Settings: Examples from Paleogene Paleosols, Bighorn Basin, Wyoming, U.S.A.

Abstract: Different modes of preservation of root traces (rhizoliths) provide information on soil-moisture regimes in alluvial paleosols in the Paleogene Fort Union and Willwood formations of the Bighorn Basin, Wyoming, U.S.A. This paper links different styles of rhizolith preservation to paleosols whose other pedogenic attributes provide information on ancient soil drainage. Consequently, the paleodrainage significance of different rhizolith preservation patterns is established more confidently. Rhizoliths consisting of elongate gray mottles (rhizohaloes) with red rims are common in relatively well-drained red paleosols. The gray zones are depleted of iron (Fe) whereas the color of the rims indicates hematite accumulation. These features are typical of surface-water gley processes that caused Fe and manganese (Mn) to move from the root channel outward to the soil matrix. Calcareous rhizocretions—either calcareous, tubular concretions or micro-accumulations of carbonate within gray rhizotubules—are also common in moderately well-drained red paleosols. More poorly drained purple paleosols also have rhizoliths consisting of Fe depletion zones; however, the rhizohaloes are surrounded by yellow-brown (goethite) rims rather than red hematitic rims. In many paleosols, the red and yellow-brown accumulation rims were partly removed by continued depletion, producing abundant irregular-shaped mottles. The nature of the rhizohalo and the color of the Fe-oxide rim provide information about the intensity of gleying that a paleosol underwent. The poorly drained paleosols also show rhizoliths preserved as goethite accumulations contained within gray depletion zones, indicating movement of reduced Fe from the matrix towards the root channel, probably as a result of groundwater-gley processes. The goethite accumulations are intermingled locally with lignite, representing preservation of the root organic material. Some rhizoliths are preserved as tube-shaped concentrations of small (0.1–0.2 mm diameter) black spheres whose color indicates a mix of Mn and Fe oxides. These are most common in paleosols with low-chroma (gray) matrix colors, indicating very poor drainage. The very poorly drained paleosols also show rhizoliths preserved in jarosite, which is an oxidation product of pyrite. Careful observational and geochemical analysis of rhizoliths, which are common in continental deposits, can help produce a clearer and more thorough interpretation of ancient drainage conditions. Information about degree of ancient soil wetness or moisture is important for understanding past climatic conditions and for reconstructing terrestrial paleolandscapes.

Stratigraphy, Sedimentary Structures, and Textures of the Late Neoproterozoic Doushantuo Cap Carbonate in South China

Abstract: The 3- to 5-m-thick Doushantuo cap carbonate in south China overlies the glaciogenic Nantuo Formation (ca. 635 Ma) and consists of laterally persistent, thinly laminated and normally graded dolomite and limestone indicative of relatively deep-water deposition, most likely below storm wave base. The basal portion of this carbonate contains a distinctive suite of closely associated tepee-like structures, stromatactis-like cavities, layer-parallel sheet cracks, and cemented breccias. The cores of tepees are composed of stacked cavities lined by cements and brecciated host dolomicrite. Onlap by laminated sediment indicates synsedimentary disruption of bedding that resulted in a positive seafloor expression. Cavities and sheet cracks contain internal sediments, and they are lined by originally aragonitic isopachous botryoidal cements with acicular radiating needles, now replaced by dolomite and silica. Pyrite and barite are common, and calcite is locally retained as a primary mineral. These features share morphological and petrographic attributes with modern and ancient methane seeps in which methane gas and fluids provide both a force for physical disruption from buoyancy and a source of alkalinity for significant cementation. The presence of δ13C values as low as −41‰ in well preserved limestone crusts and cements within and immediately above the tepee-like structures provides unequivocal evidence for methane influence, and the widespread distribution of identical sedimentary structures and paragenetic cement sequences across the entire basin at the same basal cap carbonate level is consistent with gas hydrate destabilization and the development of methane seeps as a result of postglacial warming of the ocean. Considering the broad distribution of similar features at the same stratigraphic level in other cap carbonates globally, we suggest that the late Neoproterozoic postglacial methane release may have influenced the oceanic oxygen level as well as contributed to postglacial warming via the greenhouse effects of methane.

Base-Level Buffers and Buttresses: A Model for Upstream Versus Downstream Control on Fluvial Geometry and Architecture Within Sequences

Abstract: The effects of downstream base-level control on fluvial architecture and geometry are well explored in several broadly similar sequence-stratigraphic models. Cretaceous Dakota Group strata, U.S. Western Interior, have characteristics reflecting combined downstream and upstream base-level controls that these models cannot address. Particularly, three layers of amalgamated channel-belt sandstone within this group thicken and are continuous for distances (≤ 300 km) along dip that stretch the reasonable lengths for which these models are intended to apply. As well, architecture in up-dip reaches records repeated valley-scale cut-and-fill cycles. This contrasts with equivalent strata down dip which record channel-scale lateral migration with no such valley-scale cycles apparent. We here introduce the concept of "buffers and buttresses" to address these observations. We assume that river longitudinal profiles are each anchored down dip to some physical barrier (e.g., the sea strand, etc.) that we refer to as a "buttress." Buttress shift is considered the primary downstream control on base level. Profiles extrapolated up dip from the buttress over any modeled duration of buttress shift can range widely because of high-frequency variability in upstream base-level controls (e.g., discharge, etc). All these potential profiles however are bounded above by the profile of highest possible aggradation, and below by the profile of maximum possible incision. These upper and lower profiles are "buffers," and they envelop the available fluvial preservation space. Thickness of the buffer zone is determined by variability in upstream controls and should increase up dip to the limit of downstream profile dominance. Dakota valley-scale surfaces record repeated cut-and-fill cycles driven by up-dip controls and are confined between thick stable buffers. Equivalent strata down dip record lateral reworking within a thinner channel-scale buffer zone that was positioned by downstream controls. Regression exposed slopes similar to the buffer zone, thus buffers were stable for long distances and durations. This prompted dip-extensive lateral reworking of strata into upstream valley-scale and downstream channel-scale sheets. Buffers and buttresses provide a broadly applicable model for fluvial preservation that captures upstream vs. downstream base-level controls on geometry and architecture. The model lends general insights into dip-oriented variations in fluvial architecture, production of sheet vs. lens geometry, total preservation volumes for fluvial systems, and variations in these factors related to contrasting climatic conditions and basin physiography. The model can be amended to existing sequence stratigraphic approaches in order to capture dip-oriented variations in sequence architecture.

Deltas and Sea-Level Change

Abstract: Sea-level shift from the innermost shelf out to the shelf edge produces bayhead, inner-shelf, mid-shelf, and shelf-margin deltas. We suggest that these delta types are distinguishable in the ancient record and that such distinction has advantages as compared to the conventional, entirely process-based classification. Bayhead and inner-shelf deltas tend to form thin clinoforms (a few meters to tens of meters amplitude, respectively), and as they aggrade with rising relative sea level they generate a "tail" of thick paralic deposits. Mid-shelf deltas produce clinoforms as high as the mid-shelf water depth, tend to follow a subhorizontal trajectory, generate little or no paralic tail, and are commonly thinned by transgressive ravinement. Shelf-edge deltas in a stable-to-falling relative sea level usually have no paralic tail, create by far the highest clinoforms, and can have a thick succession of sandy turbidites on the delta fronts. If sea level falls below the shelf margin, the shelf-edge delta becomes incised by its own channels and large volumes of sand can be delivered onto the slope and the basin floor. Many deltas require a strong fluvial drive to attain a shelf transit, though as they approach the outer shelf they commonly become wave dominated. Tidal influence can increase on the outermost shelf if relative sea level is falling, if the shelf-break is poorly developed, and if basinal water depth is shallow. During transgression, the system tends to be tidally and/or wave influenced. Deltas that transit back and forth on the shelf on short time scales (tens of kiloyears to 100 ky) and that are driven largely by sea-level fluctuations are referred to here as accommodation-driven deltas. Deltas that can reach the shelf edge without sea-level fall are termed supply-driven deltas. These highstand deltas deposit thick, sandy, stacked parasequences during their shelf transit, and they tend to have an extensive muddy delta front on reaching the shelf-edge area. Such deltas would not normally be incised at the shelf edge, and they would produce a progradational, shelf-edge attached, sandy slope apron (Exxonian shelf-margin systems tract) rather than basin-floor fans, except in cases of extremely high supply. Sequence boundaries are best developed on accommodation-driven deltas, and are likely to be represented on a variety of time scales (third, fourth, and fifth order). Sequence boundaries in supply-dominated deltas may be identifiable only at lower-order time scales, or they may be non-existent.

Ancient lacustrine hyperpycnites: a depositional model from a case study in the rayoso formation (cretaceous) of west-central argentina

Abstract: Hyperpycnal flows originate when sediment-laden fluvial discharges enter standing, lower-density water. Because of their excess density, the flows plunge near the river mouth and continue to travel basinward as a quasi-steady and fully turbulent underflow. The related deposits are hyperpycnites, and constitute a particular type of turbidite with poorly known facies and facies tracts. Although hyperpycnal flows seem to be quite common in present times, their occurrence in fossil strata is poorly documented. This paper addresses the characteristics and depositional processes of shallow lacustrine sandy hyperpycnites, on the basis of the field analysis of well-exposed Lower Cretaceous strata (Rayoso Formation) in the Neuquen Basin of west-central Argentina. The Rayoso Formation is composed of clastic (and minor evaporitic) red beds up to 1200 m thick, deposited in a shallow perennial lake of variable salinity affected by long-lived hyperpycnal flows. Main clastic facies are composed of fine-grained sandstones with climbing ripples and plane beds. Other common sandstone facies include massive beds and low-angle cross stratification. Most sandstone facies are related to traction plus fallout processes, and often show a vertical fluctuation between sedimentary facies originated under different traction-plus-fallout conditions within single beds. These fluctuations are interpreted to be evidence of deposition from flow fluctuations in sustained hyperpycnal flows. Most beds internally show the existence of three depositional phases, acceleration (AP), erosion plus bypass (EP), and deceleration (DP), which record the complete evolution of a single long-lived hyperpycnal flow at a fixed point. Additionally, the depositional evolution of a single long-lived hyperpycnal flow with distance records initially the progressive basinward migration of the AP and EP phases, and finally an overall deposition under the DP phase both in proximal and distal areas. This evolution provides an adequate explanation for the basinward extension of channelized features, and for the occurrence of fine-grained sandstones with climbing ripples both in proximal and distal positions within the same hyperpycnal system. Consequently, facies analysis derived from application of the Bouma sequence is not valid for deposits of quasi-steady hyperpycnal flows.

Stratigraphic Evolution of Fine-Grained Submarine Fan Systems, Tanqua Depocenter, Karoo Basin, South Africa

Abstract: The integration of correlated outcrop and newly acquired core and wireline logs, extensive paleocurrent data, and accurately mapped surfaces has enabled a common model of the stratigraphic evolution to be developed of four Permian fine-grained submarine fan systems (Fans 1–4) from the Tanqua depocenter, SW Karoo Basin, South Africa. Additionally, this data has revealed the influence of subtle seabed topography on the fan systems' boundaries, internal facies architecture, and paleocurrent directions. Furthermore, the internal stratigraphy of individual fan systems are now known to be more complex than first believed. Mapping high-frequency intrafan units reveals a progradational–aggradational–retrogradational stacking pattern common to each submarine fan, which allows the geographic and stratigraphic distribution of lithofacies and architectural elements to be predicted. Basinward of the main feeder system, fan axes are dominated architecturally by sheet turbidites with discrete zones of high amalgamation during progradational and aggradational phases, whereas isolated channel forms that cut thin-bedded turbidites are more common in the retrogradational phase. This change is interpreted to be due to local increase in the lower slope gradient through sediment aggradation increasing the potential for channel avulsion and the development of splay lobes during decreasing sediment supply. The progradational, aggradational, and retrogradational phases are assigned to the early, middle, and late lowstand systems tract of a fifth-order sequence respectively. Each phase is built of higher-frequency (sixth-order) sequences so that each fan is a composite sequence. The overall basinward-to-landward stepping of the individual fans means that moving down dip in any basin-floor fan system, the lowermost sandstone preserved is progressively younger, whilst the uppermost sandstone is progressively older. This stacking pattern imparts an important predictable control on reservoir and seal geometries. This study aids the development of predictive models for fine-grained submarine fan initiation, growth, and abandonment, and lithofacies and architectural element distributions in space and time.

Local Controls on Carbon Cycling in the Ordovician Midcontinent Region of North America, with Implications for Carbon Isotope Secular Curves

Abstract: The carbon isotope record from ancient epicontinental seas may contain much more of a local-scale carbon cycling signal than is generally appreciated. A unique opportunity exists to examine this issue in the case of the Late Ordovician Mohawkian Sea of eastern Laurentia, where the Millbrig K-bentonite stratigraphic framework has been used to delineate a time slice at 454 Ma, extending over ~ 1,500,000 km2 of the eastern United States. Across the time slice, carbonate and organic carbon δ13C vary by 4.5‰ and 7.5‰ respectively, a spatial variation that is as large as temporal (secular) changes in epeiric-sea δ13C that have been reported in the past. These new data are considered in the context of geographic variations in lithological, biological, and other geochemical sediment characteristics. Collectively, these sediment properties distinguish regions of the Mohawkian Sea which likely differed in terms of the nature and relative importance of carbon cycling processes. Water-column depth and structure, and barriers to free exchange of water across the Mohawkian Sea, may have been overarching factors in the development of these regions, raising the possibility that changes in circulation patterns, such as those caused by sea-level change, played a role in driving secular carbon isotope excursions by changing the rate of exchange of dissolved inorganic carbon between water masses. If the observed effects of local carbon cycling on the distribution of Mohawkian Sea δ13C were commonplace in ancient epicontinental marine environments, it would imply that local-scale carbon cycling may have left a nontrivial imprint on epeiric-sea records of secular variations in δ13C, in addition to the imprint left by changes in the global carbon cycle. This may have contributed to the broad scatter in δ13C values observed in the Paleozoic portion of the global carbon isotope secular curve.

Triggering and Evolution of a Giant Submarine Landslide, Offshore Angola, Revealed by 3D Seismic Stratigraphy and Geomorphology

Abstract: The source area of a large submarine landslide complex has been imaged in the subsurface off Angola using 3D seismic data. The 3D seismic data reveal erosional slide scars, landslide blocks, and debris flows involving > 20 km3 material, affecting an area > 430 km2. The basal and internal structure is imaged in detail showing basal erosion and evidence for at least three phases of landslide failure. Landslide internal structure reveals evidence for seafloor remobilization during the initial phase of submarine landslide failure. Missing strata and downslope-trending striations 15 m deep and 9 km long on the basal sliding surface record the initial failure over an area > 130 km2. The basal striations originate from and extend downslope from a major growth fault trending NW–SE. We speculate that fluid flow or seismicity associated with this growth fault triggered landsliding. The landslide deposit consists of large blocks (~ 1 to > 5 km across and ~ 100 to 150 m thick) which have slid, rotated, and deformed in the landslide within a chaotic debris-flow matrix. A smoother debris-flow morphology extends from the erosional scars and blocks to the SE beyond the boundaries of the survey area. A rough estimate of landslide geometry indicates that the deposit volume is double the volume of erosional slide scars within 5 km of source. The observations from this study are summarized in a process-oriented landslide model. Results provide insight into the triggering, evolution, and runout potential of giant submarine landslides.

High-Resolution Stratigraphy of an Underfilled Lake Basin: Wilkins Peak Member, Eocene Green River Formation, Wyoming, U.S.A.

Abstract: Lakes tend to respond noticeably to minor changes in sediment and water balance driven by climatic, tectonic, or geomorphic processes. This unique behavior of lacustrine basins can provide a high-resolution record of geologic processes within the continental setting, far from the globally averaged record of marine strata. The Wilkins Peak Member of the Eocene Green River Formation, in Wyoming, USA, is dominated by aggradation of repetitive sedimentary facies successions recording distinct lacustrine expansions and contractions. These lacustrine "cycles" consist of up to four successive facies associations: littoral, profundal–sublittoral, palustrine, and salt pan. Because they comprise disparate facies that may never have been simultaneously deposited in the basin, Wilkins Peak Member cycles are non-Waltherian successions that do not readily equate to any established sequence stratigraphic unit. The completeness of the stratigraphic record in the Wilkins Peak Member varies continuously across the basin. At least 126 cycles are present in the ERDA White Mountain #1 core near the basin depocenter, whereas only about one third as many are recognizable 53 km north, nearer to the basin margin. Cycle boundaries terminate northward by gradual amalgamation into palustrine facies, reflecting the interplay between varying lake levels and a south-dipping deposition gradient. Evidence for complete desiccation and hiatuses is commonplace even near the basin center; a truly complete record may not be present anywhere. Based on recent 40Ar/39Ar geochronology of tuffs, the average cycle duration in the White Mountain #1 core is approximately 10,000 years. However, the true average duration is shorter due to the presence of lacunae, and the time required for deposition of the thinnest cycles may have been less than 1000 years. No external driving mechanism is presently known for Eocene cycles of such short duration, raising the possibility that they are instead autogenic cycles related to geomorphic instability of the surrounding landscape. Evaporite deposition corresponded closely in time and space to maximum differential subsidence, suggesting that tectonic influences on drainage patterns and basin accommodation exerted a primary control. Differential accommodation was most pronounced during deposition of the lower Wilkins Peak Member and became progressively less so after that time. Based on structural and stratigraphic observations, a period of increased tectonic uplift commenced concurrent with deposition of the lower Wilkins Peak Member.

Seismic Facies of Incised-Valley Fills, New Jersey Continental Shelf: Implications for Erosion and Preservation Processes Acting During Latest Pleistocene–Holocene Transgression

Abstract: Incised-valley fills shallowly buried beneath the New Jersey middle–outer shelf reveal a retrogradational shift of four seismic facies, as observed in 1–4 kHz deep-towed chirp seismic data. These facies, the only preserved stratigraphic record of the latest Quaternary–Holocene drowning and infilling of fluvial drainage systems developed on this exposed shelf at or near the Last Glacial Maximum (LGM), are interpreted as (1) fluvial lag deposits, SF1; (2) estuarine mixed sand and muds, SF2; (3) estuary central bay muds, SF3; and (4) redistributed estuary-mouth sands, SF4. These fills are truncated by a transgressive ravinement, the T horizon, which is in turn overlain by Holocene marine sand deposits. The seismic facies are bounded by reflectors marking either source diastems or unconformities: (1) the Channels horizon is the lowstand fluvial incision surface, (2) B1 is a bay flooding surface, (3) B2 is an intra-estuarine depositional surface, (4) B3 is a tidal ravinement surface, and (5) the T horizon represents erosion at or near the shoreface during Holocene transgression. The Channels horizon is generally preserved only in valley axes. Elsewhere, this sequence boundary has been modified by surfaces B1 and/or B3. Dip-oriented changes in the thickness of SF3 and SF4 suggest either a stillstand in the passage of the shoreline, which allowed such spatial variations, or that local valley shape controlled hydrodynamic conditions for sediment transport and deposition. Narrower valleys may have promoted tidally dominated, fine-grained deposition within these drowning estuaries, while broader valleys attenuated tidal flow velocities and allowed the filling of the estuary to be dominated by wave and current energy, promoting more coarse-grained deposition. Our study demonstrates that wave- and tide-dominated estuarine facies can coexist within such fill strata.

Sea-Level Variation During the Holocene Deduced from the Morphologic and Stratigraphic Evolution of Morgan Peninsula, Alabama, U.S.A.

Abstract: Recent Gulf of Mexico shoreline studies interpret middle to late Holocene sea level as falling from a level above present elevation or stable at present elevation; however, the architecture of Morgan Peninsula, Alabama, does not support this. Morgan Peninsula is a beach-ridge strandplain composed of two obliquely aligned Holocene beach-ridge sets. Ground-penetrating radar profiles discriminate between parallel, even to wavy reflectors of the eolian dune environment and the underlying seaward-dipping, complex sigmoidal-oblique reflectors of the foreshore and upper-shoreface environment. The contact between foreshore and eolian facies in beach ridges can be used as a sea-level indicator. The average elevation of this contact in Morgan Peninsula rises throughout shoreline accretion, which occurred throughout the last 5.4 ka, suggesting that there was continual sea-level rise during this time. Morgan Peninsula is a useful modern analog to ancient shoreface–shelf parasequences and demonstrates the significant internal complexities that can exist in these deposits. Erosional discontinuities imaged in the Holocene foreshore–upper shoreface environment are laterally continuous, extend to elevations above mean sea level, and correlate to beach ridges and the transition between beach-ridge sets. An increase in the wave regime or fluctuations in sediment supply appear to be likely mechanisms for forming erosional discontinuities below beach ridges. The erosional surface separating beach-ridge sets may have formed by increased storm activity and associated barrier breaching, or a reconfiguration of the Mobile Bay tidal-delta complex superimposed on a gradual rate of sea-level rise. This boundary is recognized by a change in beach-ridge orientation at the surface and an increase in the aggradational component of shoreline accretion.

The Tsunamite Problem

Abstract: The genetic term tsunamite is used for a potpourri of deposits formed from a wide range of processes (overwash surges, backwash flows, oscillatory flows, combined flows, soft-sediment deformation, slides, slumps, debris flows, and turbidity currents) related to tsunamis in lacustrine, coastal, shallow-marine, and deep-marine environments. Tsunamites exhibit enormous variability of features (e.g., normally graded sand, floating mudstone clasts, hummocky cross stratification, etc.). These sedimentary features may also be interpreted as deposits of turbidity currents (turbidites), debris flows (debrites), or storms (tempestites). However, sedimentary features play a passive role when these same deposits are reinterpreted as tsunamites on the basis of historical evidence for tsunamis and their triggering mechanisms (e.g., earthquakes, volcanic explosions, landslides, and meteorite impacts). This bipartite (sedimentological vs. historical) approach, which allows here classification of the same deposit as both turbidite and tsunamite, has blurred the distinction between shallow-marine and deep- marine facies. A solution to this problem is to classify deposits solely by a descriptive sedimentological approach. The notion that tsunami waves can directly deposit sediment in the deep sea is unrealistic because tsunami waves represent transfer of energy and they are sediment starved. During tsunamis and major storms, submarine canyons serve as the physical link between shallow-water and deep-water environments for sediment transport. Tsunami-related deposition involves four progressive steps: (1) triggering stage (offshore), (2) tsunami stage (incoming waves), (3) transformation stage (near the coast), and (4) depositional stage (outgoing sediment flows). In this progression, deep-water deposition can commence only after the demise of incoming tsunami waves due to their transformation into outgoing sediment flows. Deposits of these sediment flows already have established names (e.g., debrite and turbidite). Therefore, the term tsunamite for these deposits is obsolete.

The Middle Eocene Seeb Formation of Oman: An Investigation of Acyclicity, Stratigraphic Completeness, and Accumulation Rates in Shallow Marine Carbonate Settings

Abstract: The middle Eocene Seeb Formation represents deposition on one of the earliest "modern" style carbonate platforms, i.e., influenced by seagrasses and mangroves, and presents an opportunity to explore controls on the preservation of Cenozoic carbonate lithofacies. In the study area, the Seeb Fm. is dominated by an anomalously thick (approximately 250 m) and uniform package of shallow marine, platform-interior sediments. These nodular, indistinctly bedded shallow-subtidal sediments display no evidence for relative sea-level change (such as subaerial exposure features), and lack the shallowing-upwards cyclothems that have characterized carbonate sediments deposited in platform-interior settings throughout much of the rock record (particularly during greenhouse periods). We conclude that this is a consequence of thorough bio-retexturing of the sediment by burrowing organisms and the roots of marine vegetation, which destroyed primary fabrics, facies diversity, evidence for cyclicity, and "missing time" horizons such as cycle-bounding exposure surfaces. It seems possible that the remarkable thickness of apparently acyclic Seeb Fm. sediment that built up may reflect increased bio-disturbance of the shallow marine environment following the Late Cretaceous expansion of seagrasses and mangroves. A similar lack of peritidal cyclothems may be common to many Cenozoic shallow marine carbonate deposits. "Missing time" horizons in limestones, reflecting erosion or nondeposition, have been identified by some authors to explain discrepancies between low calculated accumulation rates of ancient sediments and higher rates measured in comparable modern environments. However, our studies of the Seeb Fm. suggest that the significance of such "missing time" horizons in ancient sediments may have been overstated, and that apparent differences between modern and ancient accumulation rates are a consequence of the extrapolation of unrepresentative, localized high rates of modern sediment production to large (platform-scale) areas. Our study reinforces the idea that modern sediment production and accumulation rates may be much lower than previously thought because they are typically measured in highly productive areas such as seagrass beds and do not take into account the highly variable nature of carbonate production, storage, erosion, and destruction across platforms. Therefore, far less "missing time" needs to be inferred for ancient sediments to account for any imbalance between modern and ancient sediment accumulation rates.

Experimental Measurement of the Relative Importance of Controls on Shoreline Migration

Abstract: Shoreline position in sedimentary rocks is a sensitive recorder of the interplay of several controlling factors. The most important of these are thought to be the "stratigraphic trinity:" eustatic sea level, subsidence, and sediment supply. In ancient rock sequences, it is generally difficult to disentangle the effects of these variables. Here we analyze the relative influence of sea level, subsidence, sediment supply, and other controlling variables on shoreline migration in an experimental basin equipped with a subsiding floor. The experiment used a linear-hinge type subsidence profile for which the rate was kept constant in time, constant overall sediment supply, and base-level variation on two time scales that were first applied separately and then superimposed. Although base level was the only controlling variable that was externally varied in time, the base-level changes induced changes in other variables indirectly (e.g., by changing the partitioning of sediment between the fluvial and offshore segments of the system). We examine the relative importance of measured direct and indirect changes in all the governing variables through the use of a moving-boundary equation for shoreline migration. When measured values are used for all the variables in the equation, shoreline migration rate throughout the run can be predicted with a maximum R2 of > 0.92. Starting with this optimal prediction of the observed shoreline behavior, we successively replace variables in the equation with their run-averaged values, degrading the prediction. The relative loss of prediction accuracy as each variable is replaced is a measure of the importance of that variable in accounting for the observed shoreline migration. By this measure, base level is the most important variable, followed in turn by sediment supply to the foreset, geometry of the foreset, and the average subsidence rate over the foreset. From the shoreline migration equation, we also derive a quantitative version of the "A/S ratio" often applied in sequence stratigraphy. The new formulation reduces to a form comparable to the traditional descriptive A/S ratio if changes in foreset slope, gain or loss of sediment to the fluvial system, and spatial variation in subsidence rate are all negligible.

The Significance of Hummocky Cross-Stratification (HCS) Wavelengths: Evidence from an Open-Coast Tidal Flat, South Korea

Abstract: Although hummocky cross-stratification (HCS) is one of the most common and widely recognized structures in ancient storm-dominated successions, the stratigraphic variability and environmental significance of HCS wavelength (λ) are still not widely appreciated. New evidence from an open-coast intertidal flat where HCS might not have been expected to occur shows that the HCS becomes smaller in a landward direction because of a decrease of wave size. This confirms previous suggestions that the bedform responsible for HCS is a type of orbital ripple. A review of new and previously published data indicate that HCS wavelength is controlled by the bottom orbital diameter (d0) according to the relationship λ 0.75 d0. These observations imply that the maximum size of HCS should increase with decreasing water depth from the shelf to the surf zone (breaking point) but may then decrease landward of this point because wave size is depth limited. This suggests that it may be possible to use HCS size in paleo-environmental reconstructions to a greater degree than previously.

The Signal and the Noise: Forward Modeling of Allocyclic and Autocyclic Processes Influencing Peritidal Carbonate Stacking Patterns

Abstract: Recent carbonate sequence stratigraphic models suggest that the degree of order present in successions of carbonate parasequences is determined by temporal variations in eustatic amplitude and by varying levels of autocyclicity. Ordered parasequence stacking is suggested to occur during greenhouse periods dominated by low-amplitude eustatic oscillations. Conversely, disordered vertical successions are said to occur when amplitudes of eustatic oscillations are high, or when autocyclic processes dominate and thus drown the periodic external signal. These assertions of causal mechanism are tested here by application of a three-dimensional stratigraphic forward model. The three-dimensional model generates parasequences via a Ginsburg-type mechanism of sediment transport and shoreline progradation. Parasequence stacking is controlled by a superimposed, longer-term eustatic oscillation and parasequences generated are analyzed using runs analysis and Durbin-Watson autocorrelation. Results from the modeling suggest that autocyclic parasequences generated by shoreline progradation form ordered vertical successions when accommodation changes, driven by a low-amplitude relative sea-level oscillation, suggesting that partly autocyclic strata may not be inherently any less ordered than entirely allocyclic strata. Addition of stochastic processes such as mosaic carbonate production and fluctuations in regional sediment transport direction tend to decrease the level of measurable order present in the autocyclic strata. Increasing the frequency and amplitude of relative sea-level oscillations also tends to decrease the measurable order in stacked autocyclic peritidal parasequence thicknesses. These results suggest that controls on parasequence stacking patterns are likely to be more complex and varied than suggested by recent sequence stratigraphic models; more outcrop and modeling investigation is required, with appropriately circumspect interpretations applied.

Hyperpycnites Deposited 700 km Away from River Mouths in the Central Japan Sea

Abstract: Relatively little is known about the characteristics of deposits and potential runout distances of hyperpycnal currents. This study describes and discusses evidence of sediment deposition from sustained (long-duration), quasi-steady turbidity currents in the distal part of the Toyama deep-sea channel, which extends ca. 700 km from river mouths in the central Japan Sea. The study is based on gravity cores and airgun seismic reflection profiles obtained from the channel distal reaches. The silty turbidite beds of the channel's terminal fan show rhythmic layering that indicates sustained turbidity currents with distinct flow-strength fluctuations. Some of the rhythmite beds show a fining-upward internal trend (net flow-strength waning), whereas others show an upward coarsening (net flow-strength waxing) followed by fining. Seismic reflection profiles from the channel levees show large bedforms of climbing-dune type, attributed to the spillover of thick sustained turbidity currents. The deposition of the terminal fan rhythmites and accretionary levee bedforms is attributed to turbidity currents generated by hyperpycnal river effluent. A quantitative assessment of sediment concentrations in the coastal rivers indicates that their effluents could become hyperpycnal nearly every year or during every major seasonal flood. The density of a river-generated underflow would increase by the entrainment of saline water and seafloor sediment on the steep slope of the Toyama Bay, resulting in robust, long-runout turbidity currents. The estimated flow velocities of these currents were around 0.3 m/s, and their recurrence period for the last 1000 years was of the order of 70 years. The estimated duration of hyperpycnal flows required for the deposition of rhythmite beds 700 km away from the river mouth is of the order of several days to 3–4 weeks. The study provides new insights into the recognition and classification of hyperpycnites in distal zones of deep-marine turbiditic systems.

An Ichnological and Sedimentological Facies Model for Muddy Point-Bar Deposits

Abstract: Detailed analysis of fine-grained clastic deposits associated with the macrotidal estuarine mouth of the Shepody River, New Brunswick, Canada, concentrated on identifying the ichnological and sedimentological characteristics of tidally dominated point bars and their adjacent tidal flats. The aim of the study was to establish an ichnological facies model for similar deposits. Within the study area, the distribution of ichnological structures and sedimentary characteristics such as grain-size distribution and total organic content are associated with bar elevation (i.e., upper, middle, or lower intertidal), tidal-bank slope, and the local hydraulic processes. The distribution of animal traces is also influenced by the duration of intertidal exposure and sedimentation rates. Polykladichnus- and Skolithos-like traces characterize upper-subtidal and lower-intertidal zones of the point bars; Arenicolites-, Diplocraterion-, Polykladichnus-, Palaeophycus-, and Planolites-like forms are pervasive in middle-intertidal zones; and, Siphonichnus- and Polykladichnus-like burrows typify the upper-intertidal point bars and the tidal flats. The size and diversity of the burrowing fauna are affected by the chemistry of the depositional waters and by seasonal variations in composition and temperature of those waters. The extreme seasonality of the area favors opportunistic fauna and thereby contributes to an impoverished, brackish-water trace assemblage. Geomorphologically, the area is dominated by point-bar and tidal-flat deposits, which comprise rhythmic bedding, composed of interlaminated to thinly interbedded silty and sandy mud. Point-bar bedding dips channelwards and represents mud-dominated inclined heterolithic stratification (IHS). The IHS alternates between burrowed and laminated beds. Laminated beds contain rhythmic lamination. Cyclic variations in laminae thickness are attributed to neap–spring variation in tidal-current strength. The burrowed interbeds exhibit high degrees of bioturbation that eradicate the preexisting lamination. The intercalation of laminated and burrowed beds represent seasonal variations in the depositional system: laminated beds characterize early winter and early spring sedimentation, and the bioturbated beds represent late spring through fall deposits.

Cambrian depositional history of the zanskar valley region of the indian himalaya: tectonic implications

Abstract: A well-preserved Cambrian section in the Zanskar Valley of northern India has previously been interpreted to record the transition from a passive to an active tectonic margin related to Cambrian-Ordovician orogenesis. This interpretation has been used to support the tectonostratigraphic interpretation of other successions across the Tethyan Himalaya. Our detailed paleoenvironmental analysis significantly revises the tectonic and depositional history of these Cambrian deposits: no definitive record of impending Cambrian-Ordovician orogenesis is recorded in these late Middle Cambrian rocks. A critical transition from an , 125-m-thick, stromatolite-bearing carbonate deposit, the Karsha Formation, into shale and sandstone of the Kurgiakh Formation, was interpreted to represent tectonically induced drowning of a carbonate platform. Siliciclastic strata of the Kurgiakh Formation were thought to record deep-water flysch deposition in a tectonically active foreland basin next to an arc-trench system. This interpretation was based on sandstone beds with classic Bouma sequences. We show that these event beds in the Kurgiakh instead contain hummocky cross-stratification, quasi-planar lamination, and combined-flow ripple stratification, all of which reflect deposition in shallow-marine, storm-influenced environments. Thus, although the Karsha carbonate platform may have been drowned, it did not culminate in deep-sea flysch deposition, and this in turn eliminates a major line of evidence linking Kurgiakh deposition to the onset of Cambrian-Ordovician orogenesis. Other aspects of Cambrian-Ordovician deposits of northern India also shed doubt on the proposed link between Kurgiakh sedimentation and the Cambrian-Ordovician orogenic event. First, our improved biostratigraphic database suggests that the transition from the Karsha carbonate to the Kurgiakh Formation may have predated the main phase of Cambrian-Ordovician orogenesis, as recorded by overlying Ordovician molasse, by as much as 20-30 My. Second, published data from the Ordovician molasse indicate northward paleocurrents, which are parallel to those recorded by siliciclastic deposits of the Parahio Formation below the Karsha, and thus are at odds with standard models of foreland basin development for the Cambrian-Ordovician event. Our sedimentological analysis of depositional cycles of the Parahio Formation indicates that these strata record storm- influenced environments from offshore marine to shoreface to fluvial settings. This is at odds with previous paleoenvironmental interpretations that ranged from deep-sea flysch to intertidal deposits. Paleocurrent data for marine and fluvial facies of the Parahio Formation in both Zanskar and the Spiti Valley to the south indicate northeast sediment transport. This supports the view that the Parahio and overlying carbonate of the Karsha Formation record the ancient northern passive margin of India during the Cambrian and that these strata may be distal equivalents of the younger Cambrian deposits of the Lesser Himalaya.

The Use of the Common Pb Isotope Composition of Detrital K-Feldspar Grains as a Provenance Tool and Its Application to Upper Carboniferous Paleodrainage, Northern England

Abstract: K-feldspar is a common detrital mineral in sandstones. It contains little U or Th, and hence the common Pb isotope composition of unaltered K-feldspar is that of the source rock. Pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the Pb signature of detrital K-feldspar grains a useful provenance tool. However, it is unclear how robust this signal is, and to what extent it can be modified by weathering, transport, burial diagenesis, and/or exposure to hydrothermal fluids. This study reports an evaluation of the technique, using two granite–arkose pairs (from Helmsdale, northern Scotland, and Shap, northwest England) where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived. The results of this test have informed a pilot study using the Pb isotope composition of detrital K-feldspar in Upper Carboniferous fluvial sedimentary rocks of northern England to constrain the sediment routing. Pb isotope compositions have been measured using an in situ high-resolution single-grain technique that can distinguish original composition, heterogeneity, and subsequent alteration within individual sand grains. The results for the granite–arkose pairs show that the common Pb isotope signature of the detrital K-feldspar grains matches the in situ feldspars in the source granites. The signal is independent of grain size (medium-grained sand to granules), but alteration along a paleo-regolith at Helmsdale has resulted in perturbations in Pb composition and an increase in the errors on the analyses. The application of the technique to arkosic Carboniferous (Serpukhovian–Bashkirian) Millstone Grit Group sandstones in the Pennine Basin reveals two discrete populations of K-feldspars that cannot be distinguished petrographically. Matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far-traveled (> 500 km) source of K-feldspar from the Archean–Paleoproterozoic Lewisian rocks of northwest Scotland or their extension to the north, but also an equally important proximal source emanating from the Southern Uplands Belt of Scotland in what would have been the lower part of the hinterland drainage basin of the Millstone Grit.

Holocene Depositional History of the Burdekin River Delta of Northeastern Australia: A Model for a Low-Accommodation, Highstand Delta

Abstract: The Burdekin River of northeastern Australia has constructed a substantial delta during the Holocene (delta plain area 1260 km2). The vertical succession through this delta comprises (1) a basal, coarse-grained transgressive lag overlying a continental omission surface, overlain by (2) a mud interval deposited as the coastal region was inundated by the postglacially rising sea, in turn overlain by (3) a generally sharp-based sand unit deposited principally in channel and mouth-bar environments with lesser volumes of floodplain and coastal facies. The Holocene Burdekin Delta was constructed as a series of at least thirteen discrete delta lobes, formed as the river avulsed. Each lobe consists of a composite sand body typically 5–8 m thick. The oldest lobes, formed during the latter stages of the postglacial sea-level rise (10–5.5 kyr BP), are larger than those formed during the highstand (5.5–3 kyr BP), which are in turn larger than those formed during the most recent slight sea-level lowering and stillstand (3–0 kyr BP). Radiocarbon ages and other stratigraphic data indicate that inter-avulsion period has decreased through time coincident with the decrease in delta lobe area. The primary control on Holocene delta architecture appears to have been a change from a pluvial climate known to characterize the region 12–4 kyr BP to the present drier, ENSO-dominated climate. In addition to decreasing the sediment supply via lower rates of chemical weathering, this change may have contributed to the shorter avulsion period by facilitating extreme variability of discharge. More frequent avulsion may also have been facilitated by the lengthening of the delta-plain channels as the system prograded seaward.

Quartz Cement in the Fontainebleau Sandstone, Paris Basin, France: Crystallography and Implications for Mechanisms of Cement Growth

Abstract: Authigenic quartz cement is the most abundant form of diagenetic cement in clastic sedimentary rocks. Despite this, there are many unknowns relating to mechanisms of growth of quartz cement and the crystallography of quartz cement. The key focus of this paper is to investigate further the issues of crystallography and quartz cement growth mechanisms, using the shallowly buried Oligo-Miocene Fontainebleau sandstone, France, as a case study. We address the following points: (1) are authigenic quartz overgrowths really in crystallographic continuity with their substrate grains? (2) What is the crystallographic inter-relationship between zones of quartz cement growth? (3) Are all quartz overgrowths entirely quartz, or do other silica polymorphs exist within overgrowths? The study combines an array of techniques to answer these questions, including transmitted-light optics, cathodoluminescence (CL), and electron backscatter diffraction (EBSD), the latter of the two being performed with the use of a scanning electron microscope (SEM). The use of EBSD to this study is crucial because it provides essential crystallographic information on the grains and their overgrowths. The data revealed: (1) quartz overgrowths comprise several zones visible in optical and CL images as parallel, isopachous, alternating bright and dark bands; (2) these bands represent areas of poorly crystalline silica and fully crystalline quartz; (3) one entire zone consists only of poorly crystalline quartz; (4) the final growth stage occurred as prismatic microcrystalline quartz into the remaining porosity; (5) the crystallographic orientation across most of the overgrowth, as far as the microcrystalline quartz layer, is the same as that of the detrital grain (i.e., it is syntaxial); and (6) the microcrystalline quartz layer has crystals with different and variable orientations relative to the detrital grain. This indicates that part of the quartz cement is not in crystallographic continuity with the substrate grain and displays an epitaxial relationship. Detailed analysis of the orientation data shows that there is a rational crystallographic rotation around a variety of axes, which indicates that the orientation of the final growth stages was not random.

Natural Geomorphic Variability Recorded in a High-Accommodation Setting: Fluvial Architecture of the Pennsylvanian Joggins Formation of Atlantic Canada

Abstract: The varied form and architecture of suites of channel bodies is a product of the intrinsic geomorphic variability of the drainage network as well as extrinsic factors such as climate, eustasy, and tectonics. We investigated the balance between intrinsic and extrinsic controls on the basis of eighty two fluvial-channel bodies in the Pennsylvanian Joggins Formation, which is superbly exposed along the Bay of Fundy in Nova Scotia, Atlantic Canada. Based on their internal architecture and three-dimensional form, these channel bodies were divided into fixed, meandering, and multistory types, and each type is present in both coastal wetland and inland floodplain facies associations. Although the facies associations are organized into cyclic packages that probably represent relative changes in base level, the channel-body types are not positioned systematically within the cycles. Apart from a small group of unusually deep meandering channels within the wetland deposits, comparison of paleochannel depth and width/depth shows that active channels throughout the formation were similar in size and shape. The multistory bodies are small channel belts or valley fills that do not appear to record the basinward advance of facies belts but rather are genetically linked with the associated floodplain deposits. These observations suggest that the varied form of the Joggins channel bodies largely reflects the geomorphic variability of the original drainage network, with the largest, meandering channels in the coastal zone. Preservation of a representative drainage network was probably facilitated by rapid subsidence on bounding faults, enhanced by high-frequency, low-magnitude subsidence events caused by salt withdrawal. The effect of extrinsic factors on channel type and geometry appears to have been modest in this high-subsidence setting.

A Note on the Preservation of Offshore Tsunami Deposits

Abstract: In this contribution we explore the preservation potential of offshore tsunami deposits. The application of linear wave theory and flat-bottom conditions allows a simplified representation of the physical environment. In such an environment it is possible to compute the boundary water depth, below which the influence of storm waves on tsunami deposits is negligible. The majority of the tsunami deposits described in the literature were deposited onshore where the tsunami wave is transformed into bores and rollers. These deposits differ from tsunami sediments of the deeper shelf, where the water-sediment interaction corresponds more to wave influences. We define the boundary depth db as the depth below which tsunami deposits cannot be reworked due to storm waves. This is based on the assumption that the wave base coincides with the seabed. If the wave base is below the bed, a reworking of bed sediments occurs. If it is above, no energy is transferred to the bottom. As an example, we use tide-gauge records at Pointe de La Rue, Seychelles, and ca. 50 km offshore from Brisbane, Australia, at a water depth of 70 m, to derive the major wave parameters of the Sumatra tsunami and a Category 1 Cyclone that occurred from 4–6 March 2004 in Brisbane. The Brisbane storm wave reached the maximum wave height of 14.3 m. Wave periods varied between 6 s and 14 s and the computed maximum wavelength was 237 m. The largest height of the Sumatra tsunami was 3.01 m. Periods of between 24 min and 61 min were computed for the time between two subsequent peaks. The combination of our hydrodynamic considerations with information from a simplified Hjulstrom-Sundborg diagram implies that the most powerful storm and tsunami waves produce conditions near and at the sea bed that allow the transport of sediment grains from decimeters to meters in diameter. That means a sandy tsunami deposit in the area of the Brisbane tide gauge would be reworked by storm waves. Assuming the highest wave amplitude during the 2004 March storm at Brisbane as such a boundary wave, the water depth below which preservation of tsunami deposits is most likely is greater than 65 m.

Morphology of Molar-Tooth Structures in Precambrian Carbonates: Influence of Substrate Rheology and Implications for Genesis

Abstract: Molar-tooth (MT) is an enigmatic carbonate fabric composed of variously shaped cracks and voids filled with a characteristically uniform, equant microspar. MT is both abundant and widespread in Mesoproterozoic and early Neoproterozoic strata, where void-filling microspar comprises up to 90% of individual beds and 5–25% of preserved carbonate. The temporal restriction of this fabric suggests a potential link between MT formation and the biogeochemical evolution of marine environments. Detailed petrographic relationships among MT crack morphology, distribution of MT microspar, and composition of the surrounding substrate suggest that crack formation and microspar precipitation are intimately linked to the decomposition of sedimentary organic matter in the presence of supersaturated Proterozoic seawater. Laboratory experiments have shown that gas generated within unconsolidated mud can reproduce a variety of MT crack morphologies, yet current gas expansion and migration models do not explicitly consider the role of substrate variability in determining morphologies of MT cracks. A detailed petrographic examination of MT structures from the Mesoproterozoic Belt Supergroup, Montana, permits interpretation of the microscale relationship between crack morphology and lithologic, and potentially rheologic, variability of the surrounding substrate by tracing the distribution of petrographically distinctive MT microspar. Observations of lateral offset of MT cracks at bedding planes or within coarser-grained siltstone or sandstone layers, termination of cracks beneath clay- or organic-rich horizons, grain collapse into underlying MT cracks, and the presence of MT microspar as a pore-filling precipitate suggest that grain size, substrate lithology, and substrate cohesion all play critical roles in the development of MT cracks. By contrast, the presence of a wide range of MT crack morphologies within petrographically homogeneous substrates, and an apparent relationship between crack diameter and sinuosity, suggest that the void-forming process itself also played a role in determining the final morphology of MT cracks. Together, these petrographic observations are used to define a model of microscale gas–sediment interactions that can be used to interpret crack morphology in terms of gas pressure and the strength of sedimentary substrates. The presence of characteristic, void-filling microspar is integral to preservation of MT structures. Cathodoluminescence (CL) identification of this characteristic microspar within MT voids, in pore space of coarse-grained facies, and interstitially within fine-grained facies adjacent to MT voids suggests that MT voids and cement share a common genesis. Because microspar cores are similar in size and morphology to vaterite precipitated experimentally in the presence of a variety of dissolved organic molecules, we suggest that precipitation of MT microspar was intimately linked with gas production during organic decomposition within the host substrate. In this scenario, gas production would result in pore fluids with elevated concentrations of dissolved organic molecules, which would initiate precipitation of MT microspar when the pore fluids come in contact with supersaturated Proterozoic seawater. Restriction of MT largely to Mesoproterozoic and early Neoproterozoic strata likely reflects a critical level of carbonate saturation that limited early substrate lithification, thereby allowing void production but remained high enough that organic catalysts were able to initiate precipitation of MT microspar.

Monohydrocalcite in the Arctic Ikka Fjord, SW Greenland: First Reported Marine Occurrence

Abstract: Monohydrocalcite (MHC) has previously been known only from fresh-water and terrestrial environments. However, as documented in the present paper, MHC is a common mineral (up to 55%) in debris fragments from submarine ikaite (CaCO3·6H2O) tufa columns in the cold marine Ikka fjord, SW Greenland. The columns form where alkaline fresh water from submarine springs mixes with cold seawater and precipitates ikaite. Petrographical and geochemical data indicate that MHC represents a transitional diagenetic phase in the recrystallization process of metastable ikaite to final calcite. The stable-isotope data (δ13C and δ18O) document a closer relationship of MHC with the precursor ikaite than with calcite. Replacement of ikaite by MHC is suggested to take place by dissolution and precipitation in a semi-closed pore-water system dominated by alkaline spring water and with only limited access of seawater. In this environment, nucleation of calcite is inhibited by phosphate ions. Apart from calcite, aragonite and hydrous Mg carbonates were also observed in the tufa debris. The Mg carbonates represent late diagenetic phases, probably formed from recrystallization of MCH in normal seawater environments.

Experiments on self-channelized subaqueous fans emplaced by turbidity currents and dilute mudflows

Abstract: The passage of turbidity currents over submarine fans often results in intense channelization. The channels are typically bounded by levees, and they vary from mildly to strongly meandering. The process of self-channelization remains, however, somewhat obscure. Preliminary experiments reported here demonstrate for the first time that self-channelization of subaqueous fans by turbidity currents can be reproduced at laboratory scale. The same experiments also illustrate self-channelization by dilute mudflows. The resulting weakly sinuous channels can be predominantly depositional, predominantly erosional, or some combination of the two. The channels can elongate to the length of the entire reach available for their formation. They can show both gradual shift and avulsion. A necessary condition for the formation of intricate channelization in the laboratory experiments appears to be a turbidity current that is insufficient to cover the entire area of the fan at any given time. Loose field analogs to the experimental channelized fans (i.e., analogs that are imprecise but contain many similarities) can often be found at the distal end of submarine fans. Two possible examples are cited, one from the Pochnoi system, Bering Sea, and one from the Gulf of Cadiz.

Temporal Variations of Mesozoic Sandstone Compositions in the Qiangtang Block, Northern Tibet (China): Implications for Provenance and Tectonic Setting

Abstract: One hundred and forty-four and fifty-two Mesozoic sandstone samples from the Qiangtang block (ca. 32° 20'–35° N, 85°–93° E; northern Tibet, China) were conducted for analysis of detrital modes and heavy-mineral spectra, respectively, in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited. The Lower–Middle Triassic sandstones in central Qiangtang are characterized by complex derivations from both recycled orogen and continental block provenances. Most of the Upper Triassic and Jurassic sandstones are dominated by a recycled-orogen origin. However, sandstones derived from a continental-block source also occur in Upper Triassic strata in the southern margin of southern Qiangtang, and in Lower Jurassic strata in the southern margin of northern Qiangtang. Although the heavy-mineral spectra of the Upper Triassic and Jurassic sandstones are occupied mostly by the stable heavy minerals zircon, tourmaline, and rutile (ZTR), most of the samples are generally accompanied by unstable heavy minerals, such as apatite, chlorite, chrome spinel, amphibole, epidote, garnet, and pyroxene, except for those from Upper Triassic strata in the southern margin of southern Qiangtang and from Lower Jurassic strata in the southern margin of northern Qiangtang. The detrital modes and heavy-mineral spectra of the Mesozoic sandstones, along with the lithologic evidence, indicate that the central Qiangtang anticlinorium must have been elevated and eroding due to orogenesis and most likely has contained rolling topographic relief at least since Early Triassic time. It could have contained abundant ophiolitic fragments as evidenced by rich chrome spinel, amphibole, and other readily decomposed ferromagnesian minerals across the entire anticlinorium. These mafic minerals may point to the presence of the Shuanghu suture along the entire central Qiangtang anticlinorium. The blueschist-bearing metamorphic complexes in central Qiangtang could be accommodated in a large-scale enclave in an irregular central Qiangtang continental margin. The Upper Triassic–Jurassic recycled-orogen-derived sandstones in northeastern Qiangtang suggest continued orogenesis due to the suturing between Qiangtang and Asia. Such a contractional background does not support models that relate a tensile mechanism for back-arc spreading along the Bangong–Nujiang belt.