Postglacial Shales Research Articles

Nilsen Plateau, Transantarctic Mountains, Antarctica: an update on the Gondwana stratigraphy and structure

ABSTRACT A revised and updated geological map of the Nilsen Plateau and a new geological map of the nearby Mt. Hassel-Mt. Bjaaland region are presented. The Gondwana succession, overlying pre-Devonian basement rocks, consists of Permian glacial beds, post-glacial shales and siltstones, deltaic sandstones, and coal measures succeeded by Triassic flood plain deposits. Jurassic dolerite sills intrude basement rocks and the Permo-Triassic beds. High-angle reverse faults displace Permian strata at the southern end of the range, and post-Jurassic normal faulting affected the whole range.

Evolution of paleo-weathering during the late Neoproterozoic in South China: Implications for paleoclimatic conditions and organic carbon burial

The late Neoproterozoic was a key transition time for Earth, involving turbulent fluctuations of the atmosphere-ocean system including Snowball Earth events, supercontinent reorganisation, and revolution of the nutrient cycle, all of which could have potentially triggered the development of increasingly complex life forms. To further investigate the temporal variations of paleoclimatic conditions and to better understand the burial of organic carbon during this interval, comprehensive geochemical and mineralogical analyses were performed on upper Neoproterozoic samples from the Cryogenian Datangpo, the Marinoan-age Nantuo, and the Ediacaran Doushantuo formations in South China. Geochemical compositions indicate that sedimentary rocks from the Doushantuo Formation have a provenance from granodiorite-like rocks, and the Datangpo samples likely have a mixed provenance of basalt and granite. The chemical index of alteration (CIA) values of black shales from the Datangpo and Doushantuo formations are in a similar range (65–85), indicating moderate chemical weathering under a relatively warm and humid climate. However, conspicuous low Rb/Sr ratios and enrichment of chemically immature minerals for the uppermost Doushantuo samples, are indicative of elevated physical erosion during the terminal Ediacaran. Furthermore, a negative correlation between corrected CIA (CIAcorr) and total organic carbon (TOC), and a positive correlation between organic carbon isotopic compositions (δ13Corg) and TOC, imply that supercontinent-related nutrient supply and concomitantly elevated marine primary productivity played an important role in organic carbon burial during the Ediacaran-Cambrian transition. In contrast, elevated chemical weathering under increased temperatures and intensity of the hydrological cycle occurred during the deposition of post-glacial shales from the Datangpo Formation and Member II of the Doushantuo Formation. Enhanced primary productivity fostered by chemical weathering-released nutrients and sequestration of organic carbon by pedogenic clay minerals generated under such climatic conditions, and probably redox condition variations, are inferred to have been the main control on organic carbon burial during this interval.

Mixed carbonate–siliciclastic sedimentation in forebulge grabens: An example from the Ediacaran Bambuí Group, São Francisco Basin, Brazil

Mixed carbonate–siliciclastic successions have been described in multiple Phanerozoic sedimentary settings recording the dynamic interplay of tectonics, eustasy, climate, in situ carbonate production, and variations in siliciclastic sediment supply. The Ediacaran Bambuí 1st-order sequence (i.e., Bambuí Group) covers most of the intracratonic São Francisco basin (southeast Brazil) and encompasses thick packages of carbonate and fine- to coarse-grained siliciclastic strata. Recording a marine foreland basin stage that developed in the São Francisco plate during the Neoproterozoic–early Paleozoic West Gondwana assembly, the Ediacaran deposits unconformably overlie Archean–Paleoproterozoic metamorphic assemblages of the Sete Lagoas basement high and fill a series of kilometer-long grabens in the southern São Francisco basin. Seismic data reveal that these troughs developed through the extensional reactivation of ancient basement structures along with the tectonically driven forebulge uplift of the Sete Lagoas high, in the early evolutionary stages of the Bambuí basin cycle. Based on the detailed description of continuous drill cores of a well recently drilled during hydrocarbon exploration campaigns, we recognized two transgressive–regressive 2nd-order sequences preserved within one of the focused grabens: (i) Sequence 1 includes the glaciogenic deposits of the basal Carrancas Formation that grade upward into the carbonate ramp successions of the Sete Lagoas Formation; (ii) Sequence 2 contains the siliciclastic-dominated and deep water to deltaic strata of the Serra de Santa Helena Formation and passes upward into peritidal carbonates of the Lagoa do Jacaré Formation. These sedimentary successions encompass suites of retrogradational, aggradational, and progradational lower-rank cycles and are bounded by erosional surfaces. Regional seismic interpretation, well data, and the available literature indicate that most of these deposits and their correlatives are continuous over most of the Sete Lagoas basement high, showing remarkable variations within the studied forebulge grabens. Our analysis demonstrates that the depositional architecture of Sequence 1 and Sequence 2 marks the interplay of successive episodes of tectonically driven forebulge uplift and regional eustatic and climatic changes. The latter are mainly recorded by the lower second-order sequence (Sequence 1), which contains typical sedimentary features deposited in the aftermath of an important glaciation affecting lower latitudes during the late Neoproterozoic. An additional local accommodation control is restricted to the forebulge grabens and is associated with episodic and local subsidence pulses. As indicated by lower- to higher-rank cycles, it has been responsible for the deposition of thicker transgressive to early regressive successions, a few gravitational flow deposits, and the development of the deltaic to shallow marine carbonate cycles of Sequence 2. The overall stratigraphic architecture of the studied successions suggests that several basin mechanisms operating during the Phanerozoic were also present in the late Neoproterozoic. The close association between organic-rich post-glacial shales, potential reservoir facies, and rift-related migration paths and hydrocarbon traps indicate that forebulge grabens represent important elements to consider in hydrocarbon exploration campaigns.

Chapter 67 Neoproterozoic glacial deposits of central Australia

Abstract There are two distinct stratigraphic levels of Neoproterozoic glacigenic deposits in central Australia, both Cryogenian in age, spread over an area greater than 2.5×10 6 km 2 . They were deposited in a once continuous intracratonic sag basin and are now preserved in four major structural basins: the Officer, Amadeus, Ngalia and Georgina Basins. In all four basins there are units that correlate with the (older) Sturt Tillite and equivalent glacial deposits in the Adelaide Rift Complex – the Sturt glaciation ( Preiss et al. 2011 ) – and with the (younger) Elatina Formation (Fm.) and equivalents – the Elatina glaciation ( Williams et al. 2008 , 2011 ). The clearest evidence for glacial activity is the occurrence of diamictites that contain clasts of lithologically diverse origin which are often striated, faceted and polished, and the occurrence of dropstones. Most glacial deposits were deposited in shallow marine to fluvio-lacustrine palaeoenvironments. In all basins, the Elatina glacial deposits are overlain (at least locally) by dolostone units that mark the onset of post-glacial transgression and contain unique sedimentary and geochemical features. The cap dolomite units are distinct from dolomite beds within glaciogenic sediments, and those that occur near the top of Sturt glacial units in the Amadeus (Areyonga Fm.) and eastern Officer Basins (Chambers Bluff Tillite). None of the central Australian glacial units have direct geochronological constraints. There are, however, radiometric dates for a Sturt glacial unit in the Adelaide Rift Complex (Wilyerpa Fm.) and post-glacial shales in the Amadeus Basin (Aralka Fm.), Stuart Shelf (Tapley Hill Fm.) and Adelaide Rift Complex (Tapley Hill Fm.) that indicate a c. 660 Ma age for the Sturt glaciation in Australia ( Kendall et al. 2006 , 2007 ; Fanning & Link 2008 ). The age of the Elatina glaciation in Australia is constrained only by the age of the Sturt glaciation and the presence of the Ediacara fauna in overlying strata of all the basins except the Ngalia Basin. Consequently, correlations have been mainly established by means of lithostratigraphy, chemostratigraphy, palynology, and to a lesser extent, stromatolite biostratigraphy, mainly on the successions above and below the glacial units. Results from each of the above techniques show a remarkable consistency, and indicate that the two major Cryogenian glacial episodes are of similar age across Australia.

Analysis of late Palaeozoic glacial to postglacial sedimentary successions in South Africa by geochemical proxies – Response to climate evolution and sedimentary environment

Records of incisive climate changes during the late Palaeozoic Era are best documented in sedimentary basins of South Africa. Glacial (Upper Carboniferous–Early Permian Dwyka Group and equivalents) and postglacial deposits (Early to Middle Permian Ecca and Beaufort Groups) are known from the Main Karoo Basin in South Africa, from the Kalahari Basin (Namibia and Botswana) and other late Palaeozoic Basins of south Gondwana. Glacial deposits comprise diamictites and tillites followed by postglacial shales with variable organic matter contents and intercalated carbonate horizons. This study covers the final glaciation phase (Upper Pennsylvanian–Asselian/Sakmarian) and the transition into the postglacial stage of the Middle to Late Permian. For sedimentary successions from the Karoo and eastern Kalahari Basin element geochemical proxies (Chemical Index of Alteration, Zr/Ti, Rb/K, V/Cr and organic carbon content) supported by mineralogical data record pronounced variation in sedimentary conditions. These took place in the course of glacier retreat at the Dwyka/Ecca boundary where climate and weathering processes are known to have changed. The proxy signals indicate an increase in sea level and salinity, and a shift in redox conditions from oxygenated to temporary anoxic/euxinic conditions. Due to the synchronicity of these events, distant sampling localities in the Karoo Basin can be correlated. Despite facies differences between the Eastern Kalahari and Karoo Basin, the geochemical proxies point to a comparable climatic evolution during the postglacial phase. For the Early Permian, proxy signals indicate warm–humid conditions in the Karoo and Kalahari Basin. During the Middle Permian increased aridity hampered chemical weathering processes as indicated by decreasing CIA values. Results show that: (i) proxies reflect provenance, synsedimentary redox regime and late diagenetic alteration, (ii) by combination of different independent geochemical proxies primary conditions of sedimentation and climate can be deduced, (iii) the geochemical proxies applied are independent of local environments and (iv) the data reveal supra-regional or even hemispherical changes in climate and sedimentary conditions. Thus geochemical proxies can be used as reliable markers for basin wide correlations. Knowledge about synchronicity on basin scale contributes to an improved understanding of SW Gondwana's interaction with and influence on global climatic evolution.

Stratigraphy, correlation and sedimentary history of Adelaidean (late Proterozoic) basins in Australia

Adelaidean (late Proterozoic) time is represented by the Precambrian portion of the thick sequence in the Adelaide geosyncline (its stratotype basin) in South Australia. Comparable Adelaidean sedimentation, including the deposition of glaciogenic sediments, is recorded in other intracratonic and epicratonic basins extending northwards across Australia. In the Adelaide geosyncline, which lies to the east of the Gawler craton and its cratonic platform cover (the Stuart shelf), up to 15 km of mainly shallow-water sediments accumulated, partly in intracratonic troughs and perhaps partly on a miogeoclinal continental shelf. The earliest sedimentation is associated with rifting and basic volcanism, with an estimated age of ca. 1100 Ma. It commenced with basal blanket sands, shelf carbonates and basaltic volcanics, followed by mixed carbonate, clastic and evaporitic beds (Callanna Beds). The overlying Burra Group consists of a number of west-derived, possibly deltaic cycles and magnesite-bearing platform carbonates. The unconformably overlying late Adelaidean Umberatana Group contains the lower glacial beds (including the Sturt Tillite), interglacial siltstone (dated at about 750 Ma) and carbonate, and the upper glacial beds. The post-glacial Wilpena Group commences with a distinctive ‘cap’ dolomite above the upper glacials, and contains mainly fine to medium clastics and the Ediacara metazoan assemblage. Late Adelaidean sedimentation patterns are sufficiently similar in the Officer, Amadeus, Ngalia and Georgina basins of central Australia, and in the Kimberley Region, Western Australia, to suggest interconnection with the Adelaide geosyncline to the south. In the Amadeus basin, the early Adelaidean sequence is younger than about 1050 Ma, but cannot be directly correlated with the early Adelaidean formations of the Adelaide geosyncline. However, the late Adelaidean glacial and younger sequences are readily correlated; they include the lower glacial Areyonga Formation, and the Arumbera Sandstone I containing elements of the Ediacara assemblage. In the Georgina and Ngalia basins, mainly late Adelaidean sequences, including thick glacials, can mostly be correlated with the Amadeus basin. In the Kimberley Region, earliest Adelaidean clastics may be represented, but only the late Adelaidean glaciogenic and associated sediments can be correlated with confidence. The tillites commonly overlie glaciated pavements. The lower glacials are represented only in a graben, while the upper glacials and succeeding sequence are more widespread. Post-glacial shales have been dated at ca. 670-640 Ma. The dominantly carbonate and clastic sequences of the Bangemall basin, Western Australia, cannot be directly correlated with any type Adelaidean, but geochronology suggests an earliest Adelaidean age (ca. 1050 Ma).