Stratigraphic Framework and Heavy Minerals of the Continental Shelf of Onslow and Long Bays,North Carolina

One hundred fourteen vibracores from the Atlantic continental shelf offshore of southeastern North Carolina were opened, described, and processed over several contract years (years 6-9) of the Minerals Management Service Association of American State Geologists Continental Margins program. Reports for years 9 and 10 of the program compiled the results of the work and assembled the data for release as an interactive CD-ROM report, respectively. The continental shelf of Onslow and Long Bays consists predominantly of outcropping Cretaceous through late Tertiary geologic units. Nearshore these units are covered and incised by late Tertiary and Quaternary units. From oldest to youngest, formally recognized geologic units mapped as part of this study are the Late Cretaceous Peedee Formation a muddy, fine-to medium-grained quartz sand with trace amounts of glauconite and phosphate; the Paleocene Beaufort Forma tion a muddy, fine-to medium-grained glauconitic quartz sand with locally occurring turritelid-mold biosparrudite; the middle Eocene Castle Hayne Forma tion a sandy bryozoan biomicrudite and biosparrudite; the Oligocene River Bend Formation a sandy molluscan-mold biosparrudite; and the Miocene Pungo River Formation a medium-grained, poorly sorted slightly shelly phosphatic sand. Infor mal units include a very widespread, unnamed fine-to very fine grained, well-sorted, dolomitic muddy quartz sand that is biostratigraphically equivalent to the Oligocene River Bend Formation; several large valley-fill lithosomes composed of biomicrudite, biomicrite, and biosparrudite of Plio Pleistocene age; muddy, shelly sands and silty clays of Pliocene, Pleistocene, or mixed Plio Pleistocene age; and loose, slightly shelly, medium- to coarse-grained sands assigned a Holocene age. Heavy minerals (SG>2.96) comprise an average of 0.54 wt % (on a bulk-sam ple basis) of the sediments in 306 samples derived from the 114 vibracores. Heavy-mineral content ranges from <0.01 to 3.69 wt %. The economic heavy mineral content (EHM ilmenite zircon rutile aluminosilicates leucoxene [altered ilmenite] monazite) of the bulk samples averages 0.26 wt % in a range of <0.01-1.70 wt %. As a percentage of the heavy-mineral concentrate, the average EHM value is 45.78 % in a range of 0.27-68.60 %. The distribution of heavy minerals offshore of southeastern North Carolina is controlled by the lithostratigraphic framework. The unnamed Oligocene sand unit has the highest heavy-mineral content, averaging 0.86 wt % on a bulk-sample basis. The remaining geologic units and their heavy-mineral content (in decreasing order of abundance) are Beaufort (0.64 %), Holocene sand (0.60 %), Plio-Pleistocene muddy sand and silty clay (0.59 %), Peedee (0.42 %), River Bend (0.34 %), Plio-Pleistocene carbonate (0.12 %), and Castle Hayne (0.08 %). The heavy-mineral assemblage is fairly consistent throughout the different units. Significantly smaller percentages of heavy minerals correlate with increased amounts of CaCO3 in the sediments. The sediments analyzed in this study have significantly lower overall heavymineral content, as well as lower EHM content than sediments that are known to host commercially important heavy-mineral deposits in the southeastern United States. The potential for economic deposits of heavy minerals in the area of this study, therefore, appears to be limited.     

Characterization of marine aggregates off Waikiki,O'ahu,Hawai'i

Researchers at the University of Hawai'i at Manoa have been working for the past several years to develop the necessary techniques for finding and quantitatively characterizing offshore unconsolidated carbonate deposits with potential for beach nourishment and use in construction aggregates for tropical island communities. This article examines particular results of this research, with special attention given to the area offshore from Waikiki Beach. Acoustic surveying, water‐jet probing to measure the thickness of unconsolidated material and three different sampling methods were used in this study. Two separate seismic systems were used for the subbottom profiling survey, a Datasonics Bubble‐Pulser® system and a broad‐band, frequency‐modulated ("chirp") prototype system.

The following conclusions were reached. (1) Many different types of sediment underlie tropical island carbonate sand deposits and serve as refusing horizons to jet probing. Examples include consolidated or unconsolidated reef debris, beach rock, cemented sand, and various types of conglomerates formed from rhodoliths (coralline algae) or reef detritus. (2) Massive coral growth over clastic deposits is not a common offshore feature in this area, though it does occur in some areas off the Reef Runway. (3) Matrices of the coral Porites compressa, in‐filled with sand, may have acoustic properties similar to those of the sand bodies. Such deposits may be difficult to distinguish from unconsolidated deposits from seismic records alone. (4) Significant new prospects for offshore aggregates were found in the insular shelf offshore from Southern O'ahu. A total of 5,100,000 m³ were mapped off Waikiki. The Makua Shelf deposits in this area presently appear to be the best prospects for commercial development.     

The Skaergaard liquid line of descent revisited

There is a fundamental conflict between the suggestion that the iron content of Skaergaard liquids increases during Fe–Ti oxide fractionation and the observation that at the same time oxygen fugacity () drops by two log-units below the fayalite-magnetite-quartz oxygen buffer (FMQ). A new petrographic study of average Skaergaard gabbros shows that the total modal content of Fe–Ti oxides is about 22% in the early LZc and markedly decreases to below 5% in the UZc. Forward modeling based on these modal constraints, as well as experimental results on Skaergaard-related dikes, predicts that fractionation of troctolitic LZa gabbros drives the derivative liquid towards a high-iron content. Strong iron enrichment continues, together with a small decline in silica, during LZb crystallization due to the appearance of augite as a fractionating phase. The fractionation of Fe–Ti oxides in the LZc initially suppresses iron enrichment and reverses the silica trend to one of slight enrichment. However, continued evolution into the UZ produces liquids with maximum UZc FeO* content of 23–25 wt.% and SiO₂ content of 53 wt.% (FeO* is total iron as FeO). The maximum in FeO* is dependent on several factors of which the Fe–Ti oxide mode has the strongest effect. The during crystallization of the LZc is widely thought to have been at, or slightly below, the fayalite-magnetite-quartz oxygen buffer (FMQ). Under closed system evolution, incorporation of ferric iron into augite during formation of the LZb restricts the increase in to about 0.1 log-units above FMQ (=0.1 ΔFMQ). Likewise, crystallization of the LZc through the UZa, involving Fe–Ti oxide minerals, leads to a decline in of less than 0.1 ΔFMQ. Crystallization of the UZb-c gabbros results in oxidation to a maximum of 0.5 ΔFMQ. This behavior can account for the iron-rich character of the UZ gabbros, as well as, the low modal content of Fe–Ti oxides. Thus, evolved Skaergaard liquids are high in iron and contain a modest amount of SiO₂. Our modeling result do not account for a strong drop in through the layered series. Such a drop would require an unacceptably high proportion of Fe–Ti oxides and high-magnetite content in the fractionating assemblage. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

Jarosite growth zoning as a recorder of fluid evolution

Growth zoning in jarosite, an Fe³⁺-bearing sulfate mineral, can generally be characterized by variation of Na⁺ for K⁺ in the crystallographic A site or by Al³⁺ for Fe³⁺ in the octahedral (B) site. Growth zoning in a sample from Post Pit, NV, however, is more complicated than has been observed in other jarosite samples examined in this study and by previous work, and is characterized by varying Ba, Sr, P, and As. In this sample, these elements define a coupled substitution in which Ba²⁺ and Sr²⁺ substitute for K⁺ in the crystallographic A site, and are balanced by the substitution of P⁵⁺ and As⁵⁺ for S⁶⁺ in the tetrahedral (X) site. The Post Pit sample also exhibits a high concentration of V, which does not appear to participate in the aforementioned coupled substitution. Analysis of mineral stoichiometry and charge balance reveal that V is likely tetravalent, and represents a charge excess in the B site. The occurrence of V-bearing goethite in this sample records fluctuating fluid pH, resulting in the alternating stability of jarosite and goethite. Measurement of the REEs in jarosite by SIMS, indicate that Ce is trivalent in the Post Pit sample; reduced relative to Ce⁴⁺ found in the remainder of the sample suite. The presence of V⁴⁺ and Ce³⁺ indicate that the Post Pit sample was deposited in a less oxidizing environment than the remainder of the sample suite. This study illustrates how jarosite crystal chemistry can be used to place relative constraints on fluid conditions (pH, Eh, chemistry, etc.) in the terrestrial environment, and characterizes possible major and trace element coupled substitutions into the jarosite crystal structure. Furthermore, this study provides a framework for future studies to examine mineral-fluid partition coefficients (D values) which may place more absolute constraints on fluid chemistry in the jarosite depositional environment.     

A late Pleistocene steppe bison (Bison priscus) partial carcass from Tsiigehtchic, Northwest Territories, Canada

A partial steppe bison (Bison priscus) carcass was recovered at Tsiigehtchic, near the confluence of the Arctic Red and Mackenzie Rivers, Northwest Territories, Canada in September of 2007. The carcass includes a complete cranium with horn cores and sheaths, several complete post-cranial elements (many of which have some mummified soft tissue), intestines and a large piece of hide. A piece of metacarpal bone was subsampled and yielded an AMS radiocarbon age of 11,830 ± 45 ¹⁴C yr BP (OxA-18549). Mitochondrial DNA sequenced from a hair sample confirms that Tsiigehtchic steppe bison (Bison priscus) did not belong to the lineage that eventually gave rise to modern bison (Bison bison). This is the first radiocarbon dated Bison priscus in the Mackenzie River valley, and to our knowledge, the first reported Pleistocene mammal soft tissue remains from the glaciated regions of northern Canada. Investigation of the recovery site indicates that the steppe bison was released from the permafrost during a landslide within unconsolidated glacial outwash gravel. These data indicate that the lower Mackenzie River valley was ice free and inhabited by steppe bison by 11,800 ¹⁴C years ago. This date is important for the deglacial chronology of the Laurentide Ice Sheet and the opening of the northern portal to the Ice Free Corridor. The presence of steppe bison raises further potential for the discovery of more late Pleistocene fauna, and possibly archaeological evidence, in the region.     

Salt Marsh Sustainability: Challenges During an Uncertain Future

Many salt marshes throughout southern New England are exhibiting a trend toward submergence, as reported in this volume and other published literature. This paper provides a brief perspective on sea-level rise and the many other interacting factors that contribute to marsh submergence in this and other regions. Curtailing nutrient loading and removing or altering barriers (e.g., dams, dikes) to the delivery of suspended sediment to marshes are discussed as management or restoration techniques to consider for increasing long-term sustainability of marshes. Adaptation measures are many (e.g., thin-layer sediment application to marsh surface, facilitation of landward marsh migration, shoreline stabilization), but all require study to evaluate their potential for enhancing resilience. Research, monitoring, and dynamic modeling, coupled with appropriate management and adaptation approaches implemented at local and regional scales, will contribute to the challenge of sustaining salt marshes in an uncertain future of sea-level rise, other climate factors, and stressors associated with a developing coastal zone.     

Analytical models for velocity distributions in compound channels with emerged and submerged vegetated floodplains

The lateral distributions of depth-averaged velocity in open compound channels with emerged and submerged vegetated floodplains were analyzed based on the analytical solution of the depth-integrated Reynolds-Averaged Navier-Stokes equation with a term to account for the effects of vegetation. The three cases considered for open channels were two-stage rectangular channel with emerged vegetated floodplain, rectangular channel with submerged vegetated corner, and two-stage rectangular channel with submerged vegetated floodplain, respectively. To predict the depth-averaged velocity with submerged vegetated floodplains, we proposed a new method based on a two-layer approach where flow above and through the vegetation layer was described separately. Moreover, further experiments in the two-stage rectangular channel with submerged vegetated floodplain were carried out to verify the results. The analytical solutions of the cases indicated that the corresponding analytical depth-averaged velocity distributions agree well with the simulated and experimental prediction. The analytical solutions of the cases with theoretical foundation and without programming calculation were reasonable and applicable, which were more convenient than numerical simulations. The analytical solutions provided a way for future researches to solve the problems of submerged vegetation and discontinuous phenomenon of depth-averaged velocity at the stage point for compound channels. Understanding the hydraulics of flow in compound channels with vegetated floodplains is very important for supporting the management of fluvial processes.     

Interest in geological and palaeontological curiosities by southern African non-western societies: A review and perspectives for future study

Compared with other parts of the world, the study of geomythology in southern Africa, and the associated documentation of non-western awareness of palaeontological and geological phenomena, is in an early phase. We focus on examples of rocks and fossils as items of special interest and curiosity, and we search for evidence of an indigenous palaeontology and geology. We review twenty-one sites or cases for which published accounts exist, and we describe a newly identified trilobite manuport site. In combination these sites provide various levels of evidence of palaeontological and geological awareness exhibited by non-western cultures in southern Africa, and how these cultures incorporated this knowledge into their understanding of their world. We anticipate that in time a diverse heritage of such ‘natural knowledge’ may become evident in southern Africa, aided in part by recognition of the possibility that rock art images may be associated with awareness of body fossils and trace fossils. We suggest ways in which further analysis may bolster this contention.     

The stratigraphic implications of long‐term terrace agriculture in dynamic landscapes: Polycyclic terracing from Kythera Island,Greece

Geoarchaeological work in conjunction with the Kythera Island Project indicates that significant portions of the island are now or have at some time been terraced. Geoarchaeological observations and local historical records confirm extensive terracing during the last few centuries. Detailed stratigraphic, soil, and sediment analysis along with radiocarbon dating suggest, however, that some of the slopes and small drainages of the island were terraced more than once and that this relatively recent phase of terracing followed earlier efforts, some dating to the early second millennium B.C. (Bronze Age). During each phase of slope organization significant amounts of soil were moved locally on the slopes. Polycyclic terracing has, therefore, interesting implications for surface survey visibility, preservation of archaeological record, early agricultural landscapes and soils, and past land use, as well as for interpretation of local records of Holocene sediment mobilization and valley alluviation. Geoarchaeological investigation of terraces may reveal unique archaeological information not available in urban or village dwelling sites. Early features, such as those reported in this paper, however, usually remain hidden due to the remarkable lack of stratigraphic investigations of non‐site landscapes. © 2008 Wiley Periodicals, Inc.