Selective organic matter preservation in "burn-down" turbidites on the Madeira Abyssal Plain

Oxidized intervals of five organic-rich Madeira Abyssal Plain (MAP) turbidites deposited during the Miocene, Pliocene, and Pleistocene all displayed comparable major loss of total organic carbon (TOC) (84 ± 3.1%) accompanied by a negative isotopic (δ13C) shift ranging from −0.3 to −2.9‰. Major but significantly lower loss of total nitrogen (Ntot, 61 ± 7.1%) also occurred, leading to a decrease in TOC relative to Ntot (C/Ntot) and a +1.3 to 2.7‰ Ntot isotopic (δ15N) shift. Compound specific isotopic measurements on plant wax n-alkanes indicate the terrestrial organic component in the unoxidized deposits is 13C-enriched owing to significant C4 contribution. Selective preservation of terrestrial relative to marine organic carbon could account for the δ13C behavior of TOC upon oxidation but only if a 13C-depleted component of the bulk terrestrial signal is selectively preserved in the process. Although the C/Ntot decrease and positive δ15N shift seems inconsistent with selective terrestrial organic preservation, results from analysis of a Modern eolian dust sample collected in the vicinity indicate these observations are compatible. Regardless of the specific explanation for these isotopic observations, however, our findings provide evidence that paleoreconstruction of properties such as pCO2 using the δ13C of TOC is a goal fraught with uncertainty whether or not the marine sedimentary record considered is “contaminated” with significant terrestrial input. Nonetheless, despite major and selective loss of both marine and terrestrial components as a consequence of postdepositional oxidation, intensive organic geochemical proxies such as the alkenone unsaturation index, U37K′, appear resistant to change and thereby retain their paleoceanographic promise

Defining Petrophysical Units of the Palmer Deep Sites from Leg 178

Palmer Deep, on the inner continental shelf southwest of Anvers Island off the Antarctic Peninsula, is a glacially overdeepened basin consisting of three subbasins. Two sites, 1098 and 1099, were drilled in the Palmer Deep area. A high-resolution porosity curve has been calculated from density data and subsequently plotted against the shipboard lithologic logs. These new data correspond accurately to the lithologic logs, magnetic susceptibility, and gamma ray attenuation (GRA) density data and offer information on the heterogeneity of the sediments. Petrophysical groups have been generated to investigate interrelationships between different physical attributes. To develop these petrophysical groups, crossplots of the available physical properties data were performed. The results for the GRA density and magnetic susceptibility crossplots demonstrate distinct clusters. Plotting the magnetic susceptibility and GRA density data logs (divided into these new petrophysical groups) against lithology provided information to subdivide the lithologic unit(s) into a series of petrophysical units

Deep-water turbidites as Holocene earthquake proxies: the Cascadia subduction zone and Northern San Andreas Fault systems

New stratigraphic evidence from the Cascadia margin demonstrates that 13 earthquakes ruptured the margin from Vancouver Island to at least the California border following the catastrophic eruption of Mount Mazama. These 13 events have occurred with an average repeat time of ?? 600 years since the first post-Mazama event ?? 7500 years ago. The youngest event ?? 300 years ago probably coincides with widespread evidence of coastal subsidence and tsunami inundation in buried marshes along the Cascadia coast. We can extend the Holocene record to at least 9850 years, during which 18 events correlate along the same region. The pattern of repeat times is consistent with the pattern observed at most (but not all) localities onshore, strengthening the contention that both were produced by plate-wide earthquakes. We also observe that the sequence of Holocene events in Cascadia may contain a repeating pattern, a tantalizing look at what may be the long-term behavior of a major fault system. Over the last ?? 7500 years, the pattern appears to have repeated at least three times, with the most recent A.D. 1700 event being the third of three events following a long interval of 845 years between events T4 and T5. This long interval is one that is also recognized in many of the coastal records, and may serve as an anchor point between the offshore and onshore records. Similar stratigraphic records are found in two piston cores and one box core from Noyo Channel, adjacent to the Northern San Andreas Fault, which show a cyclic record of turbidite beds, with thirty- one turbidite beds above a Holocene/.Pleistocene faunal «datum». Thus far, we have determined ages for 20 events including the uppermost 5 events from these cores. The uppermost event returns a «modern» age, which we interpret is likely the 1906 San Andreas earthquake. The penultimate event returns an intercept age of A.D. 1664 (2 ?? range 1505- 1822). The third event and fourth event are lumped together, as there is no hemipelagic sediment between them. The age of this event is A.D. 1524 (1445-1664), though we are not certain whether this event represents one event or two. The fifth event age is A.D. 1204 (1057-1319), and the sixth event age is A.D. 1049 (981-1188). These results are in relatively good agreement with the onshore work to date, which indicates an age for the penultimate event in the mid-1600 s, the most likely age for the third event of ?? 1500-1600, and a fourth event ?? 1300. We presently do not have the spatial sampling needed to test for synchroneity of events along the Northern San Andreas, and thus cannot determine with confidence that the observed turbidite record is earthquake generated. However, the good agreement in number of events between the onshore and offshore records suggests that, as in Cascadia, turbidite triggers other than earthquakes appear not to have added significantly to the turbidite record along the northernmost San Andreas margin during the last ?? 2000 years

The Western Irish Namurian Basin reassessed

Current basin models for the Western Irish Namurian Basin (WINB) envisage an elongate trough along the line of the present-day Shannon Estuary that was infilled with clastic sediments derived from a hinterland that lay to the W or NW. This paper argues for an alternative basin configuration with source areas to the SW supplying sediment to a basin where deepest water conditions were in northern County Clare. Rapid subsidence along the present-day Shannon Estuary ponded sediment in this area throughout the early Namurian and, only with the rapid increase of sedimentation rates within the mid-Namurian (Kinderscoutian Stage), were substantial amounts of sediment able to prograde to the NE of the basin. This alternative model better explains the overwhelming predominance of NE-directed palaeocurrents in the Namurian infill, but requires fundamental revisions to most aspects of current depositional models. Deep-water black shales (Clare Shale Formation) initially accumulated throughout the region and were progressively downlapped by an unconfined turbidite system (Ross Formation) prograding to the NE. This in turn was succeeded by an unstable, siltstone-dominated slope system (Gull Island Formation) characterized by large-scale soft-sediment deformation, which also prograded to the NE. In the northern-most basin outcrops, in northern County Clare, this early phase of basin infill was developed as a condensed succession of radiolarian-rich black shales, minor turbiditic sandstones and undisturbed siltstones. The new basin model envisages the northern exposures of County Clare to be a distal, basin floor succession whereas the traditional model considers it a relatively shallow, winnowed, basin margin succession. Later stages of basin infill consist of a series of deltaic cycles that culminate in major, erosive-based sandstone bodies (e.g. Tullig Sandstone) interpreted either as axial, deltaic feeder channels or incised valley fills genetically unrelated to the underlying deltaic facies. Within the context of the new basin model the former alternative is most likely and estimated channel depths within the Tullig Sandstone indicate that the basal erosive surface could have been generated by intrinsic fluvial scour without recourse to base-level fall. The northerly flowing Tullig channels pass down-dip into isolated channel sandbodies interbedded with wave-dominated strata that suggest the deltas of the WINB were considerably more wave-influenced than hitherto proposed. The retreat of the Tullig delta during sea-level rise saw the rapid southerly retrogradation of parasequences, as may be expected if the basin margin lay to the SW of the present-day outcrops

Sedimentary processes and architecture of Upper Cretaceous deep-sea channel deposits : a case from the Skole Nappe, Polish Outer Carpathians

Deep-sea channels are one of the architectonic elements, forming the main conduits for sand and gravel material in the turbidite depositional systems. Deep-sea channel facies are mostly represented by stacking of thick-bedded massive sandstones with abundant coarse-grained material, ripped-up clasts, amalgamation and large scale erosional structures. The Manasterz Quarry of the Ropianka Formation (Upper Cretaceous, Skole Nappe, Carpathians) contains a succession of at least 31 m of thick-bedded high-density turbidites alternated with clast-rich sandy debrites, which are interpreted as axial deposits of a deep-sea channel. The section studied includes 5 or 6 storeys with debrite basal lag deposits covered by amalgamated turbidite fills. The thickness of particular storeys varies from 2.5 to 13 m. Vertical stacking of similar facies through the whole thickness of the section suggest a hierarchically higher channel-fill or a channel complex set, with an aggradation rate higher than its lateral migration. Such channel axis facies cannot aggrade without simultaneous aggradation of levee confinement, which was distinguished in an associated section located to the NW from the Manasterz Quarry. Lateral offset of channel axis facies into channel margin or channel levee facies is estimated at less than 800 m. The Manasterz Quarry section represents mostly the filling and amalgamation stage of channel formation. The described channel architectural elements of the Ropianka Formation are located within the so-called Łańcut Channel Zone, which was previously thought to be Oligocene but may have been present already in the Late Cretaceous

Petrophysical and rock physics analyses for characterization of complex sands in deepwater Niger delta, Nigeria

Characterization of complex sand reservoirs in deepwater of Niger Delta was carried out through petrophysical and rock physics evaluation of well log data from three wells. Petrophysical analysis to determine clay volume, porosity, lithologies and hydrocarbon saturation were made. Rock physics was studied in velocity-porosity plane to analyze the influence of depositional and diagenetic features on the reservoirs. Cross-plots of different elastic parameters, using linear regression and cluster analysis, were generated for lithologic and fluid fill identification and to differentiate between the hydrocarbon bearing sands, brine sands and shale. Variance attribute was extracted on seismic time slice in order to image the complex sand distribution in the area. Three reservoirs of turbidite origin were identified within the upper fan to lower fan area. Petrophysical results revealed gas bearing reservoir units with less than 20% shale volume and porosity of 25-31%. Lambda-Mu-Rho (LMR) cross-plots for the reservoirs show gas saturated data cloud and trend. Ratio-Difference (R-D) cluster analysis of elastic rock properties shows a distinct trend and data cloud that represents lithofacies units and fluid fills. The study concludes that the reservoirs simulated contact cement and friable models with properties that ranged from highly porous, well sorted and poorly consolidated sand to fairly sorted and highly cemented sands. The results provide a model that increases the possibility of finding reservoir sand, while mitigating the risk involved in finding hydrocarbons

Palaeolimnology of Lake Sapanca and identification of historic earthquake signals, Northern Anatolian Fault Zone (Turkey)

Lake Sapanca is located on a strand of the Northern Anatolian Fault Zone (NAFZ, Turkey), where a series of strong earthquakes (Ms >6.0) have occurred over the past hundred years. Identifying prehistoric earthquakes in and around Lake Sapanca is key to a better understanding of plate movements along the NAFZ. This study contributes to the development of palaeolimnological tools to identify past earthquakes in Lake Sapanca. To this end several promising proxies were investigated, specifically lithology, magnetic susceptibility, grain size (thin-section and laser analysis), geochemistry, pollen concentration, diatom assemblages, 137Cs and 210Pb. Sedimentological indicators provided evidence for reworked, turbidite-like or homogeneous facies (event layers) in several short cores (<45 cm). Other indicators of sediment input and the historical chronicles available for the area suggest that three of these event layers likely originated from the AD 1957, 1967 and 1999 earthquakes. Recent changes in sediment deposition and nutrient levels have also been identified, but are probably not related to earthquakes. This study demonstrates that a combination of indicators can be used to recognize earthquake-related event layers in cores that encompass a longer period of time

Erosion and deposition in interplain channels of the Maury channel system

Large turbidity currents originating on the insular margin of southern lceland have flowed clown a 2 500 km-long pathway comprising rise valleys, unchanneled plains and segments of erosional and depositional deep-sea channels that are collectively called the Maury Channel system. Two steep interplain reaches of the channel have been eut up to 100 m through volcanogenic turbidites of probable La te Pleistocene age. Near-bottom observations with side-scan sonars and profllers across the upper channels (at 59°24\u27N, 18°50\u27W, 2 750 m depth) and at the lower interplain channel (around 56°23\u27N, 24°25\u27W, 3 340 m depth) defmed their structure and morphology. The upper channels, and a tributary to the lower channel, start as broad, shallow depressions that deepen and narrow downstream. The lower channel bas a pattern of anastomosing branches that probably evolved by head ward extension of low-angle tribu taries to the original sinuous channel, and its branches are at different stages of development. Several hundred bottom photographs show well-indurated rocks on channel walls and floors, with such flysch-like characteristics as cyclic graded bedding, clastic dikes, and syndepositional deformation. The lower-channel branches have been eut by turbidity currents with speeds of 5- 12 rn/sec., and combined discharges exceeding 1 x 106 m3 /sec. Bedrock erosion in and around the channels bas proceeded by intense corrasion and fluid stressing, and is marked by such small-scale effects as rock polishing, fluting, pot-holing and ledge recession. Rockfalls have caused retreat of steep channel walls, and conglomerate or pcbbly mudstone deposits suggest that debris flows have been locally active. Sorne coarse debris delivered by these processes and clay halls torn from semi-lithifled outcrops remain in the channels, but the channel f1ll is generally thin, with a patch y veneer of pelagie mud that bas accumulated since the last major turbidity current event. The surfaces of the unconsolidated s~diment have been smoothed and lineated, or moulded into seo ur moats and occasional fields of ripples, by thermohaline currents

The Carboniferous Southern Pennine Basin, UK

Many of the Carboniferous outcrops located in the Derbyshire region of the Peak District National Park, UK, have provided sites for both significant and pioneering research relating to the clastic sedimentology of marine palaeoenvironments, particularly so during the 1960s and 1970s when early models describing the sedimentary architecture of fluvio-deltaic, submarine slope and deep-marine submarine-fan sedimentation were first developed. The area was subject to hydrocarbon exploration from the 1920s to 1950s, which although unsuccessful in economic terms left a legacy of sub-surface data. Despite a long-history of sedimentological research, the deposits exposed at several classic localities in the Pennine Basin continue to broaden and challenge our current understanding of sedimentary processes to this day