Search

Palaeogene passive margin sediments on the US mid-Atlantic coastal plain provide valuable insight into facies interaction and distribution on mixed carbonate–siliciclastic shelves. This study utilizes well cuttings, outcrop, core, and seismic data to document temporal and spatial variations in admixed bryozoan-rich skeletal carbonates and sandy siliciclastic units that were deposited on a humid passive margin located in the vicinity of a major marine transition zone. This zone was situated between north-flowing, warm waters of the ancestral Gulf Stream (carbonate dominated settings) and south-flowing, cold waters of the ancestral Labrador Current (siliciclastic dominated settings). Some degree of mixing of carbonates and siliciclastics occurs in all facies; however, siliciclastic-prone sediments predominate in nearshore settings, while carbonate-prone sediments are more common in more open marine settings of the inner shelf break and deep shelf. A distinctive dual-break shelf depositional profile originated following a major Late Cretaceous to Palaeocene transgression that drowned the earlier shallow platform. This profile was characterized by prominent mid-shelf break dividing the shallow shelf from the deep shelf and a major continental shelf/slope break. Incomplete filling of available accommodation space during successive buildup of the shallow shelf preserved the topographic break on this passive margin. Storm wave base also contributed to the preservation of the dual-break shelf geometry by beveling shallow shelf sediments and transporting them onto and seaward of the mid-shelf break. Sediment fines in deep shelf facies were produced in place, transported downdip from the shallow shelf by storm ebb currents and boundary currents, and reworked from adjacent areas of the deep shelf by strike-parallel boundary currents. Regional climate and boundary currents controlled whether carbonate or siliciclastic material was deposited on the shelf, with warmer waters and more humid climates favouring carbonate deposition and cooler, more arid conditions favouring glaucony and siliciclastic dominated deposition. Continuous wave and current sweeping of the shallow shelf favoured deposition of mud-lean facies across much of the shallow shelf. Skeletal components in much of the carbonate-rich strata formed in warm, nutrient-rich subtropical waters, as indicated by widespread occurrences of larger benthic foraminifera and molluscan assemblages. These indicators of warm water deposition within the bryozoan-mollusk-rich carbonate assemblage on this shelf provide an example of a warm water bryomol assemblage; such facies generally are associated with cooler water depositional settings. [ABSTRACT FROM AUTHOR]

The sequence stratigraphy and facies of the Paleogene in the subsurface of the Albemarle Basin, North Carolina was defined using 1600 thin sections of plastic impregnated well cuttings from 24 wells, wireline logs, biostratigraphic data, and seismic data. The facies formed in the transition zone between warm subtropical and temperate conditions on a swell-wave dominated, open shelf exposed to major boundary current activity. The shelf has a distinctive seismic profile consisting of a shallow inner shelf, inner-shelf break, deep shelf (depths in excess of 200 m), and the continental slope. The inner shelf was characterized by distinctive quartz sand and sandy mollusk facies inshore, passing seaward into a broad, wave-swept abrasional shelf, and then into storm-influenced bryozoan–echinoderm limestones to depths of several tens of meters. Argillaceous lime mud (marl) deposition was widespread across the deep shelf, extending onto the inner-shelf during major highstands.Sediment thickness trends were controlled by greater differential subsidence of crustal blocks within the Albemarle Basin, which considerably modified but did not obliterate the effects of eustatic sea level changes in this passive margin setting. Five supersequences were identified on seismic and in wells, each consisting of multiple regionally identifiable sequences. The Paleocene supersequence is dominated by widespread marl deposition, reflecting shelf flooding into the Late Paleocene thermal maximum. This warming corresponds with widespread inner-shelf skeletal carbonate deposition from the Late Paleocene through the Middle Eocene. The two Eocene supersequences identified are dominated by bryozoan–echinoderm-rich carbonates that formed a seismically definable sediment buildup 50 km wide by 100 m thick across the deepest inner-shelf during the Lower to early Middle Eocene. Middle to Upper Eocene supersequence highstand sequences indicate increased progradation and greater mixing of shelf carbonates with nearshore siliciclastics, likely in response to lowering sea-levels and cooling climate. The two Oligocene supersequences identified are dominated by coarse siliciclastic sand that is heavily admixed with mollusk-foraminifer-dominated carbonates. Rapid flooding, followed by extensive progradation of shallow shelf sediments in Oligocene sequences reflects continued eustatic lowering driven by the onset of icehouse climatic conditions. Increased incision and reworking of deep shelf sediments during Oligocene supersequence highstands resulted from increased ancestral Gulf Stream boundary current activity during icehouse times.Key elements controlling facies distribution through time include significant wave energy, which controls the width and distribution of inner-shelf facies belts through the degree of water bottom abrasion and winnowing. Boundary currents also are capable of large-scale reworking of deep shelf sediment; they also provide a mechanism for widespread inner-shelf hardground development during major transgressions. In addition, boundary currents play a major role in stabilizing local climate by buffering seasonal temperature variations, directly influencing which carbonate grain producers can thrive in this transition zone setting. [Copyright &y& Elsevier]

Schimmel, M., Kowalewski, M. & Coffey, BP. 2011: Traces of predation/parasitism recorded in Eocene brachiopods from the Castle Hayne Limestone, North Carolina, USA. Lethaia, Vol. 45, pp. 274-289. The Castle Hayne Limestone (Middle Eocene, North Carolina), noted for its diverse macro-invertebrate fossils, was sampled to assess if Early Cenozoic brachiopods from eastern North America record any traces of biotic interactions. Systematic surveys of two North Carolina quarries yielded 494 brachiopods dominated by one species: Plicatoria wilmingtonensis (Lyell and Sowerby, 1845). Despite subtle variations in taphonomy, taxonomy and drilling patterns, the two sampled quarries are remarkably similar in terms of quantitative and qualitative palaeoecological and taphonomic patterns. In both quarries, brachiopods contain frequent drillholes (24.5% specimens drilled). The majority of drillholes were singular, perpendicular to shell surface and drilled from the outside. Ventral valves were drilled slightly more frequently than dorsal ones, but site-selectivity in drilhole location was not evident. Larger brachiopods were drilled significantly more frequently than smaller ones. However, drillhole diameter did not correlate with brachiopod size. The drillholes are interpreted as records of 'live-live' biotic interactions, representing either predatory attacks or parasitic infestations or a combination of those two types of interactions. A notable fraction of specimens bear multiple drillholes, which is consistent with either parasitic nature of interactions or frequent failed predatory events. The high drilling frequency reported here reinforces other reports (from other continents and other epochs of the Cenozoic), which suggest that brachiopods may be an important prey or host of drilling organisms in some settings. The number of case studies reporting high frequencies of drilling in brachiopods is still limited and thus insufficient to draw reliable generalizations regarding the causes and consequences of these occasionally intense ecological interactions. □ Brachiopods, drilling parasitism, drilling predation, Eocene, North Carolina, taphonomy. [ABSTRACT FROM AUTHOR]

Examines the carbon isotope compositions of pedogenic calcite from paleosol formations spanning across the Triassic/Jurassic boundary to represent a rise in atmospheric carbon dioxide concentrations. Effects of the eruptions of the Central Atlantic Magmatic Province; Factors controlling carbon isotopic composition of soil carbonate; Stratigraphic units sampled; Calculation findings of atmospheric paleo-carbon dioxide.

Abstract: Exposed, uplifted Miocene carbonate sequences of the Cape Range, North West Cape, Western Australia, provide outcrop analogues of seismic sequences from offshore parts of the shelf. Facies include deep shelf marls (very fine and fine packstone), larger foram wackestone, floatstone and muddy rudstone, foram-coralline algal skeletal fragment packstone–wackestone (shallow seagrass facies), lagoonal wackestone–mudstone with scattered corals, and tidal flat laminites. The exposed Early Miocene units include the Mandu highstand, a sequence in the Tulki and one in the Middle Miocene Trealla Limestone. Sequences contain decameter scale (5–20 m thick), 4th-order parasequences evident on gamma ray logs and by facies stacking, that shallow and coarsen up; they appear to be due to eccentricity driven sea-level changes which may have been up to 50 m. Higher frequency meter-scale parasequences of deep water marl up into larger foram rudstone/floatstone (perhaps precession/obliquity) are evident at the base of the exposed Mandu section. These parasequences are not merely random storm deposits. This is indicated by the covariance of C and O isotopes, with the lighter values associated with deepening and deposition of deep shelf marls, and the heavier values being associated with shallowing and deposition of larger foram facies. The uplifted Miocene continental shelf sediments of the North West Cape preserve a record of eustasy, paleoclimate and paleoceanography and thus provide a window into factors affecting the shelf, that can be compared with coeval, better studied deep sea cores. [Copyright &y& Elsevier]