ALBERT

Description: The sequence boundary associated with the last glacial-eustatic lowstand was mapped across the northwestern Gulf of Mexico continental shelf. The geomorphology of incised fluvial valleys varies widely across the shelf. These differences are due to differences in shelf physiography and the interval of the eustatic cycle the valleys were occupied. Incision begins during the falling limb of sea level and results in terraced valleys. Rivers that abandoned their valleys during the fall in sea level to cut new valleys during the lowstand generally have u-shaped profiles. Incised valleys connected to turbidite systems only occurred in two valleys (the Colorado and Rio Grande), but this may be because sea level did not fall below the shelf break during the last eustatic cycle. Some valleys deepen in an offshore direction, others become shallower. The timing of fluvial incision was constrained using radiocarbon dates so that incision can be tied directly to the sea-level curve for the last glacial-eustatic cycle. The results show that the fluvial incision occurred throughout the falling limb of sea level and lowstand; however, maximum incision occurred during the lowest position of sea level. The resulting surface has significant relief, extends across the shelf, and has time significance. The associated conformable surface, on the other hand, is much harder to recognize and occurs at different stratigraphic levels relative to different shelf-margin deltas.

Description: Differences in marine terrace elevations across the Pacific coast of North America have long been assumed to be a result of differences in the rates of tectonic motion. However, other processes, particularly glacio-isostatic adjustment, lead to regional variations in sea levels. In this study, we compiled the elevations of marine isotope stage (MIS) 5e (ca. 119–129 ka), 5c (ca. 106 ka), and 5a (ca. 84 ka) terraces across the Pacific coast of central North America and compared these regional variations in elevation with model predictions of glacio-isostatic adjustment after correcting for tectonics. These predictions are generally consistent with the observed trends in the elevations of the terraces and show that this process created up to 20 m of coeval variation in local sea levels along the Pacific coast of central North America (between 20°N and 45°N) during MIS 5c and MIS 5a, but less, ~4 m, during MIS 5e. Accounting for glacio-isostatic adjustment reduces the variability in uplift rates calculated at individual locations using different-aged terraces as datums. Ignoring glacio-isostatic adjustment leads to overestimated uplift rates by an average of 40%, but up to 72%, across the Pacific coast of central North America. An understanding of regional variations in glacio-isostatic adjustment–corrected sea levels also contributes to the correct identification of marine terraces with mistaken ages.

Description: A bstract :  Bayhead deltas exist between the fluvial and central-basin depositional environments of many incised-valley-fill successions. Prior work has shown that tributary junctions in flooded incised valleys provide "pinning points" or locations of stabilization for bayhead deltas during transgression. The purpose of this study is to examine the influence of tributary junctions on the behavior of bayhead deltas during regression. This influence is examined by using a simple numerical model that incorporates established relationships for systematic changes in valley geomorphology downstream. In addition, the model incorporates important observations regarding the controls on water depth in wave-dominated estuarine central basins, which in part controls sediment accommodation. Water depth in the central basin of wave-dominated estuaries with high suspended sediment loads is a function of the width of the central basin parallel to the prevailing wind direction (fetch). A direct relationship is observed between the width of the central basin parallel to the prevailing wind direction and water depth based on data from modern estuaries in North America, Europe, and South America. Our model shows that bayhead-delta progradation rates increase at tributary confluences and larger deltas prograde at faster rates than smaller deltas. In the case of two tributary bayhead deltas fed by systems of the same size coming together, the combined bayhead delta progrades 17.6% faster. This study provides one more example of the importance of autogenic processes in influencing stratigraphic architecture and the rate of shoreline changes.

Description: : Grain shape is useful as a depositional, environmental, diagenetic, and provenance proxy. Here we conduct the first study of down-core grain-shape variation in Antarctica. This study further examines the utility of grain shape as a depositional proxy, namely of glacial influence, in the James Ross Basin and Joinville Plateau of the Antarctic Peninsula from the Eocene to the present. Fourier grain-shape analysis was used to quantify the grain shape of 31 samples spanning the glacial history of the Antarctic Peninsula Ice Sheet. A total of 6,442 quartz grain peripheries were digitized and described through the twenty-first Fourier harmonic. Sediment provenance has a greater influence on grain roughness than glacial activity in the study area. However, provided a similar sedimentary source, secondary changes in roughness correlate with changes in the extent of the Antarctic Peninsula Ice Sheet since the late Oligocene. Grain roughness increased with increased glacial influence during the middle Miocene, after an early Pliocene warm period, and just above a Pliocene regional glacial unconformity. Conversely, grain roughness decreased with less glacial influence during the late Oligocene and during an early Pliocene warm period. This study further illustrates the usefulness of grain shape in distinguishing sediment provenance and environmental conditions in Antarctic settings.

Description: The Antarctic Peninsula is considered to be the last region of Antarctica to have been fully glaciated as a result of Cenozoic climatic cooling. As such, it was likely the last refugium for plants and animals that had inhabited the continent since it separated from the Gondwana supercontinent. Drill cores and seismic data acquired during two cruises (SHALDRIL I and II) in the northernmost Peninsula region yield a record that, when combined with existing data, indicates progressive cooling and associated changes in terrestrial vegetation over the course of the past 37 million years. Mountain glaciation began in the latest Eocene (approximately 37–34 Ma), contemporaneous with glaciation elsewhere on the continent and a reduction in atmospheric CO2 concentrations. This climate cooling was accompanied by a decrease in diversity of the angiosperm-dominated vegetation that inhabited the northern peninsula during the Eocene. A mosaic of southern beech and conifer-dominated woodlands and tundra continued to occupy the region during the Oligocene (approximately 34–23 Ma). By the middle Miocene (approximately 16–11.6 Ma), localized pockets of limited tundra still existed at least until 12.8 Ma. The transition from temperate, alpine glaciation to a dynamic, polythermal ice sheet took place during the middle Miocene. The northernmost Peninsula was overridden by an ice sheet in the early Pliocene (approximately 5.3–3.6 Ma). The long cooling history of the peninsula is consistent with the extended timescales of tectonic evolution of the Antarctic margin, involving the opening of ocean passageways and associated establishment of circumpolar circulation.

Description: Although sea-level highstands are typically associated with sediment-starved continental shelves, high sea level does not hinder major river floods. Turbidity currents generated by plunging of sediment-laden rivers at the fluvial-marine interface, known as hyperpycnal flows, allow for cross-shelf transport of suspended sand beyond the coastline. Hyperpycnal flows in southern California have deposited six subaqueous fans on the shelf of the northern Santa Barbara Channel in the Holocene. Using eight cores and nine grab samples, we describe the deposits, age, and stratigraphic architecture of two fans in the Santa Barbara Channel. Fan lobes have up to 3 m of relief and are composed of multiple hyperpycnite beds ~5 cm to 40 cm thick. Deposit architecture and geometry suggest the hyperpycnal flows became positively buoyant and lifted off the seabed, resulting in well-sorted, structureless, elongate sand lobes. Contrary to conventional sequence stratigraphic models, the presence of these features on the continental shelf suggests that active-margin shelves may locally develop high-quality reservoir sand bodies during sea-level highstands, and that such shelves need not be solely the site of sediment bypass. These deposits may provide a Quaternary analogue to many well-sorted sand bodies in the rock record that are interpreted as turbidites but lack typical Bouma-type features.

Description: The mixed siliciclastic/carbonate sediments of Baffin Bay, Texas, provide a record of the evolution of the Bay for the last 10 ka. Flooding surfaces at 8 ka and 5.5 ka provided an a priori separation of sediments in a core into three groups. Discriminant analysis and interpretation of species composition of the foraminifera from these groups indicated a progression from deltaic to open-bay to hypersaline environments. This traditional paleoecological analysis, however, does not utilize the information available in the relative abundance distribution (RAD) within each community. An approach capable of assessing within community change is provided by S (species richness), H (Shannon information function) and E (evenness) analysis. Using this approach not only can communities be designated without a priori assumptions and environments identified easily, but also the RAD within each community can be evaluated, providing a record of community growth, decline or stasis with time. Stasis, or the ecological balance of nature, is mathematically defined as an equilibrium between S and E such that diversity (H) remains constant with time. This stasis requires that, as the number of individuals (N) gets larger with time, the value of H remains constant. Thus, at stasis a regression between H versus lnN will have a 0 value for the regression coefficient (β 1H ), here termed the Relative Abundance Distribution Index, RADI. A positive value of RADI indicates community growth, a negative value indicates community decline. In the Baffin Bay core 6, communities were identified from 46 samples using SHE analysis. At ~9 ka the RADI was positive, indicating the growth of a normal marine community with a high S. A second community, still largely normal marine but with a slightly negative RADI, formed at ~8 ka. Beginning at ~6.4 ka, the 3 rd and 4 th marginal marine communities with highly negative RADIs formed, indicating a sharp decline for ~1 ka during the formation of Padre Island, which may have taken ~1000 years. At ~5 ka the 5 th and 6 th marginal marine communities were established with RADIs indicating a prolonged period of stasis. The 5 th community was dominated by Elphidium with a high percentage of miliolids. The 6 th community, established at ~2 ka, was dominated by Ammonia and a moderate percentage of miliolids. These last two communities, both at stasis, were apparently responding to changes in salinity brought on by changes in rainfall.

Description: Although recent work has shown that changing interstitial fluid density within turbidity currents is a frequently overlooked factor affecting the texture and internal architecture of turbidites, little is known about its influence on submarine fan morphology. Here we present the results of three-dimensional flume experiments of turbidity currents that clearly demonstrate the role of low-density interstitial fluid, in combination with sediment concentration and basin gradient, on submarine fan geometry. The experiments show that turbidity currents with reversing buoyancy, and their resulting deposits, are narrower than those that remain ground hugging. Furthermore, wider deposits result from increases in sediment concentration and/or basin-floor gradient. We also propose that Taylor-Görtler vortices associated with currents traveling over a break in slope may lead to the deposition of wider lobes compared with those traveling over a constant gradient.

Description: Few submeter-resolution relative sea-level (RSL) proxies exist for semiarid and arid coastlines that lack well-developed marshes or coral reefs. Because 15% of the world’s nonpolar coastline is in desert and steppe precipitation regimes, a new RSL proxy is needed for these regions. In this study we show the utility of microbial mats, a common biota found along semiarid and arid coastlines, as an RSL proxy. The indicative range of microbial mats in Baffin Bay (Texas, United States) is ±0.29 m, much less than the ±2 m indicative range of typical sea-level indicators currently used along the semiarid Texas coast. The elevations of 22 buried radiocarbon-dated microbial mats plot within error of RSL data derived from the central Texas coast for the past 5.0 k.y., suggesting that microbial mats provide a robust proxy for paleo–sea levels along semiarid and arid coastlines.