An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.
Abstract Two separate and distinct diamictite‐rich units occur in the mixed carbonate‐siliciclastic Polarisbreen Group, which comprises the top kilometer of >7 km of Neoproterozoic strata in the northeast of the Svalbard archipelago. The platformal succession accumulated on the windward, tropical to subtropical margin of Laurentia. The older Petrovbreen Member is a thin glacimarine diamictite that lacks a cap carbonate. It contains locally derived clasts and overlies a regional karstic disconformity that was directly preceded by a large (>10‰) negative δ 13 C anomaly in the underlying shallow‐marine carbonates. This anomaly is homologous to anomalies in Australia, Canada and Namibia that precede the Marinoan glaciation. The younger and thicker Wilsonbreen Formation comprises terrestrial ice‐contact deposits. It contains abundant extrabasinal clasts and is draped by a transgressive cap dolostone 3–18 m thick. The cap dolostone is replete with sedimentary features strongly associated with post‐Marinoan caps globally, and its isotopic profile is virtually identical to that of other Marinoan cap dolostones. From the inter‐regional perspective, the two diamictite‐rich units in the Polarisbreen Group should represent the first and final phases of the Marinoan glaciation. Above the Petrovbreen diamictite are ∼200 m of finely laminated, dark olive‐coloured rhythmites (MacDonaldryggen Member) interpreted here to represent suspension deposits beneath shorefast, multi‐annual sea ice (sikussak). Above the suspension deposits and below the Wilsonbreen diamictites is a <30‐m‐thick regressive sequence (Slangen Member) composed of dolomite grainstone and evaporitic supratidal microbialaminite. We interpret this sabkha‐like lagoonal sequence as an oasis deposit that precipitated when local marine ice melted away under greenhouse forcing, but while the tropical ocean remained covered due to inflow of sea glaciers from higher latitudes. It appears that the Polarisbreen Group presents an unusually complete record of the Marinoan snowball glaciation.
Continental deposits of lacustrine carbonates from pre-salt reservoirs in the offshore basins of Southeastern Brazil are key contributors to the country's oil production. This study assesses the structural and geochemical properties of the Barra Velha Formation, the main hydrocarbon reservoir in the Santos Basin. The role of local/regional tectonic evolution on the deposition of the Barra Velha Formation was investigated using 2D seismic sections and structural restoration. Major element concentrations, stable isotope analyses (C, O) and radiogenic Sr isotope ratios were conducted on carbonate samples from a sidewall borehole from the Iracema region of the Tupi oil field, allowing the evaluation of diagenetic influences and structural controls on element distribution. Values of δ13C and δ18O are consistent with those found in previous studies of the Santos Basin, indicating that primary seawater isotopic signals may have been largely conserved. Other observations support a low degree of alteration, such as high Ca/Mg in most samples and low concentrations of Mn and Fe. High 87Sr/86Sr values may implicate a significant contribution of Sr from continental erosion or terrestrial sources with a minor contribution from less radiogenic sources (volcanic?). During diagenesis, Rb can be incorporated into secondary mineral phases or associated clay materials. Results indicate that the mapped fault structures may have contributed to local diagenetic alteration, as the fault system may have served as a conduit for Rb and Sr transport while simultaneously promoting the deposition of Mg-clays in the area. However, there is no indication of hydrothermal overprint.
Abstract Correlations within and between Precambrian basins are heavily reliant on precise dating of volcanic units (i.e., tuff beds and lava flows) in the absence of biostratigraphy. However, felsic tuffs and lavas are rare or absent in many basins, and direct age determinations of Precambrian basaltic lavas have proven to be challenging. In this paper, we report the first successful application of 40 Ar/ 39 Ar dating to pyroxene from a Neoproterozoic basalt unit, the Keene Basalt in the Officer Basin of central Australia. 40 Ar/ 39 Ar analyses of igneous pyroxene crystals yielded an age of 752 ± 4 Ma (mean squared weighted deviation = 0.69, p = 72%), which is underpinned by 40 Ar/ 39 Ar plagioclase age (753.04 ± 0.84 Ma) from the basalt. This age is significant because the Keene Basalt is one of the very few extrusive igneous rocks identified within the Neoproterozoic successions of central Australia and is potentially an important time marker for correlating the Neoproterozoic stratigraphy within, and beyond, the central Australian basins. Our geochronological and geochemical data show that the Keene Basalt, which is characterized by enriched elemental and Nd‐Pb isotopic signatures, is strikingly similar to, and coeval with, the 755 ± 3 Ma Mundine Well Dolerite in northwestern Australia. Here we suggest that both are part of the same large igneous province (~6.5 × 10 5 km 2 ) related to breakup of the supercontinent Rodinia. This study demonstrates the potential of pyroxene 40 Ar/ 39 Ar geochronology to date ancient flood basalts and to provide pivotal time constraints for stratigraphic correlations of Precambrian basins.
Abrupt vertical changes are commonplace in sedimentary sequences and a basis for how we subdivide the stratigraphic record. These stratigraphic shifts may be obvious, such as when they are manifested in sharp differences in lithology or an erosional surface, or they may be subtle, such as when they
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)