Showing papers in "Journal of Sedimentary Research in 2005"

Digital Outcrop Models: Applications of Terrestrial Scanning Lidar Technology in Stratigraphic Modeling

Abstract: Laser ranging is extremely accurate and efficient. Terrestrial scanning lidar (light detection and ranging) applied to outcrop stratigraphic mapping enables researchers to capture laser range data at a rate of thousands of individual X, Y, Z and laser-intensity points per second. These data, in conjunction with complementary remotely and directly sampled data, are used to conduct high-precision facies characterization and to construct 3D geological computer models. Outcrop data are presented here to explain our workflow and to discuss the construction of rock-based 3D Digital Outcrop Models (DOMs). Reproducibility and quantification are the drivers of this methodology. High-resolution terrestrial lidar acquisition, processing, interpretation, and visualization are discussed and applied to mapping of geological surfaces in three dimensions. Laser-generated models offer scientists an unprecedented visualization medium in a quantitative 3D arena. Applications of this technology include constructing and visualizing complex 3D Earth models from outcrops for improved reservoir modeling, flow simulation in hydrocarbon and aquifer systems, and property modeling to constrain forward seismic modeling.

Experiments on Oscillatory-Flow and Combined-Flow Bed Forms: Implications for Interpreting Parts of the Shallow-Marine Sedimentary Record

Abstract: Analog laboratory experiments were conducted to investigate subaqueous bed forms generated under storm-like oscillatory and combined flow. Experiments were carried out in a large wave tunnel and used a range of sand sizes (fine and very fine), wave periods (10.5 and 8 s), oscillatory velocities (0 to 125 cm/s), and unidirectional velocities (0 to 25 cm/s). At low unidirectional velocities (≤10 cm/s), addition of an increasing collinear oscillatory flow caused the bed to evolve from small-scale (wavelength 100 cm), symmetric, orbital ripples, to plane bed. At higher unidirectional velocities (>10 cm/s), a similar trend was noted, but ripples were more asymmetric. Three phase diagrams are presented to summarize the observed relationships of bed configurations to flow conditions. These diagrams are intended to assist in the interpretation of shallow marine sedimentary environments where oscillatory and combined flow are thought to be omnipresent. Distinctive features of small-scale asymmetric combined-flow ripples generated are: a 3D planform, round crest, and convex-up sigmoidal profile with local pronounced scour at the toe of the stoss side giving the ripple profile a "boxy" appearance. Similarly, large-scale asymmetric combined-flow ripples had broad and round crests, convex-up stoss sides, and "compressed profiles" due to scouring at the toe of the stoss side. Hummocky bed forms were generated under moderate to high oscillatory velocities and low unidirectional velocities. Hummocks were not observed as a distinct bed state but rather appeared to mark transitions in bed-form scale and symmetry. Hummocks were more prevalent at longer oscillatory periods and in finer-grained sediment. Stratification produced by "synthetically" aggrading hummocky bed profiles closely resembles hummocky cross-stratification. With the introduction of only a small unidirectional-flow component, hummocks evolve into downstream-migrating large-scale asymmetric ripples, and the resultant cross-stratification becomes similar to that produced by unidirectional-flow dunes. Accordingly, these experiments suggest that much of the hummocky cross-stratification observed in the stratigraphic record is produced by storm-generated long-period oscillatory-dominant combined flows. Inasmuch as long-period, high-energy waves require deep, wide basins to form, hummocky cross-stratification may therefore serve as a useful indicator of deposition in unrestricted, open-water conditions.

Discontinuity-Bounded Alluvial Sequences of the Southern Gangetic Plains, India: Aggradation and Degradation in Response to Monsoonal Strength

Abstract: Discontinuity-bounded late Quaternary sequences in the southern Gangetic Plains (Himalayan Foreland Basin) reflect floodplain aggradation and degradation in response to forcing by the powerful Southwest Indian Monsoon. The major Himalayan and cratonic rivers in this area occupy narrow, incised valleys and do not inundate the adjoining broad interfluves, which display soil development, small plains-fed rivers, lakes, eolian deposits, and badland ravines. However, these areas formerly experienced active alluviation because thick floodplain muds underlie the interfluve. An age model suggests that interfluve areas near the major rivers aggraded periodically between about 27 ka and 90 ka (Marine Oxygen Isotope Stages 3–5). They subsequently degraded or accumulated sediment only locally, probably reflecting decreased monsoonal precipitation around the Last Glacial Maximum (Marine Isotope Stage 2), when major river valleys moved to an underfit condition. Increased precipitation during the 15 to 5 ka period of monsoon recovery probably increased discharge and promoted incision and widespread badland formation. Discontinuities at deeper levels reflect earlier degradation phases, and are marked by reworked gravel lenses, thin channel bodies, and carbonate-cemented surfaces. Carbonate veins (infilled joints) that cut strata below but not above one discontinuity suggest some local tectonic influence. There is no evidence that sea-level fluctuations influenced stratal patterns in this elevated, inland region. Discontinuity-bounded sequences linked to climate change may characterize alluvial successions in many continental settings, especially along the cratonic margins of foreland basins, where subsidence rates are modest.

Causes of River Avulsion: Insights from the Late Holocene Avulsion History of the Mississippi River, U.S.A.

Abstract: The emphasis on gradient advantages in studies of avulsion is misleading. While gradient advantages are necessary for an avulsion to occur, the late Holocene avulsion history of the Mississippi River in Louisiana suggests that factors such as substrate composition and floodplain channel distributions are more important. Cross-valley to down-valley slope ratios of the modern floodplain range from 16 to 110 and are typically > 30. The slope ratio is 35 at the location of the Mississippi–Atchafalaya diversion (Old River) yet slope ratios are 83 to 110 immediately upvalley of Old River. All values of Mississippi River floodplain slope ratios are significantly larger than values of avulsion threshold calculated by numerical models. Shallow floodplain cores, 14C dating of organic remains, and geologic mapping show that the Mississippi River has avulsed only four times over the past 5 ky in the southern Lower Mississippi Valley (LMV). Gradient advantages are widespread, yet avulsions are rare. These observations indicate that factors other than gradient advantage control Mississippi River avulsion. Several examples of Mississippi and Red River avulsion by channel reoccupation support the idea that channel distributions and substrate compositions are primary influences on avulsion. Incipient Mississippi River avulsion and development of the Atchafalaya River involved reoccupation of abandoned Mississippi River channels and a Red River crevasse-splay complex. The modern Atchafalaya River also incises buried Mississippi River channel-belt sands. Abandoned channel belts and crevasse-splay complexes consist of sandy substrates that facilitate scour and the development of channels capable of capturing the Mississippi River. Abandoned channels provide ready-made conduits for Mississippi River flow that can efficiently develop into avulsive channels. Multi-storied sheet sandstones in ancient fluvial deposits may provide additional support for the idea that erodible substrates and floodplain channel distributions are critical influences on avulsion. These features record episodic reoccupation of channel belts, which at least in some cases, may simply reflect successive avulsions rather than major changes in aggradation rate or extrabasinal factors such as climate.

Using Passive Integrated Transponder (PIT) Tags to Investigate Sediment Transport in Gravel-Bed Rivers

Abstract: In gravel-bed rivers, measuring the displacement of individual grains by fluid flow is essential in order to understand sediment transport processes and to investigate changes in channel morphology. We present preliminary results of a new technique that traces pebble movements by inserting 23 mm passive integrated transponders (PIT) into individual clasts. Because each PIT has its own signal identification, this technique is ideal for tracking the individual movements of episodically transported particles in gravel-bed rivers. Two hundred and four tagged particles were inserted into a 130-m-long reach of a gravel-bed river with a 2% slope and a bed material with a D50 of 70 mm. The b axis size of the tagged particles ranged from 40 mm to 250 mm. Recovery percentages after two competent floods were 96% and 87%, clearly demonstrating the effectiveness of this new technique. Buried particles can be recovered at a depth of 0.25 m. PIT tags are also suited for long-term studies over several years.

Fingerprints of Fluid Flow: Chemical Diagenetic History of the Jurassic Navajo Sandstone, Southern Utah, U.S.A.

Abstract: Colorful outcrop exposures of diagenetic iron oxide, clay, and carbonate cements in the Jurassic Navajo Sandstone reflect a mul- ti-phase history of fluid-rock interactions. Characteristic mineralogical and geochemical variability occurs on microscopic and outcrop scales throughout southern Utah. We identify six common diagenetic facies and evaluate formation mechanisms, paragenetic relationships, and relative timing between alteration events. The diagenetic facies have distinctive visual characteristics, variations in mineralogy, major ox- ides, trace elements, and carbon and oxygen isotopes. They include red and bleached sandstone, diffuse and concretionary iron oxide precip- itates, and carbonate concretions. Development of these facies requires changes in the interstitial fluid environment and an open geochemical system with basinwide fluid flow and variations in redox conditions. Spatial and temporal changes in paleohydrologic and diagenetic con- ditions indicate complex coupling and feedbacks between stratigraphic architecture, fluid flow, and basin evolution.

Mass-Transport Complexes (MTCs) and Tectonic Control on Basin-Floor Submarine Fans, Middle Eocene, South Spanish Pyrenees

Abstract: In many deep-marine clastic systems, mass-transport complexes (MTCs) constitute a major component of the stratigraphy and represent an integral part of the evolution and depositional style. In tectonically active basins, as is the case in our study of the syntectonic deep-marine Ainsa submarine fans (Eocene of the Ainsa basin, Spanish Pyrenees), organized, predictable vertical sedimentary sequences provide a testable generic model for submarine fan development in other basins where tectonic processes provide a first-order driver on fan growth. In the Ainsa basin, the linear submarine fans were confined by lateral thrust ramps and influenced by intra-basinal growth anticlines. This study represents an integration of outcrop data from sedimentary logs and mapping, with core data from eight wells and micropaleontological and palynomorph analyses. Mapping shows the lateral stepwise migration of sandy channelized submarine fans, as a foreland-propagating clastic wedge. The stepwise foreland migration of each fan (away from the deformation front), and also the time scales involved (many hundreds of thousands of years for each fan), are interpreted within the context of a primary tectonic control. Three distinct types of MTC are recognized, each of which appears to be characteristic of stratigraphic position in relation to the evolution of individual fans. The deep-marine expression of the inferred tectonic pulses began with the large-scale basin-slope collapse as sediment slides and debris flows (type I MTCs) that formed much of the seafloor topography for each fan and contributed to their lateral confinement. The uppermost slope and any shelf edge, including the narrow shelf, then collapsed, redepositing unconsolidated sands and gravels into deep water (type II MTCs). The basal coarse clastics are overlain by an interval of mainly channelized and amalgamated sandy deposits, with major erosional events, including reincision processes associated with channel development, being characterized by pebbly mudstones and sandstones rich in angular, locally derived intraclasts (type III MTCs). The channelized sands pass up into several tens of meters of less confined, non-amalgamated, medium- and thin-bedded, fine-grained sands and marls. These deposits represent the phase of most active fan growth, initially by erosional channel development, sediment bypass, and backfill (in several cycles), giving way to nonchannelized, fine-grained sandy deposition, interpreted as a response to the flushing out of the coarser clastics from the coastal and near-coastal fluvial systems. During this latter stage in active fan growth and when sediment accumulation rates probably remained high, the degraded submarine slope was regraded and healed by fine-grained depositional events. The high amount of woody material and the high nonmarine palynomorph signal in these sandy deposits suggest direct river input as both turbidity currents and hyperpycnal flows for the silty marls. In the upper few meters, a thinning and fining-upward sequence shows a return to background marl deposition, representing fan abandonment. Many sequences are overlain by intraformational sediment slides (typically type I MTCs but, rarely type II MTCs) that attest to the increasing seafloor gradients associated with the regrading and healing stage in slope development. Our explanation for these vertical sequences provides a readily testable depositional model for other deep-marine clastic systems associated with a tectonically active hinterland and basin slopes.

Diagenesis and Reservoir-Quality Evolution of Incised-Valley Sandstones: Evidence from the Abu Madi Gas Reservoirs (Upper Miocene), the Nile Delta Basin, Egypt

Abstract: Elucidation of the distribution of diagenetic alterations in incised-valley sandstones within a sequence stratigraphic framework allows a better understanding of their reservoir-quality evolution during burial. Gas reservoirs of the upper Miocene Abu Madi Formation (present-day depth 3350 m, temperature 116°), the Nile Delta Basin, Egypt, consist of lowstand systems tract (LST) fluvial and transgressive systems tract (TST) estuarine sandstones deposited in an incised valley. These sandstones have a wide range of porosity (2 to 29%) and permeability (0.01 to 6071 mD), which reflect both depositional facies and diagenetic controls. Diagenetic events that influenced the reservoir-quality evolution include mainly the formation of extensive grain-rimming Fe-Mg chlorite, mechanical compaction of ductile grains, pressure dissolution of quartz, and quartz cementation. The chlorite rims, distributions of which are not clearly related to sequence stratigraphy, have preserved reservoir quality by inhibiting quartz cementation but have increased microporosity values in the reservoirs. Calcite cementation (18O = −13.5‰ to −6.0‰, δ13C = −14.3‰ to −1.0‰, and 87Sr/86Sr = 0.707124 to 0.708181), has not influenced the overall reservoir-quality evolution of the sandstones because of its localized occurrence. Other diagenetic events that have little influence on reservoir quality include the more frequent formation of kaolin in the LST fluvial sandstones and formation of pyrite in the TST estuarine sandstones. This study demonstrates the possibility of constraining and evaluating the impact of diagenetic alterations on reservoir-quality evolution in incised-valley deposits and their sequence stratigraphic distribution, and thus has an important impact on hydrocarbon exploration in such settings.

Where Has All the Aragonite Gone? Mineralogy of Holocene Neritic Cool-Water Carbonates, Southern Australia

Abstract: Surficial carbonate sediments on the southern continental shelf of Australia are cool-water in aspect and composed of biogenic particles produced largely during the late Quaternary. Current understanding is that such sediments are calcite-dominated, as were their older Cenozoic counterparts. The Holocene fraction of these sediments in modern open-shelf, neritic environments between 30 and 350 meters water depth is, however, 50% to 80% aragonite. Scant evidence of significant former aragonite in many cool-water carbonate sedimentary rocks implies that most aragonite is lost before such sediments exit the marine diagenetic environment. Although marine dissolution must be taking place in such settings, the conundrum is exacerbated because seawater over the shelf in southern Australia is saturated with respect to aragonite. It is proposed that the aragonite, from skeletons of gastropods, infaunal bivalves, and certain bryozoans, is dissolved in the shallow subsurface, probably as the byproduct of bacterial degradation of sedimentary organic matter. As a consequence, the geological and paleontological record of many cool-water carbonates is strongly biased, and the inferred original calcitic composition of such sediments is the product of early diagenetic taphonomic loss, not selective biogenic productivity. The net result is not only dissolution of aragonite but also neomorphism of Mg-calcite to calcite with a marine geochemical signature. Synsedimentary aragonite loss, by removing CaCO3 that is usually available for calcite cementation during meteoric diagenesis, leads to retarded lithification of these cool-water carbonates until deeply buried. Such removal of a significant carbonate fraction during deposition likely contributes to the low rates of cool-water sediment accumulation.

Origin of Dolomite in the Arab-D Reservoir from the Ghawar Field, Saudi Arabia: Evidence from Petrographic and Geochemical Constraints

Abstract: A significant proportion of oil production from the Kim- meridgian Arab-D strata in the Ghawar field, Saudi Arabia originates from dolomitized rocks. Stratigraphic, petrographic, and geochemical data suggest that at least four episodes of dolomitization affected these sediments. The lower portion of the Arab-D, Zone 3, is only partially dolomitized, with the dolomite frequently being associated with firm- grounds. We propose that these dolomites were formed on an outer- ramp setting with a maximum water depth of 50 m, during a period of nondeposition, with the dolomitization process being promoted by the oxidation of organic material and the diffusion of Mg 21 from the overlying seawater. The dolomites in Zone 2B are geochemically dis- tinct compared to those in Zone 3 in that they have relatively positive oxygen isotope compositions ( 2 1t o22‰ compared to 26.5 ‰). The relatively positive oxygen isotope composition and the geochemical sim- ilarity of Zone 2B to the dolomites in Zone 1, which are intimately associated with the overlying evaporites, has led us to conclude that the Zone 2 dolomites probably formed by the reflux of hypersaline fluids through the sediments. These hypersaline fluids bypassed Zone 2A by moving through the grain-dominated sediments. Early cemen- tation and dolomite formation made these units more susceptible to later fracturing that affected the entire Arab-D formation. This frac- turing allowed higher-temperature fluids to leach the dolostones, there- by removing any remaining calcite and partially resetting the oxygen isotope composition of some of the dolomites. As a result of this later dolomitization event, rocks that were only partially dolomitized were leached, creating units with extremely high permeability and porosity (super-k intervals). Dolomites in the lower Zone 3 were recrystallized during burial by the normal geothermal gradient, leading to the pre- sent negative oxygen isotope values. Zone 1 dolomites are petrograph- ically distinct from Zone 2 dolomite in that they are mimetic and fabric preserving, although they are geochemically similar. This mimetic style of dolomitization occurs immediately adjacent to the overlying anhy- drite and is interpreted to have formed very shortly after deposition from hypersaline brines. recovery, and their presence is inferred from a combination of production data and the diameter of the bore hole as measured using the caliper log. As a result of the importance of dolomite in the formation of the super-k intervals within the Arab-D, an understanding of the paragenesis of these zones is important not only in predicting super-k formation within the Arab-D but also as an analogue for understanding other dolomitized res- ervoirs with high permeability and porosity. Previous characterization work of dolostones from the Arab-D in the Ghawar field (Cantrell et al. 2004) has identified three types of dolomite with distinctive petrographic, stratigraphic, and geochemical characteristics: fabric-preserving, non-fabric-preserving, and baroque dolomite. Fabric-pre- serving dolomite is very finely crystalline in which petrographic details of the original limestone fabric are usually well preserved. Fabric-preserving dolomite contains low concentrations of Fe (average 247 ppm Fe) and relatively positive oxygen isotope values (average

Microbial Kinetic Controls on Calcite Morphology in Supersaturated Solutions

Abstract: Recognizing microbial imprints in the morphology of calcium carbonate minerals that form in very supersaturated solutions containing a high level of dissolved inorganic carbon (DIC) is challenging. To better define criteria for this purpose, we have analyzed the influence of sulfate-reducing bacterium Desulfovibrio desulfuricans strain G20 on the morphology of calcite in such solutions. G20 does not induce large shifts of pH or alkalinity under these conditions, but its uptake of millimolar sulfate and lactate decreases the number of anhedral crystals and stimulates growth of subhedral spar crystals relative to the abiotic controls. In addition, organic compounds associated with the basal growth medium, purified exopolymeric substances produced by G20 and lypopolysaccharide, stimulate the growth of anhedral crystals and crystals with rounded edges at low supersaturation index (SI) of calcite. The effect of organic compounds is reduced at higher SI, where rhombohedral habits dominate. Our results suggest that the local production and uptake of kinetic inhibitors within microbial biofilms may be an important control on calcite morphology in supersaturated solutions.

Impact of Longer-Term Modest Climate Shifts on Architecture of High-Frequency Sequences (Cyclothems), Pennsylvanian of Midcontinent U.S.A.

Abstract: Pennsylvanian glacioeustatic cyclothems exposed in Kansas and adjacent areas provide a unique opportunity to test models of the impact of relative sea level and climate on stratal architecture. A succession of eight of these high-frequency sequences, traced along dip for 500 km, reveal that modest climate shifts from relatively dry-seasonal to relatively wet-seasonal with a duration of several sequences (~600,000 to 1 million years) had a dominant impact on facies, sediment dispersal patterns, and sequence architecture. The climate shifts documented herein are intermediate, both in magnitude and duration, between previously documented longer-term climate shifts throughout much of the Pennsylvanian and shorter-term shifts described within individual sequences. Climate indicators are best preserved at sequence boundaries and in incised-valley fills of the lowstand systems tracts (LST). Relatively drier climate indicators include high-chroma paleosols, typically with pedogenic carbonates, and plant assemblages that are dominated by gymnosperms, mostly xerophytic walchian conifers. The associated valleys are small ( 4 km wide and >20 m deep), and dominated by quartz sandstones derived from distant source areas, reflecting large drainage networks. Transgressive systems tracts (TST) in all eight sequences generally are characterized by thin, extensive limestones and thin marine shales, suggesting that the dominant control on TST facies distribution was the sequestration of siliciclastic sediment in updip positions. Highstand systems tracts (HST) were significantly impacted by the intermediate-scale climate cycle in that HSTs from relatively drier climates consist of thin marine shales overlain by extensive, thick regressive limestones, whereas HSTs from relatively wetter climates are dominated by thick marine shales. Previously documented relative sea-level changes do not track the climate cycles, indicating that climate played a role distinct from that of relative sea-level change. These intermediate-scale modest climate shifts had a dominant impact on sequence architecture. This independent measure of climate and relative sea level may allow the testing of models of climate and sediment supply based on modern systems.

Parallel Lamination Formed by High-Density Turbidity Currents

Abstract: Experiments were conducted in order to investigate processes of sediment transport and deposition from sustained turbidity currents with initial volume-fraction sediment concentration of 0.20, 0.25, and 0.35. Parallel lamination was formed at bed-aggradation rates up to 4 mm s-1 and bedload-layer sediment concentration up to 0.36. This observation contradicts the generally held assumption that parallel lamination forms only from low-density flows and at low bed-aggradation rates. This misconception illustrates a poor understanding of threshold values of grain-support mechanisms in the bedload layer. The present results bring new insight into understanding sediment transport in the deep sea and interpreting the sedimentary record.

No change in fluvial style across a sequence boundary, cretaceous blackhawk and castlegate formations of central utah, u.s.a.

Abstract: Nonmarine sequence stratigraphic models are based largely on studies of fluvial units in the Cretaceous Western Interior of North America, including the Blackhawk and Castlegate formations of central Utah. These models suggest that fluvial units should show a transition from mudstone-prone, isolated meander-belt deposits of the transgressive and highstand systems tract into amalgamated braided-stream sandstones of the lowstand systems tract. These mudstone-rich fluvial strata are largely assumed, rather than documented, to represent the deposits of isolated meander-belt deposits, despite recent work that suggests that there is no simple relationship between the proportion of preserved floodplain mud and fluvial style. There are numerous studies documenting the braided character of the sandstone-rich Castlegate Formation, but there are no corresponding studies that document the internal fluvial facies architecture within the Blackhawk Formation to support the interpretation that the Blackhawk comprises the deposits of a meandering-stream system. We test the validity of the sequence stratigraphic model by comparing dimensions and styles of associated bar and channel deposits, as well as cross-stratal thicknesses, between the Blackhawk and Castlegate formations along Salina Canyon in Utah. Data compiled from 24 measured sections and 6 photomosaics show that the Blackhawk Formation comprises isolated sandy channel-belt sheet sandstones, 5–8 m thick, contained within thick floodplain mudstones, whereas the Castlegate Formation consists of about 80 m of amalgamated sandy channel-belt sheets, 4–7 m thick, with only minor mudstone. The average height of cross-sets is 13 cm in the Castlegate Formation and 14 cm in the Blackhawk Formation and formative dune height is estimated to be 38 cm for the Castlegate Formation and 45 cm for the Blackhawk Formation. Average bar-accretion thicknesses are 2 m in both the Blackhawk and Castlegate formations. Corresponding water depths are estimated to be 2.5–4.1 m for the Blackhawk and 2.3–3.8 m for the Castlegate. The correlation of master bedding planes in the Blackhawk Formation shows overlapping lens-shaped bar deposits and channel fills. Bar tops appear to dip in several directions, indicating both lateral and downstream accretion of mid-channel braid bars. While there is some indication that Castlegate rivers were slightly shallower, the differences do not suggest a major change in fluvial style across the sequence boundary; both are braided. The lack of change in scale of channels also is not compatible with a hypothesized increase in aridity between the Blackhawk and the Castlegate. The difference in fluvial architecture between the two formations is interpreted to reflect changes in accommodation that were likely tectonic in origin. This supports that idea that preservation of thick successions of fine-grained overbank material is not a function of the plan-view channel geometry.

Distribution of Diagenetic Alterations in Siliciclastic Shoreface Deposits within a Sequence Stratigraphic Framework: Evidence from the Upper Jurassic, Boulonnais, NW France

Abstract: The distribution of diagenetic alterations in Upper Jurassic, siliciclastic shoreface sediments from NW France has been linked to the sequence stratigraphic framework. Calcite cement in mudrocks and sandstones of the transgressive (TST) and lower part of the highstand (HST) systems tracts is microcrystalline and occurs as continuously cemented layers and stratabound concretions. The average delta(18)O(V-PDB) (-2.6%omicron) and Sr-87/Sr-86 (0.7078) compositions of microcrystalline calcite indicate precipitation from largely marine pore waters. >Calcite cement in sandstones of the forced regressive wedge (FRWST) and lowstand (LST) systems tracts is poikilotopic and occurs mainly as stratabound concretions. Complete dissolution of the carbonate grains and concomitant precipitation of poikilotopic calcite cement with low average delta(18)O (-5.3%omicron) and radiogenie Sr-isotope (0.70882) signatures suggest incursion of meteoric waters into sandstones during relative sea-level lowstand. The poorly lithified sandstones interbedded with sandstones cemented by poikilotopic calcite concretions display evidence of diagenesis under episodes of and to semiarid paleoclimate, including: (i) partial cementation by opal, chalcedony, gypsum, and minor vadose calcite cement, (ii) mechanically infiltrated clays and Fe-oxides, and (iii) secondary porosity owing to partial dissolution of carbonate grains. The integration of diagenesis into sequence stratigraphy allows better elucidation and prediction of the spatial and temporal distribution of diagenetic alterations and related reservoir-quality modifications in shoreface sediments

Storm-Influenced Prodelta Turbidite Complex in the Lower Kenilworth Member at Hatch Mesa, Book Cliffs, Utah, U.S.A.: Implications for Shallow Marine Facies Models

Abstract: Isolated sandstone bodies encased in marine mudstones have proved difficult to explain, especially those that are not easily incorporated into conventional facies models. The Hatch Mesa succession (Campanian, lower Kenilworth Member) is a marine mudstone-encased, turbiditic sandstone body, 6 to 20 m thick, that is exposed along a 7-km-long outcrop belt, approximately 15 km east of Green River, Utah, U.S.A. Various interpretations of depositional environment and regional correlation have been proposed over the past 20 years. A sedimentological analysis of the Hatch Mesa succession suggests deposition as a storm-influenced, prodelta turbidite complex on the shallow inner shelf, between fair-weather and storm wave base. This interpretation is corroborated by the high-resolution outcrop correlation and subsequent paleogeographic reconstruction, which indicates deposition 16 to 21 km basinward of the time equivalent lower-shoreface deposits, in about 20 m water depth. A variety of mechanisms are capable of generating instability in the delta front and triggering the turbid flow of sediments into deeper water, including storm events, river flooding, high rates of sedimentation, or earthquakes. This explains the complex mixture of event beds in the Hatch Mesa succession, which are dominated by wave-modified turbidites. A three-component model, consisting of delta-front, subaqueous channel, and prodelta turbidite deposits, is proposed to explain the depositional environment and setting of the Hatch Mesa succession. All three components are observed in the lower Kenilworth Member to upper Aberdeen Member stratigraphic interval. The results of this study indicate that shallow marine facies models should be revised to include marine mudstone-encased, prodelta turbidite complexes, thus adding one more possibility to the diverse suite of interpretations used to explain the generation and preservation of isolated marine sandstone bodies. These results also shed new light on the stratigraphic position and depositional setting of the Mancos Shale-encased, isolated sandstone bodies of the Prairie Canyon Member in eastern Utah and western Colorado.

Splay Formation Along the Lower Reaches of Ephemeral Rivers on the Northern Plains of Arid Central Australia

Abstract: Splays are common features of many rivers and deltas, forming as a result of overbank flooding and sediment deposition, but they have rarely been described from modern dryland settings. This paper investigates splay formation along the lower reaches of ephemeral rivers on the Northern Plains of arid central Australia. Here, splays vary from small, lobate or tongue-shaped features < 1 km long, to larger, elongate features up to ~ 4 km long, and are supplied with rare floodwater and sediment through well-defined breaches in the upper parts of the main channel banks. In proximal and medial parts, splay channels are incised up to ~ 2 m into pre-splay, fine-grained floodplain sediments but remain elevated between ~ 0.5 m and 2 m above the main channels. Only in the very distal parts are splay channels less deeply incised, either grading into the floodplain surface or terminating as sediment lobes that are deposited upon the floodplain. Aerial photographs and field observations indicate that many splays develop rapidly during large floods and are modified during infrequent, subsequent floods. Four main stages of splay development are proposed: (1) aggressive overbank flow results in bank-line breaching and channel scour on the floodplain, but little sediment deposition; (2) coarse bed sediment is diverted from the main channel, preventing further deepening at the bank-line breach and slowing channel scour; (3) increasing coarse sediment transport through the bank-line breach and scoured channels leads to the formation of distal sediment lobes; (4) subsequent floods divert around these distal lobes, sometimes leading to further channel scour and lobe formation. On the Northern Plains, splays contribute to downstream decreases in the discharge and size of the main channels, and splay deposits form local accumulations of gravelly sand in the predominantly fine-grained floodplain successions. Some splays ultimately may evolve into distributaries or anabranches. The dominantly erosional bases of these splay channels contrast with typical descriptions of many humid-region splays, but they do not display any geomorphological or sedimentological features that are necessarily diagnostic of formation in a dryland setting.

Paleopedologic and Paleohydrologic Records of Precipitation Seasonality from Early Pennsylvanian "Underclay" Paleosols, U.S.A.

Abstract: Underclays in the Crooked Fork Group (Lower Pennsylvanian, Atokan, Langsettian) in eastern Tennessee, U.S.A., are paleosols associated with fourth-order depositional sequences that record changing paleohydrologic conditions during their formation. Climate changes (possibly Milankovitch-driven) resulted in evolution of soil landscapes from well-drained, seasonally wet floodplains and delta plains dominated by vertic (Vertisol-like) paleosols, to very poorly drained, everwet swamps dominated by sideritic gley paleosols. Coals (paleoHistosols) cap most underclays and record peat-swamp development during everwet conditions, followed by marine submergence. Pedogenic slickensides and angular blocky ped structures, in conjunction with illuviated clay pore fillings and sepic-plasmic microfabric, indicate an initial better-drained phase of paleosol development (Stage I). Gley overprinting, characterized by drab, low-chroma paleosol colors (Fe reduction) in upper portions of paleosols, sideritic rhizocretions, sphaerosiderite and pyrite nodules, extensive leaching and translocation of alkali and alkaline earth elements, and kaolinitization of smectites and hydroxy-interlayer vermiculite (HIV), indicate a later poorly drained stage of paleosol development characterized by saturated conditions, moderate to low Eh, and low pH (Stage II), as well as peat-swamp development. Lastly, burial and juxtaposition of marine facies on top of terrestrial deposits terminated paleosol and peat development (Stage III). Paleosols likely formed under conditions of marked seasonality of rainfall, judging from both modern analog systems in Sumatra and Pennsylvanian paleoclimate models. The chemical index of alteration without potash (CIA-K) was used as a proxy for Atokan paleoprecipitation, estimated as 1300 mm/yr. This estimate is comparable with mean annual precipitation (MAP) in modern tropical to subtropical rainy environments.

Experimental Test of Tectonic Controls on Three-Dimensional Alluvial Facies Architecture

Abstract: In an extended series of papers by M. Leeder, J.R.L. Allen, J.S. Bridge, J. Alexander, and others, the depositional stacking patterns of channel-belt deposits were related to tectonic and other external controls (we refer to these models collectively as LAB models). These models established a basic connection between subsidence, channel avulsion, and the mesoscale (channel-belt) architecture of an alluvial deposit. Two fundamental predictions resulted from this work: (1) channel-belt stacking density and hence connectedness is inversely correlated to temporal (vertical) changes in sedimentation rate; and (2) channel-belt stacking density and hence connectedness is directly correlated to lateral changes in sedimentation rate. In this paper we make use of the Experimental EarthScape (XES) Facility at the St. Anthony Falls Laboratory to examine the effects of temporally and laterally variable subsidence rates on mesoscale alluvial architecture. We then compare the experimental results with the main predictions of the early LAB models. Regarding the predictions of the LAB models, under conditions of high sediment supply and a highly active alluvial system, lateral and downstream variation in subsidence geometry and rate have little effect on the details of alluvial architecture. We found that the principal architectural signature of changes in subsidence geometry (i.e., laterally variable subsidence rate) is stratal tilting and that channel belts are not attracted to the subsidence maximum. We hypothesize that the effect of variable subsidence geometry is felt by the fluvial system only if subsidence proceeds faster than the river can adjust to the formation of a topographic low via the deposition of overbank material in the form of splays and sheet sands. Our results lend impetus to the development of more complete three-dimensional numerical models of mesoscale alluvial architecture that couple architecture to broader, allogenic forcing mechanisms.

Earliest Triassic Claystone Breccias and Soil-Erosion Crisis

Abstract: Chemostratigraphic methods (δ13Corg) for pinpointing the Permian–Triassic boundary in nonmarine rocks of Antarctica, Australia, and South Africa have drawn attention to unusual claystone breccias at this horizon of the greatest extinction in the history of life on Earth. These rocks differ from other breccias in having a high proportion of clasts with birefringence microfabrics (sepic plasmic fabrics) characteristic of soils, and can be called sepic pedoliths in the terminology of soil science. At many localities, earliest Triassic sepic pedoliths are thin (5–15 cm) beds, but some fill shallow paleochannels, and pinch out laterally. Other non-marine breccias at the Permian–Triassic boundary are dominated by fragments of coal, pedogenic carbonate nodules, and deeply weathered volcanic rock fragments. Earliest Triassic sepic pedoliths record an unusually severe and widespread episode of soil erosion associated with forest dieback at the Permian–Triassic boundary. Comparable sepic pedoliths are found in debris flows after clear-cutting of forests in western Oregon today. Studies of these modern deposits show that sepic soil peds do not withstand weathering for more than a few months. Such clayey clasts are held together mainly by roots and fungi, which decay rapidly in soils and streams. These modern analogs demonstrate that terminal Permian soil erosion was rapid and profound. Sepic pedoliths are also known at other times of mass extinction, such as the Triassic–Jurassic boundary, but are not yet known before the Devonian advent of trees and forested clayey soils.

Sharp-Based, Flood-Dominated Mouth Bar Sands from the Burdekin River Delta of Northeastern Australia: Extending the Spectrum of Mouth-Bar Facies, Geometry, and Stacking Patterns

Abstract: Distinguishing the deposits of mouth bars from delta distributary channels in the rock record may not be as straightforward as often portrayed, because mouth-bar deposits can be more variable than usually presumed. Mouth bars with triangular plan-view subaerial geometry are well developed at the mouth of the modern Burdekin River of northeastern Australia. This planform is intermediate between the elongate, lozenge-shaped mouth bars typical of river-dominated deltas and the beach-ridge geometries characteristic of wave-dominated deltas. Surface and shallow-subsurface bar deposits are predominantly moderately sorted, coarse-grained sand, similar to that of adjacent, lower-delta-plain-channel floor. Sedimentary textures are modified at seaward sides of mouth bars by waves into a foreshore of well-sorted fine to medium-grained sand, and mouth bar sands pass distally and laterally into more mud-dominated lithologies. The Holocene section beneath the lower delta plain is dominated by 5–8 m thick, sharp-based bodies of coarse-grained sand, texturally indistinguishable from the modern mouth bar with no vertical grain-size trend, a slight upward-fining trend, or in a few cases a coarsening-upward trend. These sand bodies have low-angle seaward-dipping internal bedding surfaces and are bounded by surfaces of similar attitude. The Holocene Burdekin Delta was and is flood-dominated (rather than strongly wave-influenced, as proposed previously) and prograded by rapid deposition of mouth bars during river floods (most of which last for only a few days). Mouth-bar construction takes place over tens of years. The variable vertical grain-size profiles of mouth-bar deposits suggests formation by both aggradation and progradation, and this is supported by the cross-sectional geometry of mouth-bar clinoform sets imaged geophysically. Once a mouth bar has become emergent and is stabilized by vegetation, a new bar is initiated seaward. In this way, delta "lobes" are constructed over 100s to 1000s of years before being abandoned following an avulsion of the trunk river channel to another part of the delta. Caution is needed in the interpretation of ancient shallow-water deltaic successions, where sharp-based, fining-upward mouth bar deposits may be confused with distributary-channel facies.

Integrated provenance analysis of a complex orogenic terrane: mesozoic uplift of the bogda shan and inception of the turpan-hami basin, nw china

Abstract: We employ petrographic and advanced geochemical tech- niques to better document the evolution of the Turpan-Hami basin based on the unique geologic histories of the arc terranes that served as potential sources of Turpan-Hami deposits. First, a provenance study of Permian through Cretaceous sandstone of the Turpan-Hami basin reveals temporal and spatial changes in dominant source ter- ranes that provided detritus to the basin. Volcanic-lithic-rich Upper Permian sandstone (mean Qm 19F18Lt63 ;Q p 7Lvm89Lsm4 ;Q m 48P39K13) followed by more quartzose compositions in Triassic sandstone (mean Qm 41F19Lt40 ;Q p 20Lvm75Lsm5 ;Q m 68P21K11) indicate progressive un- roofing of the extinct northern and central Tian Shan arc terranes to the south of Turpan-Hami. A sharp change to sedimentary-lithic-rich Lower Jurassic sandstone (mean Qm 47F16Lt37 ;Q p 16Lvm42Lsm42; Qm 75P12K13) overlain by a return to volcanic-lithic-rich Middle Ju- rassic sandstone (mean Qm 39F21Lt40 ;Q p 14Lvm51Lsm35 ;Q m 65P21K14) points to the initial uplift and unroofing of the largely andesitic Bogda Shan to the north, which first shed its sedimentary cover as it emerged to become the partition between the Turpan-Hami and southern Jung- gar basins. Second, geochronological, trace-element, and Sm-Nd isotopic varia- tions among granitoids in the late Paleozoic Tian Shan orogenic belt provide a further test of Mesozoic uplift of the Bogda Shan. On the basis of previous models of crustal compositions throughout the South, Central, and North Tian Shan, Bogda Shan, and East and West Jung- gar terranes, we infer that isotopically enriched granitic cobbles (av- erage eNdi 5 20.50, n 5 6) contained in Lower Triassic deposits in the north-central Turpan-Hami basin were derived from the continen- tal crustal Central Tian Shan terrane, south of Turpan-Hami, and not from the more oceanic North Tian Shan, Bogda Shan, and East and West Junggar terranes, north of the Turpan-Hami basin. We therefore infer that the ancestral Bogda Shan had not been uplifted by the Early Triassic, and that prior to this time, a unified Junggar-Turpan-Hami basin existed during Late Permian deposition of extensive lacustrine deposits.

Remote Sensing of Geomorphology and Facies Patterns on a Modern Carbonate Ramp (Arabian Gulf, Dubai, U.A.E.)

Abstract: The occurrence of power law relationships in the spatial distribution of sedimentary lithotopes has been identified in both modern and ancient carbonate depositional bodies. In this study, facies patterns and their spatial relationships are investigated quantitatively in an Arabian Gulf shallow subtidal carbonate ramp setting, using a synergy of IKONOS satellite imagery and vessel-based acoustic bathymetry survey. Three-dimensional reconstruction of the facies distribution on the seabed enabled quantification of relationships between eight dominant facies classes and the association of particular lithotopes to water depth. Fractal behavior was investigated using a combination of boundary- and patch-based metrics, and the spatial distribution of early diagenetically cemented hardgrounds and unconsolidated carbonate sand were shown to display strong fractal properties. In contrast, we show that landscape-scale processes can be treated as essentially deterministic and facies neighborhood patterns are strongly probabilistic. Identifying that the heterogeneity of shallow subtidal carbonate facies scales with a power law within certain thresholds has the potential to serve as a tool in environmental reconstruction in the ancient, where information on the lateral persistence of facies units is difficult to obtain. The findings are relevant to the interpretation of stratigraphic sequences and paleo-depth analysis.

Radar Architecture and Evolution of Channel Bars in Wandering Gravel-Bed Rivers: Fraser and Squamish Rivers, British Columbia, Canada

Abstract: The alluvial architecture and evolution of two kilometer-scale compound bars in the wandering gravel-bed Fraser and Squamish rivers, British Columbia, Canada, are described. Integrating ground-penetrating radar, bathymetry, and aerial photographs enables the internal architecture to be linked to the evolution of the gravelly barforms over the previous 50 years. These linkages reveal the sedimentary mechanisms that formed the various architectural packages within compound bars and unit bars. Growth of compound bars is controlled by the accretion of unit bars onto discrete segments along their gravelly edges. The attachment of unit bars deflects the thalweg to impinge on and erode specific portions of bars and channel banks. Bar growth leads to the stabilization of bars, vegetation colonization of bar interiors, and island formation. It is the formation of islands, along with channel avulsions, that maintains channel division in wandering rivers. Seven styles of gravelly deposition are imaged in the alluvial architecture. Vertical-accretion deposits formed from the deposition of gravelly bedload sheets are the most common strata. A moderate abundance of slipface deposits preserves high-relief bar margins. Lateral accretion dominates point-bar deposits. Downstream-accretion deposits govern some phases of down-bar growth. Partial and complete channel-fill and chute-fill deposits are eroded into underlying sediments, as are scour-and-fill deposits. Upstream-accretion deposits are uncommon. A depositional model of gravelly channel bars in the two rivers is presented and reveals that the architecture is made up of depositional styles similar to those observed in braiding-river successions, although the sedimentary packages are preserved in different proportions. Differences in braiding-river and wandering-river sedimentology largely reflect the relatively frequent migration of channels and bars in braiding rivers, which preserve high proportions of channel fill and chute fill, and confluence scour-and-fill deposits. Conversely, the moderately stable island and channel network in wandering rivers limits channel shifting and consequently preserves a low number of channel fills. Moderate proportions of slipface strata and coherent patterns of sand deposition along bar tops provide evidence of comparatively uniform flow patterns in independent channel segments divided by islands. These patterns of bar sedimentation and channel shifting preserved in the alluvial architecture appear to be signature characteristics of wandering rivers. The occurrence of similar architecture in both the Fraser and Squamish rivers suggests that the model likely applies to most wandering gravel-bed rivers.

Terrace Inundation as an Autocyclic Mechanism for Parasequence Formation: Galveston Estuary, Texas, U.S.A.

Abstract: Architecture of late Quaternary incised-valley fills is commonly attributed to the interplay between sea-level rise, sediment supply, and hydrodynamic processes. Inundation of fluvial terraces is commonly overlooked as an autocyclic mechanism for formation of parasequences. If the rate of sea-level rise and sediment supply is constant, architecture of terraced incised-valley fills will likely show backstepping parasequences. The control that variable antecedent topography has on architecture of incised-valley fills is examined in the Trinity incised valley, Texas. The Trinity valley is characterized by a series of downward-stepping terraces, and the Galveston Estuary formed above this irregular antecedent topography. Flooding surfaces, recognized in core by a decrease in sedimentation rates and a change from delta-plain to central-basin facies, formed at ~ −14 m, 8,200 cal. yr BP and ~ −10 m, 7,700 cal. yr BP, matching depths of the relatively flat fluvial terraces. Flooding surfaces formed rapidly and represent entire reorganization of the estuarine complex. Across the −10 m flooding surface, the river mouth and bay-head delta shifted landward at a rate of ~ 6.5 km per century and the associated barrier shoreline was stranded on the inner continental shelf, forming Heald Bank. Flooding surfaces formed as the rate of sea-level rise was decreasing, and are not associated with a decrease in sediment delivery to the estuary. As sea level inundates relatively flat fluvial terraces, rates of transgression rapidly increase, resulting in a sudden increase in accommodation space and an associated landward shift in coastal facies. Backstepping parasequences are inherent to the architecture of terraced incised-valley fills.

Distinguishing Climate in the Soil Record Using Chemical Trends in a Vertisol Climosequence from the Texas Coast Prairie, and Application to Interpreting Paleozoic Paleosols in the Appalachian Basin, U.S.A.

Abstract: A suite of Vertisols (clay-rich soils with high shrink-swell potential) were examined across a climosequence (climatic transect) in twelve soil pits from the Coast Prairie of Texas in order to determine if mean annual precipitation (MAP) exerts a control on the chemistry of these soils, and if the observed chemical trends are useful for interpreting paleoclimate records of paleoVertisols in the geologic record. The precipitation regime of the climosequence spans a range between 144 and 86 cm/year, with moisture regimes classified as udic, udic-ustic, ustic, and aridic-ustic, in a general northeast to southwest direction. Other soil-forming factors, such as soil age (<35–40 ka), parent material (fluviodeltaic Beaumont Formation of late Pleistocene age), landscape (low-relief coastal plain), and vegetation (prairie or mixed woody shrubs), are relatively constant across the climosequence. Climate-sensitive chemical proxies of MAP identified include dithionite citrate-extractable Fe (Fedith), acid oxalate-extractable Fe (Feoxal), CaCO3 equivalent (CaCO3equiv), S, and ammonium acetate-extractable Na, K, and Mg (Naacet, Kacet, and Mgacet, respectively), which vary across the climosequence because of differences in effective depths of leaching and intensity of wetting and drying cycles. These standard USDA wet-chemical climate proxies are related to bulk (oxide or element) chemistry of soils and paleosols measured using XRF, which supports the use of geochemical climate proxies for interpreting the paleoclimate records of paleoVertisols. Application of the chemical index of alteration minus potash (CIA-K) geochemical climofunction to late Mississippian paleosols from the Appalachian basin of the eastern U.S. demonstrates evidence for a shift from a lower to a higher MAP paleoclimate that is consistent with previous paleoclimate models and with observed morphological changes in the paleosols. We advocate actualistic research using bulk chemistry of soils and paleosols as a means of obtaining soil information useful for interpreting paleosols in the geological record.

Sedimentology, Stratigraphic Architecture, and Ichnology of Gravity-Flow Deposits Partially Ponded in a Growth-Fault-Controlled Slope Minibasin, Tres Pasos Formation (Cretaceous), Southern Chile

Abstract: A recent resurgence in the study of slope deposits is strongly related to the complexity inherent with the depositional setting, and the continued prospectivity for hydrocarbons in associated units. Intraslope basins are particularly intriguing to explorationists, because they provide accommodation space for coarse-grained facies in areas often characterized by otherwise muddy deposits. An outstanding outcrop of a growth-fault-controlled slope minibasin and its sandy fill are exposed in Cretaceous strata of southern Chile. A turbiditic sandstone package (TSP) 60 m thick sharply overlies fine-grained deposits of the Cerro Toro Formation, and defines the base of the Tres Pasos Formation in the study area, 65 km north of Puerto Natales, Chile. Individual sedimentation units are rarely amalgamated, and are tabular for at least hundreds of meters, suggesting deposition in a relatively unconfined setting. Trace-fossil assemblages are grouped into the Zoophycos or Cruziana ichnofacies, and are consistent with slope deposition rather than deposition on the basin floor. Beds at the base of the TSP are truncated by a normal fault at the southern end of the outcrop, and lap out towards the north onto the tilted hanging wall. Stratal thickening and thinning across the fault, and unfaulted overlying deposits, suggest that the fault was active during sand deposition (a growth fault), and that it created accommodation space for the partially ponded TSP. This, and other growth faults in the underlying strata, suggest deposition in an intraslope minibasin setting. The TSP is overlain by ~700 m of amalgamated slump, slide, and debris-flow units, suggesting that deposition immediately preceded rapid slope progradation.

Holocene Reef Development Along the Northeastern St. Croix Shelf, Buck Island, U.S. Virgin Islands

Abstract: Eight cores were recovered from Buck Island Underwater National Monument (U.S. Virgin Islands). Facies were defined based on recovered coral species, fabrics observed in core slabs and thin sections, and detailed notes on drilling character. Thirty-six radiometric dates constrained the timing of reef accretion. Together, these data provide a detailed history of reef development under varying regimes of sea-level rise and physical oceanography. Holocene reefs around Buck Island initiated atop a broad antecedent bench at 13–16 meters below present sea level. Shelf flooding near Buck Island occurred as early as 9,500 years ago (CalBP), but preserved reefs lagged by as much as 1,800 years. Earliest reef development was dominated by branching Acropora palmata near the shelf edge and massive corals closer to Buck Island. By 7,200 CalBP, A. palmata apparently declined near the platform margin and was absent until ca. 5,200 CalBP throughout the study area. Over time, the reefs closer to Buck Island built upward (ca. 16 m) and seaward (ca. 50 m), as the rate of sea-level rise slowed and carbonate production increasingly exceeded the accommodation space that was being created. Reef topography and zonation became progressively more distinct, with A. palmata dominating the shallow reef crest. Branching coral apparently disappeared again between ca. 3,030 and 2,005 CalBP for reasons that are not clear. This and the previous decline of A. palmata mimic patterns seen around St. Croix and throughout the Caribbean. By 1,000 CalBP, the reefs close to Buck Island had largely assumed their present character and continued to track slowly rising sea level until the present. Around 1,200 CalBP, vertical accretion along Buck Island Bar apparently ceased. Paradoxically, the surface of this outer reef has historically been dominated by large stands of A. palmata since the area was first described, but rapid coral growth has not resulted in preservation of this species over the last millennium. Modern community structure mimics facies patterns seen in cores. Over the past 7,700 years, the southern reef crest appears to have remained slightly shallower than its northern counterpart, a condition that persists today. Observations after Hurricane Hugo in 1989 suggest that this difference in elevation is related to the piling up of debris on the broader, southern reef crest by high waves from storms passing south of St. Croix. Also, facies along the southern reef are more variable in species composition than their northern counterparts, a condition that is exhibited by the modern reef community. Coral abundance and diversity in the cores (total coral = 20–30%; dominated by A. palmata) are comparable to the community structure present in the late 1970s (Bythell et al. 1993; Hubbard et al. 1993). In contrast, fossil-coral abundance and diversity are consistently higher than what was measured in the 1980s and early 1990s (total coral = 7–14%; A. palmata ≤ 2%), after the onset of White Band Disease, a putative pathogen, which has recently decimated branching acroporids throughout the region. The dominance of branching A. palmata in the cores would seemingly reflect an absence of disease or other factors that would discourage its continued abundance. In apparent contrast, two lengthy gaps in the A. palmata record reflect previous disappearances that roughly correspond to similar lapses elsewhere in the Caribbean. Thus, the spatial persistence of a species in the fossil record cannot necessarily be equated with its temporal continuity. Comparisons between changes in modern reefs on a time scale of decades and their fossil forebears must be made with great care. Understanding the role of short-term changes and how they are reflected in the preserved record is thus critical to relating the late Holocene A. palmata gaps to the recent decline of the species. This has important implications for our understanding of how preserved community structure relates to what actually existed in the past, and could limit our ability to use the recent geologic record as a proxy to short-term, future changes in coral reefs.

Mechanisms for Forming Discontinuity Surfaces Within Shoreface–shelf Parasequences: Sea Level, Sediment Supply, or Wave Regime?

Abstract: A 2D process–response model was used to evaluate potential mechanisms involved in the formation of small-scale stratigraphic variability that is observed within progradational wave-dominated shoreface–shelf parasequences. Model experiments, which are based on scenarios that consist of a 25 ky progradational phase, suggest that discontinuity surfaces form when either sediment supply or sea level fall rapidly, or when wave-height regime increases over relatively short time periods (102–104 y) during coastal progradation. In these cases, mean deposition rate near the lower-shoreface drops, which, combined with coastal progradation, results in a discontinuity surface. Both observed and simulated discontinuity surfaces form predominantly in lower shoreface deposits and bound meter-scale stratigraphic cycles. Simulations show that wave-regime variability affects the shoreface–shelf to a much greater depth than minor sea-level change. Therefore, facies shifts along wave climate-induced bounding surfaces occur over wider reaches of the shoreface–shelf system than sea-level-induced facies shifts. High-frequency variability in sediment supply rate results in the formation of discontinuity surfaces across which there are no distinct facies shifts.

Provenance Analysis by Single-Quartz-Grain SEM-CL/Optical Microscopy

Abstract: The integration of panchromatic scanning electron microscopy-cathodoluminescence (SEM-CL) with optical microscopy analysis on single quartz grains is a new technique to interpret provenance of quartz-rich sediments. The combination of information gained from SEM-CL on textural features and CL response with information from optical microscopy allows distinction of different quartz types much more easily then with conventional microscopy or color CL analysis. Features visible by SEM-CL include zoning, microcracks, deformation fractures and lamellae, diagenetic crushing, recrystallization, and strength and texture of CL response. Features visible by optical microscopy include embayments, open fractures, extinction behavior, and mono-, poly-, and microcrystallinity. The key is to integrate CL and optical properties of the same quartz grains in order to reduce ambiguities inherent in both techniques used on their own. Our technique is well suited for distinguishing between plutonic, volcanic, and metamorphic quartz in coarse- to medium-grained sand or sandstone. This simple and inexpensive approach can easily be combined with other provenance techniques. In this paper we outline the combined SEM-CL/optical microscopy technique and demonstrate its application with examples from the South Island of New Zealand.