The type locality for the upper Oligocene Nuwok Member of the Sagavanirktok Formation (Carter Creek, North Slope, Alaska, USA) contains an abundant occurrence of glendonite, a pseudomorph after the calcium-carbonate mineral ikaite, which typically forms in the shallow subsurface of cold marine sediments. The region during the time of Nuwok Member deposition was located at a high latitude, similar to today, and the study site is characterized by sands and silty muds interpreted here to have been deposited in coastal and shelfal marine environments. Isotopic (Sr) and biostratigraphic (foraminifera) evidence presented here refine the depositional age of the outcrop to approximately 24 Ma. Glendonites occur in two basic forms: radial clusters, commonly centered around a single larger primary crystal (similar to 10 cm, Type A) and larger single blades generally without accessory crystals (similar to 15-25 cm, Type B). Microscopic examination reveals a sequence of multiple types of replacive calcite that formed as a direct result of ikaite transformation: Type 1 rhombohedral crystals characterized by microporous and inclusion-rich cores and concentric zones, Type 2A, composed of clear calcite that overgrew and augmented Type 1 crystals, and inclusion-rich, microcrystalline Type 2B, which formed a matrix surrounding the rhombs and commonly dominates the outer rims of glendonite specimens. Type 3 calcite precipitated as fibrous, botryoidal epitaxial cement atop previous phases and is not ikaite-derived. These phases are distributed in similar ways in all examined specimens and are consistent with several previously described glendonite occurrences around the world, despite differing diagenetic and geologic histories. Stable-isotope evidence (delta C-13 and delta O-18) suggests sourcing of glendonite carbon from both organic and methanogenic sources. Glendonites of the Nuwok Member can therefore assist in the determination of a more comprehensive ikaite transformation model, improving our understanding of glendonite formation and the sedimentological and environmental context of their occurrence. Oligocene glendonites are uncommon globally; the well-preserved occurrence described here can allow future studies to better reconstruct Arctic environmental conditions and paleoclimates during this time.

The middle Cretaceous greenhouse period experienced profound environmental change including episodes of enhanced global burial of organic carbon marked by carbon isotopic excursions (CIEs). However, the role and response of polar regions like the newly formed, partially enclosed Arctic Ocean Basin during middle Cretaceous carbon burial remains enigmatic. We present the first Arctic deepwater CIE record that characterizes conditions offshore of the Alaska margin north of 75 degrees N paleolatitude. Organic carbon isotopes (delta C-13(org)) and 103-82 Ma ash zircon U-Pb dates from the distal Hue Shale record multiple Albian-Campanian CIEs during slow similar to 3-15 m/Myr sediment accumulation rates. Average total organic carbon (TOC) increased substantially during large 2-3 parts per thousand CIEs of the similar to 101 Ma Albian-Cenomanian boundary event (from 7 to 18 % TOC) and similar to 94 Ma Cenomanian-Turonian boundary event (5 to 10 % TOC). Turonian TOC remained elevated (8-13 %) during high global sea levels and temperatures of the Cretaceous Thermal Maximum, followed by an increase from 7 to 11 % TOC during the similar to 90 Ma late Turonian event 1.5 parts per thousand CIE. Average TOC subsequently decreased in the Coniacian-Campanian, but relative maxima occurred during subtle 0.5-1 parts per thousand CIEs interpreted as the similar to 87 Ma late Coniacian event (increase from 4 to 7 % TOC), similar to 85 Ma Horseshoe Bay event (3.5 to 4.5 % TOC), and similar to 84 Ma Santonian-Campanian boundary event (3.5 to 5 % TOC). Increases in hydrogen index and productivity proxies (P, Ba, Nd) that accompanied each CIE episode with enhanced TOC suggest a strong link between marine productivity and organic carbon burial at short-term CIE timescales. However, long-term (>5-8 Myr) changes in trace metal redox (Mo, Fe, V) and salinity (B/Ga) proxies suggest shifts in prevailing environmental conditions at timescales longer than the CIEs. Late Albian-middle Turonian marine salinity occurred during euxinic (103-98 Ma) and suboxic (98-90 Ma) conditions with deposition interpreted to have occurred within and beneath an oxygen minimum zone, respectively. In contrast, late Turonian-early Campanian (90-82 Ma) freshening and restricted euxinic basin conditions may signal the start of widespread restriction known to characterize the Paleogene Arctic. Overall, these results highlight that middle Cretaceous Arctic deepwater remained a productive marine carbon sink coupled to the global carbon cycle despite evolving Arctic greenhouse conditions.

Detrital zircon data from modern barrier island and estuarine environments in the Outer Banks (Atlantic Coast, USA) were statistically compared to sands from nearby rivers to assist in determining source-to-sink pathways. Fluvial samples, collected from near the Fall Line contact between the Appalachian Orogen and sediments of the coastal plain, all have age unique distributions, making them ideal for tracing provenance. Three samples from the Atlantic foreshore showed high similarities to one another, as well as to three samples from the estuarine (back-barrier) Pamlico and Albemarle Sounds. Mixture modeling with multiple data reduction methods and three different statistical tests for similarity consistently indicated that the nearby Potomac River was the primary source for all Atlantic foreshore and estuarine zircons, followed by minor contributions from the James River in some models. The models indicate little or no sediment contribution from the Susquehanna, Roanoke, Tar, Neuse, Cape Fear, and Peedee Rivers. Both Atlantic foreshore and estuarine sands are therefore interpreted to have initially originated from Appalachian bedrock to the north of their present-day location, and subsequently to have been transported southward through the Chesapeake Bay watershed before deposition in Virginia and North Carolina. Prior to barrier island formation in the last several thousand years, differing geomorphology of the Chesapeake Bay facilitated southward movement of sediments from its constituent rivers via longshore drift, where they were deposited in coastal settings on the mainland. The modern barrier islands, formed during the most recent post-glacial transgression, may be reworked from these deposits, but may also include a contribution from sediments that were derived more recently from relict deposits on the shelf. Oceanographic and sedimentological evidence suggests that movement of sand-sized grains from southern rivers across the back-barrier sounds is unlikely. These findings can assist with coastal resilience planning and resource management in a region under severe threat from climate change and rising sea levels.

Fluvial sediments are the product of erosion, weathering, and transport of bedrock within a well-defined catchment area, and their constituent grains may therefore record valuable information about the lithological and geochemical properties of geologic units within the upstream drainage. Analysis of U-Pb ages and Lu/Hf isotopic values in detrital zircon grains from major rivers in the eastern United States characterizes these parameters within broad areas of the Appalachian orogen. In this study, five modern fluvial sediment samples, collected across similar to 1500 km and representing 216,000 km2 of total catchment area, reveal that the relative proportions of Mesoproterozoic to Paleozoic U-Pb crystallization ages vary widely across the former Laurentian margin. However, epsilon Hft values in the same samples are largely consistent regardless of their geographic location. Mesoproterozoic (Grenville orogen) zircons display a more limited range of epsilon Hft values (approximately 0 to +10 epsilon Hft units) compared with the more negative, more variable values (-15 to +10) found in grains from the Paleozoic orogenies. Compared with other published modern detrital samples from the Gulf Coastal Plain, Appalachian samples show more similarity to each other and to the Pleistocene of Florida than to those from the Mississippi River mouth, likely because the latter also sources sediments from the Cordillera of western North America. More negative epsilon Hft values in Paleozoic zircons may be genetically related to older Mesoproterozoic grains, as they are compatible with the continued isotopic evolution of Lu/Hf derived from the mantle between 1.2 and 2.0 Ga. Hafnium geochemistry in detrital grains may therefore have some utility in discerning sediment provenance between the Appalachians and other regions and may also provide useful information regarding the nature of crustal generation through time.

First posted February 5, 2021 For additional information, contact: Geology, Energy & Minerals Science CenterU.S. Geological Survey12201 Sunrise Valley Drive954 National CenterReston, VA 20192Contact Pubs Warehouse Using a geology-based assessment methodology, the U.S. Geological Survey estimated a mean of 1,407 billion (1.4 trillion) cubic feet of gas in conventional accumulations in Upper Devonian to Lower Cretaceous strata of the western North Slope, Alaska.

Oceanic currents can profoundly reshape the seafloor and even modify the characteristics of turbidite systems. Multiple erosional and depositional features directly formed by bottom currents (i.e. contourites), as well as by the interaction between bottom currents and turbidity currents or turbidite systems (i.e. mixed turbidite-contourite systems) have been identified in the Mozambique Channel (SW Indian Ocean) in multibeam bathymetry, seismic reflection data, sub-bottom profiler images and sediment cores. In this study, we characterise the morphology, stacking pattern and sedimentary characteristics of these sedimentary systems, and analysed the properties of bottom currents at these systems using a hydrodynamic numerical model. Modelled bottom currents are the highest at abraded surfaces and moats, but they also display a relatively high variability, suggesting that the observed erosion is not the result of a constant or persistent current but rather of episodes of intense circulation. Modelled bottom currents at contourite terraces are not significantly different from currents at related plastered drifts, where accumulation is enhanced. The formation of contourite terraces can thus not solely be explained by the mean oceanic circulation and eddies, implying that other processes such as internal waves may play a relevant role in their formation. Three different types of mixed turbidite-contourite systems were observed: one characterised by asymmetric channel-levee systems formed by the synchronous interaction of bottom currents and turbidity currents, one characterised by a phased interaction that resulted in the erosion of the channel flanks by bottom currents, and another one in which both synchronous and phased interaction played a relevant role in the evolution of the system. Finally, we propose a simplified classification of contourites that can be applied to any contourite system worldwide, and that comprises erosional and depositional features, including muddy and sandy contourite deposits.

Isolated, detached sands provide opportunities for large-volume stratigraphic traps in many deepwater petroleum systems. Here we provide a review of the different types of sandbody detachments based on published data from the modern-day seafloor and recent (generally Quaternary-present), shallow-buried strata. Detachment mechanisms can be classified based on their timing of formation relative to deposition of the detached sandbody as well as their process of formation. Syndepositional detachment mechanisms include flow transformation associated with slope failure (Class 1), turbidity current erosion (Class 2), and contourite deposition (Class 3). Post-depositional detachment is related to subsequent erosive processes and truncation of the pre-existing sandbody, either by submarine channels (Class 4), mass-transport events (Class 5), post-depositional sliding or faulting (Class 6) or bottom currents (Class 7). Examples of each of these mechanisms are identified on the modern seafloor, and show that detached sandbodies can form at different locations along the continental slope and rise (from upper slope to basin floor), and between or within different architectural elements (i.e., canyon, channels and lobes). This variation in formation style results in detached sands of highly variable sizes (tens to hundreds of kilometres) and geometries across and along the depositional profile, which are dependent upon the erosive and/or depositional processes involved, as well as the seafloor topography of the area in question. Whilst modern seafloor systems may not always represent the final stratigraphic architecture in the subsurface, they provide important insights into the development of detached sandbodies and therefore serve as potential analogues for subsurface stratigraphic traps.

The Adelaide Rift Complex is a large sedimentary superbasin in South Australia that formed resultant of Rodinia’s breakup and subsequent evolution of the Australian passive margin of the Pacific basin. It holds a globally significant and exceptionally well-preserved Neoproterozoic–early Cambrian succession. Much work has been done over the last century describing the lithostratigraphy and sedimentology of this vast basin. The rift complex contains evidence for major changes in Earth’s systems, yet, the rocks are poorly dated, and the sediment provenance, and link with tectonic evolution, is remarkably poorly known. This work provides a centralised database of the currently available, and previously unpublished, detrital zircon geochronology for the Neoproterozoic of the Adelaide Rift Complex, highlighting where the available data is from, and the stratigraphic and spatial gaps in our knowledge. By subjecting the U–Pb detrital zircon data to data analytical techniques, we provide a first look overview of the change in provenance, and subsequently (generalised) palaeo-tectonogeography that this suggests during the Neoproterozoic. These data show a change from dominantly local sources in the middle Tonian, to dominantly far-field sources as the rift-basin develops over time. The Cryogenian icesheets punctuate this with an ephemeral return to more local sources from nearby rift shoulders. This effect is particularly apparent during the Sturtian Glaciation than in the younger Marinoan Glaciation. In the Ediacaran, we see an increasingly stronger influence of younger (<700 Ma) detrital zircons from an enigmatic source that we interpret to be from southern (i.e. Antarctic) sources. We also note that we see a slight shift in the late Mesoproterozoic age peaks, from ca. 1170 Ma to ca. 1090 Ma, with a corresponding decrease in older ca. 1600 Ma detritus. This work forms the basis of continuing work to improve our understanding of the geochronology, provenance and palaeo-tectonogeography of the Adelaide Rift Complex.

The hyperthermal events of the Cenozoic, including the Paleocene-Eocene Thermal Maximum, provide an opportunity to investigate the potential effects of climate warming on marine ecosystems. Here, we examine the shallow benthic marine communities preserved in the late Cretaceous to Eocene strata on the Gulf Coastal Plain (United States). In stark contrast to the ecological shifts following the end-Cretaceous mass extinction, our data show that the early Cenozoic hyperthermals did not have a long-term impact on the generic diversity nor composition of the Gulf Coastal Plain molluscan communities. We propose that these communities were resilient to climate change because molluscs are better adapted to high temperatures than other taxa, as demonstrated by their physiology and evolutionary history. In terms of resilience, these communities differ from other shallow-water carbonate ecosystems, such as reef communities, which record significant changes during the early Cenozoic hyperthermals. These data highlight the strikingly different responses of community types, i.e., the almost imperceptible response of molluscs versus the marked turnover of foraminifera and reef faunas. The impact on molluscan communities may have been low because detrimental conditions did not devastate the entire Gulf Coastal Plain, allowing molluscs to rapidly recolonise vacated areas once harsh environmental conditions ameliorated.

The Adelaide Superbasin (Adelaide Rift Complex, Stuart Shelf, Torrens Hinge Zone, Coombalarnie Platform, and Cambrian Stansbury and Arrowie Basins) is a vast sedimentary basin in southern Australia that initiated due to the break-up of central Rodinia and, evolved into the Australian passive margin on edge of the Pacific Basin. Rocks within it contain evidence for the evolving earth system through the Neoproterozoic, including type sections of the Ediacaran fauna, Sturtian and Marinoan glaciations, and the GSSP for the base of the Ediacaran period. Much research over the last century has unravelled the lithostratigraphy and sedimentology of the basin. Despite this, the rocks are poorly dated, and their sedimentary provenance and link with tectonic geography is poorly known. This poor chronology hampers global and local efforts to gain a detailed understanding and chronological framework of the interplay between tectonics and momentous changes to the earth system during this time. This paper presents a comprehensive database of detrital zircon geochronology and review of geochronology for the Neoproterozoic of the Adelaide Superbasin, highlighting the stratigraphic, and spatial locations of available data. In the north of the basin, zircons were sourced locally in the initial stages of rifting, ca. 830 Ma—from the adjacent Gawler Craton and Curnamona Province. During the late Tonian, detritus was transported along graben from the north-west, from the Musgrave Orogen, as the rift basin developed during the opening of the nascent Pacific Ocean. Cryogenian icesheets punctuate the detrital record with an ephemeral return to more localised rift shoulder sources. In the Ediacaran, there is an increasing influence of younger (<740 Ma) detrital zircon from an enigmatic source that we interpret to be from southern (i.e. Antarctic) sources, with a corresponding shift in the late Mesoproterozoic age peaks, from ca. 1180 Ma to ca. 1090 Ma, and corresponding decrease in older, ca. 1600 Ma, detritus. These changes in sediment source reflect the changing tectonic geography and large-scale environmental influence of the Cryogenian glaciations as the basin evolved from a local rift, to a larger rift basin and finally to a continental margin, with sedimentary input becoming increasingly restricted over time.

Little is known about the geological history of the Glorieuses seamount including basic information about its age and origin related to the regional evolution of the southern tip of the Somali Basin. This study focused on describing and reconstructing the long-term stratigraphic evolution of the Glorieuses seamount (SW Indian Ocean) to identify the mechanisms that have occurred through time to finally shape the emerged modern islands. Distinct terrace levels, currently submerged along the flanks of the seamount and surrounding seamounts, have already been interpreted as resulting from successive carbonate development and back-stepping episodes over the last 62 Myr. New isotopic and biostratigraphic dating on the flanks of the seamount, coupled with sequence stratigraphic interpretation of seismic profiles acquired in the adjacent basin, provide new constraints for the Late Cretaceous and Cenozoic vertical evolution of the seamount topped by carbonate platforms and sedimentation in the surrounding deep basin. Even if starved steep slopes prevent a straightforward source-to-sink continuity between the platform and the basin domains, our findings propose a consistent chronostratigraphic framework for the identified seismic markers and sequences in the deep basin, and discuss a long-term geological model that includes the main driving factors behind deposition (volcanic events, subsidence vs uplift phases, climate and hydro-dynamism changes) and their quantitative impact on the evolution of the isolated carbonate sedimentary system. Our results show that: (i) the Glorieuses volcanic seamount emerged from two successive Late Cretaceous magmatic pulses, firstly during the Turonian, then during the Maastrichtian (ii) at least two potential uplift phases are recognized during the Tertiary (Paleogene and/or the Eocene and Tortonian); (iii) basinal sedimentation recorded an abrupt change probably related to major regional hydro-dynamical changes in Late Eocene times in the Western Indian Ocean; (iv) the export of sediments from the platform towards the basin (numerous gravity flow processes) is strongly enhanced after the Mid Miocene, and is probably linked to the onset of the Asian monsoon winds and bipolar circulation. Finally, the Glorieuses seamount, although located in the vicinity of the Comoros islands, appears to have a much longer history and is geologically more comparable to the nearby Seychelles. This long-term study has enabled us to associate the Glorieuses seamount with the SSE-NNW Madagascar-Seychelles alignment rather than with the Comoro hot spot evolution.